MEPS 312:1-13 (2006)  -  doi:10.3354/meps312001

ABSTRACT: The impact of hydrodynamic regime on rates of nutrient uptake for a seagrass community and for individual components of the community (Thalassia testudinum, epiphytes, phytoplankton, microphytobenthos) was quantified through the deployment of a field flume and application of ¹⁵N-labeled ammonium and nitrate tracers. Ammonium uptake rates for the community and for seagrass leaves and epiphytes were enhanced with increased bulk velocity (uptake rate ∝ U_b^{0.57 to 0.70}) and Reynolds shear stress at the top of the canopy (uptake rate ∝ τ_R^{0.32 to 0.40}); thus, relationships expected for mass-transfer limitation apply for the entire assemblage and individual components that form the canopy. Nitrate uptake rates for the community and for epiphytes <35 µm were also enhanced with increased bulk velocity (uptake rate ∝ U_b^{0.40 to 0.67}) and Reynolds shear stress (uptake rate ∝ τ_R^{0.19 to 0.32}), but less so than ammonium uptake rates. For all components, uptake rates for NO₃^– were lower than those for NH₄⁺, suggesting that nitrate uptake was affected by a biological factor (e.g. availability of nitrate reductase). Epiphytes and phytoplankton each accounted for 40 to 45% of the total ammonium and nitrate recovered; however, these components contributed the least to total particulate nitrogen in the community. Uptake by seagrass leaves and sediments containing microphytobenthos each represented <5% of the ammonium and <10% of the nitrate recovered, but contained the majority of particulate nitrogen. Our results emphasize the importance of epiphytes and phytoplankton in nitrogen uptake from the water column over the short term, and reaffirm that seagrasses and sediments play an integral role in the long-term retention of nitrogen within the canopy.

KEY WORDS: Ammonium uptake · Nitrate uptake · Seagrass · ¹⁵N · Isotope label · Thalassia testudinum · Epiphytes · Water flow