Use of historical isoscapes to develop an estuarine nutrient baseline

Coastal eutrophication is a prevalent threat to the healthy functioning of ecosystems globally. While degraded water quality can be detected by monitoring oxygen, nutrient concentrations, and algal abundance, establishing regulatory guidelines is complicated by a lack of baseline data (e.g., pre-Anthropocene). We use historical carbon and nitrogen isoscapes from sediment cores to reconstruct spatial and temporal changes in nutrient dynamics for a central California estuary, where development and agriculture dramatically enhanced nutrient inputs over the past century. We found strong contrasts between current sediment stable isotopes and those from the recent past, demonstrating shifts exceeding those in previously studied eutrophic estuaries and substantial increases in nutrient inputs. Comparisons of contemporary with historical isoscapes also revealed that nitrogen sources shifted from a marine-terrestrial gradient to amplified denitrification at the head and mouth of the estuary. Geo..., To examine interrelationships between nitrogen pollution and anthropogenic sources over the past century, we parametrized a model of nitrogen inputs to the watershed. Our model was based on the Nitrogen Loading Model (NLM). We applied the NLM model to calculate watershed sources of nitrogen over time in decadal increments from 1930–2010. We compiled historical data on changes in human population from census data, atmospheric deposition, homes with wastewater treatment, the areal extent of cultivated and natural lands and impervious surface cover, and estimated changes in fertilizer application rates in the Elkhorn watershed (based on annual \"Commercial Fertilizers\" and \"Fertilizing Materials\" reports published by the California Department of Agriculture 1925–2012).
Eighty-five ~ three-meter-deep sediment cores were collected during 2010 from the vertices of a 200 m x 200 m grid superimposed over the tidal and never-diked portions of the estuary. Most of the sediment cores were collected..., Excel, R, and a GIS software such as ArcGIS or QGIS.