RAPID PHOSPHORUS REMOVAL CAN PROMOTE SUSTAINED DECREASED NITROGEN CONCENTRATIONS IN LAKES, BUT HOW?

While anthropogenic impacts have historically led to increased phosphorus (P) concentrations in freshwaters, we are now beginning to reverse course. Reducing inputs and limiting internal loading of P has successfully pushed some eutrophic systems back to a clear, macrophyte dominated state. However, less focus has been given to how targeting P has impacted the nitrogen (N) cycle or whether changes in N pools could be partially responsible for maintaining improved water quality. Lake management efforts in Minnesota give us a unique opportunity to examine the ecosystem response to rapid P removal. In this study, we assess the impact of 48 whole-lake rapid P removal events (alum treatments) on the N-cycle using monitoring data and laboratory experiments. After P removal, summer epilimnetic N concentrations decreased. The magnitude of N removal was positively correlated with P removal, and N concentrations were reduced for at least 8 years after alum treatment. Hypotheses to explain the decrease in N include decreased inputs from N-fixing cyanobacteria, increased assimilation by macrophytes, and increased nitrate availability leading to increased denitrification or increased downstream export. Preliminary analyses indicate that total cyanobacterial cell counts decreased, and improved water clarity promoted macrophyte growth. Potential denitrification rates remained unchanged, and problems with variably reported detection limits made it challenging to assess N export. Ongoing work seeks to examine the relative importance of these hypotheses in explaining the observed patterns.

Primary Presenter: Catherine Polik, University of Minnesota (polik020@umn.edu)

Authors:

Catherine Polik, University of Minnesota  (polik020@umn.edu)

Makenna Tosi, University of Minnesota  (tosi0002@umn.edu)

Grace Neumiller, University of Minnesota  (neumi008@umn.edu)

Gaston Small, University of St. Thomas  (gaston.small@stthomas.edu)

Jacques Finlay, University of Minnesota  (jfinlay@umn.edu)