Macroalgae Mediation of Dissolved Organic Nitrogen Fluxes in a Temperate Coastal Lagoon

Abstract

The activity of the benthos, including benthic plants, is important in driving the overall system dynamics in shallow lagoons, due to the high ratio of sediment surface area relative to water volume. In Hog Island Bay, benthic macroalgae appear to be a key regulator of DON dynamics, both while alive and following senescence. We investigated the role of macroalgae in mediating water column concentrations and sediment-water column fluxes of DON across a nutrient gradient in Hog Island Bay, a shallow macroalgal-dominated back-barrier lagoon located on the Virginia Coast. Sediment-water column exchanges of DON, urea and DIN were measured in sediment cores with and without macroalgae (Ulva lactuca) at three subtidal sites from the mainland to the barrier islands in the fall of 1997 and the spring and summer of 1998. The summer sampling dates bracketed a large macroalgal bloom in the mid-lagoon. Dissolved organic nitrogen was an important component (52-98%) of the total dissolved nitrogen pool in Hog Island Bay waters and made up the majority of the sediment N flux to the water column. Macroalgae impacted benthic-pelagic coupling by preventing diffusion of DIN from the water column to the sediments and by intercepting urea fluxes from the sediment to the water column. Closest to the mainland and closest to the barrier islands, at sites with low macroalgal biomass, sediment-water column fluxes of DIN and urea-free DON were negligible or directed into the sediments. Fluxes of urea from the sediment to the water column were significant at both sites, and may play an important role in satisfying macroalgal N demand, especially at the low N island site. Overall, urea was 32% of the mean DON flux from the sediments to the water column. Fluxes of urea-free DON were highest in the mid-lagoon, where macroalgal biomass was highest. The highest overall flux rates of DON (>38mmol m−2d−1) and DIN (>33mmol m−2d−1) were measured following an isolated crash of a large macroalgal mat. These release rates were not sustained for long, however, and we estimated that the majority of the N contained in the decomposing macroalgal tissues disappeared in <2 weeks. In addition to release of organic N following senescence, macroalgae ‘leak’ DON into the water column during active growth; release of DON increased by 250% in cores incubated with U. lactuca relative to cores with sediment only. These algae function as a conduit whereby water-column DIN and sediment urea are taken up and released to the water column as DON over relatively short (hours) time scales. This conversion of bioavailable dissolved N to PON and DON by macroalgae is likely to be important to overall system metabolism and to the retention of N within the lagoon.