Drivers of sulfide intrusion in Zostera muelleri in a moderately affected estuary in south-eastern Australia

Abstract

The seagrass Zostera muelleri Irmisch ex Asch. is abundant in estuaries in Australia and is under pressure from coastal developments. We studied sulfide intrusion in Z. muelleri along a gradient of anthropogenic impact at five stations in the Wallis Lake estuary, Australia. Results showed differences in sediment biogeochemical conditions, seagrass metrics as well as nutrient content and sulfide intrusion along the gradient from the lower estuary (affected) to the lagoon (unaffected). Sulfide intrusion was driven by complex interactions and related to changes in seagrass morphology and sediment biogeochemistry and was modified by the exposure to wind and wave action. The sediments in the lower estuary had high contributions from phytoplanktonic detritus, whereas the organic pools in the lagoon were dominated by seagrass detritus. Despite high concentrations of organic matter, sulfide intrusion was lower at stations dominated by seagrass detritus, probably because of lower sulfide pressure from the less labile nature of organic matter. Porewater diffusive gradients in thin-film (DGT) sulfide samplers showed efficient sulfide reoxidation in the rhizosphere, with high sulfur incorporation in the plants from sedimentary sulfides being likely due to sulfate uptake from reoxidised sulfide. This is a unique adaptation of Z. muelleri, which allows high productivity in estuarine sediments.