Strengths and challenges of δ¹⁵N to identify anthropogenic nutrient loading in coastal systems

Coastal ecosystems are under increasing stress from anthropogenic nutrient loading; which is most often assessed through water quality measurements. Here, 136 published studies on the use of δ¹⁵N to identify nutrient loading in coastal systems were analyzed to identify key strengths and challenges when using this isotope technique. δ¹⁵N has been used successfully for this purpose around the globe for over 40 years. Studies have mainly used benthic macroalgae and sediment samples in estuaries and coral reefs of North America and Oceania. Strengths of this technique include timely identification of nutrient loading and its sources, even when inputs are pulsed or assimilated by biota, the benefits of varying isotope turnover rates in different types of samples, sporadic sampling efforts, simple collection and preparation of samples, and relatively low analysis costs. The shortcomings of this technique have led to a loss in popularity in recent times, mainly from isotopic overlap of potential sources and the effects of other confounding factors on isotopic compositions. These challenges can be compensated by simultaneous measurement of other key variables including additional isotopes (δ¹³C, δ³⁴S), water column nutrient concentrations, and fecal coliforms, highlighting great potential to use this tool.