Saltwater Ponds: Biogeochemically Dynamic Components of Salt Marsh Ecosystems

Saltwater ponds can be prominent features of salt marsh ecosystems. Located within the high marsh, these shallow depressions are continuously submerged and only hydrologically connected to saltwater tidal creeks on spring tides. The spatial extent of marsh ponds is expected to increase with sea level rise yet the consequences of pond expansion on marsh ecosystem processes, including organic matter production, burial and export, are largely unknown. To better understand how ponding affects the biogeochemistry of salt marshes, we characterized water column and sediment processes in three ponds within the Plum Island Ecosystems Long Term Ecological Research site (Rowley, MA). The ponds were characterized by different plant communities; two ponds were dominated by macroalgae and the submerged grass Ruppia while the third pond was unvegetated except for isolated patches of macroalgae. Sensors measuring dissolved oxygen (DO) concentrations, salinity, temperature, and light levels were deployed in each of the ponds from May – November 2014. Sediment processes were evaluated over 11 weeks in summer and fall by characterizing by organic matter composition and pore water chemistry. The ponds were shallow (~30cm) and had similar salinity and temperature regimes. Whole-pond metabolism varied with plant abundance and rates of production declined from summer to fall. In the vegetated ponds, water column DO concentrations were often supersaturated during the day and hypoxic at night. In contrast, day-to-night swings in DO were not as extreme in the unvegetated pond and concentrations rarely fell to hypoxic levels. In the sediments, concentrations bacteria-specific lipids as well as rates of respiration and sulfate reduction were higher in the vegetated ponds but varied over time. Considerable variability in plant community composition and abundance across seasons and between ponds as well as the unique hydrology of these systems make them biogeochemically dynamic components of salt marsh habitats.