Fall 2021 eNewsletter
Research Institute Spotlight: Kelly McCabe
Lake Erie Phosphorus Cycling
Harmful algal blooms (HABs) are a symptom of eutrophication, which is the enrichment of a waterbody with excess nutrients over time. Understanding nutrient dynamics in HAB-affected systems is critical for management strategies to mitigate the negative impacts of HABs, making comprehensive nutrient measurements an important component of the CIGLR and NOAA GLERL HAB monitoring and forecasting program. CIGLR biogeochemistry laboratory analyst, Kelly McCabe helps collect, process, and measure different forms of phosphorus (P) in a variety of freshwater lotic (e.g., rivers) and lentic (e.g., lakes) systems in the Great Lakes region, with a focus on the Lake Erie basin and watershed. “Our measurements help scientists better understand how P influences the size, duration, and severity of HABs, which also further promote hypoxia in Lake Erie, subsequently altering the “normal” cycling of P within the lake,” says McCabe.
Phosphorus enters Lake Erie primarily through rivers. Despite successful efforts to manage riverine total P (TP) inputs, especially from the Maumee River, HABs and hypoxia continue to plague Lake Erie every summer and fall. “Numerous hypotheses have been proposed to explain this predicament,” says McCabe. “One prevailing hypothesis is that a larger fraction of the TP entering the lake is bioavailable.” TP is composed of a variety of physical (i.e., particulate and dissolved) and chemical (i.e., inorganic and organic) fractions of which some are more readily available for algae to use for growth and energy transport. “To assist CIGLR scientists, I helped create standard operating procedures for accepted methods that allow us to measure different physical and chemical fractions of P in aquatic systems,” says McCabe. “These methods involve various chemical digestions and extractions to target each desired fraction of P.” The specialized P fractionation techniques were used by McCabe and CIGLR graduate student, Kate Laramie, to characterize the P-pool in the tributaries and main stem of the Huron River, which flows through Ann Arbor and empties into Lake Erie. P characterization in other riverine systems within the Lake Erie watershed are also underway. These data will help support more localized and riverine-specific management of nutrient inputs to Lake Erie.
The utility of the P fractionation methods goes beyond the characterization of riverine inputs. Once P reaches the lake basin it is subject to biological, physical, and chemical processes that alter and transform it. These processes collectively makeup the lake’s internal P cycle. “Measuring the various fractions of P within the basin can enhance scientists’ understanding of this internal cycling and how HABs both respond to and influence the cycle,” says McCabe. In 2021, the CIGLR and NOAA GLERL HAB monitoring team collected samples to describe the physical and chemical, as well as biological forms (e.g., DNA, RNA, lipid, and poly P) of P present in Lake Erie. For McCabe, much of the summer and fall has been spent supporting the expanded sampling effort. McCabe and her colleagues collected in situ water samples aboard the NOAA 4108 research vessel and prepared the water at the laboratory for further analyses, including the various P fractions. “We have collected thousands of samples for P analyses.” says McCabe. “They will be used to understand when, where, how much, and what type of P is actually present in the water column and intracellularly within the algae as conditions change over the course of the bloom. I have a lot of samples to analyze this winter, but the results from these analyses will be a valuable addition to our HABs monitoring dataset and will ultimately support the work of our many partners.”
McCabe’s P analyses also support studies that describe the final step in the P cycle: sediment burial. Typically, the majority of the P that enters a lake will eventually sink to the lake bottom where it is buried in the sediments, sometimes for thousands of years. Lake Erie experiences seasonal hypoxia that is partially driven by HABs and can disrupt the P burial process. Recent research by CIGLR and NOAA GLERL scientists has shown that when bottom waters in Lake Erie’s central basin become devoid of oxygen or anoxic, P is released from the sediments and returns to the water column, a phenomenon that is called internal P loading. “This internal P load may promote HABs growth; however, due to the timing of the lake’s hypoxic conditions, the whole story is still unclear,” says McCabe. “We were able to further investigate internal P loading as part of both the 2019 Cooperative Science and Monitoring Initiative (CSMI) and the Coastal Hypoxia Research Program (CHRP). CIGLR scientists measured the vertical distributions of various forms of P before, during, and after hypoxic events throughout the Lake Erie Basin to track internal P loading.” The data collected in 2019 was presented by McCabe at the 2021 International Association for Great Lakes Research (IAGLR) Conference and will soon be available to the public.
“Our phosphorus analyses help CIGLR, NOAA GLERL, and our partners better understand the P cycle in Lake Erie,” says McCabe. “Our research team is committed to supporting and improving management strategies aimed at keeping the Great Lakes awesome!”