Implications for water quality in the Wabash River

Project Title: Assessment of nutrient sources and usage during harmful algae blooms (HAB) and algae eutrophication events using stable isotopes: Implications for water quality in the Wabash River

Principal Investigator(s): Dr. Greg Michalski, Purdue University, Department of Earth, Atmospheric, and Planetary Sciences

Dates: March 1, 2018 to March 1, 2019

Total Federal Funds:  Total Non-Federal Funds:

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The proposed research will use a combination of laboratory experiments, field sampling, and community based water sampling to assess the sources of nutrients in the Wabash River watershed and how these nutrients are utilized by potentially harmful algae and denitrifying bacteria. The novelty of the research will be the use of multiple naturally occurring isotopes in nitrate and phosphate that can be used as tracers of N/P sources and as evidence of in-stream nutrient loss processes. The laboratory experiments will consist of controlled incubations of cyanobacteria and naturally occurring algae obtained from the Wabash River and ponds, ditches, and streams that funnel into the main river. They will be grown at different temperatures and variable nutrient loading and the isotope enrichment factors for 15N in nitrate and 17O, 18O in both nitrate and phosphate will be determined. The same enrichment factors will be determine for denitrification occurring in an agricultural field bioreactor and in incubation experiments using Wabash River sediments. Determining these isotope enrichment factors is important for understanding the isotopic composition of nitrate and phosphate in the Wabash. Current hypotheses suggest that the isotopic composition of nitrate (and phosphate) in a water body reflects a mixing of different N/P sources. We propose an alternative hypothesis: That N/P loss by algal uptake and denitrification impose their own isotope signal and this can result in improper source apportionment using the existing mixing paradigm. Our preliminary data suggests that high 15N and 18O values detected in the Wabash are not evidence of combination of sewage/manure and atmospheric nitrate sources, rather may be N loss by eutrophication and/or HABs. We further hypothesize that similar changes in the 18O of phosphate would be manifest during P uptake by algae. Thus, isotopes maybe useful in understanding nutrient utilization during HAB and eutrophication events.