Disentangling nitrate sources at Lake Wateree, South Carolina, using stable isotopes South Carolina

Harmful cyanobacterial blooms (HCBs) are increasing all over the world and put human, animal, and ecosystem health at risk from exposure to toxins and depleting water quality. Lake Wateree, a hydroelectric reservoir in South Carolina (SC) is used for municipal water supply and recreation and has reoccurring HCBs that are dominated by the toxin producing benthic cyanobacteria Microseira (Lyngbya) wollei. We measured water-column nutrient concentrations, collected every two months at four stations at Lake Wateree from 2015 to 2023, in collaboration with a citizen-led program (WaterWatch on Lake Wateree, run by the Lake Water Watch Association). We observed increasing nitrogen (N) to phosphate (P) ratio (N:P). The nutrients introduced by humans has increased five folds in the last century. Excessive inputs of nutrients from septic systems, agriculture, fertilizers, felling, as well as N₂ fixation by cyanobacteria leads to proliferation of HCBs. Yet, the source of essential macronutrients (i.e., N and P) sustaining HCBs are still not well characterized at Lake Wateree, impeding effective HCBs management strategies. Stable isotopes of nitrate, δ¹⁵N and δ¹⁸O is a powerful tool to identify the source of N. For example, trophic level isotopic enrichment results in organic manure to be more enriched in ¹⁵N than synthetic fertilizer and newly fixed N from N₂ fixation, the reduction of unreactive atmospheric N₂ into bioavailable nitrogen by N₂ fixing organisms. We present water-column nitrate isotope data collected every two months and analysed using a MixSIAR model from January to December 2021-2023 at 20 stations at Lake Wateree. Our results indicate a dominant source of nitrate from sewage pollution into Lake Wateree.  

Primary Presenter: Archana Venkatachari, University of South Carolina (avenkatachari@seoe.sc.edu)

Authors:

Annie Bourbonnais, University of South Carolina  (abourbonnais@seoe.sc.edu)

Margaret Gordon, University of South Carolina  (MMGORDON@email.sc.edu)