Salinization of coastal freshwater wetlands; effects of constant versus fluctuating salinity on sediment biogeochemistry

Globally, coastal lowlands are becoming more saline by the combined effects of sea level rise, land subsidence and altered hydrological and climatic conditions. Although salinization is known to have a great influence on biogeochemical processes, literature shows contrasting effects that challenge the prediction of future effects. In addition, the effects of fluctuating salinity levels, a more realistic scenario than constant levels, on nutrient cycling in coastal wetland sediments have hardly been examined. A better understanding is therefore crucial for the prediction of future effects and the definition of effective management. To test the effects of constantly brackish water (50 mmol Cl l−1, 3.2 psu) or fluctuating salinity (5–50 mmol Cl l−1), versus constantly low salinity (5 mmol Cl l−1, 0.32 psu) on nutrient biogeochemistry, we conducted a controlled laboratory experiment with either peat or clay sediments from coastal wetlands. Increased salinity showed to have a fast and large effect. Sediment cation exchange appeared to be the key process explaining both a decrease in phosphorus availability (through calcium mobilization) and an increase in nitrogen availability, their extent being strongly dependent on sediment type. Supply of brackish water decreased surface water turbidity and inhibited sediment methane production but did not affect CO2 production. Constant and fluctuating salinity levels showed similar longer term effects on nutrient and carbon cycling. The contrasting effects of salinization found for nitrogen and phosphorus, and its effects on water turbidity indicate major ecological consequences for coastal wetlands and have important implications for water management and nature restoration.