Ecosystem production and respiration in response to eutrophication in shallow temperate estuaries

Abstract

Ecosystem metabolism was measured from diel changes in free-water dissolved oxygen in 3 shallow subestuaries of Waquoit Bay on Cape Cod, Massachusetts, USA. Our goal was to relate metabolism to the different levels of nitrogen loading to the subestuaries. Automatic meters recorded O2, temperature, and conductivity continuously at 2 depths in each estuary for 5 to 25 d, 7 to 10 times throughout a year. Day-to-day rates of daytime ecosystem production and nighttime respiration were quite variable, up to ±10 g O2 m-2 d-1 for consecutive days. Daytime ecosystem production was correlated with daily irradiance. At equivalent values of daily insolation, higher production rates were measured in the more heavily N-loaded estuary. Enrichment also resulted in higher rates of respiration. Daily rates of respiration and production were uncorrelated during most deployments, yet a strong correlation emerged over the year because both rates changed with season. Annual ecosystem net and gross production (ENP and EGP) increased with N loading in the 3 estuaries. Significant ENP (about 180 g O2 m-2yr-1) occurred in the highly enriched system; in the 2 other estuaries receiving lower enrichment rates system respiration (R) and production (P) both increased and thus ENP was small (P~R). On an annual basis, metabolism in these estuaries is in balance or slightly autotrophic.