Microcystin Shows Thresholds and Hierarchical Structure With Physicochemical Properties at Lake Fayetteville, Arkansas, May Through September 2020

Abstract

Highlights Despite little to no dissolved nutrient supply in surface water, harmful algal blooms are sustained throughout the 2020 growing season. Sediment phosphorus release was high in a lake that has annual harmful algal blooms, and it is an important piece of the watershed management puzzle. Thresholds and hierarchical structure with individual physicochemical properties and pigment fluorescence at this lake explain a large portion of microcystin variability. Abstract. Harmful algal blooms (HABs) in freshwaters are a global concern, and research has focused on the nutrient drivers of cyanobacterial growth and toxin production. We explored the importance of nutrients on sustained cyanobacterial HABs producing measurable microcystin at Lake Fayetteville, Arkansas, USA. The specific objectives were to (1) quantify sediment phosphorus (P) flux and estimate potential equilibrium P concentrations (EPC0) in July 2020, (2) assess water quality conditions in the lake from March through September 2020, and (3) evaluate physicochemical thresholds (or change points, CPs) and hierarchical structure with total microcystin concentrations. The sediments were a potential P source under both oxic and anoxic conditions, and the SRP concentrations in the lake water were continuously less than the EPC0 estimated for bottom sediment (~0.03 mg L-1); sediments are likely a potential P source for cyanobacterial HABs at Lake Fayetteville. The physicochemical changes at Lake Fayetteville over the 2020 growing season were typical of small, hypereutrophic reservoirs, with low biomass in winter when nutrient supply was greatest and the greatest cyanobacterial growth and microcystin toxin as nutrient supply diminished into the growing season. Microcystin concentrations were elevated above 1 µg L-1 from mid-June through mid-August 2020, and most physicochemical parameters in this study showed thresholds or change points with microcystin. Hierarchical structure existed with total microcystin concentrations, showing the potential importance of cyanobacterial biomass, N supply, and total P on elevated microcystin. Nutrients and algal pigment raw fluorescence explained 83% of the variation in total microcystin concentrations at Lake Fayetteville during the 2020 growing season. Nutrients (both N and P) from external and internal sources are likely important drivers of these blooms and toxicity at Lake Fayetteville. Keywords: Harmful Algal Blooms, Nutrient Drivers, Sediment Phosphorus Release, Water Quality.

Chlorophyll and Phycocyanin Raw Fluorescence May Inform Recreational Lake Managers on Cyanobacterial HABs and Toxins: Lake Fayetteville Case Study

“…More information about physicochemical thresholds with total microcystin concentrations at Lake Fayetteville using this cyanoHAB database from March through September 2020 is available in Haggard et al (2023).…”

“…In 2019, the first lake advisory for cyanobacterial toxins (i.e., microcystin) was issued after the AWRC observed total microcystin concentrations exceeding recreational guidelines (8 µg L ‐1 ; EPA 2019). The lake has been a research focus for watershed and lake nutrient budgets (Grantz et al 2014), sediment phosphorus (P) sources (Haggard et al 2023), and cyanobacterial HABs (Wagner et al 2021; Haggard et al 2023). However, cyanobacteria have dominated the phytoplankton community since 1968 or since initial impoundment (Meyer 1971).…”

“…Water samples have been collected since 2019; however, this study focused on water samples collected during calendar year 2020. Water samples were collected at three sites accessible by foot along the north shore, where the public has relatively easy access to the lake (see Haggard et al 2023). Upon return to the AWRC water quality lab, water samples were analyzed for raw fluorescence of chlorophyll and phycocyanin using a CyanoFlour (hand‐held fluorometer, Turner Designs, San Jose, CA, USA).…”

“…However, many studies focus on microcystin and its various forms in lakes and reservoirs because microcystins generally occur in lake waters when other classes of cyanobacterial toxins are present (Graham et al 2010). Microcystin and its physicochemical and environmental drivers in lakes and reservoirs have been a research focus for the last couple decades, showing the importance of water temperature (Walls et al 2018), nitrogen (N) supply (Wagner et al 2021), and cyanobacterial biomass (Haggard et al 2023). Recently, the Environmental Protection Agency (EPA) (2019) has released guidance on cyanobacterial toxins in recreational lakes and reservoirs for states and tribes, providing 8 µg L ‐1 as the target guideline for total microcystin concentrations.…”

Total Microcystin Concentration Variability in Water Samples and Recommended Minimum Volume (20 mL) for Freeze Thaw Cycles

“…Lake Fayetteville is a small recreational lake with a surface area of ~0.6 km 2 and catchment area of 24 km 2 , which is managed by the City of Fayetteville, Arkansas. The first recorded study on this lake was in 1968 (Meyer 1971), which showed that the phytoplankton community was dominated by cyanobacteria at that time, and the annual pattern in dissolved nutrient supply was the same as today (Haggard et al 2023a). Total microcystins were first measured on November 19, 2018 by a First‐Year Engineering Honors Research Team under our mentorship and we were surprised to see total microcystins (0.442 µg L ‐1 ) greater than MDL in late fall with colder water temperatures.…”