DOI: 10.5061/dryad.3ffbg79q6
title: Use of historical isoscapes to develop an estuarine nutrient baseline

description: 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.
description: 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 using a Russian peat
 borer to minimize compaction; in a few locations, a vibracorer was
 necessary to penetrate sands. Six focal cores were selected for
 high-resolution analyses and were collected using a piston corer with
 polycarbonate liners to obtain intact core sections for scanning and
 archiving. Focal cores were split into 1-cm sections; the remaining cores
 were sectioned into 10 cm intervals for 0–50 cm depths, and into 25-cm
 intervals for 50–100 cm depths. Core splits were archived at the LacCore
 repository at the University of Minnesota. Chronologies were created using
 downcore profiles of <sup>210</sup>Pb,
 <sup>137</sup>Cs, and <sup>226</sup>Ra measured
 with a low-energy germanium multichannel gamma spectrometer. Historical
 geochemical markers included Pb concentrations measured using ICP-AES
 following four-acid extractions, AMS radiocarbon dating of fossil peat,
 and magnetic susceptibility and imaging using a Geotek Multi-Sensor core
 logger. The maximum depth of radiocesium was assigned an age of 1953,
 radiocesium peaks were assigned an age of 1963, and total lead
 concentration peaks were assigned an age of 1974. Lead-210, radiocesium, and
 radiocarbon dating were combined in an age-depth model using a Bayesian
 approach to construct chronologies for seven cores. The age model
 <sup>210</sup>Pb
 <em>Plum</em> in R version 4.0.5 uses the same statistical
 approach as the previous model Bacon, but incorporates radionuclide dating
 including parameters of deposition of <sup>210</sup>Pb,
 supported <sup>210</sup>Pb, and accretion rates. The
 <em>Plum </em>model was selected because it can account for
 incremental <sup>210</sup>Pb data over depth in the cores, as
 opposed to using the analytical approach of the continuous rate of supply
 model. Additionally, this model has been used previously for chronologies
 of estuarine sediments. Within Elkhorn Slough, sediment accumulation rates
 varied little from site to site over the past century and were similar to
 values reported previously; thus, to estimate ages for the 85 undated
 cores, we compiled a composite core chronology using the seven cores to
 represent mean age-date model for the entire estuary. This composite core chronology was
 then applied to the 85 undated cores, using the composite age-depth
 relationship to estimate dates for the depth segments utilized for
 isotopic and stoichiometric measurements. We report the mean
 year output of the model and 95% confidence intervals around the
 mean. For the six
 high-resolution focal sites, cores were analyzed at 1-cm increments (for 0
 to 50 cm depths) for stable carbon and nitrogen isotopic composition using
 a Finnegan Delta Plus
 continuous flow isotope ratio mass spectrometer using standard methods,
 and for carbon and nitrogen concentration using a Flash 1112 EA. For the
 85 coarser resolution cores, sediments were analyzed for carbon and
 nitrogen abundance and stable isotope ratios using a Vario
 Cube elemental analyzer interfaced to an Isoprime 100 IRMS. Isotope ratios
 for carbon and nitrogen are reported in permille notation as:  where <em>R</em> is the
 abundance ratio of the less common (<em>a</em>)<em>
 </em>to more common isotope. The standard for nitrogen is
 atmospheric nitrogen gas; the standard for carbon is PeeDee Belemnite; by
 definition, standards have δ=0. Sediments were
 not pretreated to remove inorganic carbon, as acidification did not
 quantitatively shift ratios. Previous studies suggest little effect of
 diageneses on sediment δ<sup>15</sup>N ratios in coastal marine settings, but shifts of ~ -1.5‰ in δ<sup>13</sup>C ratios are expected and C/N ratios are thought to decrease over time. Furthermore, atmospheric δ<sup>13</sup>C ratios have declined by about -1.5‰ since 1850 associated with the Suess Effect – the release of lighter C from fossil fuel combustion. Whole estuary isoscape and stoichioscape maps were produced using sedimentary stable isotope (δ<sup>13</sup>C and δ<sup>15</sup>N) and molar nutrient stoichiometric (C/N) ratios interpolated from the 85 core locations using ordinary kriging in ArcGIS version 10.2.2 (ESRI, Redlands, CA, USA) to the spatial extent of cored areas in Elkhorn Slough. Maps were created for six depth intervals dated using the composite chronology (ca. 1726–1839, 1839–1885, 1885–1951, 1951–1963,1963–1981, and 1981–2010). Different interpolation variogram models including spherical, circular, exponential, Gaussian, linear interpolation with linear drift, and linear with quadratic drift were tested. Leave-one-out cross validation of 15% of the points was used to choose the model which yielded the smallest root mean square error between predicted and actual values. To ensure that historical differences in interpolation maps were a function of data differences rather than variogram methodology, the spherical kriging method was used for all timepoints. We also applied data from monthly water quality sampling at a network of (~26) stations across Elkhorn Slough since 1988. Monthly nitrate data from the sites were averaged during the full year of 1995 and mapped using ordinary kriging for comparison to spatial patterns of the isoscape and stoichioscape maps. Trends in isotopic and stoichiometric signatures since the 1850s were examined for the six high-resolution cores. Timeseries analysis of the high-resolution data investigated the statistical significance of trends during the period of increasing fertilizer application, as well as offsets in the signatures associated with the timing of marine inlet construction for the harbor. Statistically significant change points in the timeseries were determined using the Pettitt Test, a nonparametric test that identifies the year of a step change and assigns significance to the selection. Datasets of δ<sup>15</sup>N, δ<sup>13</sup>C, and C/N for each of the six high-resolution coring sites were separately tested for the period 1850–2010 (n = 45 time points each). Next, the timeseries were split at the significant step change points that were statistically identified, forming two datasets “before” and “after” the year of change. Trend analysis was performed using linear regression on the split datasets, to model the slope after the split as well as the difference of y-intercept at the step change year (Fig. S1 diagrams the slope and intercept of our statistical models). The difference of y-intercept at the step change year is interpreted as an offset in the timeseries, consistent with construction of the harbor inlet when the step occurred at the same time as the construction (1946 ± 10 years). The slope after this step change year is attributed to increasing fertilizer addition to the watershed from 1940–1980. To compare sediment isotope results to dissolved nutrient concentrations, we compared water quality monitoring data to the high-resolution sediment cores during a 20-year period. Monthly water sampling of parameters (including salinity) were measured at the sites, and water samples were also collected into brown Nalgene bottles; stored on ice; filtered; and analyzed for nutrients, including nitrate (NO<sub>3</sub><sup>−</sup>), within 48 hours, or frozen for later analysis in accordance with standard methods. Three of the high-resolution sediment cores were collected at the same locations as water quality monitoring sites. For these three water quality sampling stations (Portero Road North, Kirby Park, and Hudsons Landing West), we compared annual mean water column dissolved NO<sub>3</sub><sup>− </sup>(mM) and salinity (ppt) to sedimentary δ<sup>15</sup>N values during the same year from 1990–2010.
description: Excel, R, and a GIS software such as ArcGIS or QGIS.