Unionid shells (Hyriopsis cumingii) record manganese cycling at the sediment-water interface in a shallow eutrophic lake in China (Lake Taihu)

Aquatic eutrophication is becoming a serious environmental problem throughout the world. The utility of bivalves as bio-filters to improve water quality and reduce algal blooms has been widely acknowledged, but the potential usefulness of bivalve shells as retrospective monitors of eutrophication-induced environmental change has received little attention. Here, we present the first multi-year, high-resolution Mn/Cashell records of the freshwater mussel, Hyriopsis cumingii (Lea, 1852) from a shallow eutrophic lake (Lake Taihu, China). Mn/Cashell time-series of the two studied shells exhibit a high degree of synchrony after being placed in a precise temporal context by means of growth pattern analysis. There is a large inter-annual variability of Mn/Cashell records during 2011-2015, with the highest value occurring in 2013. Mn/Cashell also displays a pronounced intra-annual variability with maxima consistently occurring during late spring/early summer. The high reproducibility of Mn/Cashell time-series among contemporaneous specimens highlights the existence of strong environmental rather than biological control on the incorporation of Mn into the shells. In particular, the striking feature of late spring/early summertime Mn/Cashell maxima is synchronous with the occurrence of reducing conditions beneath the sediment-water interface (SWI). The latter results in substantial increases of biologically available Mn2 + in the sediment pore water and organic particles, the element being rapidly taken up from these sources by the mussels with subsequent incorporation of Mn into their shells. Therefore, Mn/Cashell can potentially serve as a high-resolution proxy of the mobility of Mn at the SWI. As demonstrated by the present study, documenting and understanding geochemical properties of bivalve shells can help to retrospectively monitor eutrophication-induced environmental change in aquatic ecosystems.