| dc.description.abstract | 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 | en_US |
