Nitrogen stable isotope values of large-bodied consumers reflect urbanization of coastal catchments

Anthropogenic nitrogen inputs to aquatic ecosystems can detrimentally affect ecosystem function; therefore, we need methods that identify nitrogen sources consistently among catchments. Nitrogen sources may be distinguished with stable isotope values (δ15N). This study tested if the isotopic values of biota and inorganic oxides of nitrogen (NO₃⁻ + NO₂⁻, hereafter NOₓ⁻) within estuaries were predictably influenced by anthropogenic land use in catchments distributed across a coastline. The δ15N values of NOₓ⁻, autotrophs and consumers (fish) were measured. Increased catchment urbanization was reflected in higher δ15N values of larger-bodied fish consumers but not smaller-bodied consumers, autotrophs or NOₓ⁻, which may reflect the time-averaging effect of longer tissue equilibration times in the larger fish. The proportion of the catchment subject to intensive agriculture did not relate to isotopic values of estuarine biota or NOₓ⁻. The lack of relationships between intensive agriculture and δ15N in the estuaries studied may result from the mixing of nitrogen sources within the catchments studied, non-uniformity in biogeochemical transformations of nitrogen during transport from agricultural sources to estuaries, or insufficiencies of broad land-use data for describing impacts of agriculture on nitrogen isotopic pools. The δ15N values of consumers with slow tissue turnover appear most useful for identifying the incorporation of urban nutrient inputs into estuarine food webs at catchment scales.