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

## TL;DR

This study examines how phosphorus is stored and released in sediments of a shallow lake in Mississippi, showing that low oxygen conditions can trigger high phosphorus release, potentially worsening water quality.

## Contribution

The study provides new insights into phosphorus dynamics in oxbow lake sediments under varying redox conditions and quantifies legacy phosphorus storage capacity.

## Key findings

- Sediments in Beasley Lake have a high phosphorus storage capacity and can sorb an additional 2000 mg-P kg−1.
- Anaerobic conditions significantly increase phosphorus release rates compared to aerobic conditions.
- Legacy phosphorus in sediments can become bioavailable during dry months with low oxygen levels, impacting lake productivity.

## Abstract

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.

Sediments in Beasley Lake were high in phosphorus (P) with capacity to sorb an additional 2000 mg‐P kg−1.Under anaerobic conditions, P release rate peaked at 102 mg m
−

2 day−1 on the second day.Sediments at shallow location had slower release rates that peaked on day 4 under anaerobic conditions.P release rates were much lower (2.4–3.7 mg m
−

2 day−1) with aerobic conditions, but still positive overall.Mississippi lakes may be high in legacy P, which can be released when oxygen levels decrease in the dry season.

Sediments in Beasley Lake were high in phosphorus (P) with capacity to sorb an additional 2000 mg‐P kg−1.

Under anaerobic conditions, P release rate peaked at 102 mg m
−

2 day−1 on the second day.

Sediments at shallow location had slower release rates that peaked on day 4 under anaerobic conditions.

P release rates were much lower (2.4–3.7 mg m
−

2 day−1) with aerobic conditions, but still positive overall.

Mississippi lakes may be high in legacy P, which can be released when oxygen levels decrease in the dry season.

Lakes are common features in agricultural areas of the Mississippi Delta. Erosion around these areas can cause nutrients and soils to wash into lakes, causing water quality problems. High concentrations of phosphorus from soils and lake sediments in shallow oxbows can be a major concern. We looked at shallow and deep sediments from Beasley Lake, Mississippi, to better understand these connections and how phosphorus can be both stored and released in these types of sediments. We found the sediment in Beasley Lake could store a large amount of phosphorus, but it could also be available for algae to use, which could impact the quality of the lake water.