Carbon Exports to the Gulf of Maine: Temporal Variations of Spatial Patterns from Hours to Years

Abstract:

The export of terrestrial carbon into the coastal waters of the Gulf of Maine was investigated from an extended field program that spanned three years and sampled four of the largest Maine and New Brunswick watersheds. Monthly observations of optical, biogeochemical and water quality parameters were collected at 67 stations during ice-free conditions. Optical proxies for biogeochemical properties were quantified with the emphasis on investigating three pools of organic carbon: algal carbon from chlorophyll fluorescence, particulate organic carbon from particle backscattering, and dissolved organic carbon from fluorescent dissolved organic matter. These proxies remained statistically robust throughout the sampling period and between watersheds. Seasonal and interannual patterns in the carbon pools varied predictably with latitude, land cover and discharge. Monthly observations indicated that dissolved organic carbon dominated total carbon export and further, that its concentration exhibited a strong coherence with river discharge, particularly the fall discharge peak. Watersheds with significant wetland were associated with the highest dissolved organic carbon concentrations over other land coverage types. While algal carbon from riverine primary production is not significant in terms of carbon export, it does significantly reduce the inorganic nutrient flux from rivers to coastal waters. Non-algal particulate organic carbon was the smallest component with isolated tributaries contributing non-negligible contributions to total organic carbon flux. In situ optical sensors deployed at the mouths of each river yielded hourly estimates of organic carbon concentration and export allowing elucidation of the variability in flux dynamics such as a hysteresis in the relationship between carbon concentration and river discharge that was not observed on weekly or monthly sampling regimes. Such observations are used to improve modeled estimates of annual carbon export.