The Effects of Tidal Export from Salt Marsh Ditches on Estuarine Water Quality and Plankton Communities

Abstract

Salt marshes are an important transition zone between terrestrial and marine ecosystems, and in their natural state, they often function to cycle or trap terrestrially derived nutrients and organic matter. Many US salt marshes were ditched during the twentieth century, potentially altering their functionality. The goal of this 4-year study was to assess the impact of water from ditches within seven salt marshes on estuarine water quality and plankton communities within four estuaries on Long Island, NY, USA. We found that concentrations of inorganic nutrients (ammonium, phosphate), dissolved and particulate organic nitrogen and carbon (POC, PON, DOC, DON), and total coliform bacteria were significantly enriched in salt marsh ditches compared to the estuaries they discharged into. In addition, concentrations of ammonium and DON became more enriched in ditches as tidal levels decreased, suggesting these constituents were generated in situ. Quantification of nitrogen sources in Flanders Bay, NY, suggested salt marsh ditches could represent a substantial source of N to this estuary during summer months. Experimental incubations demonstrated that water from salt marsh ditches was capable of significantly enhancing the growth of multiple classes of phytoplankton, with large diatoms and dinoflagellates displaying the most dramatic increases in growth. Experiments further demonstrated that salt marsh ditchwater was capable of significantly enhancing pelagic respiration rates, suggesting discharge from ditches could influence estuarine oxygen consumption. In summary, this study demonstrates that tidal draining of salt marsh ditches is capable of degrading multiple aspects of estuarine water quality.