Expanding the history of estuarine nitrogen load and source: decadal to millennial-scale approaches

Lena Kae Champlin

doi:10.17918/00001804

Eutrophication from anthropogenic nitrogen (N) pollution is a prevalent threat to estuaries globally, however challenges of assessing eutrophication persist. Direct measurement of N levels may not capture highly dynamic diel, seasonal, and spatial patterns and cannot distinguish sources from terrestrial runoff vs. background marine inputs. Furthermore, the lack of long-term data to establish N baselines complicates regulatory efforts. This dissertation examines proxies of nutrient dynamics over multiple decades, centuries, and millenniums in a central Californian estuary, Elkhorn Slough, where agriculture dramatically enhanced nutrient inputs during the past century. First, multidecadal datasets spanning 20 years were used to investigate temporal cycles and spatial patterns of water quality. In addition to previously recognized impacts of eutrophication on hypoxia, emerging research has found the compounding impact of N pollution on pH trends caused by rising anthropogenic CO₂. Therefore, in Chapter 1, the effect of local nutrient sources on acidification was modeled using 20 years of monitoring data and patterns of aragonite saturation. Results revealed pronounced acidification trends near the marine inlet, however, large cycles from ecosystem metabolism were observed to also drive drops of pH in tidally restricted areas. Next, in Chapter 2, stable isotope analysis and maps (isoscapes) spanning ~300 years were used to assess marine vs. terrestrial N sources. Comparisons of contemporary verses historical isoscapes (ca. 1900) from sediment cores revealed that N sources shifted from a historical gradient with heavier isotopes near the marine endmember to a spatial pattern of amplified N levels at both the head and mouth of the modern estuary. Additionally, isotopes in the shells of modern bivalves Mytilus and Ostrea, suspended particulates, and nitrate showed enrichment further landward from the marine inlet indicative of a watershed N source. Geospatial analysis of historical records suggests that an increase in fertilizer application is chiefly responsible for this signal of increased nitrogen during the 20th century. Strong contrasts were observed between current sediment stable isotopes and those from the recent past (~150 years), demonstrating baseline shifts exceeding those in previously studied eutrophic estuaries. Finally, in Chapter 3, [delta]15N, [delta]13C and [delta]18O in bivalve shells spanning ~1500 years showed greater changes of nutrient sources during the past century (1933 - 1999 C. E.) than the proceeding millennium (ca. 630 - 1830 C. E.). This dissertation demonstrates the ability of using multiple parameters including pH and aragonite saturation, isoscapes from sediments, and isotopes in historical bivalve shells, to provide important perspectives on spatial patterns and long-term shifts of eutrophication.

Expanding the history of estuarine nitrogen load and source: decadal to millennial-scale approaches

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Expanding the history of estuarine nitrogen load and source: decadal to millennial-scale approaches

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