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Using Stable Isotopes to Assess Connectivity: the Importance of Oceanic and Watershed Nitrogen Sources for Estuarine Primary Producers
Citation:
Brown, C., Jim Kaldy, W. Rugh, K. Willard, P. Fong, TChris MochonCollura, AND C. Fong. Using Stable Isotopes to Assess Connectivity: the Importance of Oceanic and Watershed Nitrogen Sources for Estuarine Primary Producers. To be Presented at 2017 AWRA Spring Specialty Conference, Connecting the Dots: Emerging Science of Aquat. Connectivity, Snowbird, UT, April 30 - May 03, 2017.
Impact/Purpose:
Nutrients are the leading cause of water quality impairments in the United States, and as a result tools are needed to identify the sources of nutrients. We assembled a regional scale dataset to evaluate the utility of using stable isotopes to identify nitrogen sources to estuaries along the Pacific Coast of North America (spanning from Alaska to Mexico). In California estuaries, the elevation of isotope data of macroalgae are similar to nitrate isotope from the watershed and appeared to be related to anthropogenic nitrogen sources. In contrast, nitrate coming from the watersheds for Oregon systems has low isotope ratio reflective of the presence of red alder trees in the watershed which have symbiotic nitrogen fixing communities. In Oregon estuaries, the nitrogen isotope ratio of macroalgae suggested that the oceanic source. In some Oregon estuaries, there was an elevation of the isotope ratio above marine end members in the vicinity of wastewater treatment facility discharge locations, suggesting isotopes may be useful for distinguishing inputs along an estuarine gradient. This abstract contributes to SSWR 4.02B.
Description:
Estuaries located at the interface of terrestrial and oceanic ecosystems receive nutrients from both ecosystems. Stable isotopes of primary producers and consumers are often used as an indicator of nutrient sources. We assembled natural abundance nitrogen stable isotope (δ15N) data for dissolved inorganic nitrate, green macroalgae, seagrass (Zostera marina) and mussels in the nearshore and in estuaries along the west coast of North America to assess the relative importance of terrestrial and oceanic nutrient sources in these systems. We found a latitudinal gradient in nearshore δ15N of nitrate of -0.2 ‰ per degree latitude from Mexico to British Columbia with more depleted isotope ratio to the north. Primary producers (green macroalgae and Zostera marina) located in the nearshore and the marine dominated portion of Pacific Coast estuaries exhibited a similar latitudinal gradient in δ15N of -0.3 ‰ per degree latitude. This latitudinal gradient is similar to δ15N observed for intertidal mussels (Mytilus californianus), which are known to reflect the isotope ratio of the phytoplankton they feed on. The consistent latitudinal gradient for multiple primary producers and a consumer, and the agreement with the gradient in nearshore δ15N of nitrate, suggests that it is a result of oceanic source waters. On the watershed side, there is a gradient in the δ15N of nitrate with southern California systems receiving nitrate with a δ15N-NO3 of about +12 ‰, while for Oregon estuaries the δ15N-NO3 is less than +1 ‰ for most systems. The low isotope ratio for Oregon systems is a result of the presence of red alder (Alnus rubra) trees in the watershed which have symbiotic nitrogen fixing communities associated with them. To assess the importance of watershed and oceanic sources, we compared the isotope ratio at all sites (not just those from marine dominated sections) within the estuaries to oceanic and watershed endmembers. In southern California estuaries, the isotope ratio of the macroalgae suggests that watershed sources are fueling primary productivity; while in most Oregon systems the ocean is the primary source of nitrogen. For some Oregon estuaries (e.g., Rogue, Coos, and Necanicum) macroalgae isotope ratios are elevated in the vicinity of point source inputs suggesting local importance of anthropogenic sources. It is important to understand spatial patterns in isotope ratios and drivers to track nutrient sources and understand their importance in supporting and structuring food webs