Nitrogen sources and cycling in the San Francisco Bay Estuary: A nitrate dual isotopic composition approach

We used the dual isotopic composition of nitrate (d15N and d18O) within the estuarine system of San Francisco (SF) Bay, California, to explore the utility of this approach for tracing sources and cycling of nitrate (NO 3 ). Surface water samples from 49 sites within the estuary were sampled during July–August 2004. Spatial variability in the isotopic composition suggests that there are multiple sources of nitrate to the bay ecosystem including seawater, several rivers and creeks, and sewage effluent. The spatial distribution of nitrate from these sources is heavily modulated by the hydrodynamics of the estuary. Mixing along the estuarine salinity gradient is the main control on the spatial variations in isotopic composition of nitrate within the northern arm of SF Bay. However, the nitrate isotopic composition in the southern arm of SF Bay exhibited a combination of source mixing and phytoplankton drawdown due mostly to the long residence time during the summer study period. Very low dONO3 values (as low as 25.0%) at the Sacramento–San Joaquin River delta region give rise to a wide range of dONO3 values in the SF Bay system. The range in dONO3 values is more than twice that of dNNO3, suggesting that dONO3 is an even more sensitive tool for tracing nitrate sources and cycling than d NNO3. San Francisco Bay (SF Bay) is one of the most anthropogenically altered estuaries in the United States (Nichols et al. 1986) and some of the world’s largest ecosystem restoration efforts are currently under way to mitigate these alterations (Jassby and Cloern 2000; Kimmerer 2004). Changes that have been recorded in the SF Bay estuary over the past decades include dramatic reductions in the area of tidal marshland (by ,95%), reduction in the amount of freshwater inflow (by ,65%), and high loading of agricultural, industrial, and domestic waste (Nichols et al. 1986; SFBWQCB 1995). Although nutrient loading to the estuary is quite high, it is remarkable that so little eutrophication has occurred as a result of this loading (Cloern 2001). In particular, SF Bay receives significant annual nitrogen (N) loads, yet does not experience large-scale eutrophic or hypoxic conditions. Thus, an understanding of how the SF Bay estuary functions to moderate the effects of nutrient loading represents an important research area for those focusing on coastal and estuarine restoration and rehabilitation