Journal of Coastal Research      24      1A      250–259      West Palm Beach, Florida      January 2008




Nutrient Pollution and the Molluscan Death Record: Use of
Mollusc Shells to Diagnose Environmental Change
Chad Allen Ferguson

Department of Geology
University of Cincinnati
500 Geology Physics Building
Cincinnati, OH 45221-0013, U.S.A.
ferguscd@email.uc.edu

             ABSTRACT
             FERGUSON, C.A., 2008. Nutrient pollution and the molluscan death record: use of mollusc shells to diagnose envi-
             ronmental change. Journal of Coastal Research, 24(1A), 250–259. West Palm Beach (Florida), ISSN 0749-0208.

             Anthropogenic alteration is a persistent and growing problem in coastal marine ecosystems. Changes may have oc-
             curred and gone undetected in coastal systems, and analysis of the molluscan death assemblage provides a useful
             tool for detecting changes where long-term observational monitoring of living organisms has not been conducted.
              During the early 1980s, a series of experiments were conducted along Cross Bank, Florida Bay (inside Everglades
             National Park) to assess the effects of nutrient pollution by seabirds roosting on implanted marker posts on underlying
             sea grass beds. Over time, Halodule wrightii replaced Thalassia testudinum as the dominant sea grass at these altered
             sites. The present study focuses on the effects of this nutrient pollution on the molluscan death assemblage. Two
             parallel transects were established, one using altered sites as sampling stations and the other using unaltered, Thal-
             assia-dominated localities. A series of surficial push cores were collected along these transects (24 in all) to compare
             compositionally the assemblages on both transects. Results demonstrate detectable differences in the composition of
             molluscan death assemblages between altered and unaltered sites. Several molluscs were virtually excluded by the
             change in sea grass composition, and results suggest that differences in root and blade morphology of the two sea
             grass species are causal factors. Therefore, the results demonstrate that molluscan death assemblages effectively
             recorded environmental change through some two decades in the study area and that similar environmental changes
             may be diagnosed elsewhere in coastal ecosystems on the basis of assemblage composition.

             ADDITIONAL INDEX WORDS: Molluscan death assemblages, sea grass, carbonate bank, Florida Bay.




           INTRODUCTION                    al., 2002; BREWSTER-WINGARD and ISHMAN, 1999; BREW-
                                     STER-WINGARD, ISHMAN, and HOLMES, 1998; BREWSTER-
 In general, historical investigations of biotic change in ma-
                                     WINGARD, STONE, and HOLMES, 2001; CRONIN et al., 2001;
rine ecosystems have been limited to assessments of living
                                     KOWALEWSKI et al., 2000). In a similar vein, while others
organisms. However, because of the time-intensive nature of
                                     have recognized and attributed compositional differences in
this work, habitat monitoring may be infrequent and, even
                                     subfossil assemblages to physical or biological causes (PE-
in well-studied settings, observational records are only avail-
                                     TERSON, 1976; TREWIN and WELSH, 1972), the present in-
able for the second half of the last century. A ready source of
                                     vestigation is the first empirical attempt to assess the benthic
untapped ecological data, spanning both short- and long-term
timescales, is available in the form of molluscan shells de-       marine molluscan response to a known, temporally con-
posited in the accumulating sedimentary record. The com-         strained environmental change resulting from an instance of
position of molluscan death assemblages have been shown to        anthropogenically induced nutrient pollution.
reflect, with high fidelity, the living biotas from which they         Previous ecological research conducted on Cross Bank, lo-
were derived (e.g., FERGUSON, 2003; KIDWELL, 2001; KO-          cated at the southeastern margin of Everglades National
WALEWSKI et al., 2003; MILLER, 1988; PANDOLFI and MIN-          Park inside Florida Bay, caused a nutrient enrichment in lo-
CHIN, 1995; PETERSON, 1976). When a persistent environ-          cal benthic marine environments (POWELL, KENWORTHY,
mental change occurs, compositional changes should take          and FOURQUREAN, 1989). During experiments designed to
place in the associated biota, and evidence of this transition      assess the nocturnal feeding habits of wading birds (POWELL,
should in turn be recorded in the death assemblage.            SOGARD, and HOLMQUIST, 1987), it was observed that sea-
 In the emerging field of applied paleoecology (K OWALEW-         birds roosting on marker posts affected the productivity and
SKI, 2001), several researchers have used novel approaches        composition of underlying sea grass beds (POWELL, KENWOR-
to address ecological and environmental questions for which        THY, and FOURQUREAN, 1989). Original marker posts were
paleoecological techniques are well suited (e.g., ARONSON et       removed after the initial work was completed and a new tran-
                                     sect was established along Cross Bank, consisting of polyvi-
DOI:10.2112/06-0650.1 received 10 February 2006; accepted in revi-    nyl chloride posts topped with wooden blocks, to measure the
sion 11 September 2006.                          effects of nutrient pollution via excrement from pesciverous
                        Environmental Change and Molluscan Response                            251


avian occupants of the marker posts, royal terns (Sterna max-
ima) and double-crested cormorants (Phalacrocorax auritus).
This enrichment induced replacement of local populations of
turtle grass, Thalassia testudinum, with the more thinly
bladed sea grass, Halodule wrightii (POWELL et al., 1991).
The effects of this type of anthropogenic alteration of the
landscape were recognized during the mid-1980s and, as long
as the posts remain intact, birds supply the nutrients nec-
essary to maintain Halodule populations below. Areas in
Florida Bay where Halodule is present tend to have higher
levels of dissolved organic phosphorus in sedimentary pore
waters (FOURQUREAN, ZIEMAN, and POWELL, 1992). Exclu-
sion of Thalassia at nutrient-enriched sites is likely the result
of superior competition for light resources by Halodule
(DAWES and TOMASKO, 1988; FOURQUREAN et al., 1995).
 Molluscs are among the most promising members of the
benthos for historical investigations of marine coastal ecology
because they are (1) highly abundant; (2) possess robust skel-
etons; and (3) have well-understood ecological tolerances. Al-
                                     Figure 1. Map of the Cross Bank study area. Cross Bank is a carbonate
though groups such as benthic foraminifera have also been
                                     bank measuring 3000 m in length and up to 200 m in width. Experi-
shown to be highly effective indicators of salinity (BREWSTER-      mental sites consist of Halodule ‘‘halos’’, orientated toward the northwest,
WINGARD, ISHMAN, and HOLMES, 1998; HAYWARD et al.,            that are exaggerated spatially in this figure (actually measure 2 m by 4
2004), molluscs are known to be correlated with substrate         m). Control and experimental transect localities are paired at 200-m lat-
type and benthic vegetation (HECK, 1979; JACKSON, 1973;          eral intervals running from southeast to northwest along the bank crest.
                                     The background vegetation state of Cross Bank is moderate Thalassia
MILLER, 1988). Environmental factors such as dissolution of        coverage with occasional barren mud zones where sea grass has been
calcite, transportation, and breakage via predation may in-        removed by storm activities.
fluence the fidelity of a subfossil record for historical analysis.
For the purposes of the present investigation, these issues
are mitigated because Florida Bay is a low-energy, protected
carbonate setting and sea grass beds have been shown ex-         served background level (Figure 2). After 3 years of enrich-
perimentally to be centers of trapping and binding of sedi-        ment, Halodule became more abundant than Thalassia, mea-
ment, minimizing taphonomic loss, and providing refuge from        sured in grams of dry weight per square meter, and has
predation (ALMASI et al., 1987; JACKSON, 1973; PETERSON,         maintained that dominance at localities where nutrient ad-
1982; PRAGER and HALLEY, 1999; SCOFFIN, 1970). Molluscan         dition continues (i.e., where bird excrement addition persists).
death assemblages have been shown to provide faithful in-         Sampling for the current investigation was conducted in Oc-
dicators of the benthic environments from which they were         tober 2004. Therefore, the expectation is that conditions fa-
derived, recording transitions in the relative abundances of       vorable to altered molluscan fauna may have persisted for as
taxa on decadal timescales (e.g., FERGUSON, 2003; KIDWELL,        many as 18 years (1986–2004). Previous work in shallow ma-
2001). However, because of time averaging, the long-term         rine carbonate settings has shown that changes in the com-
preservation of transitions on this fine a scale may be com-        position of molluscan communities occurring over similar in-
promised (CARROLL et al., 2003; KIDWELL and BOSENCE,           tervals have been successfully recorded in the sedimentary
1991; KOWALEWSKI et al., 2000; MELDAHL, FLESSA, and            record (FERGUSON, 2003).
CUTLER, 1997). Therefore, the goal of this investigation is to        During the work of POWELL, KENWORTHY, and FOURQUR-
assess the usefulness of the molluscan death assemblages at        EAN, (1989), a 3000-m transect with bird posts at 100-m in-
the nutrient-enriched sites in Florida Bay as indicators of        tervals was developed along the crest of Cross Bank. For the
historical transitions in sea grass community composition as-       purposes of the present investigation, a 1200-m experimental
sociated with pollution.                         transect, with stations at 200-m intervals (six stations total
                                     per transect), was established using the sites of POWELL,
            STUDY AREA                    KENWORTHY, and FOURQUREAN (1989), along with a corre-
                                     sponding parallel ‘‘control’’ transect, with stations situated 50
  This investigation is set along the shallow ( 30-cm water       m N-NW of the control sites (Figure 1). This experimental
depth) crest of Cross Bank, inside the Everglades National        design was used to assess the nature of molluscan death as-
Park, in south-central Florida Bay (Figure 1). An investiga-       semblages at the Halodule-dominated experimental sites in
tion of the response of sea grass communities to nutrient en-       POWELL, KENWORTHY, and FOURQUREAN (1989) in compar-
richment (FOURQUREAN et al., 1995) showed that less than         ison to the unaltered, Thalassia-dominated sites. However,
a decade after placement of the bird posts, the standing crop       there is a 400-m gap between control and experimental sta-
of Thalassia at experimental, nutrient-enriched sites had         tions 5 and 6 because of an anomalous spit (likely the result
nearly been eliminated, while Halodule had attained a stand-       of a storm blowout [WANLESS, 1981]) protruding from Cross
ing crop density equal to that of Thalassia at its highest ob-      Bank where station 6 would have otherwise been.



                    Journal of Coastal Research, Vol. 24, No. 1A (Supplement), 2008
252                                   Ferguson


                                        posited in this upper decimeter of the sedimentary veneer on
                                        the basis of estimated sedimentation rates ranging between
                                         1 cm/y derived from Pb210 dating (HOLMES et al., 2001) and
                                        0.3–3 mm/y derived from C14 (STRASSER and SAMANKASSOU,
                                        2003) in this part of Florida Bay. Therefore, this sampling
                                        protocol is designed to obtain a sample of molluscan material
                                        that can be used to assess two decades of accumulated dif-
                                        ferences in surficial composition between death assemblages
                                        collected along the control and experimental transects.

                                        Sample Treatment
                                          Push-core samples, with an average volume of about 1.6 L
                                        of sediment, were wet-sieved to retrieve coarse material
                                        ( 150 m). This coarse fraction was dry-sieved through 2-
                                        mm mesh to extract all but the smallest shell material visible
                                        to the naked eye and it is from this 2-mm-size fraction that
                                        the data considered here were collected. All samples were
                                        uniformly sieved and picked so as to reduce the chance of bias
                                        in removing the smallest size class of taxa, such as those of
                                        the genera Bittiolum, Rissoina, and Schwartziella.
                                          Specimens were identified using standard malacological
                                        references (e.g., ABBOTT, 1974; REDFERN, 2001) and the au-
                                        thor’s previous experience working with tropical West Atlan-
                                        tic molluscs. The numbers of individual species were tallied
                                        for each sample and bivalve counts were halved to account
                                        for two retrievable skeletal parts per individual (GILINSKY
                                        and BENNINGTON, 1994). This is a more conservative ap-
                                        proach to assessment of abundance than counting left and
                                        right valves individually but is appropriate for counting shell
                                        material from bivalves that do not display taphonomic differ-
                                        entiation between valves and that are not subject to a high
                                        degree of postmortem transport.
                                          To be counted as a specimen, a shell had to (1) possess an
                                        umbo, for bivalves, and either an apex or aperture, depending
Figure 2. Mean standing crop of Thalassia testudinum and Halodule
                                        on the species, for gastropods; (2) possess greater than three-
wrightii. Mean values come from five experimental and control sites along
Cross Bank and are displayed against a time series. Values are reported     fourths of its original shell or valve; and (3) be preserved well
in grams dry weight of sea grass biomass per square meter and illustrate    enough to identify. The objective of this strict counting is to
a pattern of turnover of these two sea grass species at experimental sites   control for potential worker bias in identification and mini-
and stasis of Thalassia populations at control localities. Halodule       mize artificial differences between control and experimental
achieved equal biomass to Thalassia at experimental sites within 4 y of
initial nutrient loading and became dominant thereafter, while Halodule     samples.
was only rarely reported at control localities. Error bars about the mean     Shell counts make up the data matrices used during this
represent calculated standard error. Figure modified from Fourqurean et     investigation and, for multivariate analyses, abundance-
al. (1995).                                   based data were limited to the top 20 specimens common to
                                        both experimental and control series. The top 20 species were
                                        determined by the rank order of the sum of abundance totals
               METHODS                      from control and experimental groups and represent 89% of
                                        total specimens considered. Patterns illustrated here are ro-
Sampling
                                        bust at both higher and lower levels of inclusion; below the
 Sampling for this investigation was designed to assess the          top 20 threshold are taxa with a total abundance of less than
state of molluscan death assemblages deposited at experi-            50 specimens aggregated from all 24 push-core samples.
mental, Halodule-dominated sites of POWELL, KENWORTHY,             Abundance values for the top 20 molluscan taxa were trans-
and FOURQUREAN (1989) over some two decades vs. control             formed to a percentage of total within-sample specimen abun-
samples from a Thalassia-dominated environment that               dance for multivariate comparisons of samples (i.e., Q-mode
should more faithfully represent background conditions along          analysis), and this initial percentage transformation was fol-
Cross Bank. In all cases, a replicate pair of shallow push           lowed by a transformation to a percentage of the maximum
cores with diameters of 5.1 cm and a length of 15–25 cm were          abundance of each species found among all samples for com-
taken at each station, for a total of 12 per transect (24 in all).       parisons of species distribution (R-mode analysis). Use of per-
The molluscan response to environmental change is likely de-          centage data here is justified because overall per-sample



                       Journal of Coastal Research, Vol. 24, No. 1A (Supplement), 2008
                            Environmental Change and Molluscan Response                             253


Table 1. Mean specimen abundance and species richness from control
and experimental samples along Cross Bank. Specimen abundance and
species richness ( standard error) are both higher along the control tran-
sect, but only the difference in species richness is statistically significant.
Therefore, while abundance is comparable, species richness is negatively
affected by the transition to Halodule via nutrient enrichment.

            Control       Experimental      p-value

Richness      37.58  1.62     29.83   1.70     0.003*
Abundance     383.75  59.18    283.75   55.48     0.231
On the basis of independent samples t test, * indicates significance at p
 0.01, two-tailed test.



abundance varies despite holding sediment volume roughly
constant, and this likely reflects postmortem processes. This
second, ‘‘percent-maximum’’ transformation has been shown
previously to be effective in R-mode analyses for comparing
the abundance distributions of taxa with widely varying total
abundances (e.g., MILLER, 1988). Similarity matrices were de-
rived from these transformed data using the Bray–Curtis
similarity coefficient (B RAY and CURTIS, 1957). Nonmetric
                                         Figure 3. Ordination of molluscan death assemblages. Multidimensional
multidimensional scaling (MDS) and cluster analysis (un-
                                         scaling (MDS) analysis of push-core samples illustrates differences in as-
weighted pair-group method) were both used for comparisons            semblage composition of experimental vs. control samples in ordination
of samples (Q-mode), and cluster analysis was used for com-            space. Results show two distinct groups of samples, almost entirely de-
parisons of species (R-mode). The PRIMER 5 for Windows              pendent on whether a sample came from a control (C) or experimental
software package (PRIMER-E, Ltd.; Plymouth, United King-             (X) site, with the exception of the push-core samples from control station
                                         4. Stress values are calculated on the basis of the extent to which an
dom) was used for all multivariate analyses and SPSS 11.0             arrangement of samples is representative of the calculated distance
for Windows (SPSS, Inc.; Chicago, IL) was used for correla-            among samples. Calculated two-dimensional stress of this MDS analysis
tion procedures, such as Spearman Rank, and comparison of             is 0.12 (lower stress indicates a ‘‘better fit’’), which is below the threshold
means using independent samples t tests for normally dis-             of 0.20 generally accepted as indicative of whether an arrangement of
                                         ordinated data is representative of variation among samples (Clarke and
tributed variables and the Mann-Whitney U test for non-
                                         Warwick, 2001), and this conclusion is corroborated by similar patterns
parametric data.                                 displayed in Q-mode cluster analysis.

                 RESULTS
Specimens
                                         Cerithium muscarum, caused control station 4 to be more
  A total of 24 push-core samples from sites along Cross             similar compositionally to experimental stations (see below).
Bank yielded 8011 specimens, representing 74 species. Mean             Further dissecting the faunal patterns, a two-way cluster
richness among push-core samples from control sites was 38            analysis was constructed using both R-mode and Q-mode
taxa (from 4605 specimens), whereas mean richness at ex-             dendrograms, from the same top 20 data set as MDS analy-
perimental sites was 30 species (from 3406 specimens). Mean            sis, populated with symbols representing the percentage of
richness observed between control and experimental sites             maximum abundance attained for each species throughout
shows a significant difference, whereas differences in mean            the study area corresponding to a station (Figure 4). In this
abundance are insignificant (Table 1).                       analysis, replicate samples for each station were combined.
                                         Three groups of species are expressed in the R-mode cluster
Assemblage Results                                analysis, from top to bottom in the dendrogram:

 Assessment of samples from nutrient-enriched experimen-             ● Species that occur readily in the control, Thalassia-domi-
tal sites illustrates a molluscan death assemblage that is al-           nated, environments, represented most reliably by Ceri-
tered relative to assemblages collected at control sites, as            thium muscarum, but also including Carditamera flori-
demonstrated with MDS (Figure 3). Control and experimen-              dana and Pitar fulminatus.
tal samples plot separately from one another, indicating a            ● Species that are generally present at both control and ex-
clear compositional difference between the two transects.              perimental sites but demonstrate slight habitat preferenc-
 Both of the replicate samples from control station 4 were             es as illustrated by percentage maximum abundance val-
compositional outliers relative to other control samples and            ues arrayed in two-way cluster analysis. This second
bear a strong similarity to samples from the experimental              grouping includes important species such as Brachidontes
transect. The substrate at station 4C was, indeed, covered by            exustus and Bittiolum varium, which occur at higher abun-
the naturally dominant sea grass, T. testudinum, but the near            dance at nutrient-enriched, Halodule-dominated experi-
absence of a key diagnostic species, the high-spired gastropod           mental sites, and Chione cancellata and Modulus modulus,



                        Journal of Coastal Research, Vol. 24, No. 1A (Supplement), 2008
254                                   Ferguson


                                        Table 2. Life habits of common bivalves and gastropods from push-core
                                        samples collected on Cross Bank. Bivalves: Ep, epifaunal; Si, semiinfaunal;
                                        Ne, nestling; Is, infaunal siphonate; Ia, infaunal asiphonate; Su, suspen-
                                        sion feeding; Sd, surface deposit feeding; Sed, sedentary; Mo, actively mo-
                                        bile; Im, immobile; Sw, swimming; By, byssate; Un, unattached.
                                        Gastropods: Hrb, herbivorous; Pc, carnivorous predator; Ps, parasitic; Pb,
                                        browsing predator. All gastropod codes relate to feeding because all species
                                        discussed here are epifaunal. Length is maximum dimension reported from
                                        common references. References: (1) Abbott, 1974; (2) Redfern, 2001; (3)
                                        Todd, 2001a; (4) Todd, 2001b; and (5) Mikkelsen et al., 2004. Pinctada
                                        longisquamosa (*) had, until recently, been assigned to the genus Pteria;
                                        see Mikklesen et al., 2004 for a review of the most recent taxonomy. Life
                                        habit codes modified from Neogene Marine Biota of Tropical America pro-
                                        ject (NMITA: http://eusmilia.geology.uiowa.edu/).

                                                                   Attach- Length Refer-
                                            Species     Substrate  Diet   Mobility ment  (mm)  ence

                                        Bivalves
                                          Arcopsis adamsi   Ep     Su   Sed    By   13     2,3
                                          Brachidontes
                                           exustus      Ep/Si/Ne Su    Sed/Mo   By   19     1,3
                                          Carditamera
                                           floridana     Ia     Su   Sed/Mo   By   38     1,3
                                          Chione cancellata  Si/Is    Su   Mo     Un   44     1,3
                                          Cumingia
                                           vanhyningi    Is/Ne    Sd   Sed    Un   16.5    2,3
                                          Laevicardium
                                           mortoni      Is     Su   Mo     Un  27     2,3
                                          Limaria pellucida  Ep     Su   Mo/Sw   Un/By 26     2,3
                                          Pinctada
Figure 4. Two-way cluster analysis. Q- and R-mode clusters arrayed          longisquamosa*  Ep     Su   Im     By   63    2,3,5
with data points representing percentage of maximum abundance for          Pitar fulminatus  Is     Su   Mo     Un   22.5    2,3
each species. Sample replicates from each station were combined for the       Tellina alternata  Is     Sd   Mo     Un   58     2,3
Q-mode cluster analysis. Two Q-mode clusters are recognized with sam-        Transennella
ples (replicates combined) grouped according to whether a sample came         stimpsoni     Is     Su   Mo     Un    9    2,3
from an experimental (X) or control (C) site, with the exception of control
station 4, which incorporates species indicative of both transects, with           Species         Diet    Length (mm)    Reference
Cerithium being the strongest differentiating taxon. Three R-mode clus-
                                        Gastropods
ters are defined on the basis of the occurrence of taxa within Thalassia-
                                         Bittiolum varium            Hrb      6.5       2,4
dominated environments, Halodule-dominated environments, or both;
                                         Cerithium muscarum           Hrb     22        2,4
this is illustrated by abundance values arrayed on the figure. Maximum
                                         Costoanachis avara           Pc      22        1,4
abundance, within a sample, for each species is provided in parentheses
                                         Crepidula sp.             Su      13        1,4
next to the species name and is useful for determining which species are
                                         Eulithidium affine           Hrb      9        2,4
most important for comparisons of assemblage composition among sam-
                                         Modulus modulus            Hrb     11        2,4
ples. Species in the Thalassia-only cluster, with Cerithium muscarum be-
                                         Prunum apicinum            Pb      13.5       2,4
ing the most important, are present near their maximum abundance
                                         Rissoina sp.              Hrb      7        2,4
throughout the control transect but are not found above 20% of their
                                         Schwartziella bryerea         Hrb      6        2,4
maximum abundance (if at all) at experimental transect sites.


                                        above a certain size threshold cannot inhabit Halodule
 which are more likely to occur in higher abundance at con-
                                        blades. In addition to abundance, there are differences in
 trol sites.
                                        mean size between Cerithium specimens pooled from control
● Species that are present more ubiquitously at Halodule-
                                        and experimental sites. The maximum dimension of Ceri-
 dominated experimental sites and more sparsely at control
                                        thium was measured (apex to siphonal canal), and the mean
 sites. This third group is best represented by the micro-
                                        size of specimens collected from control sites is more than a
 grazing gastropod, Schwartziella bryerea, and the swim-
                                        millimeter larger than mean size ( standard error) from
 ming and actively burrowing bivalves, Limaria pellucida
                                        experimental sites: 6.96 0.49 mm vs. 5.95 1.14 mm with
 and Tellina alternata. The abundance distributions of mi-
                                        significance of p   0.022, derived from a Mann-Whitney U
 crograzing gastropods (Schwartziella, Rissoina, Eulithi-
                                        test. Cerithium specimens collected at experimental localities
 dium, and Bittiolum) at experimental stations suggest that
                                        rarely attained a size 10 mm (6.25% of experimental spec-
 exclusion of Cerithium is not related to its grazing feeding
                                        imens), which appears to be a threshold for those living on
 habits (Table 2).
                                        Halodule, whereas specimens of this size were comparatively
 Rather, dramatically low abundance of Cerithium at ex-            abundant at control localities (16% of control specimens). It
perimental sites (often present, but always 20% of it max-           is important to note that the sample size of Cerithium spec-
imum studywide abundance), while its far smaller guild-             imens was low, as a result of its reduced abundance, at ex-
mates seem unaffected, indicates that grazing gastropods            perimental sites (n   32 specimens) relative to the sample



                       Journal of Coastal Research, Vol. 24, No. 1A (Supplement), 2008
                       Environmental Change and Molluscan Response                       255


size from control sites (n   194 specimens) and is reflected      difference relates to relative densities of root structure. T.
in the higher standard error reported for the experimental        testudinum, the slow-growing, long-lived climax sea grass in
population.                               this region (DEN HARTOG, 1971), possesses a dense root–rhi-
  Overall, therefore, three trends emerge from consideration      zome layer capable of binding sediment to a depth of 10–20
of the biotic patterns: (1) Cerithium muscarum is highly         cm below the sediment–water interface. Large-scale biotur-
abundant in the death assemblage at control group sites but       bation is not possible in dense Thalassia beds and sediment
is never present at 20% of its maximum abundance at ex-         is stabilized during all but the most intense storm events
perimental sites; (2) Brachidontes exustus is highly abundant      (KALBFLEISCH and JONES, 1998). By contrast, H. wrightii, a
throughout the study area, but is consistently more abundant       fast-growing opportunist (FOURQUREAN et al., 1995), pos-
in the death assemblage at experimental sites than at control      sesses only a slight root system that is not suited to stabiliz-
localities; and (3) Small grazing gastropods, such as Bittiolum     ing sediment and as a result, experimental sites in the pres-
varium and Eulithidium affine, show the opposite pattern to        ent study possessed a soft substrate that was easily disturbed
Cerithium in that they were consistently more abundant in        and penetrated.
experimental than control locales.                     In light of these differences, it is apparent that larger, mo-
  As in MDS, sample site 4C groups outside of control sam-       bile molluscs preferring large, strong blade structures or sta-
ples in cluster analysis, and it can be seen here that this       ble sediment would be less likely to inhabit a Halodule-dom-
relationship is based primarily on the low abundance of C.        inated environment than those that are adaptable to small
muscarum and the relatively higher abundance of B. exustus        blades and active substrates. The characteristics of the mol-
(Figure 4). Interestingly, however, station 4C also clusters       luscs discussed here, and described in Table 2, are useful in
outside of the experimental group, suggesting that it is com-      assessing the reasons for the differential molluscan response
positionally unique relative to other samples.              to changes in sea grass community composition. Of interest
                                     are the feeding strategies of gastropods and the life habits of
            DISCUSSION                    certain bivalves. Six herbivorous gastropods of different sizes
                                     occur in the top-20 species from Cross Bank: C. muscarum,
  The results of this study illustrate the molluscan response      M. modulus, B. varium, Rissoina sp., S. bryerea, and E. affine.
to change in sea grass community composition, and demon-         Cerithium is the largest of this group and, as noted earlier,
strate that such a response is recorded in the accumulating       is the only highly abundant member of this guild that shows
molluscan death assemblage. The faunal differences de-          a strong decrease in abundance at experimental sites relative
scribed above are the basis for sample differentiation in the      to controls (Figure 4). The dramatic difference in Cerithium
multivariate analyses and demonstrate that assemblages de-        abundance, coupled with the observation that a significantly
posited at nutrient-enriched experimental sites are different      higher proportion of Cerithium from experimental sites falls
from those at control sites. The explanation for this difference     below the 10-mm size threshold, supports the argument that
likely relates to the structural characteristics of blades of H.     blade size is a likely control on the distribution of large graz-
wrightii relative to T. testudinum. In this case, nutrient en-      ing gastropods. On the basis of these results, it appears that
richment itself can largely be discounted as a controlling        Halodule blades cannot support the mass of a full grown Cer-
mechanism on the basis of recent research that has estab-        ithium and might not provide adequate blade surface area
lished bivalves, mussels and oysters in particular, as poten-      for foot attachment. Future work on this topic will include a
tial tools for pollution remediation. Representatives of these      comparison of size and mass among Cerithium and Modulus,
groups have been shown to remove heavy metal pollution and        the two most abundant large grazing gastropods in the re-
excess nutrient content from estuarine systems in both tem-       gion.
perate and tropical settings (GIFFORD et al., 2005). This          The infaunal bivalves, Carditamera floridana and Pitar ful-
seems intuitive when one considers that nutrient increases        minatus, which are generally found in Thalassia beds at low
would cause blooms in the microorganisms and algae that         abundances (Figure 5), are asiphonate, byssate and siphon-
grazing and filter-feeding molluscs consume and, therefore,        ate, nonbyssate, respectively (Table 2). Other byssate bi-
would provide a favorable habitat so long as extreme eutro-       valves Brachidontes exustus, Arcopsis adamsi, and Pinctada
phication did not develop.                        longisquamosa (previously identified as Pinctada radiata by
  Halodule and Thalassia possess distinct morphologies and       TURNEY and PERKINS [1972] and later reported as Pteria lon-
sediment controlling abilities in shallow marine environ-        gisquamosa by BREWSTER-WINGARD, STONE, and HOLMES
ments. An assessment of the average blade surface area of        [2001]; see MIKKELSEN et al. [2004] for the current taxonomic
Thalassia and Halodule was undertaken by MILLER (1988),         placement), either epifaunal and attached or of variable life
who found that, in his study area in St. Croix, U.S. Virgin       habit, seem to be numerically unaffected by the switch to
Islands, Thalassia averaged 0.25 cm2 of blade surface area,       Halodule. Bivalves with an infaunal life habit may be ill
whereas Halodule averaged 0.02 cm2. Although the absolute        equipped for the increased sediment mobility that is associ-
values of blade size are likely to vary considerably among        ated with the far less developed root–rhizome network of Hal-
different settings, the numbers from St. Croix reflect what is      odule relative to the dense network possessed by Thalassia.
certainly appreciable, a consistent difference in relative sur-       In summary, two trends are demonstrated: (1) Cerithium
face area of the two kinds of blades. Thalassia blades are        muscarum, the largest gastropod collected among the top-20
more robust relative to the wispy blades of Halodule and are       specimens, decreases dramatically in abundance with tran-
capable of supporting larger invertebrate grazers. A second       sition of the sea grass community to Halodule coverage,



                    Journal of Coastal Research, Vol. 24, No. 1A (Supplement), 2008
256                               Ferguson


whereas other smaller herbivorous gastropods were not af-        Table 3. Rank abundances of mollusc taxa from aggregate experimental
                                     vs. control samples. Species, aggregated from control and experimental
fected in the same way; and (2) bivalves, in general, were not
                                     transects, are ranked in order by percentage abundance within these spec-
dramatically affected by the loss of Thalassia as was Ceri-       imen pools. Any change to a species rank order between control and exper-
thium. However, some of the more abundant infaunal forms         imental settings is illustrated by the rank change column. Important
appear to have been ill-suited to Halodule-dominated set-        changes in rank, such as the decline in Cerithium abundance at experi-
tings, while epifaunal bivalves were largely unaffected.         mental sites, are apparent in both percentage abundance and change in
                                     rank. Spearman rank correlation coefficient shows a moderately strong
                                     positive correlation between lists, which illustrates a high degree of simi-
Detection of Faunal Change                        larity between faunas despite the decline of Cerithium muscarum at ex-
                                     perimental sites. This analysis illustrates that only a few of the common
  The results of this study suggest that assessments of mol-      species from Cross Bank were appreciably affected by the transition to a
                                     Halodule-based habitat.
luscan death assemblages can be used as indicators of tran-
sitions among sea grass species in the historical record. That
                                          Control              Experimental      Change
said, the signal of faunal change, in this case, is evidenced                                         in Rank
mainly by faunal subtraction, primarily the loss of C. mus-         %                             %    From
carum, rather than development of new faunal combinations        Abundance    Species    Rank    Species    Abundance Control
or the incorporation of ecological immigrants. Comparison of        19.90  Brachidontes     1   Brachidontes     28.50    0
rank-order species abundance compiled from push-core sam-               exustus          exustus
ples along control and experimental transects illustrates the        11.98  Cerithium      2   Bittiolum varium   12.87    1
                                            muscarum
high degree of taxonomic similarity between Thalassia and
                                       7.81  Bittiolum varium   3   Schwartziella     6.93    6
Halodule habitats (Table 3). Among the top-20 most abun-                             bryerea
dant species, notable shifts in rank abundance are recognized        7.77  Pinctada       4   Pinctada        6.37    0
for C. muscarum, P. fulminatus, C. floridana, Crepidula sp.,              longisquamosa       longisquamosa
E. affine, L. pellucida, and S. bryerea. A Spearman rank co-         5.68  Modulus molulus   5   Modulus modulus    4.77    0
                                       4.96  Chione        6   Eulithidium      4.54    5
efficient value of 0.714 (p   0.001 two-tailed test) was cal-            cancellata         affine
culated by comparing the differences in rank between mem-          4.02  Pitar fulminatus   7   Rissoina sp.      3.55    3
bers of two faunal lists pooled for assemblages from both the        3.35  Laevicardium     8   Chione cancellata   3.46    2
experimental and control transects. This value demonstrates              mortoni
                                       3.28  Schwartziella    9   Laevicardium      3.35    1
a moderately strong positive correlation (on a scale of 0 to 1,
                                            bryerea          mortoni
where 1 would indicate identical ranks and 0 would be no           3.25  Rissoina sp.     10  Cerithium       2.65    8
similarity), which indicates that assemblages display a high                           muscarum
degree of similarity despite the important individual differ-        2.36  Eulithidium     11  Crepidula sp.     2.65    9
ences in abundance highlighted by earlier analyses. This re-              affine
                                       2.12  Cumingia       12  Limaria pellucida   1.87    6
sult speaks to the nature of change at experimental sites over             vanhyningi
the duration of nutrient enrichment: differences in the death        1.84  Prunum apicinum   13  Prunum apicinum    1.66    0
assemblage are derived from local modifications of the re-          1.56  Arcopsis adamsi   14  Transennella      1.53    2
gional molluscan community via exclusion from habitat rath-                            stimpsoni
                                       1.56  Carditamera     15  Pitar fulminatus    1.51    8
er than wholesale reorganization of community structure.
                                            floridana
  TURNEY and PERKINS (1972) defined several environmen-           1.54  Transennella     16  Cumingia        1.30    4
tal regions in and around Florida Bay, each with a unique               stimpsoni         vanhyningi
faunal assemblage. Cross Bank is situated on the boundary          1.18  Costoanachis     17  Arcopsis adamsi    1.24    3
between two of these regions, the Interior Bay and the At-               avara
                                       0.85  Limaria pellucida  18  Tellina alternata   1.24    1
lantic region (tidal influence from Tavernier Creek supplies         0.85  Tellina alternata  19  Costoanachis      1.03    2
open marine conditions). The array of fauna recorded here in                           avara
subfossil form shows that some of the Atlantic region species        0.66  Crepidula sp.    20  Carditamera      0.18    5
of TURNEY and PERKINS (Tegula fasciata, Astralium phoe-                              floridana
bium, Carditamera floridana) occur in low abundances, but
are generally found only at control localities. The Interior
Bay, a broad, shallow Thalassia-dominated zone of inter-         tecting the Thalassia–Halodule transition in Florida Bay be-
spersed banks and ‘‘lakes’’ (deeper areas [1–2 m] between        cause of the low abundance of these species, it suggests that
banks where only a thin veneer of sediment overlies bedrock),      in regimes with more stable salinity, these species could
houses species that include Cerithium muscarum, Brachidon-        prove useful for diagnosing environmental change.
tes exustus, and Bittiolum varium; of which only Cerithium is       By itself, a comparison of the percentage abundance of C.
negatively affected in the present study by the transition to      muscarum to B. exustus seems to be a useful means of as-
Halodule at experimental sites. Atlantic region molluscs are       sessing whether a substrate is Thalassia or Halodule covered
restricted to areas that experience normal marine salinities       (Figure 5). Brachidontes is highly adaptable to a wide array
(TURNEY and PERKINS, 1972), but they may also be more          of environmental conditions, often thrives in stressed envi-
suited to Thalassia-dominated habitats, as they are not gen-       ronments, and is capable of inhabiting virtually any sub-
erally found in assemblages from experimental sites. Al-         strate (BREWSTER-WINGARD, ISHMAN, and HOLMES, 1998;
though the loss of Atlantic region taxa is not useful for de-      BREWSTER-WINGARD, STONE, and HOLMES, 2001). These



                   Journal of Coastal Research, Vol. 24, No. 1A (Supplement), 2008
                          Environmental Change and Molluscan Response                       257


                                       mension of this study will be the analysis of piston-core sam-
                                       ples already collected from the upper meter of sediment on
                                       Cross Bank, subsampled at 2-cm intervals, to assess the pres-
                                       ervation potential and stratigraphic resolution of changes in
                                       assemblage composition. Taphonomy and time-averaging
                                       permitting, this will allow detailed construction of long-term
                                       ecological and environmental histories of near-shore environ-
                                       ments during the Holocene from molluscan death assemblag-
                                       es.

                                                    CONCLUSION
                                         Molluscan death assemblages at experimental, nutrient-
                                       enriched sites along Cross Bank reflect alteration in the liv-
                                       ing molluscan community caused by transition between dom-
Figure 5. Comparison of the percentage abundance Cerithium vs. Brach-
                                       inant sea grass species and ultimately driven by human ac-
idontes. The relationship in abundance of these two species is indicative
of the molluscan response to the Thalassia–Halodule transition. Ceri-     tivities. These changes are apparent through comparison of
thium has a consistently higher abundance at control sites than at ex-    molluscan skeletal composition with unaltered environments
perimental sites. Brachidontes is more abundant on a percentage basis at   on Cross Bank and should be apparent in other settings
experimental sites. Because the abundance of Cerithium drives major dif-   where the wide-bladed Thalassia is replaced by Halodule be-
ferences between assemblages from control and experimental sites, the
relationship between these two highly abundant taxa should be a useful
                                       cause of nutrient loading. Cerithium muscarum, the species
indicator of the type of sea grass coverage below the uppermost sedimen-   most strongly affected, appears to be the best indicator for
tary horizon and at other localities. Error bars represent 95% confidence   this type of transition in south Florida, the most likely mech-
and were calculated using a program written by Raup (1991) for use with    anism being the difference in blade size and strength differ-
percentage values.
                                       ences between Thalassia and Halodule rather than nutrients
                                       directly. Furthermore, initial work in deeper piston cores
                                       sampled at a fine vertical scale indicates a decrease toward
characteristics make Brachidontes a useful taxon for compar-         the surface in the percentage abundance of C. muscarum at
ison with Cerithium as it behaves rather differently with re-         experimental sites and suggests that the stratigraphic tran-
spect to substrate sensitivity in the study, and it exhibits         sition to the Cerithium-depleted biota can be recognized in
moderate abundance increases at experimental sites relative          the sedimentary record. Therefore, despite potential for in-
to controls in this investigation.                      terference by bioturbation, the results of this study add to
                                       the growing body of evidence indicating that molluscan death
Long-Term Ecological History                         assemblages have great promise for diagnosing recent envi-
                                       ronmental transitions on coastal seafloors.
  Results of these surficial analyses suggest that a record of
change may be preserved in the broader sedimentary column
and, if true, would demonstrate the diagnostic power of mol-
                                                  ACKNOWLEDGMENTS
luscan death assemblages for discerning ecological and en-           This work was conducted under National Park Service per-
vironmental changes through extended intervals of time. Re-          mit no. EVER-00225 (accession no. EVER-773) in cooperation
ported sedimentation rates and observed differences in Cer-          with the Everglades National Park and would not have been
ithium abundance between samples from the upper sedimen-           possible without the generous contribution of time and equip-
tary veneer at control and experimental sites suggest that if         ment by G.L. Brewster-Wingard and J. Murray of the U.S.
a stratigraphic record of change associated with the Thalas-         Geological Survey, Reston, VA. I also appreciate the assis-
sia–Halodule transition at experimental sites is to be found,         tance of J. Murray, A. Hendy, J.W. Fourqurean, and D. Szy-
it will be located in the uppermost two decimeters. One lim-         manski in field work. Financial support for this project came
itation will be the role of time averaging, which is one im-         from Sigma Xi and the K.E. Caster Memorial Fund of the
portant difference between control and experimental sites, re-        University of Cincinnati. Comments from M. Zuschin and an
lated to sea grass root–rhizome morphology. This has been           anonymous reviewer, as well as A. I. Miller, C.E. Brett, and
discussed here in terms of the effects on less actively burrow-        D.L. Meyer, who read earlier drafts of this manuscript, have
ing infaunal bivalves relative to more active or epifaunal va-        greatly improved the final outcome. I thank L.C. Ivany, R.B.
rieties. However, elevated bioturbation is also expected at          Aronson, A. Hendy, K. Bulinski, D. Buick, J. Bonelli, and V.
Halodule-dominated sites because of low root density relative         Dryfhout for helpful discussions along the way and T. Phillips
to Thalassia, which has a dense root–rhizome, and is capable         for assistance in drafting figures.
of binding sediment and excluding many bioturbating organ-
isms (SUCHANEK, 1983). The effect on the stratigraphic sed-                     LITERATURE CITED
imentary record might be a more gradual and blurred record
                                       ABBOTT, R.T., 1974. American Seashells: The Marine Mollusca of the
of change at experimental sites, which is supported by an-           Atlantic and Pacific Coasts of North America. New York: Van
ecdotal observations that the sediment was much less stable           Nordstrand Reinhold Company, 663p.
and more actively eroded at altered sites. An additional di-         ALMASI, M.N.; HOSKIN, C.M.; REED, J.K., and MILO, J., 1987. Ef-




                      Journal of Coastal Research, Vol. 24, No. 1A (Supplement), 2008
258                                 Ferguson


 fects of natural and artificial Thalassia on rates of sedimentation.    JACKSON, J.B.C., 1973. The ecology of molluscs of Thalassia com-
 Journal of Sedimentary Research, 57, 901–906.                munities, Jamaica, West Indies. I. Distribution, environmental
ARONSON, R.B.; MACINTYRE, I.G.; PRECHT, W.F.; MURDOCH, T.J.T.,         physiology, and ecology of common shallow-water species. Bulletin
 and WAPNICK, C.M., 2002. The expanding scale of species turn-        of Marine Science. 23, 315–347.
 over events on coral reefs in Belize. Ecological Monographs, 72,      KALBFLEISCH, W.B.C. and JONES, B., 1998. Sedimentology of shal-
 233–249.                                   low, hurricane-affected lagoons; Grand Cayman British West In-
BRAY, J.R. and CURTIS, J.T., 1957. An ordination of the upland for-      dies. Journal of Coastal Research, 14, 140–160.
 est communities of Southern Wisconsin. Ecological Monographs,       KIDWELL, S.M., 2001. Preservation of species abundance in marine
 27, 325–349.                                 death assemblages. Science, 284, 1091–1094.
BREWSTER-WINGARD, G.L. and ISHMAN, S.E., 1999. Historical          KIDWELL, S.M. and BOSENCE, D.W.J., 1991. Taphonomy and time-
 trends in Central Florida Bay: a paleoecological reconstruction us-     averaging of marine shelly faunas. In: ALLISON, P.A., and BRIGGS,
 ing modern analogue data. Estuaries, 22, 369–383.              D.E.G. (eds.), Taphonomy: Releasing the Data Locked in the Fossil
BREWSTER-WINGARD, G.L.; ISHMAN, S.E., and HOLMES, C.W., 1998.         Record. New York: Plenum Press, pp. 115–210.
 Environmental impacts on the South Florida coastal waters: a his-     KOWALEWSKI, M., 2001. Applied marine paleoecology: an oxymoron
 tory of change in Florida Bay. Journal of Coastal Research, 26,       or reality? Palaios, 16, 309–310.
 162–172.                                  KOWALEWSKI, M.; AVILA SERRANO, G.E.; FLESSA, K.W., and GOOD-
BREWSTER-WINGARD, G.L.; STONE, J.R., and HOLMES, C.W., 2001.          FRIEND, G.A., 2000. Dead delta’s former productivity: two trillion
 Molluscan faunal distribution in Florida Bay, past and present:       shells at the mouth of the Colorado River. Geology, 28, 1059–1062.
 An integration of down-core and modern data. In: WARDLAW, B.R.       KOWALEWSKI, M.; CARROLL, M.; CASAZZA, L.; GUPTA, N.S.; HAN-
 (ed.), Paleoecological Studies of South Florida. Bulletins of Ameri-     NISDAL, B.; HENDY, A.; KRAUSE, R.A., JR.; LABARBERA, M.; LAZO,
 can Paleontology, 361, 199–231.                       D.G.; MESSINA, C.; PUCHALSKI, S.; ROTHFUS, T.A.; SALGEBACK,
CARROLL, M.M.; KOWALEWSKI, M.; SIMOES, M.G., and GOOD-             J.; STEMPIEN, J.; TERRY, R.C., and TOMASOVYCH, A., 2003. Qual-
 FRIEND, G.A., 2003. Quantitative estimate of time-averaging in        itative fidelity of brachiopod-mollusk assemblages from modern
 terebratulid brachiopod shell accumulations from a modern trop-       subtidal environments of San Juan Islands, U.S.A. Journal of Ta-
 ical shelf. Paleobiology, 29, 381–402.                    phonomy, 1, 43–65.
CLARKE, K.R. and WARWICK, R.M., 2001. Change in Marine Com-         MELDAHL, K.H.; FLESSA, K.W., and CUTLER, A.H., 1997. Time-av-
 munities: An Approach to Statistical Analysis and Interpretation,      eraging and postmortem skeletal survival in benthic fossil assem-
 2nd edition. Plymouth, U.K.: PRIMER-E, 172p.                 blages: quantitative comparisons among Holocene environments.
CRONIN, T.M.; HOLMES, C.W.; BREWSTER-WINGARD, G.L.; ISHMAN,          Paleobiology, 23, 207–229.
 S.E.; DOWSETT, H.J.; KEYSER, D., and WAIBEL, N., 2001. Histor-       MIKKELSEN, P.M.; TEMKIN, I.; BIELER, R., and LYONS, W.G., 2004.
 ical trends in epiphytal ostracodes from Florida Bay: implications      Pinctada longisquamosa (Dunker, 1852) (Bivalvia: Pteriidae), an
 for seagrass and macro-benthic algal variability. In: WARDLAW,        unrecognized pearl oyster in the western Atlantic. Malacologia,
 B.R. (ed.), Paleoecological Studies of South Florida. Bulletins of      46(2), 473–501.
 American Paleontology, 361, 159–198.                    MILLER, A.I., 1988. Spatial resolution in subfossil molluscan re-
DAWES, C.J. and TOMASKO, D.A., 1988. Depth distribution of Thal-        mains: implications for paleobiological analyses. Paleobiology, 14,
 assia testudinum in two meadows on the west coast of Florida; a       91–103.
 difference in effect of light availability. Marine Ecology, 9, 123–    PANDOLFI, J.M. and MINCHIN, P.R., 1995. A comparison of taxonom-
 130.                                     ic composition and diversity between reef coral life and death as-
DEN HARTOG, C., 1971. The dynamic aspect in the ecology of sea-        semblages in Mandang Lagoon, Paupa New Guinea. Palaeogeog-
 grass communities. Thalassia Jugoslavica, 7, 101–112.            raphy, Palaeoclimatology, Palaeoecology, 119, 321–341.
FERGUSON, C.A., 2003. Spatial and Temporal Fidelity of Subfossil       PETERSON, C.H., 1976. Relative abundances of living and dead mol-
 Molluscan Assemblages in a Modern, Shallow Marine Carbonate         lusks in two California lagoons. Lethaia, 9, 137–148.
 Setting. Cincinnati, Ohio: University of Cincinnati, Master’s the-     PETERSON, C.H., 1982. Clam predation by whelks (Busycon spp.):
 sis, 128p.                                  experimental tests of the importance of prey size, prey density,
FOURQUREAN, J.W.; POWELL, G.V.N.; KENWORTHY, W.J., and ZIE-          and seagrass cover. Marine Biology, 66, 159–170.
 MAN, J.C., 1995. The effects of long-term manipulation of nutrient     POWELL, G.V.N; FOURQUREAN, J.W.; KENWORTHY, W.J., and ZIE-
 supply on competition between the seagrasses Thalassia testudin-       MAN, J.C., 1991. Bird colonies cause seagrass enrichment in a sub-
 um and Halodule wrightii in Florida Bay. OIKOS, 72, 349–358.         tropical estuary: observational and experimental evidence. Estu-
FOURQUREAN, J.W.; ZIEMAN, J.C., and POWELL, G.V.N., 1992. Re-         arine, Coastal and Shelf Science, 32, 567–579.
 lationships between pore water nutrients and seagrasses in a sub-     POWELL, G.V.N.; KENWORTHY, W.J., and FOURQUREAN, J.W., 1989.
 tropical carbonate environment. Marine Biology, 114, 57–65.         Experimental evidence for nutrient limitation of seagrass growth
GIFFORD, S.; DUNSTAN, H.; O’CONNOR, W., and MACFARLANE, G.R.,         in a tropical estuary with restricted circulation. Bulletin of Marine
 2005. Quantification of in-situ nutrient and heavy metal remedi-       Science, 44, 324–340.
 ation by a small pearl oyster (Pinctada imbricata) farm at Port      POWELL, G.V.N.; SOGARD, S.M., and HOLMQUIST, J.G., 1987. Ecol-
 Stephens, Australia. Marine Pollution Bulletin, 50, 417–422.         ogy of Shallow-Water Habitats in Florida Bay. Final Report, NPS
GILINSKY, N.L. and BENNINGTON, J B., 1994. Estimating numbers         contract CX5280-3-2339, Homestead, Florida: South Florida Re-
 of while individuals from collections of body parts: a taphonomic      search Center, 405p.
 limitation of the paleontological record. Paleobiology, 20, 245–258.    PRAGER, E.J. and HALLEY, R.B., 1999. The influence of seagrass on
HAYWARD, B.W.; GRENFEL, H.R.; HORROCKS, M.; NICHOLSON, K.;           shell layers and Florida Bay mudbanks. Journal of Coastal Re-
 PARKER, R.; SABAA, A.T.; SWALES, A., and WILMHURST, J., 2004.        search, 15(4), 1151–1162.
 Foraminiferal record of human impact on intertidal estuarine en-      RAUP, D.M., 1991. The future of analytical paleobiology. In: GILIN-
 vironments in New Zealand’s largest city. Marine Micropaleontol-       SKY, N.L., and SIGNOR, P.W. (eds.), AnalyticalPaleobiology. Knox-
 ogy, 53(1–2), 37–66.                             ville, Tennessee:Paleontological Society. Short Courses in Paleon-
HECK, K.L., 1979. Some determinants of the composition and abun-        tology, 4, 207–216.
 dance of motile macroinvertebrate species in tropical and temper-     REDFERN, C., 2001. Bahamian Seashells: A Thousand Species from
 ate turtlegrass (Thalassia testudinum) meadows. Journal of Bio-       Abaco, Bahamas. Boca Raton, Florida: Bahamianseashells.com,
 geography, 6, 183–200.                            280p.
HOLMES, C.W.; ROBBINS, J.; HALLEY, R.; BOTHNER, M.; TEN BRINK,        SCOFFIN, T.P., 1970. The trapping and binding of subtidal carbonate
 M., and MAROT, M., 2001. Sediment dynamics of Florida Bay mud        sediments by marine vegetation in Bimini Lagoon, Bahamas.
 banks on a decadal timescale. In: WARDLAW, B.R. (ed.), Paleoeco-       Journal of Sedimentary Petrology, 40, 249–273.
 logical Studies of South Florida. Bulletins of American Paleontol-     STRASSER, A. and SAMANKASSOU, E., 2003. Carbonate sedimenta-
 ogy, 361, 31–40.                               tion rates today and in the past; Holocene of Florida Bay, Baha-



                     Journal of Coastal Research, Vol. 24, No. 1A (Supplement), 2008
                         Environmental Change and Molluscan Response                         259


 mas, and Bermuda vs. Upper Jurassic and Lower Cretaceous of         Tropical America (NMITA). University of Iowa. Available from:
 the Jura Mountains (Switzerland and France) Geologia Croatica,       http://porites.geology.uiowa.edu/database/bivalves/Bivalve eco.html
 56, 1–18                                  (accessed 29 January 2006).
SUCHANEK, T., 1983. Control of seagrass communities and sediment      TREWIN, N.H. and WELSH, W., 1972. Transport, breakage and sort-
 distribution by Callianassa (Crustacea, Thalassinidea) bioturba-      ing of the bivalve Mactra corallina on Aberdeen Beach, Scotland.
                                       Palaeogeography, Palaeoclimatology, Palaeoecology, 12, 193–204.
 tion. Journal of Marine Research, 41, 281–298.
                                      TURNEY, W.J. and PERKINS, B.F., 1972. Molluscan Distribution in
TODD, J.A., 2001a. Bivalve Life Habits [online]. Neogene Marine Biota     Florida Bay. Miami, Florida: Rosensteil School of Marine and At-
 of Tropical America (NMITA). University of Iowa. Available from:      mospheric Science, The University of Miami, 37p.
 http://porites.geology.uiowa.edu/database/bivalves/Bivalve eco.html    WANLESS, H.R., 1981. Fining upward sedimentary sequences gen-
 (accessed 29 January 2006).                         erated in seagrass beds. Journal of Sedimentary Petrology, 51(2),
TODD, J.A., 2001b. Gastropod Diets [online]. Neogene Marine Biota of     445–454.




                     Journal of Coastal Research, Vol. 24, No. 1A (Supplement), 2008