Light regulation of benthic sulfate reduction rates mediated by seagrass (Thalassia testudinum) metabolism

The effects of light reduction on community metabolism and sediment sulfate reduction rates (SRR) were assessed experimentally in a shallow (< 2.0 m) seagrass (Thalassia testudinum) meadow along Florida's north-central Gulf coast. Nine experimental plots (1.5 m x 1.5 m) were shaded differentially to achieve a 0-90% gradient in light reduction within the seagrass meadow. Gross primary production and net community production (NCP), estimated with in situ benthic chamber incubations, decreased with increasing light reduction. The compensation irradiance for community metabolism, i.e., the shading level at which NCP shifted from net autotrophic to net heterotrophic, was determined to be 52.5% of the incoming irradiance at canopy height in the seagrass bed (308.7 μE m^-2 s^-1 PAR at noon). Sediment SRR, determined with the use of a ³⁵S-SO₄^2- radiotracer technique, increased quickly (within 5 d) and markedly with increased shade, i.e., simulated light reduction. SRR increased 50-fold when shading exceeded the light compensation point for the seagrass community, rendering the community net heterotrophic. Five days after restoring ambient light conditions, SRR had decreased sharply for all shading treatments. The observed decrease in NCP, coincident with the increase in the SRR with light reduction, suggests that light reduction has an indirect influence on sediment SRR mediated through its effect on seagrass metabolism.