Phosphorus storage and release dynamics in an oxbow lake within the Mississippi Alluvial Plain

Oxbow lakes are common features within the agricultural landscape of the Mississippi Alluvial Plain (MAP). Yet it is unclear the role that internal nutrient loads may play in perpetuating eutrophic conditions in shallow MAP lakes. Eight years of depth-integrated water column monitoring in Beasley Lake suggests periods of high benthic PO₄-P during dry months corresponded with periods of lower benthic DO, but there is uncertainty in the magnitude of internal contributions and drivers. To address this knowledge gap, we collected sediments from deep and shallow habitats within Beasley Lake. We characterized sediment properties with sequential P fractions and P sorption curves and measured nutrient and metal concentrations. We quantified P fluxes from intact sediment cores under anaerobic and aerobic conditions over 14 days. Sediment total P ranged from 660 to 980 mg-P kg^-1 with sorption capacity for an additional 2000 mg-P kg^-1. Bioavailable P fractions represented less than 0.1% of total P, while redox-sensitive fractions associated with iron or aluminum oxides and/or organic material dominated more than 70% of sediment P. Sediments were high in iron with over 20,000 mg kg^-1. Anaerobically treated cores had higher P release rates (30.9-102.4 mg m^-2 day^-1) than aerobic cores (2.4-3.7 mg m^-2 day^-1). Our findings demonstrate high capacity for legacy P storage in MAP lake sediments, which can become available for water column productivity under temporally explicit conditions, potentially impacting management and assessment of nutrients in shallow lakes.