Effects of nutrient enrichment and shading on sediment primary production and metabolism in eutrophic estuaries

The impact of anthropogenic eutrophication on the productivity and metabolism of estuarine sediments has received relatively little attention. In this study, we investigated the separate and combined effects of decreased light availability and sediment nutrient enrichment, 2 of the most important impacts of anthropogenic eutrophication, on sediment primary production and metabolism in 2 eutrophic subestuaries of Weeks Bay (Alabama, USA), seasonally, over a year. We found a significant effect of shading on both sediment primary production and metabolism, mostly due to decreased photosynthetic rates of benthic microalgae coupled with simultaneous increases in sediment respiration rates. The ratio of mean production in non-shaded plots to that in shaded plots was close to 1 and tended to occur at natural bottom light intensities <100 μmol m–2 s–1 during the summer experiments, whereas higher light intensities tended to increase the ratio (i.e. higher values under non-shaded conditions). The ratio tended to decrease and become closer to 1 at the highest values of light intensity recorded (>600 μmol photons m–2 s–1). Nutrient enrichment had little effect on sediment primary production and metabolism in the estuarine sites studied; however, in some instances nutrient enrichment influenced the negative effects of reduced light on sediment net production. The results demonstrate that the impact of light reduction on sediment primary production and metabolism in a turbid, nutrient-rich estuary is greater than that of additional sediment nutrient enrichment.