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Use of historical isoscapes to develop an estuarine nutrient baseline.
Citation:
Champlin, L., A. Woolfolk, A. Oczkowski, A. Rittenhouse, A. Gray, K. Wasson, F. Rahman, P. Zelanko, N. Quintana Krupinski, R. Jeppesen, J. Haskins, AND E. Watson. Use of historical isoscapes to develop an estuarine nutrient baseline. Coastal & Estuarine Research Federation Conference, Portland, OR, November 12 - 16, 2023.
Impact/Purpose:
Coastal eutrophication is an important issue in marine science. Here we measure stable isotope values in sediment to better understand how nutrients, which drive the eutrophication, have changed over time in Elkhorn Slough in California. Overall, fertilization has contributed large amounts of nutrients which appear to be driving current eutrophication in the estuary.
Description:
Coastal eutrophication is a prevalent and increasing threat to the healthy functioning of ecosystems globally. While degraded water quality can be detected by monitoring oxygen, dissolved nutrient concentrations, chlorophyll, and algal abundance, establishing regulatory guidelines for water quality targets can be complicated by a lack of baseline (e.g., pre-Anthropocene) water quality data and the complex nutrient dynamics of estuaries. We used historical carbon and nitrogen isoscapes to reconstruct spatial patterns and temporal changes in nutrient dynamics for an estuary in central California, where development and agricultural practices have dramatically enhanced nutrient inputs over the past century. We found strong contrasts between current sediment stable isotope signatures and those from the recent past, demonstrating extreme shifts far greater than those in previously studied eutrophic estuaries. These results suggest substantial increases in nutrient inputs over the past century. In addition, comparisons of contemporary with historical isoscape maps revealed spatial patterns of nitrate sources which shifted from a marine-terrestrial estuarine gradient to amplified denitrification at the head and mouth of the estuary. Geospatial analysis of historical land use, population, and fertilizer application data suggests that an increase in fertilizer application – rather than population growth or increases in the extent of land under cultivation – is chiefly responsible for increasing anthropogenic nutrient loads during the 20th century. This study demonstrates the ability of isotopic and stoichiometric data to provide important perspectives on long-term shifts and spatial patterns in nutrient inputs and sources that can be used to improve management of nutrient pollution in estuaries.
