| Coastal lagoons are a common land-margin feature world-wide. The shallow nature of lagoons leads to substantial benthic-pelagic coupling and dominance by benthic autotrophs. Increased inputs of nitrogen (N) from coastal watersheds may cause nuisance macroalgal blooms. However, little is known about the dynamics of dissolved organic nitrogen (DON), an important component (50–95%) of the total dissolved N (TDN) pool. The objectives of this dissertation were to: (1) quantify benthic-pelagic fluxes of specific dissolved organic and inorganic N (DIN) compounds along an environmental gradient in Hog Island Bay, an algal-dominated lagoon at the Virginia Coast Reserve LTER site, (2) determine how uptake and release by benthic macro- and microalgae impacts DON cycling and (3) estimate the turnover and retention of N by macroalgae.; Sediment-water column DON fluxes were highly variable but comparable to DIN fluxes; fluxes of individual compounds (urea and dissolved free and combined amino acids [DFAA, DCAA]) often proceeded concomitantly in different directions. Where sediment metabolism was net autotrophic due to microalgal activity, TDN fluxes, mostly comprised of DIN, urea and DFAA, were directed into the sediments. Heterotrophic sediments, particularly beneath macroalgal mats, were a net source of TDN, mostly as DIN. Isolated crashes of dense macroalgal mats resulted in an order of magnitude increase in DIN and DON release. When present, living macroalgae controlled benthic-pelagic coupling by intercepting DIN, urea and DFAA fluxes and releasing DON, mostly as DCAA. Separate estimates from 15N isotope dilution field experiments showed that macroalgae release ∼50% of total N uptake. Ulva lactuca took up DIN, urea and DFAA throughout the lagoon, but DON uptake was only important where DIN was low. In the laboratory, urea and AA uptake rates were consistently higher for U. lactuca than for Gracilaria tikvahiae . Uptake and 15N and 13C assimilation rates varied for individual amino acids, suggesting different uptake mechanisms. Overall, macroalgae act as a conduit whereby both organic and inorganic N are taken up, transformed, and re-released to the water column on short time scales (minutes-hours). Benthic algae thus clearly influence benthic-pelagic coupling and the retention of N moving across the land-sea interface. |
