Benthic algae control sediment–water column fluxes of organic and inorganic nitrogen compounds in a temperate lagoon

Coastal lagoons are a common land-margin feature worldwide and function as an important filter for nutrients entering from the watershed. The shallow nature of lagoons leads to dominance by benthic autotrophs, which can regulate benthic–pelagic coupling. Here we demonstrate that both microalgae and macroalgae are important in controlling dissolved inorganic as well as organic nitrogen (DIN and DON) fluxes between the sediments and the water column. Fluxes of nitrogen (NH , NO , DON, urea, and dissolved free and combined amino acids [DFAA, 1 2 4 3 DCAA]) and O2 were measured from October 1998 through August 1999 in sediment cores collected from Hog Island Bay, Virginia. Cores were collected from four sites representing the range of environmental conditions across this shallow lagoon: muddy, high-nutrient and sandy, low-nutrient sites that were both dominated by benthic microalgae, and a mid-lagoon site with fine sands covered by dense macroalgal mats. Sediment–water column DON fluxes were highly variable and comparable in magnitude to DIN fluxes; fluxes of individual compounds (urea, DFAA, DCAA) often proceeded simultaneously in different directions. Where sediment metabolism was net autotrophic because of microalgal activity, TDN (total dissolved nitrogen) fluxes, mostly comprised of DIN, urea, and DFAA, were directed into the sediments. Heterotrophic sediments, including those underlying macroalgal mats, were a net source of TDN, mostly as DIN. Macroalgae intercepted sediment–water column fluxes of DIN, urea, and DFAA, which accounted for 27–75% of calculated N demand. DON uptake was important in satisfying macroalgal N demand seasonally and where DIN concentrations were low. Up to 22% of total N uptake was released to the water column as DCAA. Overall, macroalgae assimilated, transformed, and rereleased to the water column both organic and inorganic N on short (minutes–hours) and long (months) time scales. Microalgae and macroalgae clearly regulate benthic–pelagic coupling and thereby influence transformations and retention of N moving across the land–sea interface. Coastal lagoons, like deep estuaries, process nutrients traveling from coastal watersheds across the land margin to the open ocean. The shallow nature of lagoonal estuaries leads to a high ratio of surface area to water volume and the benthos is usually within the photic zone. As a result, benthic primary production is often more important than pelagic production, and sediment mineralization of nutrients may drive overall biogeochemical cycling (Martens 1982; SandJensen and Borum 1991; Anderson et al. 2003). Seagrasses, 1 To whom correspondence should be addressed. Present address: Department of Environmental Science and Policy, University of California at Davis, One Shields Avenue, Davis, California 95616 ([email protected]).