Effects of magnetite on phosphorus storage and carbon cycling in Lake Michigan shoreline sediments

Lake Michigan, one of the Laurentian Great Lakes, hosts the world's largest freshwater sand dune system and is ecologically and economically important. Phosphorus (P) plays a key role in the lake biota, while industrial P runoff, pollution controls, and invasive species add to the complexity of P dynamics in the shoreline ecosystem. Rich in iron oxide minerals, the Lake Michigan shoreline has been little studied with respect to P bioavailability and cycling. In this study, we analyzed magnetite concentrations in samples collected from 19 beach and sand dune sites located along the eastern shore of Lake Michigan, and we examined the mineral effects on P bioavailability through adsorption experiments, sediment fractionation and extraction, and microcosm incubations. We find that magnetite is widely distributed in the sand dune ecosystem and its content can reach very high levels (>30% dry weight) in natural beach sediments. P has the strongest adsorption affinity to magnetite when compared to prominent sediment minerals calcite and quartz, and the native magnetite holds up to 10 and 28 times more moderately and strongly bound P, respectively, than the bulk sediments. Microcosm incubations with native sediments showed that the addition of P significantly increased microbial decomposition of labile organic carbon, whereas the added synthetic magnetite did not significantly reduce the P bioavailability or decrease the carbon degradation. This research highlights the multiple effects of magnetite on P storage, bioavailability, and labile organic carbon degradation in the shoreline sand dunes, which may provide new insights into understanding nutrient and carbon cycling in the Great Lakes ecosystem. • Magnetite is widely distributed in Lake Michigan shoreline sand dunes and can reach high levels (>30% dry weight). • Magnetite strongly sorbs phosphorus and serves as a major phosphorus sink. • Phosphorus stimulates dune microbial degradation of labile organic carbon. • Addition of magnetite may not reduce the P bioavailability in the beach sediments.