Phosphate oxygen isotope insights into lacustrine phosphorus cycling

The biogeochemistry of phosphorus (P) is key to the health of our freshwaters, and perturbations of the lacustrine P cycle entail severe ecological complications associated with eutrophication: harmful algae blooms, oxygen deficits, fish kills, and decline of water quality. Despite decades of eutrophication research, critical controls in the lake phosphorus cycle are not conclusively understood.

The stable oxygen isotope signature in phosphate (δ¹⁸O_P) has been used to detect reactions associated with biological phosphate turnover and utilization, and to track the contribution of isotopically different sources to lacustrine phosphate stocks. Many studies have evaluated δ¹⁸O_P isotope effects in the lab, but it remains to be seen how these findings relate to the growing number of environmental measurements of this signature.

We here present two case studies – Lake Arendsee and Lake Stechlin from NE Germany –where we obtained comprehensive data on phosphate oxygen isotopes in lake and ground water and interpreted these with respect to P transport via lacustrine groundwater discharge and phosphorus turnover in the water column. We address problems associated with the interpretation of δ¹⁸O_P data from "open" aquatic ecosystems, postulate that integrative studies in aquatic ecosystems of reduced complexity can help identifying critical benchmarks for the δ¹⁸O_P concept, and discuss how these can eventually be transferred to enhance our understanding of P cycling in large water bodies such as lakes and estuaries.