Using sedimentary phosphorus/nitrogen as indicators of shallow lake eutrophication: concentrations or accumulation fluxes

Inland lakes have been suffering from eutrophication and nutrient sedimentation plays an important role in the eutrophication history of shallow lakes. Sedimentation processes and accumulation fluxes of nutrients were studied in the Central Lake zone (CL) of Taihu Lake (Taihu), a large shallow lake in China, with multi-sampling by the ²¹⁰Pb dating method and determination of sediment mass accumulation rates (MARs) and sedimentary phosphorus, nitrogen, carbon and iron concentrations, in order to track the anthropogenic eutrophication history of Taihu and to explore the available sedimentary indicators of trophic states of shallow lakes. It was found that MARs in CL varied from 0.016–0.048 to 0.078–0.339 g cm⁻² a⁻¹ with a turning point in 1980, corresponding to the rapid developing trend of society and economy in Taihu Basin after 1980. Rather than sedimentary nutrient concentrations, nutrient accumulation fluxes increased by 0.6–7.1-fold (P) and 2.3–12.2-fold (N) acting as paleolimnological indicators for the eutrophication explosion of Taihu since 1980. Yearly elevated TN in sediment cores and more greatly increased N accumulation fluxes than P with 350–1906 t a⁻¹ in recent years (2009–2012), suggest that nitrogen should get more attention to control for eutrophication risk of Taihu in future. There were spatial differences in nutrient accumulation fluxes and in weights of MARs and sedimentary nutrient concentrations contributing to the nutrient accumulation fluxes in CL. TOC in modern sediment (<1 m), responsible for increments in sedimentary P/Fe and TN and nutrient accumulation fluxes, acts as a key factor for controlling nutrient sedimentation and needs studies further for nutrient mass balance theory in Taihu.