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Gelatinous zooplankton in a marine reserve and surrounding waters: diversity, seasonal dynamics, oceanography and trophic importance
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Date
2024
Authors
McGuinness, Maria
Journal Title
Journal ISSN
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Publisher
University College Cork
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Abstract
The marine area off the southwest coast of Ireland is dynamic, highly productive and contains a rich and diverse marine ecosystem. The oceanography of the region, such as the Celtic Sea front and the Irish coastal current, heavily influence the distribution of plankton, often creating abrupt boundaries which separate plankton communities. The area is considered biologically sensitive and has recently been proposed as a potential marine protected area. Lough Hyne (Europe and Ireland’s first marine reserve) is located in the southwest of Ireland and is known for its remarkable biodiversity. Despite over 100 years of research conducted in Lough Hyne and surrounding waters, limited studies have focused on gelatinous zooplankton and several knowledge gaps remain. Firstly, while the influence of the coastal current on the distribution of phytoplankton has been assessed, no study has investigated how this current may influence the abundance of zooplankton or gelatinous zooplankton. Secondly, gelatinous zooplankton was last monitored in Lough Hyne 30 years ago, and in the intervening years, numerous ecosystem changes have taken place in the urchin, algal and sponge communities among others in this marine reserve. It is not known how gelatinous zooplankton have responded to these changes but given that there is global concern that altered and perturbed ecosystems give rise to increases in gelatinous zooplankton, this needs to be addressed. Thirdly, no study has investigated the trophic ecology of plankton using molecular tools such as fatty acid analysis. Lastly, limited knowledge exists globally on the larval stage of phoronid worms which are known to occur in Lough Hyne. To address these questions, a 7-day research survey was carried out around southwest Ireland to investigate the Irish coastal current and the vertical and horizontal distribution of gelatinous zooplankton in this current. In situ measurements (temperature, salinity, density) were used to identify the coastal current and a multinet with mesh size 300 µm was used to collect zooplankton samples. A key finding was that the current was acting as a boundary to offshore waters and was involved in moving coastal neritic species around the southwest of Ireland. From an aquaculture perspective, the coastal current had higher abundances of the harmful jellyfish Muggiaea atlantica than the offshore waters and supports the hypothesis that M. atlantica is advected around the south and southwest coast of Ireland by the coastal current into aquaculturally sensitive areas. This study also carried out a detailed examination of gelatinous zooplankton abundance, diversity and seasonality over 3 years in Lough Hyne. Zooplankton samples from 58 sampling dates from January 2021 to December 2023 were collected using a 200 µm HydroBios ring net every month from November to March and fortnightly from April to October. The abundance of gelatinous zooplankton was exceptional (six times higher than other nearby coastal locations) and was dominated by four taxa: the appendicularians Oikopleura sp. and Fritillaria sp. and the hydrozoans Lizzia blondina and Obelia sp. It was noteworthy that the species richness of gelatinous zooplankton (n=32) in Lough Hyne was similar to other areas in southwest Ireland, contrary to previous reports. This indicates that southwest Ireland in general and not Lough Hyne specifically, has a high species richness of gelatinous zooplankton. Fatty acid analysis was conducted on phytoplankton and zooplankton samples from Lough Hyne over a 6-month period. This study found evidence for two phytoplankton communities, each of which had distinct fatty acid signatures. The first community was dominated by a single genus, Chaetoceros sp. and had low fatty acid content (5.05 ± 2.26 (µg/mg DW), while the second community was more diverse and had a much higher fatty acid content (19.26 ± 0.26 (µg/mg DW). This may have a significant bottom-up effect on the food web as fatty acids can regulate growth and reproduction rates in higher trophic levels, such as planktivorous fish. From a gelatinous zooplankton perspective, the study suggested that appendicularians may be far more important to the food web in Lough Hyne at the start of the year in February and March than previously thought. It was found that appendicularians may contain high levels of important fatty acids, which would benefit predators such as fish. Finally, two species of phoronid worms were recorded in Lough Hyne, Phoronis muelleri and P. hippocrepia. Their larvae were present in plankton samples from April to October with abundances in the same range as echinoderm and fish larvae. In summary, this thesis demonstrated that the southwest of Ireland has a high diversity of gelatinous zooplankton, very high abundances of gelatinous zooplankton (making up on average 16.5% of zooplankton communities in Lough Hyne) and that gelatinous zooplankton may be distributed by the coastal current. It also demonstrated that gelatinous zooplankton may be rich in essential nutrients, such as fatty acids, and thus play a more significant role in the food web of southwest Ireland than previously thought. Gelatinous zooplankton is therefore very important in southwest Ireland and requires regular monitoring.
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Keywords
Gelatinous zooplankton , Ecology , Jellyfish , Aquaculture , Lough Hyne marine reserve , Plankton
Citation
McGuinness, M. 2024. Gelatinous zooplankton in a marine reserve and surrounding waters: diversity, seasonal dynamics, oceanography and trophic importance. PhD Thesis, University College Cork.