Effects of Water Velocity and Canopy Morphology on Ammonium Uptake by Seagrass Communities

In the research presented here, we examine the effects of water velocity and canopy morphology on rates of nutrient uptake by seagrass communities. Ammonium uptake rates for two types of seagrass communities, Halodule wrightii and Thalassia testudinum, are measured over a range of velocity using a field flume. The field flume allows independent measurements of uptake by communities of natural composition and condition. We compare our results with those estimated using empirically derived engineering equations that describe transport processes to rough surfaces in order to explore the possibility that uptake rates can be predicted from these equations. We also investigate the possibility that the seagrass canopy alters the characteristics of water flow within the community, which is reflected by the friction imposed by the canopy (the friction coefficient) on the moving water. Our results indicate that ammonium uptake by seagrass communities is dependent on water velocity. Further, seagrasses affect characteristics of water flow within the community that are reflected in rates of ammonium uptake. Empirically derived engineering equations used with measured friction coefficients yield expected Stanton numbers (a non-dimensional ratio of flux to a surface to advection by a surface) that are within 95% of those measured in the field flume. Thus, the capacity of these communities to remove ammonium from the water column can be predicted using empirically derived engineering equations that describe the transport of chemicals between a fluid and a rough surface.