The Ecology of the Seagrasses of South Florida - USGS National ...

grass, and shoal grass in various 10cations
from Texas to St. Croix and Jamaica.
embayments with restricted circulation,
such as southwest Biscayne Bay, nany
Populations from south Florida were inter- algal species are reduced during summer
mediate in tolerance between plants from high temperatures and some of the nore
Texas and the northern Florida coast sensitive types such as Cauler a
and those from St. Craix and Jamaica in hora and Laurencia nay d9F@=
e ki ed Zienan
the Caribbean. In south Florida, the 1975a).
most chill-tolerant plants were from the
shallow bays, while the populations that
were least tolerant of cold temperatures 2.4 SALINITY
were from coral reef areas, where less
fluctuation and greater buffering would be While all of the common south Florida
expected. During winter, the cold north- seagrasses can to1 erate considerable saern
winds quickly cool off the shallow linity fluctuations, all have an optimum
(0.3 to 1 m or 1 to 3.3 ft) waters of range near, or just below, the concentra-
Florida Fay. The deeper waters, however, tion of oceanic water. The dominant seain
the area below the Keys and the reef grass, turtle grass, can survive in sal inline
(up to 15 m or 50 ft) not only have a ities fron 3.5 ppt (Sculthorpe 1967) to 60
much greater mass to be cooled, hut are ppt (McVillan and Moseley 1967), but can
also flushed daily with warmer Gulf Strea~t tolerate these extremes for only short
water which further tends to buffer the periods. Even then, severe leaf loss is
envi ronmental fluctuations. common; turtle grass lost leaves when
sa1 inity was reduced below 2C ppt (den
The anount of direct evidence for the Warton 1970). The optimuin sal ini ty for
temperature ranges of shoal grass and man- turtle grass ranges from 24 ppt to 35 ppt
atee grass is far less than for turtle
grass, Phil 1 Ips (1960) suggested that
(Phil1 ips 1960; HcMillan and Poseley 1Q67;
Zieman 1975h). Turtle grass showed maxinum
shoal grass generally prefers temperatures photosynthetic activity in full -strength
af 20 to 30°C (68' to 86aF), but that it seawater and a linear decrease in activity
is somewhat marc ecrrythermal than turtle ti th decreasing sal ini ty (Hammer lP68b).
grass, This fits its ecological role as a At 5QX strength seawater, the photosynthepioneer
or colonizing species. Shoal grass tic rate was only one-third of that in
is comnonly Found in shallower water than full-strength seawater. Fo1 lowing the
either turtle grass ar manatee grass, passage of a hurricane in south Florida in
wherc then~al variation would tend to be 1960, Thornas et a1 . (1951) considered the
greater, FAcP"i1 lan (1979) found that shoal damage to the turtle grass hy freshwater
grass had a greater chill tolerance than runoff to have been more severe than the
turll~ grass, while manatee grass showed
less resistance to chi1 I ing,
pl~ysical effects of the high winds and
water surge.
Seagrasses are partially huf'fercd The tolerance of local seagrass spefral
tetnpcrdture extrelscs in the overlying cies to sal a'nity variation is simildr to
water because of the scctir~en ts covcring their temperature to1 erances. Shoal Grass
the roots and rhizotiles. Sedi~?cnts are is thc nost broadly euryhaline, turtle
poorer conductors of heat than seawater grass is intermediate, and vanatee grass
and they absorb heat morc slowly. In a and - fJalophi1a -- have the narrowest tolerance
study by Redfield (19651, changes in the ranges, with -- tialghila being even rqore
ter:;perature of the water cel urnn decrease stenohal ine than manatee grass (F-?clJil 1 an
exponent? at ly ni th depth in sedir??n ts. 1979).
Hxtroa? ~ ae assaci;tcd wi kt? grass beds
exist totally it? the water coluinn, and 2.5 5EDIYCP!TS
thus will be af fccted at a rate thdt is
dcpcndent upon their indivitlual tellper- Scagrassec qrow in a wide variety of
attlrc tolerances. Ilost algae associated sediments froir fine mud.; to coarse sands,
with tropical seagrass beds are more depending on the type of source rf?ateriii'I,
sensitive to t"n~7tii stress than the the prevailing physical flaw regi4n@, and
.;@itgrasses (Zieman lQ75a). Ira shallow the density of the Seagrdss blad~s. AS
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