Stable isotope tracers: Enriching our perspectives and questions on sources, fates, rates, and pathways of major elements in aquatic systems

Stable isotope applications have evolved from simple characterizations of isotope composition in organisms and organic matter, to highly complex methodologies on scales ranging from individual compounds and cells, to broad ecosystem-level approaches. New techniques are rapidly evolving, allowing novel, difficult, and inconvenient questions to be addressed. This article aims to provide an overarching perspective on how the field has evolved and where it is going with regard to aquatic systems, some of the oceanographic and limnological concepts derived from these approaches, and important challenges. To this end, we highlight a selection of natural abundance and tracer enrichment studies that represent a wide range of stable isotope applications. These include studies of rate processes and biogeochemical cycling, source tracking, food webs, and paleoenvironments. Our coverage of stable isotope applications is by no means complete, but by highlighting a mixture of classic and new applications across a wide range of research areas, our goal is to convey the power of stable isotope tools and the excitement in this rapidly expanding field, while also encouraging the scrutiny and healthy respect for limitations and assumptions necessary to take full advantage of these powerful tools.