Spatial and temporal patterns of benthic nutrient cycling define the extensive role of internal loading in an agriculturally-influenced oxbow lake

Authors: Nifong, Rachel; Taylor, Jason; Devilbiss, Jonathan

Submitted to: Biogeochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/25/2022
Publication Date: N/A
Citation: N/A

Interpretive Summary: Shallow oxbow lakes are a common feature of the Mississippi Alluvial Plain. draining nearby agricultural fields. Implementation of best management practices has sought to reduce external loads and improve water quality within oxbow lakes. We condusted 14 diferent sedimant core incubations over the course of a year to examine patterns of nutrient flux and deitrification, a permanent N removal mechanism, across three habitat zones in Beasly Lake. Using models, we found dentrification to be influenced by temperature, dissolved inorganic nitrogen, sediment oxygen demand, and sediment carbon-to-nitrogen ratios. However results show Beasly Lake to be a net source of both nitrogen and phosphorus on an annual basis with internal nad external loads approximately equal to each other. This suggests that external loading must be reduced in order to improve water quality in oxbow lakes, such as Beasly, within heavily farmed alluvial landscapes.

Technical Abstract: Within shallow lakes, benthic denitrification can be an important pathway of reactive N (nitrogen) removal. However, these benthic sediments can also release substantial amounts of ammonium and phosphorus, influencing lake nutrient cycles. Seasonal and habitat specific patterns of temperature, organic matter (OM), and nutrient availability effects on sediment oxygen demand (SOD), dissolved nutrient flux, and n2-n flux were explored within Beasly Lake, and oxbow in the Mississippi Alluvial Plain. Time series models indicate a higher probility of positive n2-n fluxes in fall through the spring and significant negative summer fluxes, but timming and magnitude of positive and negative fluxes varied with habitat. Sediments from dominant open water habitats had a net negative annual N2-N fluxes (-2.34gm-2Y-1, sediments within cypress habitatshad a slightly negative net fluxes, while shoreline habitat had a positive net N2-N fluxes (0.26 g m-2Y-1).Predictive models explain similar amounts of variation (Adj. R2 = 0.57 vs0.47) in benthic N2-N fluxes associated with changes in tempature, dissolved inorganic N, SOD, and sediment C:N ratios, but the best performing models demonstrated that relationships with predictor variables varied with habitat. Results show substantial annual release of ammonium and phosphorus from sediments with uptake of nitrateoccuring during the spring. Results indicate that legacy nutrient cycling in Beasly sediments genrates nutrient loads proportional to external loading from the watershed. This poses a significant challenge for improving water quality in oxbow lakes within heavily farmed alluvial landscapes.