Composition of Gelatinous Zooplankton Communities in Korsfjord and Fanafjord: seasonal dynamics and change over 20 years

Abstract

Marine zooplankton are vital components of pelagic ecosystems. Gelatinous zooplankton refers to a polyphyletic group of soft-bodied gelatinous organisms including the pelagic cnidarians and ctenophores. Despite being an integral part of pelagic marine and coastal ecosystems and communities, jellyfish, a highly diverse and evolutionary successful ensemble, are still heavily understudied and often neglected in marine life monitoring surveys. Hence, little is known of their seasonal dynamics, with the only previous seasonality-specific jellyfish- focused study from western Norway conducted in 2003 by Hosia & Båmstedt (2007). This study aimed to collect monthly GZ samples from two of the same stations used by Hosia & Båmstedt (2007), in Korsfjord and Fanafjord, from November 2022 to October 2023. The main objective of the study was to establish a new baseline for the community composition and seasonal variation of gelatinous zooplankton in the two fjords, by 1) describing the structure, diversity and spatiotemporal variation of the gelatinous zooplankton assemblages in Korsfjord and Fanafjord, and 2) by comparing these results with Hosia & Båmstedt’s (2007) study to investigate interannual differences. Additionally, a jellyfish-specific protocol for eDNA sampling and extraction was developed to contribute to future jellyfish monitoring approaches. A total number of 7989 specimens belonging to 49 different taxa were found and recorded across both fjords over the entire course of the study. Korsfjord consistently had a higher species richness than Fanafjord throughout the sampling period with maximum richness number recorded in May. In Fanafjord the maximum richness was reached in July. Statistical analysis revealed difference in species richness between the fjords but not between months, with the same pattern for the Shannon-Wiener Diversity Index. For both fjord systems, the highest values of abundance were for most species observed during spring and summer months. Also, cluster analysis supported the presence of six seasonal groups of samples across the two fjords, with warm season samples from April – July constituting one group, regardless of fjord, with low similarity to groups representing late summer, autumn and winter. The samples from Fanafjord showed a larger number of groups with low similarity (four seasonal groups) than the samples from Korsfjord (two seasonal groups). Considering Hosia & Båmstedt (2007) and the known community structure of similar fjord systems, the diversity and richness patterns discovered were generally typical of the north-eastern Atlantic Ocean, with all species recorded having previously been observed in the greater region. However, some interannual differences were discovered locally, with particularly Fanafjord displaying a shift in community structure with the introduction of the invasive ctenophore Mnemiopsis leidyi, and the exclusion Staurodiosarsia gemmifera in 2023, the single most abundant species found in 2003.