Watershed Symposium 2019

N vs P: Nutrient Limitation of Harmful Algal Blooms on Utah Lake

Summary:
Utah Lake is naturally high in nutrients, but there is potential for nutrient pollution to exacerbate toxic harmful algal blooms. In July, we used bioassays to test nutrient limitation and found chlorophyll, a surrogate of algal growth, was highest in treatments with both nitrogen and phosphorus added, compared to control and N or P individually.

Full Abstract:
Water bodies across the Wasatch Front are experiencing harmful algal and cyanobacterial blooms, possibly with greater frequency and severity due to anthropogenic inputs of nitrogen (N) and phosphorus (P). These blooms pose a threat to food and water security and human health due to cyanotoxin production. Utah Lake is a shallow, hypereutrophic lake garnering recent public attention for repeated advisories and closures due to cyanobacterial blooms. Its naturally P-rich sediments combined with cyanobacteria’s ability to fix atmospheric nitrogen have brought into question the need for management of external anthropogenic nutrient inputs. To quantify the effects of additional nutrients on algal growth, we are conducting seasonal bioassay experiments across three different lake locations to capture the spatial component of blooms (i.e., West Side, East Side, Provo Bay). We are testing four nutrient treatments: N, P, N+P, and a control added at a 16:1 ratio of DIN:SRP that reflect the new EPA limitations of P in wastewater effluent. We measured a suite of indicators to identify responses in bloom physiology, including cyanotoxin concentrations. In July, N+P additions in Provo Bay waters during a bloom, and to a lesser extent N additions in east-side waters, induced photosynthetic growth measured as higher levels of chlorophyll and volatile suspended solids concentration than the other treatments. Microcystis loosely related to blooms induced by N+P and anatoxin-a related to N additions. To date, our results indicate a N and P co-limitation and potential N limitations elicits summer blooms. as a possible mechanism affecting bloom growth and attenuation. In the future, we are conducting bioassays throughout the summer, fall and spring; conducting nutrient dilution treatments; and manipulating grazer density. Our results will help policy makers select ecologically-relevant water quality standards for Utah Lake.