Spatial and temporal patterns of benthic nutrient cycling reveal extensive role of internal loading

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

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 11/9/2021
Publication Date: N/A
Citation: N/A

Interpretive Summary: Shallow lakes are a common feature across alluvial, low relief landscapes, draining adjacent lands, and potentially processing excess nutrients present in runoff. Implementation of best management practices has sought to reduce external loads and improve water quality within shallow lakes. Lake water quality may also be influenced byinternal removal of excess nutrients through uptake or denitrification, but also through release of excess nutrients stored in sediments. We conducted 14 diferent sediment core incubations over the course od a year to examine the potential for nutrient release from sediments or denitrification, a permanrnt N removal mechanism, across three habitat zones in Beasley Lake. Using models, we found evidence for nutrient removal through uptake or denitrification occurring in lake sediments but removal mechaism observed during winter and spring were outweighed by release of nitrogen and phosphorus from Beasley Lake sediments during the summer. Overall, results show Beasley Lake sediments are a net source of both nitrogen and phosphorus on an annual basis, potentially equaling external sources of nutrients from runoff. Our results provide critical insight into managing lakes receiving nutrient runoff and suggest that while long-term data suggests significant water quality improvements have been made in Beasley Lake, External loading must continue to be reduces to improve water quality in shallow lakes within heavily disturbed landscape.

Technical Abstract: Within shallow lakes, benthic denitrification can be an important pathway of reactive N (nitrogen) remova. 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), disolved nutrient flux, N2-N Flux were explored within Beasley Lake, a low-gradient oxbow lake. Time series models indicate a higher probability of positive N2-N fluxes in fall though spring and significant negative summer fluxes, but timing and magnitude of positive and negative fluxes varied with habitat type. Sediments from dominant open water habitats had a negative annual N2-N fluxes (-2.34g m-2 Y-1) sediments within cypress habitiats had slightly negative net fluxes, while shoreline habitat had positive annual net N2-N fluxes (0.26 g m-2Y-1).Predictive models explain similar amounts of varition (Ady. R2 = 0.57 vs. 0.47) in benthic N2-N fluxes associated with chages in temperature, dissolved inorganic N, SOD, sediment C:N ratios, but the best perforning models demonstrated that relationships with predictor variables varied with habitat. Results show substantial annual release of ammonium and phosphorus from sediments with uptake of nitrate occurring during the spring. Results indicate that legacy nutrient cycling in Beasley Lake sediments generates nutrient loads proportional to external loading from the watershed. This poses a significant challenge for improving water quality in oxbow lakes within disturbed landscapes.