Uptake of dissolved inorganic nitrogen by shallow seagrass communities exposed to wave-driven unsteady flow

摘要

ABSTRACT: In natural seagrass systems, regular oscillatory motion—like that caused by surface wind-waves—enhances uptake of dissolved inorganic nitrogen (DIN) relative to current-driven, unidirectional flows. A mobile field flume was deployed to measure the uptake of DIN by intact shallow seagrass communities exposed to unidirectional, oscillatory, and combined flows. The flume volume was spiked with 15N-labeled DIN to measure nutrient uptake rate constants for the entire system (S) and to determine height-specific nutrient uptake rates of seagrass epiphytes (ρ). In oscillatory and combined flows, S depended positively on the water speed, but also inversely on a modified Keulegan-Carpenter number (KC). This ratio compares the wave-orbital excursion distance to canopy element spacing. Experiments characterized by relatively low KC values were associated with significantly higher uptake efficiencies than experiments characterized by high KC values. Uptake efficiencies for high KC conditions were similar to those measured in comparable unidirectional flows. Measured canopy flow attenuation was also found to increase with KC, a result in line with expectations from a parameterized conceptual model. Damping variability alone could not explain the observed oscillatory flow uptake enhancement however, a result that seemingly highlights the role of both element flexural response and unsteady boundary layer mechanics in the exchange process. In all flow conditions, DIN uptake rates of epiphytes harvested from the bottom of the blades (ρ_bot) were lower than those for epiphytes harvested from the upper portion of the blades (ρ_top). While uptake by the top epiphytes exhibited flow dependency, uptake by bottom epiphytes did not.