Seasonal observations of surface waters in two Gulf of Maine estuary-plume systems: Relationships between watershed attributes,optical measurements and surface pCO2

The partial pressure of carbon dioxide (pCO₂) in estuary-plume systems is related to the internal processes of net organic metabolism and physical mixing, but is also strongly influenced by biogeochemical inputs from the land and ocean. Surface layer pCO₂, stimulated fluorescence of chlorophyll (f-chl) and colored organic matter (f-com), and beam attenuation at 660 nm (c-660) were measured during three seasonal surveys of the Kennebec (ME) and Merrimack (MA) estuary-plume systems. These estuaries are both supplied by large New England Rivers and separated by less than 150 km, but significant differences were often observed in the distributions of surface pCO₂ and optical variables. High pCO₂ concentrations were generally associated with high f-com, while lower pCO₂ concentrations were associated with high f-chl and c-660. Using simple regression models, optical measurements were used to estimate chlorophyll and total organic carbon concentrations. Results suggest that labile riverine carbon is responsible for sustaining supersaturated pCO₂ conditions and that phytoplankton productivity, likely driven by inputs of riverine dissolved inorganic nitrogen, is responsible for pCO₂ undersaturation. Although optical variables are often related to surface pCO₂, the results suggest that efforts to retrieve pCO₂ in complex waters using optical data may be enhanced using a site-specific, multivariate approach.