Ó Springer 2006
Wetlands Ecology and Management (2006) 14:435–444
DOI 10.1007/s11273-006-0006-3
-1


The interplay between mangroves and saltmarshes at the transition between
temperate and subtropical climate in Florida

Philip W. Stevens1,3,*, Sandra L. Fox1,2 and Clay L. Montague1
1
 Department of Environmental Engineering Sciences, University of Florida, P.O. Box 116450, Gainesville, FL
32611, USA; 2St. Johns River Water Management District, 4049 Reid Street, Palatka, Florida 32177, USA;
3
 Present address: Florida Fish and Wildlife Conservation Commission, Fish and Wildlife Research Institute,
Charlotte Harbor Field Laboratory, 1481 Market Circle, Unit 1, Port Charlotte, FL 33953, USA; *Author
for correspondence (e-mail: philip.stevens@myfwc.com; phone: +1-941-255-7403; fax: +1-941-255-7400)

Received 23 November 2005; accepted in revised form 17 January 2006



Key words: Avicennia, Cedar Keys, Florida, Freezes, Mangrove crabs, Spartina, Succession


Abstract

The interplay between mangroves and saltmarshes at the temperate to subtropical transition in Florida
results in dramatic changes to the appearance of the coastal landscape. In the 1980s, freezes killed entire
mangrove forests dominated by black mangroves, Avicennia germinans (L.). Following the freezes, salt-
marshes dominated by smooth cordgrass, Spartina alterniflora Loisel, revegetated the intertidal zone. After
a decade of mild winters, however, mangroves are beginning to reclaim the area. The rate of mangrove
expansion was determined by comparing aerial photography (change from 1995 to 1999), and from
monitoring transects (over a 3 year period) on three of the Cedar Keys, Florida (Lat. 29°08¢). The rate of
mangrove expansion varied among islands, and the mechanism of expansion ranged from propagule-
trapping by saltmarshes along the edges of mangrove clumps to widespread dispersal and growth of
existing or newly imported propagules. A freeze occurred during the study, which may have set back
mangrove expansion by defoliating mangrove trees and resetting mangrove reproduction. Mangrove
expansion was projected to take 20–30 years for complete seedling cover. Given the possibility of global
climate change and its potential influence on the distribution of coastal vegetation, the timeframes and
implications to coastal wetland ecosystems involved in this regular interplay will provide valuable baseline
information for future studies.




                                 mangrove survival and growth. At the transition
Introduction
                                 between temperate and subtropical climate,
The distribution of mangroves in the northern          however, an interplay occurs between mangroves
Gulf of Mexico is controlled primarily by freezing        dominated by black mangroves, Avicennia
temperatures (Davis 1940; Lugo and Patterson-          germinans (L.), and saltmarshes dominated by
Zucca 1977; Kangas and Lugo 1990). Saltmarshes          smooth cordgrass, Spartina alterniflora Loisel.
dominate intertidal zones in more temperate           Freezes favor the marsh, but mangroves prevail
climates, but mangroves replace saltmarshes at          during periods of mild winters. In Florida, this
lower latitudes where warm temperatures allow          transition zone occurs along the east coast north of
436

Cape Canaveral to St. Augustine, and along the     vegetation, quantifying the timeframes involved in
west coast north of Tampa Bay to the Cedar Keys    this regular interplay would provide valuable
(Kangas and Lugo 1990).                baseline information for future studies. The pur-
  During periods of mangrove succession, a      pose of this study is to document the ongoing
variety of factors influence the distribution of    succession from saltmarshes to mangrove forests
mangrove and saltmarsh ecotones. Saltmarsh veg-    within the temperate to subtropical transition in
etation cannot grow in the shade of mangrove trees   Florida and to estimate the time frame for man-
(Kangas and Lugo 1990). Freezes open the forest    grove replacement.
floor to light, which allows fast pioneering salt-
marsh species to vegetate the intertidal zone
(Patterson et al. 1993). In the process, saltmarshes
remove available nutrients, possibly causing root   Materials and methods
competition for nutrients among the saltmarsh
species and new mangrove seedlings (Patterson     Study location
et al. 1993). Saltmarshes, however, may also facil-
                            The Cedar Keys are located in the Big Bend region
itate mangrove colonization by trapping mangrove
                            of Florida (Lat. 29°08¢ N; Figure 1), which has a
propagules (Lewis and Dunston 1975). The com-
                            low-energy coastline with a mean tidal range of
bination of propagule trapping and nutrient com-
                            1 m (NOAA 1988). Salinity is influenced by the
petition makes the overall effect of the marsh on
                            Gulf of Mexico, local rainfall, surface runoff, and
mangrove recolonization unclear. Other factors
                            freshwater discharge from the Suwannee River
influencing the distribution of mangrove and salt-
                            (average 296 m3 sÀ1) 26 km to the north (NOAA
marsh ecotones at local scales are hydroperiod,
                            1985). Mangrove forests persist on the Cedar Keys
salinity, sediment characteristics, and propagule
                            because the intensity of freezes is reduced by the
predation (e.g., Patterson and Mendelssohn 1991;
                            surrounding water (Laessle and Wharton 1959;
Clarke and Allaway 1993; Clarke and Myercough
                            Lugo and Patterson-Zucca 1977; Montague and
1993; Patterson et al. 1993). At regional scales,
                            Odum 1997). Patches of closed-canopy mangrove
mangrove transgressions into saltmarshes occur-
                            forest on the Cedar Keys exist as nearly mono-
ring in South Florida (Wanless et al. 1994) and
                            specific stands of A. germinans. Red mangrove,
Australia (Saintilan and Williams 1999) have been
                            Rhizophora mangle L., and white mangrove,
explained by changes in precipitation patterns, in-
                            Laguncularia racemosa (L.), only occur as scat-
creases in nutrient levels and sedimentation,
                            tered individuals, probably because they do not
revegetation of disturbed areas, altered tidal re-
                            tolerate low-temperatures as well as A. germinans
gimes, and sea-level rise; however, the overall dis-
                            (Markley et al. 1982; McMillan and Sherrod
tribution of mangroves in the northern Gulf of
                            1986). The canopy height of A. germinans forests
Mexico appears to be dominated by freeze fre-
                            prior to the mangrove kill in the 1980s was
quency (Kangas and Lugo 1990), with other fac-
                            approximately 3–4 m (Lugo and Patterson-Zucca
tors explaining local variation (e.g., Patterson and
                            1977; Watson 1986).
Mendelssohn 1991).
                             Small intertidal wetlands on Snake Key, North
  Recent climate history in Florida provides an
                            Key, and Seahorse Key (Figure 1) were selected
opportunity to investigate the interplay between
                            for study. Sites on Snake Key and North Key were
mangroves and saltmarshes. Hard freezes during
                            in pockets of marsh nearly surrounded by upland
the 1980s killed entire mangrove forests in the
                            forests and naturally occurring sand berms. These
northern Gulf of Mexico (McMillan and Sherrod
                            barriers effectively block wind in the marshes.
1986; Watson 1986; Montague and Weigert 1990;
                            Water enters these pocket marshes via creeks. The
Montague and Odum 1997). Saltmarshes replaced
                            site on Seahorse Key, on the other hand, was fully
the intertidal zone within 4–5 years. However,
                            exposed to open water and north wind; water filled
6 years of mild winters have allowed mangroves to
                            and drained the marsh from an adjacent lagoon.
begin reclaiming the area. Mangroves now grow in
                            The saltmarsh vegetation on the islands consisted
clumps among saltmarsh vegetation. Given the
                            almost entirely of S. alterniflora except on Sea-
possibility of global climate change and its
                            horse Key, which had a mixture of S. alterniflora,
potential influence on the distribution of coastal
                                                      437




                Figure 1. Map of the Cedar Keys showing study sites.


saltwort, Batis maritima L., and glasswort, Sali-      pink, and beach and spoil appeared white. Hard
cornia virginica L.                     copies of the same US Geological Survey aerial
                              photography were obtained from local resource
                              managers and placed under a stereoscope to fur-
                              ther assist in the delineation of the vegetation.
Aerial photography/change analysis
                                From the digitizing process, maps were gener-
Gross changes in vegetation were determined by       ated depicting mangrove and saltmarsh distribu-
comparing 1995 and 1999 digital orthophotos.        tion within the study area for 1995 and 1999.
Digital orthophotos (1-m resolution digital ortho-     Mangrove area for each study site was calculated
quads) taken by US Geological Survey National        for each year based on the total area digitized for
Aerial Photography Program at the Cedar Keys        each vegetation class. Change analyses were per-
were obtained from the Florida Land Boundary        formed in ArcMap using the Union feature, which
Information System (www.labins.org). For each        computes a geographic intersection of the input
study site, the digital orthophotos were displayed     features from both years producing output fea-
at 1:1000 scale in ArcMap 8.3. Mangrove and         tures attributed with the vegetation class of both
saltmarsh vegetation were digitized based primar-      time periods. The changes in mangrove distribu-
ily on height, color and texture variance (e.g.,      tion that occurred over the 5-year period were
Higinbotham et al. 2004). The vegetation was        displayed in ArcMap using symbology based on
easily distinguished in the orthophotos; marsh       the attribution for both time periods (e.g., persis-
vegetation appeared black, mangrove vegetation       tent mangrove, mangrove expansion, mangrove
appeared dark red, upland vegetation appeared        loss). For example, mangrove areas present in
438

both 1995 and 1999 aerial photographs were      regional differences (Wilcoxon signed rank test,
termed persistent mangroves, and mangrove       0.05 level of significance).
expansion referred to areas where mangroves were    An estimate of mangrove seedling recruitment
present in 1999 but not in the earlier 1995 photos.  was used as an indicator of mangrove expansion
Polygons less than 10 m2 were dropped from the    along the transects to supplement results from
change analyses because these small areas were    aerial photography. The estimate of seedling
considered to be within the error of the digitizing  recruitment was determined by the following
process. The results of the change analyses were   equation: Seedling Recruitment = D seedling
used to provide a mangrove expansion rate (new    density + D tree density + tree death + seedling
mangrove area yearÀ1) at each study site.       death. Changes in seedling and tree densities were
                           calculated by subtracting densities in November of
                           one year from densities in November of the fol-
                           lowing year. Tree death was easily measured by
Ground level transects
                           counting the number of leafless trees within a
Thirty-meter transects oriented perpendicular to   quadrat, but seedling death was not measured. As
mangrove/saltmarsh ecotones were established at    a result, seedling recruitment was underestimated.
the Cedar Keys to provide a finer scale for moni-
toring changes in mangrove and saltmarsh densi-
ties, and to supplement results of GIS analyses. A  Results
total of five transect locations were established,
which were placed in areas where definitive man-    Freeze observations
grove and saltmarsh ecotones were apparent from
                           A severe winter occurred between the 1995 and
1995 aerial photography. The approximate eleva-
                           1996 growing seasons (minimum air temperature
tions along transects with respect to Mean Lower
                           of À8 °C in January 1996 at a station near the
Low Water (MLLW) ranged between 65 and
                           Cedar Keys; NOAA 1915–1997). Many trees had
88 cm, which resulted in daily inundation by the
                           lost some or all of their leaves when measured in
1-m tides characteristic of the area.
  Increases in mangrove tree and seedling densi-   May 1996, but most recovered by November 1996.
                           Tree mortality along transects following the freeze
ties were important indicators of mangrove
                           was 12% on Seahorse Key, 5% on North Key, and
expansion, and saltmarsh stem densities were
                           4% on Snake Key. Although mangroves in this
expected to decrease as mangroves expanded and
                           region typically flower during spring and drop
shaded the underlying saltmarsh. Mangrove trees
                           propagules during fall, flowering and propagule
and seedlings (identified by lack of branching)
were counted in 4-m2 quadrats at 5-m intervals    drop was not observed on the islands during 1996.
                           Mangroves were flowering again by May 1997.
along the transects for 2 years (1995 and 1996) in
May and November, which approximate the usual
growing season for this region. Densities were also
measured in May 1997 along Seahorse Key and      Aerial photography/change analysis
North Key transects and in November 1997 along
                           The study sites were at different stages of man-
transects on Snake Key. Densities of saltmarsh
stems were counted in 0.25-m2 quadrats at 5-m     grove succession at the beginning of the study
                           period in 1995 (3% cover on Seahorse Key, 20%
intervals along the transects. Saltmarsh stem den-
sities within the seven 0.25-mÀ2 quadrats along    cover on North Key, and 33% cover on Snake
                           Key, Table 1). Mangrove cover expanded to
each transect were summed to give number of
stems 1.75 mÀ2. Seasonal and annual changes in    occupy ca. another 20% of the intertidal zones at
                           each study site during the 5-year period. From the
saltmarsh stem density was calculated for each
                           change analyses, the site on North Key expanded
transect by subtracting May values from November
                           at a rate of about 300 m2 yearÀ1, and the sites on
values (seasonal) or November values from sub-
                           Seahorse Key and Snake Key expanded at rates
sequent November values (annual). The five tran-
                           exceeding 840 m2 yearÀ1. Mangrove expansion
sects were used as replicates (for non-parametric
                           on North Key and Snake Key occurred along
analysis) to evaluate statistical significance of
                                                          439

the boundaries of persistent mangrove clumps      season was defined as winters in which the mini-
                            mum temperature fell below À5 °C at several
(Figure 2). The mangrove expansion on Seahorse
Key also occurred along the boundaries of per-     temperature stations within an area. During 1932–
sistent mangrove clumps, but much of the expan-
sion at this site was represented by many small
mangrove clumps distributed widely throughout
the marsh (Figure 2). Some mangrove loss was
evident at the North Key and Snake Key study
sites, but this loss was greatly outweighed by
mangrove expansion.

Ground level transects

Initial canopy height (used simply as an indicator of
where mangrove and saltmarsh boundaries occur)
and seedling recruitment for the five transects are
shown in Figure 3. Seedling recruitment along the
two Snake Key transects was highest around
mangrove clumps and declined with distance.
Mangrove seedling recruitment along Snake Key–
Transect 2 occurred up to 5 m from mangrove
clumps into saltmarsh areas. Seedling recruitment
along Snake Key–Transect 1 was as high as
76 seedlings mÀ2 yearÀ1 and seedlings recruited at
least 15 m into saltmarsh areas. Although seedling
recruitment on Seahorse Key occurred, it primarily
occurred within the mangrove clump (Transect 1),
and along the area fringing the lagoon (Transect 2).
On the North Key transect, seedling recruitment
did not occur. Seedling recruitment did not occur
along any transect in 1997.
  Seasonal changes in saltmarsh stem density
along transects were not significant during 1995
(p = 0.35), but were significant during 1996
(p = 0.04). Annual changes in saltmarsh stem
density along transects were higher in 1996 than in
1995 (p = 0.04). These increases occurred along
transect locations that were farthest from man-
grove clumps.


Discussion

Frequency of freezes in Northern Florida
                            Figure 2. GIS change analysis between 1995 and 1999 on Snake
Freeze cycles in Florida have been explored by     Key (a), North Key (b), and Seahorse Key (c). White refers to
                            upland or beach, blue refers to water, pink refers to persistent
scientists interested in the risks associated with
                            mangrove, red refers to mangrove expansion, opaque refers to
citrus production. In one such study, Miller and
                            mangrove loss, grey refers to remaining intertidal zone, dark
Downton (1992) defined a local killing freeze as     grey refers to change in intertidal zone (low tide in 1995 aerial
winters in which the minimum temperature fell      photography vs. high tide in 1999 aerial photography). Loca-
below À6.7 °C, and a regionwide severe freeze      tions of 30-m transects are also shown.
440

Table 1. Total intertidal area, mangrove area, and percent mangrove cover at each study site for 1995 and 1999 determined from aerial
photography. Also shown are mangrove expansion rates at each study site determined from GIS change analysis.

Aerial photography

Location               1995                  1999                  Change analysis
                                                          mangrove
           Intertidal    Mangrove      Mangrove      Mangrove      Mangrove      expansion
           area (m2)    area (m2)               area (m2)
                             cover (%)               cover (%)     rate (m2 yearÀ1)

Snake Key      15,333      5114        33         9044        59         862
North Key       6948      1356        20         2565        37         300
Seahorse       21,186      609         3         4822        23         842
Key




1980, a regionwide severe freeze season occurred           catastrophic freeze in the 19th century, for
approximately every 8 years in Florida. The freeze          example, involved a series of severe winters dur-
frequency changed substantially during the 1980s           ing 1895–1905, and the series of severe winters
as tree killing freezes occurred in 4 out of 5 years         during 1977–1989 comprised the catastrophic
between 1981 and 1985 (Miller and Downtown              freeze of the 20th century.
1992).
  The freezes that affect the citrus industry appear
to coincide with freezes that affect mangroves and           Impact of freezes on mangroves
probably subtropical species in general. Major
impacts to mangroves resulting from freezes              According to Lugo and Patterson-Zucca (1977),
occurred in 1962 (Dr Frank Maturo, University of           freeze damage to mangroves is greatest in areas
Florida, personal communication), 1977 (Lugo             more exposed to cold wind. The Seahorse Key
and Patterson-Zucca 1977), 1981, 1983, 1985, 1989           marsh is exposed to cold north winds that follow
(personal observation) and 1996 (this study).             cold fronts in this region. This may explain the
These accounts are consistent with an annual             high mangrove mortality (12%) at Seahorse Key
minimum temperate of less than À6.7 °C (defini-            following the January 1996 freeze (compared to
tion of a local killing freeze for citrus; Miller and         5% on North Key and 4% on Snake Key). The
Downtown 1992) recorded at the Cedar Key               marshes on Snake Key and North Key, where
temperature station (NOAA 1915–1997). How-              mangroves are more extensive, are isolated from
ever, temperatures on the outermost islands              north wind by natural barriers such as berms and
(Snake Key, North Key, and Seahorse Key) are             upland forests. Even on these islands, however,
slightly warmer than Cedar Key (Laessle and              some mortality was evident from direct observa-
Wharton 1959). Therefore, the lowest temperature           tions along transects and from results of the
                                   change analyses that show some small net losses
that can be tolerated by mangroves at this latitude
may be close to Davis’ (1940) original suggestion           of mangrove, which likely occurred from areas
of À4 °C.                               affected by freeze that had not yet fully recovered.
  A single severe winter, however, may only set             Although Seahorse Key appears to be the
back mangrove development without killing a              most vulnerable to freezes, the impacts of the
substantial number of trees. At the Cedar Keys,            January 1996 freeze reset mangrove development
mangroves have recovered within one year fol-             on all three islands. The failure of mangroves to
lowing a severe freeze (Lugo and Patterson-Zucca           flower and produce propagules during the 1996
                                   growing season presumably resulted from deple-
1977; personal observation 1981). However, a
series of severe freezes is catastrophic, as evi-           tion of the plants’ resources during recovery from
denced by the mangrove kill in the 1980s. Such a           the previous winter. The lack of seedling recruit-
catastrophic series of severe winters may recur            ment in 1997 suggests that mangrove expansion at
only once every century in Florida (Winsberg             the Cedar Keys depends largely on a local source
1990; Miller and Downton 1992). The                  of propagules rather than imports from more
                                                                     441

                      Snake Key - Transect 1
             2.5                            100




                                              (Seedlings m-2 yr-1)
Canopy Height (m)                                  80
              2




                                               Recruitment
                                          60
             1.5                                             canopy height
                                          40
                                          20                 recruitment 96
              1
                                          0                 recruitment 97
             0.5
                                          -20
              0                            -40
                0m  5m   10m   15m    20m   25m   30m

             2.5                             80
                      Snake Key - Transect 2
Canopy Height (m)




                                              (Seedlings m-2 yr-1)
              2                             60




                                                Recruitment
                                                            canopy height
             1.5                             40
                                                            recruitment 96
              1                             20
                                                            recruitment 97
             0.5                             0
              0                             -20
                0m  5m   10m   15m   20m   25m    30m

             2.5                             80
                          North Key



                                                (Seedlings m-2 yr-1)
Canopy Height (m)




              2                             60
                                                 Recruitment      canopy height
             1.5                             40
                                                            recruitment 96
              1                             20
                                                            recruitment 97
             0.5                             0
              0                            -20
                0m  5m   10m   15m   20m   25m   30m

                     Seahorse Key - Transect 1
             2.5                             80
                                               (Seedlings m-2 yr-1)
Canopy Height (m)




              2                             60
                                                Recruitment




                                                           canopy height
             1.5                             40
                                                           recruitment 96
              1                             20
                                                           recruitment 97
             0.5                             0
              0                             -20
                0m  5m   10m   15m   20m   25m    30m


             2.5                             80
                     Seahorse Key - Transect 2
   Canopy Height (m)




                                               (Seedlings m-2 yr-1)




              2                             60
                                                Recruitment




                                                           canopy height
             1.5                             40
                                                           recruitment 96
              1                             20
                                                           recruitment 97
             0.5                             0
              0                             -20
                0m  5m   10m   15m   20m   25m    30m
                      Distance Along Transect

                   Figure 3. Canopy height and seedling recruitment along each transect.
442

southerly locations, such as Tampa Bay (a net      scattered throughout the marsh that grew to be-
northward transport of Gulf of Mexico waters      come trees during the study period. Some of the
occurs in this region; Clarke 1997). Hence, freezes   smallest ‘clumps’ apparent in the digital ortho-
that impact mangrove reproduction at the Cedar     photos may result from one or two trees that
Keys cause a major setback to mangrove succession.   developed a broad canopy (possibly an advanta-
  Freezes that impact mangroves, however, may     geous strategy when competing alone with salt-
indirectly affect saltmarshes. Saltmarsh stem densi-   marsh). Some of these small clumps among
ties at the Cedar Keys were higher in 1996 than in   saltmarsh vegetation were also evident on Snake
1995, even though mangrove expansion was        Key.
expected to result in lower saltmarsh stem densities.
Although the increases followed a freeze that defo-
                            Time-frame for mangrove succession
liated mangroves, increased light availability cannot
explain the changes in saltmarsh stem density
                            The freezes in 1983 and 1985 killed 98% of man-
because they occurred primarily in areas away from
                            groves at the Cedar Keys (Montague and Odum
mangrove clumps. However, recycled nutrients
                            1997), which may have eliminated the local man-
from fallen mangrove leaves might explain the
                            grove propagule source. It is not known whether
increase. Freezes may have resulted in greater
                            local seedlings survived, which would have pro-
amounts of detritus as mangrove biomass was
                            vided a seed bank for subsequent mangrove suc-
defoliated or killed, and subsequent detrital break-
                            cession, or if new recruitment was initially
down increased nutrients available for plant growth.
                            dependent on propagules from areas further south.
                            Regardless, as more mangroves continue to reach
                            reproductive maturity at the Cedar Keys, the rate
Mangrove expansion
                            of mangrove colonization should increase. In the
                            absence of major freezes, the time it will take for
The mechanism of mangrove seedling recruitment
                            mangroves to completely cover the marsh on Snake
and expansion appeared to differ among the study
                            Key is another 8 years (15,333 m2 total area –
sites. On Snake Key, trapping of mangrove propa-
                            9044 m2 1999 mangrove area/mangrove expansion
gules by saltmarsh (Lewis and Dunston 1975) could
                            rate of 862 m2 yearÀ1). Thus, the time frame for
account for the high seedling recruitment near
                            complete mangrove recolonization on Snake Key
mangrove clumps. Propagules produced by the
                            since the freeze kill during the 1980s is nearly
mangrove clumps on Snake Key were immediately
                            20 years. Using the same calculation for North
trapped in saltmarsh within 5–15 m of their source.
                            Key and Seahorse Key, these study sites will take
Without the adjacent saltmarsh, more mangrove
                            25–30 years for mangroves to again reclaim the
propagules may have been exported rather than
                            marsh. Periodic freezes, however, ensure that some
being retained in the vicinity of the parent trees.
                            saltmarsh cover will persist. This mixture of man-
High rates of seedling recruitment along the edges
                            grove forests and saltmarshes may continue for
of the mangrove clumps and subsequent growth of
                            several decades until another catastrophic series of
these mangroves into trees probably explains why
                            freezes kill mangrove forests, and allow saltmarshes
Snake Key has the greatest mangrove cover and
                            to once again occupy the intertidal zone.
highest expansion rates among the study sites.
  On Seahorse Key, mangrove seedling recruit-
ment occurred within the existing mangrove clump    Implications for coastal wetland communities
and along the transect adjacent to the lagoon also
                            As the intertidal zone changes between mangrove
suggesting that mangrove seedlings may remain
                            forests and saltmarshes, faunal changes should
near parent trees (Blanchard and Prado 1995;
                            occur. Both mangroves and saltmarshes provide
McKee 1995). However, numerous mangrove
                            food and refuge for fiddler crabs, periwinkles,
clumps on Seahorse Key appeared to ‘sprinkle in’
                            clams, and oysters (Odum et al. 1982; Stout 1984).
throughout the marsh (apparent from change
                            However, the greater volume and structural com-
analysis), particularly at the southern end of the
                            plexity of mangroves supports additional fauna
study site adjacent to the lagoon. This mechanism
                            such as the mangrove tree crab, Aratus pisonii
of expansion may result from single seedlings
                                                       443

(Milne Edwards), the coffee-bean snail, Melampus    of severe winters, however, the combined energy
coffeus (Linnaeus), the mangrove crab, Goniopsis    drains of salinity and frost limit the amount of
cruentata (Latreille), the ladder-horn snail, Ceri-  structure that can be maintained. As a result,
thidea scalariformis (Say), the mangrove rivulus   mangroves are killed and the coastal wetlands
fish, Rivulus marmoratus Poey, and the mangrove    rapidly retrogress to saltmarshes. Following a
cuckoo, Coccyzus minor (Gmelin) (Odum et al.     mangrove kill, saltmarshes occupy the intertidal
1982; Davis et al. 1995; Taylor et al. 1995). In   zone within a period of only 5–10 years. The
mangroves, the periwinkle Littoraria angulifera    saltmarsh vegetation, however, may stabilize
(Lamarck) replaces the Littoraria irrorata (Say) of  marsh soils and maintain certain wetland func-
saltmarshes (Odum et al. 1982). Also, mangrove    tions (e.g., juvenile fish habitat, detritus produc-
forests may attract nesting colonial birds (Watson  tion). Whether the overall effect of saltmarshes
1986). Conversely, saltmarsh-dependent bird spe-   facilitates mangrove colonization by trapping
cies such as seaside sparrows, Ammospiza maritima   mangrove propagules or retards mangrove rees-
(Wilson), and long-billed marsh wrens, Cistotho-   tablishment by outcompeting mangrove seed-
rus palustris (Wilson), may leave the area when    lings remains equivocal. Nevertheless, saltmarshes
saltmarsh is overtaken by mangroves (Post and     maintain emergent habitat in the intertidal zone in
Greenlaw 1994).                    the interim between occupations by mangroves.
  During the present study, saltmarsh-dependent   Although coastal wetlands alternate between
A. maritima and C. palustris were only encoun-    mangroves and saltmarshes at the temperate to
tered at the Seahorse Key study site where salt-   subtropical transition in Florida, overall produc-
marsh cover remains extensive (personal        tion and survival of wetland-dependent biota are
observation). Although pelicans were observed     maintained despite periodic disturbances.
nesting in mangroves on Seahorse Key prior to the
catastrophic freezes in the 1980s (Watson 1986),   Acknowledgements
colonial birds were not observed in the developing
mangroves of the present study. The only fauna    We are grateful to F. Maturo and the University
unique to mangroves on the islands was A. pisonii.  of Florida Marine Lab at Seahorse Key for pro-
Aratus pisonii reached densities of 1 crab mÀ3    viding logistical support during this study. We also
prior to the 1996 freeze (unpublished data), which  thank K. Litzenberger, refuge manager of the
are within the range reported for mature mangrove   Lower Suwannee National Wildlife Refuge, for
forests of South Florida (1–4 crabs mÀ3; Beever    permitting us to conduct research within the Cedar
et al. 1979). Mangrove fauna may not only be     Keys National Wildlife Refuge. Florida Sea Grant
dependent on the habitat present, but also on     and the Aylesworth Foundation provided financial
other environmental factors, especially tempera-   support. T. Crisman and S. Vince gave thoughtful
ture (e.g., R. marmoratus, Taylor 1993). A long    advice and guidance throughout this study, and
period of time without freezes may allow a richer   commented on early drafts. Finally, we acknowl-
mangrove fauna to develop. Alternatively, the lag   edge all of the volunteers who assisted with
in development of mangrove fauna may exceed the    the fieldwork especially G. Stevens, J. Stevens,
frequency of freezes at this latitude.        B. Clarke, and A. Wilson.

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