The interannual variability of gelatinous zooplankton abundance and diversity in North East Atlantic shelf seas

Abstract

Gelatinous zooplankton are a natural and globally important group of marine organisms, as they provide regulating, provisioning and supporting services to ecosystems. However, gelatinous zooplankton are still a major issue for human activities in several parts of the world, and there is concern that in these regions, the abundance of gelatinous zooplankton may be increasing over time. Added uncertainty comes from the lack of information for the abundance of gelatinous zooplankton at a regional level, particularly in offshore waters, and for gelatinous mesozooplankton that are < 2 cm in size. This thesis sought to improve these knowledge gaps by generating two decade-long datasets for the abundance and diversity of two poorly represented groups of gelatinous zooplankton in the Celtic Seas region of the North East Atlantic (NEA), using fisheries surveys as a research platform. To generate baseline information for gelatinous mesozooplankton species in the Celtic Seas, we identified two fisheries research surveys that were actively collecting zooplankton samples in the region, that could be re-analysed for gelatinous mesozooplankton. However, the net methods used to sample zooplankton on the two types of fisheries survey were different. To evaluate whether each net type (i.e. a Gulf VII sampler and a ring net) estimated the abundance and diversity of gelatinous mesozooplankton similarly, simultaneous deployments of the Gulf VII and ring net were made at 15 sites in July 2017 in Irish and UK shelf waters, and their gelatinous catches were compared. The difference in the estimates of gelatinous mesozooplankton abundance and taxon richness of the Gulf VII and the ring net samples were not significant. This confirmed that zooplankton samples from these two fisheries surveys could be re-analysed to generate a novel long-term data set for gelatinous mesozooplankton in the Celtic Seas. Using zooplankton samples from the two aforementioned fisheries surveys, species abundance data was generated for gelatinous mesozooplankton in the Celtic Sea for seven summers over a 13-year period (2007 – 2019). Total abundance was highly variable each summer, but did not increase or decrease linearly over time. However, subtle compositional changes occurred interannually, including a gradual shift in the abundance ratio of two siphonophore species Muggiaea atlantica and Agalma elegans. Holoplanktonic species dominated the abundance of the gelatinous mesozooplankton community (93.27%) and their abundance was negatively associated with sea surface temperature in July (represented by the position of the 16°C isotherm), and this was underpinned by natural climate variability (represented by the Eastern Atlantic Pattern index). Importantly, these findings suggest that gelatinous mesozooplankton abundance in the Celtic Sea may be lower as a consequence of future ocean warming. The aggregations of one oceanic jellyfish, Pelagia noctiluca, have negatively affected the aquaculture industry in the NEA, especially in Ireland and Scotland. Despite this, there is very little long-term information for their abundance and distribution in the NEA. To investigate the frequency and scale of P. noctiluca aggregations in this region, we collected semi-quantitative data for this species from an autumn-winter fisheries survey, which deployed 1,948 trawls over 11 years (2008 – 2018) covering most of the Celtic Seas. P. noctiluca was present on the Irish shelf in every year of the study (which is much higher than the historical record), and large aggregations of this warm-temperate species occurred in 5 of 11 years, as isolated events. When aggregations occurred, the highest by-catch density was recorded in the northern Irish shelf (maximum catch of P. noctiluca was 195 kg in 2009). P. noctiluca occurrence and abundance was related to wind patterns and two modes of hydroclimatic variability (the North Atlantic Oscillation index and the Eastern Atlantic Pattern index), which could reflect changes in advective processes that transport offshore populations onto the Irish shelf. A recent increase in the occurrence of P. noctiluca detected in the present work may pose a rising threat to coastal enterprise in the North East Atlantic, namely the aquaculture industry.

Together, the present work revealed that different groups of gelatinous zooplankton species displayed different trends in abundance over time in the NEA, and this was likely a result of differences in specific biological traits such as life history type, geographical distribution and temperature preference. The diversity of trends (and associated mechanisms) described here further emphasises the complexity and diversity of gelatinous zooplankton, and the need to study this group of organisms at a high taxonomic resolution and over regional or sub-regional scales.