Microscale effects of light on H2S and Fe2+ in vegetated (Zostera marina) sediments

Interactions between seagrasses and associated sediment pore water chemistry in response to light–dark conditions were investigated in this study. Vertical profiling using a solid-state, gold–mercury amalgam made possible the observation of fine-scale (<1 mm) chemical changes, both spatial and temporal, for Fe2+ and H2S. Sediments vegetated with Zostera marina and nearby (<1 m) unvegetated sediments in Yaquina Bay, OR were studied. The composition of the pore water varied both laterally and vertically, and lateral and vertical variability were similar. The variability was an order of magnitude greater in vegetated sediments. Pore waters in vegetated sediments exhibited a major decrease in concentrations of H2S and Fe2+ in the root zone, relative to dark conditions, with seagrass exposure to light (200 μE m−2•s−1). Similar responses to light–dark conditions were not observed in the unvegetated sediments. These results demonstrate the major impact seagrasses can have on redox-reactive components of sediments near the sediment–water interface.