Observing the Biogeochemistry of Flue Gas Desulfurization Gypsum in Aquaculture Ponds as Well as its Impact on Zooplankton Communities

Abstract

Harmful algal blooms (HABs) are detrimental to water quality as they deplete oxygen levels, produce harmful toxins, and disrupt aquatic ecosystems. Flue gas desulfurization (FGD) gypsum, a byproduct of coal combustion made up of calcium sulfate, is commonly used within aquaculture to improve water quality. However, recent studies have suggested potential concerns that applying this product could lead to increased risk of HABs. This thesis used large scale in-situ field mesocosm experiments as well as a multi-year survey of 17 active aquaculture ponds to observe the application of FGD gypsum leading to the release of phosphorus (P) from pond sediments and causing shifts in zooplankton communities. Through an 8-week mesocosm experiment as well as a multi-year survey of 17 active aquaculture ponds we observed the long-term effects of FGD gypsum on nutrient dynamics. Studying the potential of soluble reactive phosphorus (SRP) increase allowed for a better understanding of the biogeochemistry post FGD gypsum application. Results showed that within systems, such as aquaculture ponds that experience anoxic periods within sediment-water interface (SWI), FGD gypsum led to large SRP spikes. While environments that never experienced anoxic periods, such as our mesocosm experiment, no SRP increases were observed. These findings suggest that redox reactions occurring between iron (Fe), P, and sulfur (S) cause P release from sediments after FGD gypsum is applied. An additional 21-day field mesocosm experiment was conducted to help understand how FGD gypsum applications impacts zooplankton communities. Using two ponds at the E.W. Shell Fisheries Center that consisted of differing zooplankton communities, the effects of two FGD gypsum doses, low (500 mg/L) and high (2000 mg/L), were evaluated. Results showed that cladocerans biomass as well as the size of an organism is negatively impacted by FGD gypsum. Results also found that two common orders of copepods are impacted differently by FGD gypsum, namely that larger calanoids are more affected than smaller cyclopoids. These findings suggest FGD gypsum creates an environment that shifts zooplankton communities to favor less grazing efficient smaller zooplankton that could negatively impact water quality. These experiments showed that FGD gypsum applications can potentially lead to higher risk of HABs. Increased nutrient input through P release from sediments can lead to increased algal growth. Additionally, the shift to smaller zooplankton creates greater risk for HABs as smaller zooplankton are less efficient at grazing a wide range of phytoplankton. With these findings, aquaculture farmers should note that the benefits of FGD applications, such as increased hardness, may come at the cost of increased HABs risk.