Abstract

Excess nutrients from human activity trigger toxic cyanobacterial and algal blooms, creating expansive hypoxic dead zones in lakes, damaging ecosystems, hurting local economies, undermining food and water security, and directly harming human health. To identify when and where nutrients limit phytoplankton and cyanobacterial growth, and cyanotoxin concentrations across Utah Lake, USA we conducted four in-situ bioassay studies (563 cubitainers or experimental units) that experimentally added N, P or N+P over the spring, early summer, summer, late summer, and fall in lake water from the top 20 cm of the water column. For our purpose, we defined total phytoplankton as all prokaryotic or eukaryotic organisms containing chlorophyll-a. We evaluated changes in chlorophyll-a and phycocyanin concentrations; the abundance of cyanobacterial species and total phytoplankton species or divisions; cyanotoxin concentrations of the microcystin, anatoxin-a, and cylindrospermopsin; DIN, SRP, TP, and TN concentrations; and other water chemistry parameters. We found that the nutrient limitation of cyanobacteria, and to a lesser extent phytoplankton, was influenced by season and space. Cyanobacteria were often co-limited in the spring or early summer, limited by a single nutrient in the summer, and not limited by N or P in the late summer and fall. Alternatively, phytoplankton were co-limited from the summer into the fall in the main body of the lake and either N limited or co-limited continually in Provo Bay. Microcystis, Aphanocapsa, Dolichospermum, Merismopedia, and Aphanizomenon spp., and Aulacoseira and Desmodesmus spp. and two taxonomical categories of algae (i.e., unicellular and colonial green algae) were primarily associated with cyanobacteria and phytoplankton nutrient limitations. Concentrations of the three cyanotoxins demonstrated a seasonal signal and loosely followed the growth of specific cyanobacteria but was not dependent on total cyanobacterial cell density. The DIN and SRP were biologically available in all water and nutrient treatments with nutrient concentrations declining over the incubation period, suggesting that nutrient levels were not oversaturated. Our results offer insights into specific nutrient targets, species, and, and cyanotoxins to consider in the future to manage Utah Lake.

Degree

College and Department

Life Sciences; Plant and Wildlife Sciences

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