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Gelatinous zooplankton dynamics in Bantry Bay and the development of a harmful jellyfish forecast for the salmon aquaculture industry
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Date
2024
Authors
Brownlow, Hannah
Journal Title
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Publisher
University College Cork
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Abstract
It is well recognised that gelatinous zooplankton can have negative socioeconomic impacts in areas where they interact with human activities. However, knowledge gaps on the ecology of gelatinous zooplankton and the influence of physical oceanographic processes on their distribution are substantial barriers to successfully mitigating these impacts. Therefore, this thesis sought to review the current state of the art on modelling jellyfish, develop an early warning system for a harmful jellyfish species, Muggiaea atlantica, which has been implicated in high mortality events of farmed salmon, and investigate the ecology of gelatinous zooplankton in detail.
A literature review was carried out to determine the scope and applications of modelling jellyfish. The models identified could be broadly categorised into two different types: empirical and mechanistic. A distinct geographic bias was displayed with the majority of models developed in Europe (n = 15), Asia (n = 7) and the USA (n = 6). A common approach used was Lagrangian particle tracking coupled with regional hydrodynamic models. However, challenges to modelling jellyfish were identified including a lack of quantitative abundance data and the coarse resolution of many hydrodynamic models, which fail to capture the fine scale processes that influence jellyfish distribution, especially near the coast.
High resolution sampling revealed further insights into the seasonality of M. atlantica and other gelatinous zooplankton. The seasonality of M. atlantica exhibited a distinct pattern, likely driven by local oceanographic processes and wind exchange events, with maximum abundances recorded in September or October each year. Abundances of M. atlantica showed considerable differences between years with maximum abundances of 322 ± 9.1, 39.3 ± 2.2 and 3105.4 ± 71.5 individuals m-3 in 2020, 2021 and 2022 respectively. Since the mechanism transporting aggregations of M. atlantica into the bay is likely wind driven coastal/shelf exchange, correlations with oceanic species (chaetognaths and Tomopteris spp.) were examined which revealed strong positive relationships in each year of the study. This further highlights that intrusions of oceanic water masses are influencing the composition and abundance of zooplankton in Bantry Bay.
This project sought to test an early warning system for harmful jellyfish in Bantry Bay, which is based on a successful Harmful Algal Bloom (HABs) model. To do this we carried out high temporal resolution zooplankton sampling and measured environmental parameters over three and a half years at a site in proximity to a salmon farm in southwest Ireland. The hindcast model successfully predicted all M. atlantica events (n = 3) over the three and a half year study. An important finding was that the model did not generate any false positives and each M. atlantica event (defined as the first substantial increase of at least 100 individuals m-3 within a two-week period) was hindcast by the model. This suggests that wind driven exchanges between the bay and adjacent shelf waters is the likely mechanism advecting aggregations of harmful gelatinous zooplankton into Bantry Bay.
A research cruise in 2021, CoCoJel, enabled a targeted study of vertical distribution in the bay. Differences in the abundance of gelatinous zooplankton species were observed between the inshore and offshore stations, with a higher mean abundance inside the bay (30.8 ± 10.9 indiv. m-3) compared to outside the bay (16.9 ± 9.7 indiv. m-3). The abundance of M. atlantica was similar outside and inside the bay, however, it was notably absent from the surface strata (0 – 10 m) for all inshore stations, with highest abundances at 20 – 30 m. The presence of a well-defined thermocline during the study indicates that species such as M. atlantica and Nanomia cara are likely targeting populations of prey species which are known to aggregate at these depths. The potential impact these findings have on mitigating harmful impacts on salmon aquaculture are discussed.
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Keywords
Jellyfish , Gelatinous zooplankton , Aquaculture , Muggiaea atlantica , Early warning system , Modelling , Vertical distribution , Ecology
Citation
Brownlow, H. I. 2024. Gelatinous zooplankton dynamics in Bantry Bay and the development of a harmful jellyfish forecast for the salmon aquaculture industry. PhD Thesis, University College Cork.