Factors Governing Light Attenuation in Upper Segments of the James and York Estuaries and Their Influence on Primary Producers

Upper estuarine segments are characterized by mixing of diverse source waters with a variety of constituents that may influence water clarity (e.g., algae, inorganic particulates, dissolved color). We measured turbidity, total suspended solids (TSS), chlorophyll-a (CHLa), dissolved organic carbon (DOC), and chromophoric dissolved organic matter (CDOM) in upper segments of the James and York Estuaries to better understand their role in light attenuation. Turbidity and TSS were found to be the best predictors of inter-site and intra-site variations in light attenuation. CHLa was not found to be a strong predictor of light attenuation, indicating that suspended particulate matter was largely non-algal. CDOM played a greater role in light attenuation in the Pamunkey and Mattaponi sub-estuaries, which derive a greater proportion of their inflow from lowland (Coastal Plain) sources where extensive wetlands and floodplain forest likely serve as a source of CDOM. Although dissolved and particulate components of light attenuation were derived from external (watershed) sources, variation in external inputs (river discharge) was not a strong predictor of estuarine light attenuation. Analysis of long-term (25-year) data indicate trends of decreasing turbidity and increasing water clarity at some sites, coinciding with decreases in river sediment inputs. In the James Estuary, underwater light conditions remain below targets for successful SAV colonization and favor phytoplankton-dominated primary production. Management actions that reduce sediment loads are likely to be the most effective means for improving water clarity in upper estuarine segments.