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

Currently the main 1 imitations of the
carbon isotope method are equipment and
interpretation. It requires use of a mass
spectrometer which is extremely costly,
a1 though today a number of labs will process
samples for a reasonable fee. The
interpretation can become difficul t when
an organism has a 613c value in the middle
ranges. If the 613c value is at one extreme
or another, then interpretation is
straightforward. However, a mid-range
value can mean that the animal is feeding
on a source that has this sL3c value or
that it is using a mixed food source which
averages to this value. Recent studies
util izing both isotopes of carbon and sulfur
(Fry and Parker 1982) and nitrogen
(Macko 1981) show much promise in determining
the origin of detrital material as
well as the organic natter of higher
organi sms. Know1 edge of the feeding ecology
and natural history of the organism is
needed, as is an a1 ternate indicator.
3.6 PLANT CONSTITlfENTS
Recognition of the high productivity
of seagrasses and the relatively low level
of direct grazing has led to questions
regarding their value as food sources.
Proximate analyses of seagrasses in south
Florida, particularly turtle grass, have
been performed by many authors (Burkhol der
et a1 . 1959; Eauersfeld et a1 . 1969; Walsh
and Grow 1072; Lowe and Lawrence 1976;
Vicente et a1 . 1978; Bjorndal 1980; Dawes
and Lawrence 1980); their results are
summarized in Table 7. As noted by Dawes
and Lawrence (1980), differences in the
preparation and analysis of samples, as
well as low nunbers of samples trsed in
sorne studies, make data comparison dif-
Ficul t.
The reported ash content of turtle
grass leaves ranges fron 45% dry weight
for unwashed samples down to around 25%
for sanples washed with fresh water.
Leaves wasqed in seawater contained 29%
+/- 3.52 to r14Z +/- 6.7:; ash (Dawes and
Larrrence 1380).
Values for the protein content of
leaves vary from a low of 37 of dry weight
for unwashed turtle grass leaves with
epiphytes (Dawes et al. 1979) to 29.7% for
leaves washed in distilled water (Walsh
and Grow 1972), a1 though numbers typically
fall in the range of 10% to 15% of dry
weight. Protein values rnay be suspect if
not measured directly, but calculated by
extrapol ating from percent nitrogen. In
grass beds north of Tampa Bay, Dawes and
Lawrence (1980) found that protein 1 eve1 s
of turtle grass and manatee grass leaves
varied seasonally, ranging from 8% to 22%
and 8% to 13%, respectively, with the
higher levels occurring in the summer and
fall. The protein content of shoal grass
ranged from a low of 14% in the fall up to
19% in the winter and summer. Tropical
seagrasses, particularly turtle grass,
have been compared to other plants as
sources of nutrition. The protein content
of turtle grass leaves roughly equaled
that of phytoplankton and Bermuda grass
(Burkholder et a1 . 1959) and was two to
three times higher than 10 species of
tropical forage grasses (Vicente et al.
1978). Wal sh and Grow (1972) compared
turtle grass to grain crops, citing studies
in which 114 varieties of corn contained
9.M to 16% protein; grain sorghum
contained between 8.62 and 16.5%; and
wheat was lowest at 8.3% to 12%. A1 though
several studies have included measurements
of carhohydrates (Table 7), it is impractical
to compare much of the data because
various analytical methods were employed.
Studies using neutral detergent fiber
(NDF) analyses found that cell wall carbohydrates
(cell ul ose, hemicell ul ose, and
1 ignin) made up about 45% to 602 of the
total dry weight of turtle grass leaves
(Vicente et al. 1978; Bjorndal 1980).
Dawes and Lawrence (1980) reported that
insoluble carbohydrate content in the
leaves of turt7e grass, manatee grass, and
shoal grass was 34% to 46%. The rhizones
of seagrasses are aenerally higher in
carbohydrates than are the leaves. Oawes
and Lawrence (1980) found that soluble
carhohydrates in turtle grass and manatee
grass rhizomes varied seasonally, indicating
the production and storage of starch
i~ summer and fa1 1 . These authors, however,
#ere working in an area north of
Tampa Bay, where such seasonal changes
would he more pronounced than in the
southern part of Florida and the Keys.