Investigating phytoplankton fluctuations and drum filter effectiveness on an abalone farm in Hermanus, South Africa.
Ponton, Timothy John
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Aquaculture is a growing industry in South Africa, with the production of abalone, Haliotis midae, at the forefront. The Western Cape Province hosts 12 of the 18 abalone farms in South Africa, with a concentration of farms in the Walker Bay region of Hermanus and Gansbaai. Walker Bay is situated on the western side of the Agulhas bank, which allows warm water from the Agulhas current, as well as cold water from the Benguela current to mix. This area experiences seasonal upwelling pulses in summer, which provide the environment with a high nutrient load. This encourages the prevalence of harmful algal blooms (HAB) that can consist of toxin-producing dinoflagellate species. These species have the ability to kill organisms in the nearshore. This poses a problem for aquaculture farms situated in the area, where HAB events have caused the death of millions of abalone and has decreased productivity of farms in previous years. Farms therefore need to implement stringent phytoplankton monitoring schedules, as well as develop better filtering methods to reduce the density of phytoplankton that may flow into the farm. This study aimed to understand the phytoplankton community assemblages that may be pumped into an abalone farm (Abagold Ltd) over a 16 month period. This was achieved by investigating how phytoplankton community metrics such as abundance, species diversity, richness and evenness fluctuated over a 16 month time period. The frequency of HABs were investigated, comparing the peaks of blooms and how they differed between seasons and the subsequent impact on monitoring activities by the farm until the bloom passes. Secondly, a study was done to determine the efficiency of drum filters to reduce the density of phytoplankton cells from the water that is sourced from the ocean and pumped through the farm. Phytoplankton community assemblages were sampled and identified to genus level, and species level when possible, once a day for 16 months, from September 2018 to December 2019. As the risk of potential HAB formation rises, the number of sample collections increased to assist in the decision making process of the operational manager of the abalone farm to mitigate negative impacts originating from HAB events. The species richness, Shannon-Weiner diversity index and Pielou’s index of evenness were calculated. The number of phytoplankton samples collected each day were tallied to understand the change in monitoring frequency with regard to HAB abundance. Phytoplankton community samples were collected before and after a 15 μm drum filter during bloom events, after which the densities were then compared. The peak mean monthly cell density occurred in late early autumn of 2019 (March: 721 179 ± 226 473 cells/l). During this time, the diversity (Shannon-Weiner Index) of species was lower than that of mid spring and this trend is supported by literature, where a decrease in diversity occurs with an increase in HAB density. The relative abundance of species was calculated to quantify the dominant species present