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Hale et al 04
Estuaries Vol. 27, No. 1, p. 36-43 February 2004
Changes in the Distribution of Seagrass Species along Florida's
Central Gulf Coast: Iverson and Bittaker Revisited
J. A. HALE~*, T. K. FRAZER 1, D. A. TOgCASKC_)Z,and M. O. HALLs'
1Depart~nent of Fisheries and Aquatic Sciences, University of Florida, 7922 A;W 71st Street,
Gainesville, Florida 32653
Southwest Florida Water Management District, 7601 Highway 310 North, Ta~zpa, Florida
336377481
s Florida Marine Research Institute, Florida Fish and Wildlife Conservation Commission, 100
Eighth Avenue Southeast, St. Petersbu,rg; Florida 55701
ABSTRACT: A broad-scale survey of seagrass species composition and. distribution along Florida's central Gulf Coast
(known as tbe Big" Bend reg~ion) was conducted in the s u m m e r of 2000 to address growing concerns over the potential
effects of increased nutrient loading f r o m adjacent coastal rivers. Iverson and Bittaker (1986) or@inallysurveyed seagrass
distribution in tbis reg~ion between 1974-1980. We revisited 188 stations f r o m the or@inal survey, recording tbe presence
or absence of all seagTass species. Although factors sucb as accurac3~ of station relocation, differences in s a m p l i n g e f f o r t
a m o n g studies, and length of time between surveys preclude statistical comparisolts, several interesting p a t t e r n s emerged.
While the total n u m b e r of stations occupied, by the three m o s t c o m m o n seagra~s species, Thalassia testudinum, Sytqr~godium
filiforme, and Halodule w~-ightii, was similar between the two time periods, we o b s e r v e d a change in the n u m b e r of records
of each species as well as changes in distribution with depth. T. testudinum and Halophila er~gelman~i occurrence declined
in the deepest areas of the region, while the n u m b e r of statioiLs occupied by S. filiforme and. H. wNghtii increased in
nearby areas. We o b s e r v e d several localized areas of seagT~ss loss, f r e q u e n d y associated with the moutks of coastal rivers.
Tb~se r~sults suggest that increased nutrient loading to coastal rivers that discharge into tbe Big Bend area may be
affecting seagTass~s by increasing phytoplankton abundance in the water column, tbus changing water clarity cbaracter-
istics of the region.
Introduction 1991). In many of these cases, increased nutrient
Florida's n o r t h central Gulf coastline, c o m m o n l y loading or other a n t h r o p o g e n i c insult is blamed
r e f e r r e d to as the Big Bend (Anclote Key n o r t h to for the loss.
Apalachee Bay), includes the second largest ex- Seagra~ss survival, growth, and p r o d u c t i o n are in-
panse of seagrass habitat in the eastern Gulf of fluenced by irradiance levels (reviewed by I)uarte
Mexico. Iverson and gittaker (1986) c o n d u c t e d a 1991), sediment n u t r i e n t availability (Short 1987;
broad-scale survey of seagrass distribution of this Williams 1990), water flow (Fonseca et al. 198S;
region in the late 1970s. In that report, the authors Fonseca and Bell 1998), t e m p e r a t u r e (Masini et al.
r e c o r d e d the distribution of six of seven Florida 1995), and salinity (Walker and M c C o m b 1990).
species of seagrass within this area, and estimated A m o n g these, light is most often recognized as the
total seagrass coverage at about S,000 km 2. major d e t e r m i n a n t of seagrass distribution (Duarte
Seagrass beds are essential to the ecological in- 1991). T h e a m o u n t of light available for photosyn-
tegrity and health of Florida's estuarine and near- thesis is influenced by water clarity, which is largely
shore coastal ecosystems. Along Florida's Gulf a function of turbidity. R e d u c e d water clarity
Coast, seagrasses provide refuge and forage habitat caused by increased p h y t o p l a n k t o n a b u n d a n c e or
for many ecologically and economically i m p o r t a n t suspended sediment, often associated with anthro-
fauna, such as scallops, shrimps, blue crabs, man- pogenic disturbance of n e a r s h o r e ecosystems, ap-
atees, and turtles (Killam et al. 1992). A n u m b e r p e a r s to be the m o s t w i d e s p r e a d m e c h a n i s m
of researchers have d o c u m e n t e d degradation or threatening seagrass health (Short and Wyllie-
destruction of many of Florida's seagrass ecosys- Echeverria 1996). Epiphytic algae growth may af-
tems, including areas in Apalachee Bay (Zimmer- fect seagrass photosynthetic potential by intercept-
man and Livingston 1976), T a m p a Bay" (Lewis and ing incident light (Den H a r t o g 1979; Tomasko and
Estevez 1988), and Florida Bay (e.g., Robblee et al. Lapointe 1991).
T h o u g h the addition of nutrients alone does not
* C o r r e s p o n d i n g author; tele: 352/392-9617 ext. 265; e-mail: always lead to a decline in seagrass health (e.g.,
jah@ffas.ufl.edu Erftemeijer et al. 1994), increases in phytoplank-
9 2004 Estuarine Research Federation 36
Trends in ,Seagrass Distribution in Florida 37
ton p r o d u c t i o n and epiphytic a b u n d a n c e on sea- Materials and M e t h o d s
grass lem, es generally result fi'om increased nutri- Iverson and Bittaker (1986) c o n d u c t e d surveys
ent delivery to a system. N u m e r o u s r e s e a r c h e r s of seagrass distribution along the northwest G u l f
have d o c u m e n t e d changes in seagrass ecosystems coast of Florida f r o m A p a l a c h e e Bay south to An-
across large g e o g r a p h i c extents due to various clote Key in O c t o b e r of each year between 1974-
causes. Den H a r t o g and P o l d e r m a n (1975) postu- 1980. Between July and S e p t e m b e r 2000, we revis-
lated that a n t h r o p o g e n i c pollution, expressed as a ited 188 of these s a m p l i n g stations, r e c o r d i n g the
c h a n g e in water quality, was b e h i n d the widespread p r e s e n c e or absence of all seagrass species along
effects of the seagrass wasting disease and regional 250 km of coastline w-ithin an a r e a of approxi-
declines in seagrass coverage observed in the 1930s mately 6,000 kmL T h e exact coordinates of the
a n d later. Livingston (1984) described n u m e r o u s Iverson a n d Bittaker sampling stations were n o t
l o n g - t e r m d e t r i m e n t a l effects of pollution (as a available (Bittaker personal c o m m u n i c a t i o n ) , but
c h a n g e in water quality) on seagrass ecosystems were estimated by two methods: digitization of the
along the n o r t h e r n G u l f Coast of Florida, Denni- original m a p f r o m Figure 1 of their text, a n d t h e n
son et al. (1998) linked physical habitat require- assigning m a p coordinates to the i m a g e using the
m e n t s of s u b m e r s e d aquatic vegetation to a variety available latitude-longitude graticules, or estima-
of water quality standards, including n u t r i e n t con- t i o n via t r i a n g u l a t i o n f r o m w e l l - d e f i n e d l a n d
tent, a n d a r g u e d that the c o n n e c t i o n between sea- forms, O n e s h o r t SCUBA dive or up to S snorkel
grass condition a n d water quality p a r a m e t e r s was dives were m a d e at each sampling station, and the
so consistent that changes in seagrass cover, partic- p r e s e n c e or a b s e n c e of any s u b m e r s e d aquatic veg-
ularly along their d e p t h limit, may be an early sign etation was noted, If any vegetation was present,
of d e g e n e r a t i n g water quality. F o u r q u r e a n et al. additional search time was devoted to detect as
(1995) r e p o r t e d a c h a n g e in seagrass species com- m a n y species of seagrass as possible. Surveyors
were not told w h e r e to expect to find seagrass f r o m
position of a seagrass bed in Florida Bay due to
the original survey to standardize the survey effort
increased nutrient input, Short and Burdick
at each station.
(1996) provided evidence relating loss of seagrass
Water d e p t h was r e c o r d e d to the nearest 0,5 m
cover a n d increased f r a g m e n t a t i o n ofseagrass beds
at each station, To c o m p a r e d e p t h distributions of
to u r b a n d e v e l o p m e n t a n d increased n u t r i e n t in-
species between the historical a n d c u r r e n t surveys,
put into receiving waters, while T o m a s k o et al.
we assumed the d e p t h at each station to be the
(1996) r e p o r t e d a negative correlation between s a m e b e t w e e n time periods. No m e n t i o n of stan-
seagrass biomass and productivity a n d watershed dardizing d e p t h to an), elevation d a t u m was m a d e
n u t r i e n t loading, in the original report, M e a n tidal r a n g e for the Big
N u t r i e n t c o n c e n t r a t i o n s are increasing in m a n y Bend coastline is 0.85 m (National O c e a n Service
of the rivers that feed the coastal waters along Flor- u n p u b l i s h e d material).
ida's central G u l f Coast ( H a m a n d Hatzell 1996;
J o n e s et al. 1997), and increases in n u t r i e n t loads Results
to coastal waters in the region potentially t h r e a t e n
T h e p r e s e n c e or a b s e n c e of four species of sea-
the seagrass beds along this p o r t i o n of Florida's
grasses, Thalassia testudir~u~n, Syringodiu~n fiSfor~,
G u l f Coast (Frazer et al. 2001). Because seagrasses
Halodule w,rightig and Halophila engel, manng was re-
are often used to ascertain impacts resulting f r o m
c o r d e d at 188 s a m p l i n g stations. We cataloged a
increased a n t h r o p o g e n i c n u t r i e n t inputs, we re- cumulative total of 158 records of p r e s e n c e at 85
peated, in large part, the broad-scale survey com- stations of the three m o s t c o m m o n seagrass spe-
pleted over 25 yr ago by Iverson and Bittaker to cies, T t~t.~di,~,~,,~, s. ~lir a n d hr. ~,,~ightii, a
d e t e r m i n e if a c h a n g e in seagrass distribution a n d slight increase f r o m 149 records at 79 stations
species had occurred. A p p r o x i m a t e l y two h u n d r e d m a d e by Iverson and Bittaker (1986) (Table 1). We
stations along the Big Bend coast of Florida, f r o m m a d e fewer observations of the three less c o m m o n
Piney Point ( n o r t h of the S t e i n h a t c h e e River) to species, including no records of Halophila de@lens
Anclote Key (just n o r t h of T a m p a Bay) were revis- or R'utypia ma:Hti,ma in 2000.
ited, and the p r e s e n c e or absence of seagrasses was We r e c o r d e d T testudinum at 54 of 188 stations
recorded. R e t u r n i n g to these stations enabled us (29% occurrence; Fig. 1) b e t w e e n depths of 0.5 m
to u p d a t e i n f o r m a t i o n r e g a r d i n g the distribution (station 161) a n d 7.8 m (station 45), c o m p a r e d
a n d species c o m p o s i t i o n of seagrass c o m m u n i t i e s with 63 total observations (34% o c c u r r e n c e ) m a d e
along Florida's central G u l f coast, and provided in the previous study. Fewer observations of T. tes-
the o p p o r t u n i t y to assess changes that m a y have tudinum at depths greater than S m were r e c o r d e d
o c c u r r e d as a result of increased n u t r i e n t inputs. in 2000 than 25 yr ago. In addition, o u r deepest
38 J . A . Hale et al.
TABLE 1. S m u m a r y of observations of seagTass species at 188 s a m p l i n g locations. The d e p t h s r e p r e s e n t the m a x i m u m d e p t h each
species ~r observed. The d e p t h s listed m the 1974-1980 c o l u m n were d e t e r m i n e d f r o m the d e p t h s we m e a s u r e d at the s a m e stations
in 2000.
Number of Records Maximum Depth (m)
1974-1980 2000 % Change 1974-1980 2000
Th,alassia testu,din'e~m 63 54 16% 8.5 7.3
,S)~ring'odiuvaill,brine 4.3 51 + 19 % 7.3 6.7
HalM,'ede wrlgf~,tii 43 53 +23% 10.6 8.3
Halophila eng'elmanni 2.3 16 -30% 10.6 7.0
Halophila decT/)iens 1 0 100% 9.0 --
R~q~pia marithna 4 0 -100% 2.0 --
~ ' ...
..I
. ~ '
.....'--,_~<,,~
:'9 ,3-o~
/ ~ I
" ' . o ~.Oo ......... L___ I
.... % "...rL ~ ~
... , ^ ~
..
.. -a.x0, "9 1o~ a,~,
Lo'..C ;o. % , < :
-a:aO 6 ~ 0 0 0 0 ~Oo~oO~--o ~ ........
,&,{. o ~ o
.~.:'o o~,,. ,oo oo
9
o %'
:';:~
~r .:,,..jaoo %1 ~ot 9
9~ . . . ~ % . a & ~
; .%_~,
I 9 ,..y ,~ ~.t .-~
V ~'='~'~176 ':::J;~, i 7--
I~ . . . . :<~ t :t ' 1
I~ 0,,o :-';-od" "" ,~ .... , 0,: I ::t,od" ":
t ~ '.'~o...a. l ~'~
i;i" f--
......... }...,11,o ~ ~ . . ~-e.~l
"'-"':-,'.,.. 04
...'., ''~ 7- ,?. ~.go.
..... "'~,-%o-,~
~-8 ~. ~ I
\ . ........ 0.0 -o~ ~ .......
"%' ~ I
,,< ~o!%O-& y . . . . .
~,.o o Ljor %.,.~ , oO@K
'o -moo *~'
.e,:. "~.g 9 ,
9
::'~2.~176
9.~.~ a,%.oo~oe,~l~,,..~~ /
r
:.>~ ~',~.?.. "..
"~" o % o oil ". -':% o "'
- a.0~,. ;'.@ ;,-' :' o"., ,7o,. ~ ~ ~'~,
m~o~hao,<e . . . . . :;- .~':,"" ' ~ ~ 1 7 6
;s
iv...:. { oo
i Presr m bokhsue,'eys ,tr ..e' "l~ 9 Pr~smtin bothsm'veys ~. x .
9 P. . . . . . . . ys 0 ~0~0~ 0 0 .'~
o Absmttmmt,o~,~*w1~ '" ':~ 8 9
, .
t oi. %t'it .... it' ....
^ &~":-#..~' A .i4 "oo~
.'~'): ". o
:#' - ! ( I
Fig. 1. Differences in distribution b e t w e e n surveys of four species of seagrass, Thalassia test~tdinum, Syringodi~tmfiliformr Halodule
vsig'htii, a n d Halophil~, engeba,an,ni. Solid circles i n d i c a t e p r e s e n c e in b o t h surveys, o p e n circles i n d i c a t e a b s e n c e f r o m boi l : surveys,
solid u p - p o i n t i n g arrows i n d i c a t e n e w r e c o r d , and o p e n d o w n - p o i n t i n g arrows i n d i c a t e loss or failure to observe. D o t t e d l i ne s d e l i n e a t e
fl~e 6- a n d 9-m isobaths.
Trends in ,Seagrass Distribution in Florida 39
20-
previous study, at least 9 m d e e p e r than our deep-
15 []. c.u.e. n. . . s u e
. . akerl 86 est record of approximately 8.5 m (Fig. 2).
H. engehnanni was r e c o r d e d at 16 of 188 stations
10 Thalassia testudinum (8% o c c u r r e n c e ) , only two-thirds of the 2S stations
in the previous stud}, (12% o c c u r r e n c e ) . In gen-
5 eral, the loss of records o c c u r r e d either along the
coastline of the n o r t h e r n half of our study area, or
0
from the deepest, ofs regions of the s o u t h e r n
half (Fig. 1). T h e latter observation accounts for a
20 loss of at least S m from the m a x i m u m d e p t h of
distribution of this species (Fig. 2).
T h e o c c u r r e n c e of T testudinum along the 6-m
isobath from 20 to S0 km offshore in the s o u t h e r n
Syringodium filiforme third of our study area decreased considerably.
These contiguous records of absence occur in
some of the deepest areas where this species was
r e c o r d e d between 1974 and 1980. In addition to
the absence of T. testudinum, we r e c o r d e d an in-
8 crease in H. wrightii o c c u r r e n c e at 8 of our stations
20
and an increase in S. filifmme at 5 stations (Fig. 3).
These changes resulted in 5 stations where T. tes
Z
= 15
tudinu~n wa~s not r e c o r d e d , but either S. ,fiSfo~wze or
10 Halodule wrightii
H. wrightii or both was observed.
Discussion
Because Big Bend b a t h y m e t r y is characterized by
a very gentle slope, increasing a b o u t 1 m in d e p t h
per 5 km distance from shore, relatively large dis-
tances from shore result in only subtle changes in
depth. T h o u g h the}, observed seagrasses in d e e p e r
waters, Iverson and gittaker (1986) described an
" o u t e r limit" of seagrass bed development, where
seagra~ss occupied at least 80% of the bottom. For
m u c h of the s o u t h e r n portion of our stud}, area,
this outer limit c o r r e s p o n d e d with the 6-m isobath.
It follows that even a small decrease in water clarity
0 could decrease the potential habitat of dense sea-
I 2 3 4 5 6 7 8 9 10 II grass coverage by several h u n d r e d square km off
Depth (In) the coasts of counties south of Crystal Privet. O u r
Fig. 2. Comparison of the frequency of distribution of four data suggest the extent of a b u n d a n t seagrass dis-
species of seagrass by depth: Thalassia test*~din*~m, Syri~godh~m tribution may have retreated some distance shal-
filifom~e, Halod~le wrig'htii, and Hal@hila eng'dmanrd. lower than the 6-m isobath.
O u r observations indicate that the four most
c o m m o n seagrasses within our sampling area along
observation of this species was approximately 1 m Florida's central Gulf Coast have experienced a re-
shallower than in the previous study (Fig. 2). duction in their m a x i m u m depth of o c c u r r e n c e
S. filifor,me o c c u r r e d at 51 of our stations (27% over the last 25 yr. Light is generally recognized as
o c c u r r e n c e ; Fig. 1), an increase f i o m 4.8 records as the factor most often controlling seagrass d e p t h
r e p o r t e d in the previous study (23% o c c u r r e n c e ) . distributions (e.g., Duarte 1991), and r e d u c e d wa-
In 2000, S. fi/,iJ~r~w was distributed in water of ter clarity due to increased p h y t o p l a n k t o n abun-
depths that r a n g e d from 2 m to 7 m. In the earlier dance and s u s p e n d e d solids seems to be the most
study S. Jilif~r,~w was r e c o r d e d at depths between 2 widespread m e c h a n i s m t h r e a t e n i n g seagrass
m and 8 m (Fig. 2). health (Short and Wyllie-Echeverria 1996). We hy-
We r e c o r d e d Iv[. zorightii at 5.8 stations (28% oc- pothesize that increased n u t r i e n t loading to the re-
currence; Fig. 1), while it was r e c o r d e d at 43 sta- gion may have resulted in an increase in phyto-
tions (23% o c c u r r e n c e ) 25 yr ago. H. wrightii oc- p l a n k t o n a b u n d a n c e a n d possibly p e r i p h y t o n
curred at depths between 2 m and 11 m in the a b u n d a n c e on seagrass blades (Den H a r t o g 1979;
40 a.A. Hale et al.
Fig. 3. Locations where Thalassia test,cdin~vz was p r e s e n t in the historical study, b u t replaced by S3,9"ing'odhcm filiforme (S) or Halod~le
varig'htii (H), or b o t h species m the c u r r e n t survey. Dotted lines delineate the 6- and 9-m isobaths.
Tomasko and L a p o i n t e 1991) which has changed duced light availabilit?~ at depth. This i n f o r m a t i o n
the light regime available to seagrasses at depth. should be a priority for subsequent investigators
T h e r e have b e e n several regional reports indicat- whose aim is to m o r e fully u n d e r s t a n d these chang-
ing that n u t r i e n t concentrations (nitrate, in partic- es in d e p t h distribution of seagrasses.
ular) in rivers that discharge directly into the Gulf In several locations, the distribution of H. u,Hglz
of Mexico along Florida's Big Bend coast have in- tii and S. ,fiS]'o~",me increased where T. test'~l, di'n'u,m ex-
creased over the last several decades (Ham and p e r i e n c e d m a r k e d losses. H. z~,rightii has higher nu-
Hatzell 1996;Jones et al. 1997), and our hypothesis trient r e q u i r e m e n t s than T testudmum (Fourqm--
is consistent with the findings in these reports. e a n e t al. 1995), and the loss of the latter species
T h e r e is no comparable water quality data available (perhaps due to decreased available light) may
for the coastal region of interest to c o r r o b o r a t e the have allowed for the successful re-colonization by
hypothesis that increased n u t r i e n t loading has re- H. wrightii given adequate n u t r i e n t conditions. Ga-
sulted in increased p h y t o p l a n k t o n biomass and re- llegos et al. (1994) described how p i o n e e r seagrass
Trends in Seagrass Distribution in Florida 41
species--i.e., S..fiSfo~wze and H. wrfg'htii exhibit blages of seagrass species are n o t u n c o m m o n in
h i g h e r growth and turnover rates than T testudin- our area, rather the presence of 2 or ,g species of
u~n, s u p p o r t i n g the c o n c e p t that both the f o r m e r seagrass in the same location is the rule instead of
species are colonizers of habitats with higher nu- the exception.
trient availability or lower light. In an example O u r results also indicate that seagrasses were n o t
where light was not likely a factor, F o u r q u r e a n et necessarily present at the same locations between
al. (1995) r e p o r t e d how H. zm~ightii out-competed the historic and c u r r e n t surveys. We r e c o r d e d T.
T test'udinum in the presence of increased or excess testudinu'm at 54 stations, but only 59 stations were
nutrients. U n d e r those conditions, H. wrightii re- c o m m o n to both surveys; and we r e c o r d e d /-/.
m a i n e d the d o m i n a n t (biomass) seagrass up to 8 wrightii at a total of 53 stations, but only 16 stations
yr after the removal of the nutrient source. These were c o m m o n to both surveys. While the net re-
observations s u p p o r t our idea of a regional shift cords of seagrass presence were similar between
fi-om a high-light, n u t r i e n t - p o o r system character- surveys, the actual locations of seagrass presence
ized by T. testudinu~n to a lower light, h i g h e r nutri- were variable. This suggests a dynamic that war-
ent system favoring 5'. fiSfo~wze and H. wrightii. rants further investigation. It is possible that sedi-
O u r proposition of the effects of decreased light m e n t characteristics are m o r e highly variable, b o t h
are further s u p p o r t e d by a review of the life his- in space and time, than previously recognized or
tories of these seagrasses. 77. testudinu~n, t h o u g h ca- appreciated. It is also possible that sexual repro-
pable of rapid blade growth, also has long-lived duction a m o n g the seagrasses observed in this re-
short shoots, up to 7 yr in the n o r t h e r n Gulf of gion is a c o m m o n o c c u r r e n c e and may be an un-
Mexico (Eleuterius 1987), while S. filiJb,vne and H. der-appreciated aspect of their life history. Quan-
wr4ghtii have comparatively shorter-lived shoots, titative data c o n c e r n i n g the f i e q u e n c y and inten-
i.e., a b o u t 1-8 yr (Eleuterius 1987; Gallegos et al. sit,/ of sexual r e p r o d u c t i o n , seed dispersal and
1994). This suggests that T. testudinu~n may exhibit s u b s e q u e n t g e r m i n a t i o n of new plants are sparse
less rapid a p p e a r a n c e or disappearance than S. fiL in this region (Moffler and Durako 1987; Mattson
ir or H. wrightii. O u r observed decrease in the 2000).
offshore extent of 77. testudinu~n suggests longer- We r e c o r d e d m o r e stations with seagrasses in the
term and m o r e chronic stress, perhaps as a con- s o u t h e r n half of our study area than in the n o r t h
s e q u e n c e of decreased light availability. half. Examples of the c o m m u n i t y shift from T tes-
It is i m p o r t a n t to note that we r e c o r d e d m o r e tudinu~n to H. w,r~ghtii or 51 fili~rwze were m o r e nu-
locations with S. filiJ~r~ne and H. wrightii than did m e r o u s in the south than in the north. Iverson and
Iverson and Bittaker (1986). This suggests light is Bittaker (1986) m easured a higher light extinction
not limiting the distribution of all seagrasses in coefficient at n o r t h e r n stations ( n o r t h of Crystal
some parts of our study area. Williams (1987) dem- River, approximately 29~ than s o u t h e r n stations.
onstrated that T. testudinum may o u t c o m p e t e S. fiL While they did not r e p o r t which c o m p o n e n t of
i/brine given plentiful nutrients and light. Buesa light was most responsible for this difference in
(1975) rep orted that S. jqlif~rww grows well in lower- attenuation (e.g., scattering, color; see Kirk 1994),
lit areas where T. testudinu~n may be light-limited, there are differences in characteristic land cover
an observation s u p p o r t e d by the m e a n m a x i m u m and watersheds which may partially explain this dif-
d e p t h limits of colonization summarized by Duarte ference. Rivers draining into the n o r t h e r n half of
(1991). T h e r e is considerable overlap in the re- our stud), area are h i g h e r in color (Frazer et al.
c o r d e d depth ranges of these species, and Williams 1998), a reflection of differences in watershed size
(1987) observed how seagrasses partition sedi- and d o m i n a n t vegetation. As a result of tropical
ments (and thus n u t r i e n t availability) given ade- storm events, the p l u m e from the Suwannee River
quate light for photosynthesis. T. testudinu~n oc- (approximately 29~ may extend 20 km or
curred w-ith o t h e r seagrass species in almost 90% m o r e from the coast, exporting colored dissolved
of our observations, while Iverson and gittaker re- organic material and light-scattering particles into
c o r d e d only 75% c o - o c c u r r e n c e in their stud),. the ofs e n v i r o n m e n t (gledsoe and Phlips
A r o u n d F l o r i d a , Phillips (1960) n o t e d small 2000), and further increasing light attenuation.
a m o u n t s of S..fil:ifor, me and H. wrigh.tii in seagrass Most of the rivers in the s o u t h e r n half of the re-
areas d o m i n a t e d by 77. testudinu~n, n o r t h of the An- gion are short and fed by artesian springs, contrib-
clote Keys, and Zieman et al. (1989) r e c o r d e d spe- uting little colored dissolved organic material or
cies c o - o c c u r r e n c e at only 45 of 108 stations (42% suspended particles to nearshore areas. This nat-
stations) in Florida Bay. In light of Williams' ex- ural difference within the study area is i m p o r t a n t
perinaental results and the differences between our to consider w hen designing future research plans
observations and those of the original Iverson and aimed at describing seagrass ecology in the region.
gittaker surveys, we c o n c l u d e that mixed assem- Changes in the land cover and land use s u r r o u n d -
42 J.A. Hale et al.
ing these coastal rivers should be monitored and seagrass populations of the Dutch Waddenzee. Aqt~atic Botan7
1:141-147.
managed with care.
DENNZSON,\,V. C., R. J. ORTH, K. A. MOORE,J. C. STEVENSON,V.
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s e n t v a l u e , e v e n if t h e s p e c i e s was n o t r e c o r d e d at EP,~TF~,mWX, R L. A., J. STAPEL,M.J.E. SMF~a~NS,AND W. M. E.
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BITrAI~X, H. E Personal C o m m u n i c a t i o n . South Florida Water
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SHORT, E T. 1987. Effects of s e d i m e n t n u t r i e n t s on seagrasses: U.S. Depm-tment of C o m m e r c e . Available: h t t p : / / ~ - . c c - o p s .
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SHORT, F. T. AND S. ~,VYr,I,w,-EcHE'~IA. 1996. N a ~ ' a l a n d h u - Accepted, March 27, 2003
Changes in the Distribution of Seagrass Species along Florida's
Central Gulf Coast: Iverson and Bittaker Revisited
J. A. HALE~*, T. K. FRAZER 1, D. A. TOgCASKC_)Z,and M. O. HALLs'
1Depart~nent of Fisheries and Aquatic Sciences, University of Florida, 7922 A;W 71st Street,
Gainesville, Florida 32653
Southwest Florida Water Management District, 7601 Highway 310 North, Ta~zpa, Florida
336377481
s Florida Marine Research Institute, Florida Fish and Wildlife Conservation Commission, 100
Eighth Avenue Southeast, St. Petersbu,rg; Florida 55701
ABSTRACT: A broad-scale survey of seagrass species composition and. distribution along Florida's central Gulf Coast
(known as tbe Big" Bend reg~ion) was conducted in the s u m m e r of 2000 to address growing concerns over the potential
effects of increased nutrient loading f r o m adjacent coastal rivers. Iverson and Bittaker (1986) or@inallysurveyed seagrass
distribution in tbis reg~ion between 1974-1980. We revisited 188 stations f r o m the or@inal survey, recording tbe presence
or absence of all seagTass species. Although factors sucb as accurac3~ of station relocation, differences in s a m p l i n g e f f o r t
a m o n g studies, and length of time between surveys preclude statistical comparisolts, several interesting p a t t e r n s emerged.
While the total n u m b e r of stations occupied, by the three m o s t c o m m o n seagra~s species, Thalassia testudinum, Sytqr~godium
filiforme, and Halodule w~-ightii, was similar between the two time periods, we o b s e r v e d a change in the n u m b e r of records
of each species as well as changes in distribution with depth. T. testudinum and Halophila er~gelman~i occurrence declined
in the deepest areas of the region, while the n u m b e r of statioiLs occupied by S. filiforme and. H. wNghtii increased in
nearby areas. We o b s e r v e d several localized areas of seagT~ss loss, f r e q u e n d y associated with the moutks of coastal rivers.
Tb~se r~sults suggest that increased nutrient loading to coastal rivers that discharge into tbe Big Bend area may be
affecting seagTass~s by increasing phytoplankton abundance in the water column, tbus changing water clarity cbaracter-
istics of the region.
Introduction 1991). In many of these cases, increased nutrient
Florida's n o r t h central Gulf coastline, c o m m o n l y loading or other a n t h r o p o g e n i c insult is blamed
r e f e r r e d to as the Big Bend (Anclote Key n o r t h to for the loss.
Apalachee Bay), includes the second largest ex- Seagra~ss survival, growth, and p r o d u c t i o n are in-
panse of seagrass habitat in the eastern Gulf of fluenced by irradiance levels (reviewed by I)uarte
Mexico. Iverson and gittaker (1986) c o n d u c t e d a 1991), sediment n u t r i e n t availability (Short 1987;
broad-scale survey of seagrass distribution of this Williams 1990), water flow (Fonseca et al. 198S;
region in the late 1970s. In that report, the authors Fonseca and Bell 1998), t e m p e r a t u r e (Masini et al.
r e c o r d e d the distribution of six of seven Florida 1995), and salinity (Walker and M c C o m b 1990).
species of seagrass within this area, and estimated A m o n g these, light is most often recognized as the
total seagrass coverage at about S,000 km 2. major d e t e r m i n a n t of seagrass distribution (Duarte
Seagrass beds are essential to the ecological in- 1991). T h e a m o u n t of light available for photosyn-
tegrity and health of Florida's estuarine and near- thesis is influenced by water clarity, which is largely
shore coastal ecosystems. Along Florida's Gulf a function of turbidity. R e d u c e d water clarity
Coast, seagrasses provide refuge and forage habitat caused by increased p h y t o p l a n k t o n a b u n d a n c e or
for many ecologically and economically i m p o r t a n t suspended sediment, often associated with anthro-
fauna, such as scallops, shrimps, blue crabs, man- pogenic disturbance of n e a r s h o r e ecosystems, ap-
atees, and turtles (Killam et al. 1992). A n u m b e r p e a r s to be the m o s t w i d e s p r e a d m e c h a n i s m
of researchers have d o c u m e n t e d degradation or threatening seagrass health (Short and Wyllie-
destruction of many of Florida's seagrass ecosys- Echeverria 1996). Epiphytic algae growth may af-
tems, including areas in Apalachee Bay (Zimmer- fect seagrass photosynthetic potential by intercept-
man and Livingston 1976), T a m p a Bay" (Lewis and ing incident light (Den H a r t o g 1979; Tomasko and
Estevez 1988), and Florida Bay (e.g., Robblee et al. Lapointe 1991).
T h o u g h the addition of nutrients alone does not
* C o r r e s p o n d i n g author; tele: 352/392-9617 ext. 265; e-mail: always lead to a decline in seagrass health (e.g.,
jah@ffas.ufl.edu Erftemeijer et al. 1994), increases in phytoplank-
9 2004 Estuarine Research Federation 36
Trends in ,Seagrass Distribution in Florida 37
ton p r o d u c t i o n and epiphytic a b u n d a n c e on sea- Materials and M e t h o d s
grass lem, es generally result fi'om increased nutri- Iverson and Bittaker (1986) c o n d u c t e d surveys
ent delivery to a system. N u m e r o u s r e s e a r c h e r s of seagrass distribution along the northwest G u l f
have d o c u m e n t e d changes in seagrass ecosystems coast of Florida f r o m A p a l a c h e e Bay south to An-
across large g e o g r a p h i c extents due to various clote Key in O c t o b e r of each year between 1974-
causes. Den H a r t o g and P o l d e r m a n (1975) postu- 1980. Between July and S e p t e m b e r 2000, we revis-
lated that a n t h r o p o g e n i c pollution, expressed as a ited 188 of these s a m p l i n g stations, r e c o r d i n g the
c h a n g e in water quality, was b e h i n d the widespread p r e s e n c e or absence of all seagrass species along
effects of the seagrass wasting disease and regional 250 km of coastline w-ithin an a r e a of approxi-
declines in seagrass coverage observed in the 1930s mately 6,000 kmL T h e exact coordinates of the
a n d later. Livingston (1984) described n u m e r o u s Iverson a n d Bittaker sampling stations were n o t
l o n g - t e r m d e t r i m e n t a l effects of pollution (as a available (Bittaker personal c o m m u n i c a t i o n ) , but
c h a n g e in water quality) on seagrass ecosystems were estimated by two methods: digitization of the
along the n o r t h e r n G u l f Coast of Florida, Denni- original m a p f r o m Figure 1 of their text, a n d t h e n
son et al. (1998) linked physical habitat require- assigning m a p coordinates to the i m a g e using the
m e n t s of s u b m e r s e d aquatic vegetation to a variety available latitude-longitude graticules, or estima-
of water quality standards, including n u t r i e n t con- t i o n via t r i a n g u l a t i o n f r o m w e l l - d e f i n e d l a n d
tent, a n d a r g u e d that the c o n n e c t i o n between sea- forms, O n e s h o r t SCUBA dive or up to S snorkel
grass condition a n d water quality p a r a m e t e r s was dives were m a d e at each sampling station, and the
so consistent that changes in seagrass cover, partic- p r e s e n c e or a b s e n c e of any s u b m e r s e d aquatic veg-
ularly along their d e p t h limit, may be an early sign etation was noted, If any vegetation was present,
of d e g e n e r a t i n g water quality. F o u r q u r e a n et al. additional search time was devoted to detect as
(1995) r e p o r t e d a c h a n g e in seagrass species com- m a n y species of seagrass as possible. Surveyors
were not told w h e r e to expect to find seagrass f r o m
position of a seagrass bed in Florida Bay due to
the original survey to standardize the survey effort
increased nutrient input, Short and Burdick
at each station.
(1996) provided evidence relating loss of seagrass
Water d e p t h was r e c o r d e d to the nearest 0,5 m
cover a n d increased f r a g m e n t a t i o n ofseagrass beds
at each station, To c o m p a r e d e p t h distributions of
to u r b a n d e v e l o p m e n t a n d increased n u t r i e n t in-
species between the historical a n d c u r r e n t surveys,
put into receiving waters, while T o m a s k o et al.
we assumed the d e p t h at each station to be the
(1996) r e p o r t e d a negative correlation between s a m e b e t w e e n time periods. No m e n t i o n of stan-
seagrass biomass and productivity a n d watershed dardizing d e p t h to an), elevation d a t u m was m a d e
n u t r i e n t loading, in the original report, M e a n tidal r a n g e for the Big
N u t r i e n t c o n c e n t r a t i o n s are increasing in m a n y Bend coastline is 0.85 m (National O c e a n Service
of the rivers that feed the coastal waters along Flor- u n p u b l i s h e d material).
ida's central G u l f Coast ( H a m a n d Hatzell 1996;
J o n e s et al. 1997), and increases in n u t r i e n t loads Results
to coastal waters in the region potentially t h r e a t e n
T h e p r e s e n c e or a b s e n c e of four species of sea-
the seagrass beds along this p o r t i o n of Florida's
grasses, Thalassia testudir~u~n, Syringodiu~n fiSfor~,
G u l f Coast (Frazer et al. 2001). Because seagrasses
Halodule w,rightig and Halophila engel, manng was re-
are often used to ascertain impacts resulting f r o m
c o r d e d at 188 s a m p l i n g stations. We cataloged a
increased a n t h r o p o g e n i c n u t r i e n t inputs, we re- cumulative total of 158 records of p r e s e n c e at 85
peated, in large part, the broad-scale survey com- stations of the three m o s t c o m m o n seagrass spe-
pleted over 25 yr ago by Iverson and Bittaker to cies, T t~t.~di,~,~,,~, s. ~lir a n d hr. ~,,~ightii, a
d e t e r m i n e if a c h a n g e in seagrass distribution a n d slight increase f r o m 149 records at 79 stations
species had occurred. A p p r o x i m a t e l y two h u n d r e d m a d e by Iverson and Bittaker (1986) (Table 1). We
stations along the Big Bend coast of Florida, f r o m m a d e fewer observations of the three less c o m m o n
Piney Point ( n o r t h of the S t e i n h a t c h e e River) to species, including no records of Halophila de@lens
Anclote Key (just n o r t h of T a m p a Bay) were revis- or R'utypia ma:Hti,ma in 2000.
ited, and the p r e s e n c e or absence of seagrasses was We r e c o r d e d T testudinum at 54 of 188 stations
recorded. R e t u r n i n g to these stations enabled us (29% occurrence; Fig. 1) b e t w e e n depths of 0.5 m
to u p d a t e i n f o r m a t i o n r e g a r d i n g the distribution (station 161) a n d 7.8 m (station 45), c o m p a r e d
a n d species c o m p o s i t i o n of seagrass c o m m u n i t i e s with 63 total observations (34% o c c u r r e n c e ) m a d e
along Florida's central G u l f coast, and provided in the previous study. Fewer observations of T. tes-
the o p p o r t u n i t y to assess changes that m a y have tudinum at depths greater than S m were r e c o r d e d
o c c u r r e d as a result of increased n u t r i e n t inputs. in 2000 than 25 yr ago. In addition, o u r deepest
38 J . A . Hale et al.
TABLE 1. S m u m a r y of observations of seagTass species at 188 s a m p l i n g locations. The d e p t h s r e p r e s e n t the m a x i m u m d e p t h each
species ~r observed. The d e p t h s listed m the 1974-1980 c o l u m n were d e t e r m i n e d f r o m the d e p t h s we m e a s u r e d at the s a m e stations
in 2000.
Number of Records Maximum Depth (m)
1974-1980 2000 % Change 1974-1980 2000
Th,alassia testu,din'e~m 63 54 16% 8.5 7.3
,S)~ring'odiuvaill,brine 4.3 51 + 19 % 7.3 6.7
HalM,'ede wrlgf~,tii 43 53 +23% 10.6 8.3
Halophila eng'elmanni 2.3 16 -30% 10.6 7.0
Halophila decT/)iens 1 0 100% 9.0 --
R~q~pia marithna 4 0 -100% 2.0 --
~ ' ...
..I
. ~ '
.....'--,_~<,,~
:'9 ,3-o~
/ ~ I
" ' . o ~.Oo ......... L___ I
.... % "...rL ~ ~
... , ^ ~
..
.. -a.x0, "9 1o~ a,~,
Lo'..C ;o. % , < :
-a:aO 6 ~ 0 0 0 0 ~Oo~oO~--o ~ ........
,&,{. o ~ o
.~.:'o o~,,. ,oo oo
9
o %'
:';:~
~r .:,,..jaoo %1 ~ot 9
9~ . . . ~ % . a & ~
; .%_~,
I 9 ,..y ,~ ~.t .-~
V ~'='~'~176 ':::J;~, i 7--
I~ . . . . :<~ t :t ' 1
I~ 0,,o :-';-od" "" ,~ .... , 0,: I ::t,od" ":
t ~ '.'~o...a. l ~'~
i;i" f--
......... }...,11,o ~ ~ . . ~-e.~l
"'-"':-,'.,.. 04
...'., ''~ 7- ,?. ~.go.
..... "'~,-%o-,~
~-8 ~. ~ I
\ . ........ 0.0 -o~ ~ .......
"%' ~ I
,,< ~o!%O-& y . . . . .
~,.o o Ljor %.,.~ , oO@K
'o -moo *~'
.e,:. "~.g 9 ,
9
::'~2.~176
9.~.~ a,%.oo~oe,~l~,,..~~ /
r
:.>~ ~',~.?.. "..
"~" o % o oil ". -':% o "'
- a.0~,. ;'.@ ;,-' :' o"., ,7o,. ~ ~ ~'~,
m~o~hao,<e . . . . . :;- .~':,"" ' ~ ~ 1 7 6
;s
iv...:. { oo
i Presr m bokhsue,'eys ,tr ..e' "l~ 9 Pr~smtin bothsm'veys ~. x .
9 P. . . . . . . . ys 0 ~0~0~ 0 0 .'~
o Absmttmmt,o~,~*w1~ '" ':~ 8 9
, .
t oi. %t'it .... it' ....
^ &~":-#..~' A .i4 "oo~
.'~'): ". o
:#' - ! ( I
Fig. 1. Differences in distribution b e t w e e n surveys of four species of seagrass, Thalassia test~tdinum, Syringodi~tmfiliformr Halodule
vsig'htii, a n d Halophil~, engeba,an,ni. Solid circles i n d i c a t e p r e s e n c e in b o t h surveys, o p e n circles i n d i c a t e a b s e n c e f r o m boi l : surveys,
solid u p - p o i n t i n g arrows i n d i c a t e n e w r e c o r d , and o p e n d o w n - p o i n t i n g arrows i n d i c a t e loss or failure to observe. D o t t e d l i ne s d e l i n e a t e
fl~e 6- a n d 9-m isobaths.
Trends in ,Seagrass Distribution in Florida 39
20-
previous study, at least 9 m d e e p e r than our deep-
15 []. c.u.e. n. . . s u e
. . akerl 86 est record of approximately 8.5 m (Fig. 2).
H. engehnanni was r e c o r d e d at 16 of 188 stations
10 Thalassia testudinum (8% o c c u r r e n c e ) , only two-thirds of the 2S stations
in the previous stud}, (12% o c c u r r e n c e ) . In gen-
5 eral, the loss of records o c c u r r e d either along the
coastline of the n o r t h e r n half of our study area, or
0
from the deepest, ofs regions of the s o u t h e r n
half (Fig. 1). T h e latter observation accounts for a
20 loss of at least S m from the m a x i m u m d e p t h of
distribution of this species (Fig. 2).
T h e o c c u r r e n c e of T testudinum along the 6-m
isobath from 20 to S0 km offshore in the s o u t h e r n
Syringodium filiforme third of our study area decreased considerably.
These contiguous records of absence occur in
some of the deepest areas where this species was
r e c o r d e d between 1974 and 1980. In addition to
the absence of T. testudinum, we r e c o r d e d an in-
8 crease in H. wrightii o c c u r r e n c e at 8 of our stations
20
and an increase in S. filifmme at 5 stations (Fig. 3).
These changes resulted in 5 stations where T. tes
Z
= 15
tudinu~n wa~s not r e c o r d e d , but either S. ,fiSfo~wze or
10 Halodule wrightii
H. wrightii or both was observed.
Discussion
Because Big Bend b a t h y m e t r y is characterized by
a very gentle slope, increasing a b o u t 1 m in d e p t h
per 5 km distance from shore, relatively large dis-
tances from shore result in only subtle changes in
depth. T h o u g h the}, observed seagrasses in d e e p e r
waters, Iverson and gittaker (1986) described an
" o u t e r limit" of seagrass bed development, where
seagra~ss occupied at least 80% of the bottom. For
m u c h of the s o u t h e r n portion of our stud}, area,
this outer limit c o r r e s p o n d e d with the 6-m isobath.
It follows that even a small decrease in water clarity
0 could decrease the potential habitat of dense sea-
I 2 3 4 5 6 7 8 9 10 II grass coverage by several h u n d r e d square km off
Depth (In) the coasts of counties south of Crystal Privet. O u r
Fig. 2. Comparison of the frequency of distribution of four data suggest the extent of a b u n d a n t seagrass dis-
species of seagrass by depth: Thalassia test*~din*~m, Syri~godh~m tribution may have retreated some distance shal-
filifom~e, Halod~le wrig'htii, and Hal@hila eng'dmanrd. lower than the 6-m isobath.
O u r observations indicate that the four most
c o m m o n seagrasses within our sampling area along
observation of this species was approximately 1 m Florida's central Gulf Coast have experienced a re-
shallower than in the previous study (Fig. 2). duction in their m a x i m u m depth of o c c u r r e n c e
S. filifor,me o c c u r r e d at 51 of our stations (27% over the last 25 yr. Light is generally recognized as
o c c u r r e n c e ; Fig. 1), an increase f i o m 4.8 records as the factor most often controlling seagrass d e p t h
r e p o r t e d in the previous study (23% o c c u r r e n c e ) . distributions (e.g., Duarte 1991), and r e d u c e d wa-
In 2000, S. fi/,iJ~r~w was distributed in water of ter clarity due to increased p h y t o p l a n k t o n abun-
depths that r a n g e d from 2 m to 7 m. In the earlier dance and s u s p e n d e d solids seems to be the most
study S. Jilif~r,~w was r e c o r d e d at depths between 2 widespread m e c h a n i s m t h r e a t e n i n g seagrass
m and 8 m (Fig. 2). health (Short and Wyllie-Echeverria 1996). We hy-
We r e c o r d e d Iv[. zorightii at 5.8 stations (28% oc- pothesize that increased n u t r i e n t loading to the re-
currence; Fig. 1), while it was r e c o r d e d at 43 sta- gion may have resulted in an increase in phyto-
tions (23% o c c u r r e n c e ) 25 yr ago. H. wrightii oc- p l a n k t o n a b u n d a n c e a n d possibly p e r i p h y t o n
curred at depths between 2 m and 11 m in the a b u n d a n c e on seagrass blades (Den H a r t o g 1979;
40 a.A. Hale et al.
Fig. 3. Locations where Thalassia test,cdin~vz was p r e s e n t in the historical study, b u t replaced by S3,9"ing'odhcm filiforme (S) or Halod~le
varig'htii (H), or b o t h species m the c u r r e n t survey. Dotted lines delineate the 6- and 9-m isobaths.
Tomasko and L a p o i n t e 1991) which has changed duced light availabilit?~ at depth. This i n f o r m a t i o n
the light regime available to seagrasses at depth. should be a priority for subsequent investigators
T h e r e have b e e n several regional reports indicat- whose aim is to m o r e fully u n d e r s t a n d these chang-
ing that n u t r i e n t concentrations (nitrate, in partic- es in d e p t h distribution of seagrasses.
ular) in rivers that discharge directly into the Gulf In several locations, the distribution of H. u,Hglz
of Mexico along Florida's Big Bend coast have in- tii and S. ,fiS]'o~",me increased where T. test'~l, di'n'u,m ex-
creased over the last several decades (Ham and p e r i e n c e d m a r k e d losses. H. z~,rightii has higher nu-
Hatzell 1996;Jones et al. 1997), and our hypothesis trient r e q u i r e m e n t s than T testudmum (Fourqm--
is consistent with the findings in these reports. e a n e t al. 1995), and the loss of the latter species
T h e r e is no comparable water quality data available (perhaps due to decreased available light) may
for the coastal region of interest to c o r r o b o r a t e the have allowed for the successful re-colonization by
hypothesis that increased n u t r i e n t loading has re- H. wrightii given adequate n u t r i e n t conditions. Ga-
sulted in increased p h y t o p l a n k t o n biomass and re- llegos et al. (1994) described how p i o n e e r seagrass
Trends in Seagrass Distribution in Florida 41
species--i.e., S..fiSfo~wze and H. wrfg'htii exhibit blages of seagrass species are n o t u n c o m m o n in
h i g h e r growth and turnover rates than T testudin- our area, rather the presence of 2 or ,g species of
u~n, s u p p o r t i n g the c o n c e p t that both the f o r m e r seagrass in the same location is the rule instead of
species are colonizers of habitats with higher nu- the exception.
trient availability or lower light. In an example O u r results also indicate that seagrasses were n o t
where light was not likely a factor, F o u r q u r e a n et necessarily present at the same locations between
al. (1995) r e p o r t e d how H. zm~ightii out-competed the historic and c u r r e n t surveys. We r e c o r d e d T.
T test'udinum in the presence of increased or excess testudinu'm at 54 stations, but only 59 stations were
nutrients. U n d e r those conditions, H. wrightii re- c o m m o n to both surveys; and we r e c o r d e d /-/.
m a i n e d the d o m i n a n t (biomass) seagrass up to 8 wrightii at a total of 53 stations, but only 16 stations
yr after the removal of the nutrient source. These were c o m m o n to both surveys. While the net re-
observations s u p p o r t our idea of a regional shift cords of seagrass presence were similar between
fi-om a high-light, n u t r i e n t - p o o r system character- surveys, the actual locations of seagrass presence
ized by T. testudinu~n to a lower light, h i g h e r nutri- were variable. This suggests a dynamic that war-
ent system favoring 5'. fiSfo~wze and H. wrightii. rants further investigation. It is possible that sedi-
O u r proposition of the effects of decreased light m e n t characteristics are m o r e highly variable, b o t h
are further s u p p o r t e d by a review of the life his- in space and time, than previously recognized or
tories of these seagrasses. 77. testudinu~n, t h o u g h ca- appreciated. It is also possible that sexual repro-
pable of rapid blade growth, also has long-lived duction a m o n g the seagrasses observed in this re-
short shoots, up to 7 yr in the n o r t h e r n Gulf of gion is a c o m m o n o c c u r r e n c e and may be an un-
Mexico (Eleuterius 1987), while S. filiJb,vne and H. der-appreciated aspect of their life history. Quan-
wr4ghtii have comparatively shorter-lived shoots, titative data c o n c e r n i n g the f i e q u e n c y and inten-
i.e., a b o u t 1-8 yr (Eleuterius 1987; Gallegos et al. sit,/ of sexual r e p r o d u c t i o n , seed dispersal and
1994). This suggests that T. testudinu~n may exhibit s u b s e q u e n t g e r m i n a t i o n of new plants are sparse
less rapid a p p e a r a n c e or disappearance than S. fiL in this region (Moffler and Durako 1987; Mattson
ir or H. wrightii. O u r observed decrease in the 2000).
offshore extent of 77. testudinu~n suggests longer- We r e c o r d e d m o r e stations with seagrasses in the
term and m o r e chronic stress, perhaps as a con- s o u t h e r n half of our study area than in the n o r t h
s e q u e n c e of decreased light availability. half. Examples of the c o m m u n i t y shift from T tes-
It is i m p o r t a n t to note that we r e c o r d e d m o r e tudinu~n to H. w,r~ghtii or 51 fili~rwze were m o r e nu-
locations with S. filiJ~r~ne and H. wrightii than did m e r o u s in the south than in the north. Iverson and
Iverson and Bittaker (1986). This suggests light is Bittaker (1986) m easured a higher light extinction
not limiting the distribution of all seagrasses in coefficient at n o r t h e r n stations ( n o r t h of Crystal
some parts of our study area. Williams (1987) dem- River, approximately 29~ than s o u t h e r n stations.
onstrated that T. testudinum may o u t c o m p e t e S. fiL While they did not r e p o r t which c o m p o n e n t of
i/brine given plentiful nutrients and light. Buesa light was most responsible for this difference in
(1975) rep orted that S. jqlif~rww grows well in lower- attenuation (e.g., scattering, color; see Kirk 1994),
lit areas where T. testudinu~n may be light-limited, there are differences in characteristic land cover
an observation s u p p o r t e d by the m e a n m a x i m u m and watersheds which may partially explain this dif-
d e p t h limits of colonization summarized by Duarte ference. Rivers draining into the n o r t h e r n half of
(1991). T h e r e is considerable overlap in the re- our stud), area are h i g h e r in color (Frazer et al.
c o r d e d depth ranges of these species, and Williams 1998), a reflection of differences in watershed size
(1987) observed how seagrasses partition sedi- and d o m i n a n t vegetation. As a result of tropical
ments (and thus n u t r i e n t availability) given ade- storm events, the p l u m e from the Suwannee River
quate light for photosynthesis. T. testudinu~n oc- (approximately 29~ may extend 20 km or
curred w-ith o t h e r seagrass species in almost 90% m o r e from the coast, exporting colored dissolved
of our observations, while Iverson and gittaker re- organic material and light-scattering particles into
c o r d e d only 75% c o - o c c u r r e n c e in their stud),. the ofs e n v i r o n m e n t (gledsoe and Phlips
A r o u n d F l o r i d a , Phillips (1960) n o t e d small 2000), and further increasing light attenuation.
a m o u n t s of S..fil:ifor, me and H. wrigh.tii in seagrass Most of the rivers in the s o u t h e r n half of the re-
areas d o m i n a t e d by 77. testudinu~n, n o r t h of the An- gion are short and fed by artesian springs, contrib-
clote Keys, and Zieman et al. (1989) r e c o r d e d spe- uting little colored dissolved organic material or
cies c o - o c c u r r e n c e at only 45 of 108 stations (42% suspended particles to nearshore areas. This nat-
stations) in Florida Bay. In light of Williams' ex- ural difference within the study area is i m p o r t a n t
perinaental results and the differences between our to consider w hen designing future research plans
observations and those of the original Iverson and aimed at describing seagrass ecology in the region.
gittaker surveys, we c o n c l u d e that mixed assem- Changes in the land cover and land use s u r r o u n d -
42 J.A. Hale et al.
ing these coastal rivers should be monitored and seagrass populations of the Dutch Waddenzee. Aqt~atic Botan7
1:141-147.
managed with care.
DENNZSON,\,V. C., R. J. ORTH, K. A. MOORE,J. C. STEVENSON,V.
T w o c h a r a c t e r i s t i c s o f t h e d a t a sets p r e c l u d e d a COARTER, S. KOImAR, R kW. BEROSTROM,AND R. A. BATrUK.
r i g o r o u s statistical c o m p a r i s o n : p r e c i s e s t a t i o n lo- 1993. Assessing water quality, with submersed aquatic vegeta-
c a t i o n s w e r e n o t a v a i l a b l e to us, a n d t h e o r i g i n a l tion. Bioscienee 43:86-91.
a u t h o r s p o o l e d t h e i r d a t a a c r o s s 6 yr (to m i n i m i z e DUARTE, C. M. 1991. Seagrass depth limits. Aq~atic Botany 40:
363-377.
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s e n t v a l u e , e v e n if t h e s p e c i e s was n o t r e c o r d e d at EP,~TF~,mWX, R L. A., J. STAPEL,M.J.E. SMF~a~NS,AND W. M. E.
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