Magnitude and variability of benthic and pelagic metabolism in a temperate coastal lagoon

In shallow coastal systems where most of the seafloor lies within the photic zone, benthic photoautotrophy is likely to play a key role in regulating carbon and nitrogen cycling. We measured dissolved inorganic carbon exchanges in seasonal microcosm incubations to determine the relative importance of benthic (with and without macroalgae) and water column metabolism at 3 sites located along a nutrient gradient in a coastal barrier-island lagoon on the eastern shore of Virginia. When coupled with data on in situ biomass, the incubations clearly indicated a seasonality in the dominance of primary producers at the sites, with benthic primary producers being the most important early and late in the growing season and phytoplankton dominating following a mid-summer period of macroalgal decay and the release of algal-bound nutrients to the water column. The benthos was always net autotrophic, and differences in community metabolism at the 3 sites appeared to be largely due to variation in macroalgal biomass. Macroalgae accumulated to a greater extent in the mid-lagoon and accounted for up to 96% of benthic production. The water column was net heterotrophic throughout the lagoon except following the mid-summer macroalgal collapse, when phytoplankton biomass was highest at the 2 sites closest to the mainland. Benthic microalgal production also increased in importance following the macroalgal decline, suggesting that competition for light and possibly nutrients limited benthic microalgal production when macroalgal densities were high. Overall, there was a distinct seasonality in patterns of total metabolism within the bay, where all sites were net autotrophic in the spring and summer and net heterotrophic in the fall. Up to 8 g N m -2 accumulated in macroalgal biomass throughout spring and early summer at the mid-lagoon shoal site, slowing nutrient transport through the lagoon. Nutrient turnover rates were higher during the period when phytoplankton and benthic microalgae were the dominant primary producers. This study illustrates the functional importance of different primary producer communities to carbon metabolism and to the temporary retention of nutrients in lagoonal land-margin ecosystems.