Harmful algal blooms (HABs) cause severe economic and environmental impacts, including hypoxic events and the production of toxic and off-flavor compounds. Chemical treatments, such as copper sulfate pentahydrate (CuSO4.5H2O), are often used to mitigate the damaging effects of algal blooms. However, treatment effects are usually short-lived leading to waterbodies requiring repeated CuSO4.5H2O applications to control persistent algal blooms, particularly in highly eutrophic systems, such as aquaculture ponds or small agricultural impoundments. We hypothesized phytoplankton communities routinely treated with Cu develop community tolerance to treatment making algal blooms more difficult to manage over time. Pollution-induced community tolerance (PICT) is a method for measuring how a community can withstand selective pressures to a toxicant. To test whether phytoplankton develop community tolerance to algaecidal treatment, the toxic effects of Cu were evaluated at a standard dose (1.37 mg/L CuSO4.5H2O or 0.35 mg/L total Cu) and a low dose (0.69 mg/L CuSO4.5H2O or 0.17 mg/L total Cu) relative to untreated controls. Treatments were applied once to 1300 L mesocosm enclosures installed in a productive aquaculture pond and monitored for 28 days. Acute toxicity bioassays measured photosynthetic efficiency across a wide range of Cu concentrations (0.05-300 mg/L). The PICT bioassay results were used to generate dose-response curves for median effective concentrations (EC50s) to assess phytoplankton community tolerance to Cu toxicity. The results of this study showed that both doses of Cu led to over 99% removal of cyanobacteria in the first seven days and maintained a reduction in cyanobacterial abundance by at least 70% throughout the experiment. After three days of exposure, the phytoplankton communities in the standard and low-dose treatments exhibited a 12.4x and 5.2x increase in Cu community tolerance, respectively, compared to controls. This increase in community tolerance was driven by Cu-tolerant chlorophyte species. These findings suggest that, while community tolerance to Cu may alter the perceived effectiveness of treatment over time, it can promote a beneficial shift from cyanobacteria to chlorophyte species, ultimately contributing to a more sustainable system.

Catfish aquaculture ponds are at high risk of experiencing excessive algal growth, especially cyanobacteria, that can lead to negative water quality issues due to their consistent input through regular feedings and internal cycling of nutrients. Conventional algaecides used in aquaculture, such as copper sulfate, may be effective in the short-term but can potentially lead to developed resistance in phytoplankton over time or harmful effects on non-target species. An alternative nutrient management strategy, which has traditionally been used in land-based agriculture, is via flue gas desulfurization (FGD) gypsum, a form of calcium sulfate that is created as a by-product of carbon-based energy sources. A six-month field experiment was conducted on active catfish aquaculture ponds at a farm in west Alabama to test the effects of FGD gypsum on water quality, including its ability to manage excessive algal blooms throughout the growing season. In stark contrast to previous results from FGD gypsum application in eutrophic waters, it was discovered that FGD gypsum-treated ponds experienced a large increase in soluble reactive phosphorus (SRP) that led to a spike in phytoplankton abundance, specifically cyanobacteria. Supplementary microcosm experiments aided in determining that FGD gypsum could extract legacy phosphorus out of the nutrient-rich flocculant sediment found in aquaculture ponds. While these results were unexpected and ultimately undesired, we did discover that the use of FGD gypsum at the 500 mg/L concentration does not produce any trace metal contamination in the water column or in the fish tissue. Overall, this experiment discovered new results that FGD gypsum could produce when used in hypereutrophic waters with nutrient-rich sediments and provides the foundation for future research directions to determine the specific mechanisms behind the interaction of FGD gypsum with the sediment.

Flue gas desulfurization (FGD) gypsum, a by-product of carbon-based energy sources, has typically been incorporated as a component of concrete mixes and wallboard and beneficially used as an agricultural amendment to enhance terrestrial crop production and improve the quality of runoff. These various uses for the by-product aid in reducing the amount that is ultimately landfilled. Limited studies have investigated its benefits when used directly in aquatic settings, such as ponds and lakes, to increase hardness and potentially mitigate eutrophication. A 36-day field mesocosm experiment tested a larger range of FGD gypsum concentrations (500-2000 mg/L) than those previously tested in the literature to investigate its desired and potentially undesired impacts on water quality, including the algal community. High FGD gypsum concentrations, 1000 and 2000 mg/L, were found to have more undesired impacts than the 500 mg/L treatment, including an initial spike in cyanobacteria, a decrease in total zooplankton abundance, and an increase in certain trace metals in the highest treatment. Ultimately, the 500 mg/L FGD gypsum treatment was found to have fewer undesired impacts while still resulting in significant desired effects, including those on hardness and pH, as well as moderate reductions in algal abundance. This experiment provides a better understanding of the effects of FGD gypsum when directly used in an aquatic setting, determines an optimal dose for future field experiments, and helps provide the groundwork for developing an upper threshold on FGD gypsum so as to not have the negative effects outweigh the positive.