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

4.5 STRUCTURAL AND PROCESS SUCCESSION XM
SEAGRASSES
As species succession occurs in a
shallow marine syste1.1, important structural
changes occur. Because seagrass
systems do not have woody structural components
and only possess re1 atively sirqp-
1 i stic canopy structure, the main structural
features are the leaf area and biomass
of the leaves as well as the root and
rhizome material in the sediment. The
most obvious change with community development
is the increase in leaf area. This
provides an increase in surface area for
the colonization of epiphytic a1 gae and
fauna, with the surface area of the cl inax
community being many times that of either
the pioneer seagrass, shoal grass, or the
initial algal colonizers. In addition to
providing a substrate, the increasing 1 eaf
area also increases the current baffl in!:
and sediment-trappi ng effects, thus enhancing
internal nitrogen cycl ing .
As orga~isms grokr and reproduce in
the environr?erit, they bring about changes
in their surroundings. In doing so these
organ1 srlis Frequently nodi fy the environ-
~wnt in a way that no longer favors their
continual growth. McArthur and Connel 1
(1366) stated that this process "gives us
d clue to all of the true replacerrents of
succession: each species alters the environnent
in such a way that it can no
longer grow so successfully as others".
In a shallow hrater successional sequence
leading to turtle grass, the early
stages are often characterized by a low
supply of organic matter in the sedir'lent
and open nutrient supply; that is, the
cornunity re1 ies on nutrients bein9
brought in froin adjacent areas by water
movement as opposed to in si tu regeneration.
Mi th the development from rhizophytic
algae to turtle grass, there is a progressi
ve devel aprnent in the he1 owground
biomass of the co~tmunity as well as the
portion exposed in the water column. With
the progressive increase in leaf area of
the plants, the sedirnent trapping and particle
retention increase. This material
adds organic matter to further fuel the
sedimentary microbial cycles. A1 though
various segments of thi s successional
sequence have been rr~easured by nutaerous
authors, the most complete set of data has
recently been compiled by Will iams (1981)
in St. Croix (Table 8). In St, Croix,
where the data were collected, as on many
low, small islands with little rainfall,
the clinax is commonly a rnixture of turtle
grass and manatee grass, In south Florida,
with its higher rainfall and runoff, the
climax more commonly is a pure turtle
grass stand. In turtle grass beds in
south Florida, Capone and Taylor (1977,
1980) found that nitrification was highest
on the developing periphery of the beds
and lower in the centers where particulate
trapping and retention were grea ter. Addi
tional ly , mature ecosystems, both marine
and terrestrial, seem to he based primarily
on the detrital food weh which aids in
conserving both carbon and nitrogen, as
direct grazing is quantitatively low in
these systens.