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

Abstract

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 sea- grasses, 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 ben- thic 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 colo- nized seagrass beds. Mass-transfer coefficients (S) for ammonium, predicted using velocity data and estimates of bottom friction, ranged between 0.35 × 10 4 and 1.91 × 10 4 m s -1 for current velocity between 0.03 and 0.39 m s -1 . Values of S measured 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 -2 s -1 ) and highlighted the importance of temporal variation in both current velocity and nutrient con- centration in driving rates of nutrient uptake. Additional field flume experiments using 15 N-labeled ammonium enabled us to examine flow-dependent uptake for a number of organisms within the com- plex 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.