High frequency measurements using in situ sensors in a coupled watershed-estuary reveal factors driving DOC variability

Estuaries are recipients of large inputs of organic matter and nutrients from coastal river systems and together form a vital link between two of the largest pools of carbon, the terrestrial and ocean environment, at the same time actively cycling carbon. The Great Bay situated in New Hampshire/Maine is a nitrogen(N)-impaired estuary with a suburbanizing watershed of the Lamprey River its largest source of DOC. Long term deployment of continuously monitoring sensors are changing the way biogeochemical studies of rivers, streams and estuaries are conducted. We linked simultaneous and continuous in situ measurements of CDOM and associated measures of DOC quality (e.g. absorption coefficient, spectral slope ) in both the Great Bay estuary and its largest source of DOC the Lamprey River between April and December 2011. These sensors allowed us to examine the continuous dynamics of CDOM from inland to the coastal waters not only in short-term hydrologically varying (storm pulses) conditions, but also the longer term seasonal variability. We also collected a suite of other relevant parameters in both the watershed and estuary, including NO3, PO4, Turbidity, Chlorophyll, Conductivity/Salinity to help understand the dynamics of DOC in the river and estuary. Preliminary time series analysis indicates that DOC in the Great Bay estuary co-varies with discharge of the Lamprey River, especially in spring and fall. Freshwater discharges and its variations explained the variability in estuarine DOC. UV- absorbance at 254 nm (the precursor to SUVA) co-varies in periods of high flow during spring and fall, consistent with the bulk DOC results This suggests that hydrology is the more important driver of variability of coastal CDOM. In light of climate change, suburbanization and changing land use patterns this emphasizes the need to examine continuous measurements of DOC quantity and quality in coupled watershed-estuarine systems.