Optical observations and distribution modeling of gelatinous zooplankton in the Arctic Ocean

Abstract

One of the most understudied components of the rapidly changing Arctic ecosystems is the gelatinous zooplankton, comprising cnidarian medusae (Hydrozoa and Scyphozoa), ctenophores, pelagic tunicates, and sometimes also including chaetognaths. Although these organisms play important roles in marine ecosystems, occupying multiple trophic levels, they have been historically neglected due to the difficulties associated with sampling them and the paradigm of them representing a "dead end" in food webs. However, representatives of the different groups were recently shown to serve as a food component for commercially important fish species, act as versatile predators, and contribute significantly to the biological carbon pump. The hypothesis of an ocean "jellification", i.e., a worldwide increase in gelatinous zooplankton biomass, proposed more than a decade ago, is still debated today. For the Arctic Ocean, the questions whether gelatinous zooplankton will increase in abundance, and whether biogeographic shifts in their distributions will take place, have remained largely unanswered. In order to understand the likelihood of such distributional shifts, reliable data are needed on species diversity and abundances and to identify the key physical and biological factors that determine the distribution of gelatinous zooplankton in the Arctic at the local, meso- and pan-Arctic scale. To do so, I leveraged an extensive dataset of historical biological data and analyzed newly collected optical data from recent expeditions to study the diversity, distribution, and abundance of gelatinous zooplankton in several types of ecosystems of the Arctic Ocean. I employed species distribution modeling techniques on both large-scale datasets and regional optical datasets, to evaluate changes in species distributions in space and time, under various climate change scenarios. For addressing these questions on the Pan-Arctic scale, I compiled extensive datasets for gelatinous zooplankton taxa from four public databases: the Ocean Biodiversity Information System (OBIS), the Global Biodiversity Information Facility (GBIF), the Jellyfish Database Initiative (JeDI), and PANGAEA, spanning six decades and comprising over 24,000 observations. Rigorous data cleaning and taxonomic examination narrowed the focus to eight dominant gelatinous zooplankton taxa with solid identification bases, including two Hydrozoa (Aglantha digitale and Sminthea arctica), two Appendicularia (Oikopleura vanhoeffeni and Fritillaria borealis), two Scyphozoa (Cyanea capillata and Periphylla periphylla), and two Ctenophora (Mertensia ovum, Beroe spp.). Three-dimensional species distribution models were applied to these datasets, revealing a pan-Arctic trend of polar shifts in the distribution of gelatinous zooplankton. The projections indicated for most studied species an expansion of suitable habitat, with the largest one for the scyphozoan Cyanea capillata (180% increase of its niche from 1950-2014 to 2050-2099). The largest niche contraction was found for the hydrozoan Sminthea arctica (15% decrease). I further focused in-depth on different ecosystems that are at the core of the ongoing Atlantification, the open waters of the Fram Strait, the shelf system of the southern Barents Sea, and the western fjords of the Svalbard archipelago. In situ observations of gelatinous zooplankton were collected by conducting depth transects with the Pelagic In situ Observation System (PELAGIOS, for which I annotated over 3200 gelatinous zooplankton observations). For the Fram Strait, I assessed the diversity of the water column from 20 to 2,400m, revealing seasonal migration patterns of gelatinous zooplankton communities, providing major additions to our understanding of the regional bathypelagic diversity. A significant population of Sminthea arctica was observed in the bathypelagic layers of Fram Strait, indicating its important, but so far neglected role, and I recorded the southernmost observation for the hydrozoan species Bathykorus bouilloni. Based on the optical datasets of Fram Strait, I carried out a community distribution modeling approach that was used to model gelatinous zooplankton species abundance and community richness. It was projected that environmental changes in Fram Strait will result in less diverse but more abundant gelatinous zooplankton communities. In terms of species-specific responses, the abundance of the hydrozoan Aglantha digitale is projected to increase by 2% in the water column by 2050, the hydrozoan Sminthea arctica is projected to experience a decline in abundance of up to 60%. The analysis of optical surveys also allowed me to document large aggregations of ctenophore species. In the southwestern part of the Barents Sea, I recorded one of the largest aggregations of adults of Bolinopsis infundibulum. This aggregation was most likely a seasonal phenomenon, supported by a large phytoplankton bloom, and may have extended over several tens of kilometers. Similarly, in a western fjord of Svalbard, Van Mijenfjorden, I found the largest number of individuals ever recorded for the species Beroe sp. and could be linked with oxygen-rich waters. These findings indicate the interplay of physical and biological factors for influencing small-scale distribution patterns of gelatinous zooplankton. A general trend in gelatinous zooplankton community structure was found shared between the results of the in- situ observational studies in Fram Strait and in the Svalbard fjords: Atlantic and transformed Atlantic waters were more abundant in gelatinous zooplankton, whereas the highest taxonomic richness was found in the intermediate and Arctic water masses. These findings hint towards a potential jellification with progressing Atlantification in some Arctic regions. With an overall trend toward niche expansions for most of the arcto-boreal and cosmopolitan species modeled, I anticipate major shifts in the distribution of gelatinous zooplankton in the Arctic regions. These changes are likely to have profound impacts on ecosystem dynamics, affecting fish stocks, biogeochemical cycles and the efficiency of the biological carbon pump.