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

Abstract

We used the dual isotopic composition of nitrate (d 15 N and d 18 O) 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 2 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.

Nitrite consumption and associated isotope changes during a river flood event

“…It differs slightly from unity, which is associated with phytoplankton assimilation only (Deutsch et al, 2009;Granger et al, 2004), but this might be due to nitrification, which would lower the δ 18 O-NO − 3 values and thus lead to a slope above 1 (e.g. Wankel et al, 2006).…”

Nitrate sources and dynamics in a salinized river and estuary – a δ<sup>15</sup>N–NO<sub>3</sub><sup>−</sup> and δ<sup>18</sup>O–NO<sub>3</sub><sup>−</sup> isotope approach

“…DO concentrations were relatively enriched in this study (higher than 7.2 mg L −1 ), excluding the DO depleted area upstream of the HH River (lower than 5.0 mg L −1 ). (Dai et al, 2008) in South China Sea, but higher than those in the Elbe Estuary (Dähnke et al, 2008) in Europe and the San Francisco Bay estuary (Wankel et al, 2006) in the United States. The specific reasons causing such variations could potentially be linked to internal/external N source contributions and different N dynamics in the rivers and the estuaries.…”

“…When an enriched external source or biological transformation (e.g., mineralization, nitrification) contributes to the river, DIN distribution is expected to fall above the mixing line. In turn, when a depleted external source or the internal removal process (e.g., denitrification, assimilation) appears in the river, DIN distribution is expected to fall below the mixing line (Wankel et al, 2006). The curvilinear mixing curves of determined δ 15 N and δ 18 O of NO − 3 provide better information for transformation processes: an isotopic enriched NO − 3 source or internal removal processes (e.g., denitrification, assimilation) will result in a distribution of δ 15 N and δ 18 O falling above the mixing lines, while an isotopic depleted nitrate source or internal nitrification will result in a distribution of δ 15 N and/or δ 18 O falling below the mixing line.…”

“…More than concentration data alone, the combined use of N (δ 15 N) and O (δ 18 O) isotopes of NO − 3 has provided a powerful tool to investigate NO − 3 dynamics and identify NO − 3 sources in estuaries (Middelburg and Nieuwenhuize, 2001;Sebilo et al, 2006;Wankel et al, 2006;Dähnke et al, 2008;Miyajima et al, 2009). Therefore, in the present study, a combined approach based on the mixing curves of DIN concentration versus salinity and δ 15 N-and δ 18 O-NO − 3 is applied to (1) identify potential dominant NO …”

“…Many estuarine studies have focused on tracing N sources and assessing N dynamics in large estuarine systems, such as the Elbe Estuary (Dähnke et al, 2008) and the Atlantic coast (Middelburg and Herman, 2007) in Europe, the San Francisco Bay estuary (Wankel et al, 2006), the Mississippi River estuary (Rabalais et al,1996), and the mid-Atlantic coast (Dafner et al, 2007) in the United States, and the Yangtze River estuary (Chai et al, 2009) and Pearl River estuary (Dai et al, 2008) in China. Compared to these large estuarine systems with high discharge of freshwater, the levels of freshwater discharge are relatively low in the small estuaries, which are characterized by salinization from seawater intrusion for rather long distances upstream (Graas and Savenije, 2008).…”