Understanding sediment nutrient cycling in a hypersaline coastal lagoon using hydrogel-based passive sampling techniques.

The Coorong, South Australia, is a Ramsar-listed hypersaline coastal lagoon system of significant ecological and cultural importance. Despite facing environmental challenges such as reduced river flow and eutrophication, there is limited knowledge of organic matter fate and nutrient cycling in this system. Understanding nutrient movement and transformation within the Coorong is crucial for effective ecosystem management. This study applied diffusive equilibrium in thin films (DET) and diffusive gradient in thin films (DGT) techniques to measure dissolved nutrients (ammonium and phosphate), dissolved ferrous iron, and sulfide in sediment porewaters across a strong salinity gradient. Sampling sites were selected to capture different environmental conditions, including areas colonised by microphytobenthos and the aquatic macrophyte Ruppia tuberosa (hereafter called Ruppia). Concentration profiles were examined in both light and dark conditions to assess biotic influences on nutrient dynamics and sediment redox conditions. Sediment porewaters in deeper sites of the South Lagoon exhibited high ammonium (941-1718 μmol/L), phosphate (83-171 μmol/L), and sulfide (66-79 μmol/L) concentrations, with negligible nitrate, indicating highly anoxic conditions unsuitable for most benthic fauna. Shallow sediments showed varying concentrations of ammonium, phosphate, dissolved ferrous iron, and sulfide across different sites and depths, influenced by Ruppia and microphytobenthos. The site colonised by Ruppia contained the lowest porewater ammonium concentrations (∼30 μmol/L), suggesting that macrophyte presence can mitigate nutrient buildup in sediments. These findings indicate that sediment quality and nutrient cycling in the South Lagoon may be improved by enhancing the presence and biomass of aquatic macrophytes and macroinvertebrates.