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. Geospatial analysis of historical data suggests that an increase
in fertilizer application – rather than population growth or increases in
the extent of cultivated land – is chiefly responsible for increasing
nutrient loads during the 20th century. This study demonstrates the
ability of isotopic and stoichiometric maps to provide important
perspectives on long-term shifts and spatial patterns of nutrients that
can be used to improve management of nutrient pollution.