Exoenzyme activity and algal biomass responses to experimental nutrient gradients identify factors driving nutrient limitation of algal biomass in an agriculturally influenced oxbow lake

Author(s): Taylor, J.; Jenkins, M.; Lizotte, R.; Locke, M.; Dillard, K.; McChesney, D.; Testa III, S.; Bryant, C.

The development of robust algal biomass-nutrient enrichment relationships for lakes within the Mississippi Delta is challenging due to a lack of undisturbed conditions. However, examining long-term water quality datasets linked to best management practices, combined with nutrient enrichment experiments may provide insight into factors that limit algal biomass. We examined seasonal patterns in nutrients, sediments and chlorophyll α (chl. α) over several years in Beasley Lake. Implementation of conservation practices within the watershed resulted in significant reductions in sediments and nutrients, particularly phosphorus (P), within the lake. However, chl. α concentrations continue to remain higher than recommended criteria during the growing season despite reductions in lake P concentrations. Seasonal patterns in activity of exo-enzymes associated with mineralization of nitrogen (N) and P suggest that nutrient demand is greatest at the end of the growing season when algal biomass is high and nutrient inputs are low. Experimental algal bioassays indicated that both chl. α and FDA (a measure of overall microbial activity) increased according to Michaelis-Menten dynamics with increasing dissolved N and P. We only observed maximum chl. α below recommended 20 µg/L concentrations when dissolved inorganic N (DIN = NH4-N + NO2-N + NO3-N) concentrations were below 0.2 mg/L despite high soluble reactive P (SRP) of 0.345 mg/L. In contrast, chl. α concentrations were always higher than the management target when DIN was not limiting (1.6 mg/L) despite very low SRP concentrations (0.015 mg/L) which suggests that N may be the primary limiting nutrient in Delta oxbows. Acid Phosphatase Activity (APA), an exoenzyme associated with mineralization of P, decreased with SRP and increased with DIN enrichment suggesting that P demand was related to availability and N:P ratios. In contrast, NAG and LAP, two exoenzymes associated with mineralization of N, increased rather than decreased with DIN enrichment and were correlated with chl. α. Chlorophyll α and FDA peaked at N:P ratios of 16:1 indicating that optimal elemental ratios for eutrophic oxbow lakes are similar to those observed in marine environments and some freshwater lakes. From a water resource management and monitoring perspective, our results suggest that exoenzyme activities may provide additional insight into complex relationships between nutrients and algal biomass in oxbow lakes. Furthermore, management practices that reduce N inputs to agricultural water bodies may be key to reducing chl. α concentrations below recommended goals for maintaining ecological integrity of Mississippi Delta oxbow lakes.es.

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