The role of hydrodynamics in structuring in situ ammonium uptake within a submerged macrophyte community

In low-nutrient, macrophyte-dominated coastal zones, benthic ammonium (NH4+) uptake may be influenced by the structural properties of plant canopies via their effect on near-bed hydrodynamics. Using a dual-tracer (uranine and 15NH4+) method that does not require enclosures, we examined how this process affects nutrient uptake rates within a tidally dominated, patchy Caulerpa prolifera–Cymodocea nodosa landscape. NH4+ uptake was determined by calculating tissue 15N excesses and correcting for 15N enrichment as derived from uranine concentration. Vertical hydrodynamic profiles were measured in the downstream flow direction from outside to inside of the C. nodosa bed by using an array of acoustic Doppler velocimeters. The transition from a C. prolifera to a C. nodosa bed included a change in both benthic canopy properties (short and dense to tall and sparse) and sediment topography (0.2-m increase in water column depth) that resulted in an increase in longitudinal advection and turbulent diffusivity within the C. nodosa canopy between 0.5 and 1.5 m from the leading edge. Vertical differences in canopy water exchange appeared to explain variations in uptake between biotic functional groups; however, no clear differences in longitudinal uptake were found. Using in situ labeling, this study demonstrated for the first time the role of hydrodynamics in structuring NH4+ uptake within an undisturbed, patchy macrophyte landscape.