Seagrass communities are composed of numerous
organisms that depend on water-column nutrients for
metabolic processes. The rate at which these
organisms remove a nutrient from the water column can
be controlled by physical factors such as hydrodynamic
regime or by biological factors such as speed of
enzyme reactions. The impact of hydrodynamic regime
on rates of nutrient uptake for seagrass (Thalassia
testudinum) communities and for organisms that
comprise the community (seagrass, epiphytes,
phytoplankton, and microphytobenthos) was quantified
in a series of field flume experiments employing the use
of 15N-labeled ammonium and nitrate.

Rates of ammonium uptake for the entire community
and for seagrass leaves and epiphytes were
significantly dependent on bulk velocity, bottom shear
stress, and the rate of turbulent energy dissipation.
Relationships between uptake rates and these
parameters were consistent with mass-transfer theory
and suggest that the effect of water flow on ammonium
uptake is the same for the benthos as a whole and for
the organisms that form the canopy. In addition,
epiphytes on the surface of T. testudinum leaves were
shown to depress leaf uptake by an amount
proportional to the area of the leaf covered by
epiphytes. Water flow influenced rates of nitrate uptake
for the community and the epiphytes; however, uptake
rates were depressed relative to those for ammonium
suggesting that uptake of nitrate was also affected by
biological factors such as enzyme activity. Epiphytes
reduced uptake of nitrate by the leaves; however, the
amount of reduction was not proportional to the extent
of epiphyte cover, which provided further evidence that
nitrate uptake by T. testudinum leaves was biologically
limited.

As an additional component of the research,
hydrodynamic regime of a mixed seagrass and coral
community in Florida Bay was characterized using an
acoustic Doppler velocimeter. Hydrodynamic
parameters estimated from velocity data were used in
mass-transfer equations to predict nutrient uptake by
the benthos over a range of water velocity. Measured
rates of uptake from field flume experiments conducted
in the same community confirmed that hydrodynamic
data could be used to accurately predict nutrient
transport to the benthos under natural flow conditions.