Bivalve shells reflect 15N enrichment in a fertilizer-dominated estuary

Agricultural nitrogen (N) contributes a dominant percentage to global N pollution in the coastal zone. Emerging research on N isotopes in bivalve shells has shown value for reconstructing historical increases in estuarine wastewater inputs. However, applications for fertilizer N are understudied. Here, we integrate the study of organic N isotopes, in concert with δ18O and δ13C, in estuarine bivalve shells to investigate spatial and long-term change in nitrogen inputs and sources. Modern, museum collection, and subfossil specimens of the genera Mytilus and Ostrea were profiled in a California estuary with an intensely agricultural watershed. Spatial patterns in bivalve isotopic composition reflected gradients in watershed nutrient inputs and productivity parameters. Furthermore, comparison of modern and historical periods revealed changes in nutrient source or processing over the last 1000 years. The N isotope values from shells offer perspective on agricultural pollution in estuaries., Elkhorn Slough (36° 48' 25.18\"N, 121° 47' 23.24\"W) is a shallow estuary (2.5 m deep) that extends inland 10 km from Monterey Bay in central California, USA. Elkhorn Slough drains a watershed of 585 km2, and while a portion has been protected, 24% of the watershed is cultivated, generally with crops that have moderate to high fertilizer demands and/or are multi-cropped. Outside of tidally restricted areas or during infrequent winter storms, the salinity and temperature of the Elkhorn Slough’s estuarine waters are similar to those found in Monterey Bay (S=30 PSU; T=14oC). It serves as a model system for studying agricultural N inputs. Water sampling has frequently documented high nitrate levels exceeding 1000  μM, an order of magnitude greater than many other well-studied eutrophic estuaries. Furthermore, historical changes in the hydrology of the estuary, including creation of an artificial oceanic inlet at the harbor in 1946, may have also altered marine nutrient exchange.
In October of..., , # Data from: Bivalve shells reflect 15N enrichment in a fertilizer-dominated estuary

[https://doi.org/10.5061/dryad.cvdncjtcp](https://doi.org/10.5061/dryad.cvdncjtcp)

## Description of the data and file structure

This dataset includes nitrogen, carbon, and oxygen stable isotopes measured on modern and historical bivalve shells from Elkhorn Slough estuary, CA. Datasets include the age-depth relationship of sediment cores used to date historical shells, the isotopic data from modern shells and historical shells as well as shell parameters, and also includes the code to generate figures and statistical analysis (ANOVA and PLS) of the change of isotopes over time and relationship to water column chemistry.

### Files and variables

#### File: Water\_Column\_Isotopes.csv

**Description:** : Isotopes of the water column nutrients and suspended particulate material collected across a tide cycle from low to high tide (hourly for 6 hours) on 16 August 2022 and 20 February 2023 at four sites ...