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

1 eaves pl us their epiphytes averaged 1.4%
N. Bjorndal found that grazed turtle
grass leaves averaged 0.35% N (AFDw)
higher than ungrazed leaves, and Thayer
and Engel (MS. in preparation) found a
nitrogen content of 1.55% (DM) in the
esophagus of Chelonia. Zieman and Iverson
(in preparat'found that there was a
decrease in nitrogen content with age and
epiphytization of seagrass 1 eaves. The
basal portion of turtle grass leaves from
St. Croix contained 1.6% to 2.0% N on a
dry weight basis, while the brown tips of
these leaves contained O.G% to 1.1% N,
and the epiphytized tips ranged from 0.52
to 1.7% N. Thus the current evidence
would indicate that the green seagrass
leaves contain more nitrogen than either
the senescent leaves or the leaf-epiphyte
covpl ex. By successively recropping
leaves from a plot, the turtle maintains
a diet that is consistently higher
in nitrogen and lower in fiber content
than whole 1 eaves (6jorndal 1980).
Grazing on seagrasses produces
another effect on sea turtles. In the
Gulf of California (Felger and Moser 1973)
and Nicaragua (Mortimer, as reported by
Bjorndal 1980), witnesses reported that
turtles that had been feeding on seagrasses
were considered to be good tasting,
while those that were caught in areas
where they had fed on algae were considered
to be "stinking" turtles with a defini
te inferior taste.
Thayer and Engel (gS. in preparation)
suggested that grazing on seaorasses can
short-circuit the time frame of decornposition.
They showed that an intermediatesized
green turtle which consumes about
300 g dry weight of leaves and defecates
about 70 g dry weight of feces daily, does
return nitrogen to the environment at a
%ore rapid rate than occurs for the decomposition
of a similar amount of leaves.
They point out that this very nutrientrich
and high nutritional quality fecal
matter should be readily available to
tritivores. It is also pointed out that
is matter is probably not prodaced
entirely at the feeding site and thus
provides an additional interconnection
between grassbeds and adjacent hahi tats.
common throughout the Caribbean, especially
in the mainland areas, but is now
greatly reduced in range and population.
Manatees live in fresh or marine waters;
and in Florida, most manatee studies have
focused on the manatee's ability to control
aquatic weeds. Panatees, which weigh
up to 500 kg (1,102 Ib), can consume up to
20% of their body weight per day in aquatic
plants.
When in marine waters, the manatee
apparently feeds much like its fellow
sirenians, the dugongs. The dugongs use
their rough facial bristles to dig into
the sediment and grasp the plants, These
are uprooted and shaken free of adhered
sediment. Husar (1975) stated that feeding
patches are typically 30 by 60 cm (12
by 24 inches) and that they form a conspicuous
trail in seagrass beds. This author
has observed manatees feeding in Thal assia
beds in much the same manner. The patches
cleared were of a similar size as those
described for the dugongs, and rhizome
removal was nearly complete. The excess
sediments from the hole were mounded on
the side of the holes as if the manatee
had pushed much of it to the side before
attempting to uproot the plants.
Manatees would seem to be more
linited in their feeding range because of
sediment properties, as they reauire a
sediment which is sufficiently unconsol i-
dated that they may either root down to
the rhizome or grasp the short shoot and
pull it out of the sediment. Areas where
manatee feeding and feeding scars were
observed were characterized by soft sediments
and lush growth of turtle grass and
Halimeda in mounded patches. Nearly a11
areas in which sediments were more cons01 -
idated showed no signs of feeding. In the
areas where the manatees were observed,
the author found that he could readily
shove his fist 30 cn (12 inches) or more
into the sediments, while in the adjacent
ungrazed areas, maximum penetration was
only a few centineters and it was impossible
to revove the rhizomes without a
shovel .
Like the turtles, the Caribbean For the majority of animals that
rlanatee (Trichechus manatus) fortnerly was derive all or part of their nutrition fron
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