MEPS 542:25-37 (2016)  -  DOI: https://doi.org/10.3354/meps11543

ABSTRACT: 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 NO_x^) within estuaries were predictably influenced by anthropogenic land use in catchments distributed across a coastline. The δ¹⁵N values of NO_x^, 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 NO_x^, 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 NO_x^. 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.

KEY WORDS: Estuary · Watershed · Non-point source · Landwater coupling · Nitrate · Fish · Seagrass · Zostera · Phragmites australis