FLOW AND BIOGEOCHEMICAL FLUXES IN THE ABAGADASSET RIVER, MID-COAST MAINE

The Abagadasset River is a tidal freshwater tributary that empties into Merrymeeting Bay, mid-coast Maine. During summer 2004, a hydrological and biogeochemical assessment of the river system included high-resolution sampling across the lower channel over spring and neap tidal cycles. An Acoustic Doppler Current Profiler provided discharge estimates that were reproducible to within several percent, as well as records of backscatter that can be correlated to suspended-sediment concentrations. Depth-integrated water samples and CTD profiles, taken hourly at 5 stations across the 25-m-wide channel, demonstrated that flow is vertically and horizontally well mixed throughout the tidal cycle. Nutrient analyses of water samples indicate that, relative to the larger bay, the Abagadasset watershed is enriched in total organic carbon, nitrogen, and phosphorus, but lower in dissolved silica. Concentrations of silica were low overall (0-1.6 mg/l), as were those of dissolved inorganic nitrogen (generally <0.05 mg/l). Thus dissolved organic nitrogen was the primary form of N. Continuous sampling of water level, temperature, specific conductivity, dissolved oxygen and turbidity with an automated datalogger provided longer-term data on water exchange through the section. During spring tides with low freshwater input to Merrymeeting Bay by the Androscoggin and Kennebec Rivers, specific conductivity within the Abagadasset rose with the incoming tide. During neap tides, however, salinity in the Abagadasset fell with rising tide, demonstrating the complex circulation in the larger bay. Turbidity was generally higher during outgoing tides, and increased quickly when local rain events impacted intertidal surfaces. Semidiurnal fluctuations in dissolved oxygen decreased from summer into fall, as temperature fluctuations and biological activity diminished.