Prediction and validation of flow-dependent uptake of ammonium over a seagrass-hardbottom community in Florida Bay

Hydrodynamic surveys and field flume experiments were carried out to characterize water flow and measure nutrient uptake over a shallow hardbottom flat sparsely colonized by seagrasses, a complex community type commonly found along corridors linking Florida Bay and the Florida reef tract. Acoustic Doppler velocimeter profiles collected in tide-driven flows revealed benthic hydrodynamic conditions indicative of disturbed boundary layer flow; attenuation of flow near the benthos and measures of bottom friction were considerably less than observed in densely colonized seagrass beds. Mass-transfer coefficients (S) for ammonium, predicted using velocity data and estimates of bottom friction, ranged between 0.35 × 10⁴ and 1.91 × 10⁴ m s–1for current velocity between 0.03 and 0.39 m s–1. Values ofSmeasured using a field flume were within the same range as predicted values, validating that ammonium uptake by the community is occurring near the mass-transfer limit. Mass-transfer coefficients fell slightly above those previously measured for low-relief coral rubble and below those for dense seagrass canopies, thereby confirming a close link between bottom roughness and mass transfer. Predicted ammonium uptake based on ambient velocity and nutrient concentrations varied considerably over the tidal cycle (range = 0.014 to 0.094 μmol NH₄ m–2s–1) and highlighted the importance of temporal variation in both current velocity and nutrient concentration in driving rates of nutrient uptake. Additional field flume experiments using15N-labeled ammonium enabled us to examine flow-dependent uptake for a number of organisms within the complex community. Uptake rates were found to vary among seagrasses, macroalgae, and finger corals, perhaps due to physiological or morphological differences or varying locations within the canopy.