The apparent rise in gelatinous species is one aspect of global marine change that has captured the attention of both the scientific community and the general public. Based on the Scopus citation database, over 300 hundred peer-reviewed articles on gelatinous zooplankton have been published over the past decade2. In addition, mainstream media reports of harmful gelatinous species expanding their distribution patterns (primarily Irukandji jellyfish) are also now commonplace (Foster 2018). While there is undoubtedly a “hype” surrounding trends in gelatinous zooplankton,
interpretations of findings are somewhat clouded by public misconceptions. Myths skewing common perception of the ecosystem role of gelatinous zooplankton are discussed at length by Condon et al. (2012), who stress that, while some abundance trends in these organisms can be associated with anthropogenic environmental change, they are not a new phenomenon in global oceans. It is thought that Ctenophora, the oldest gelatinous phylum, appeared around 540 million years ago (Chen et al. 2007), while pelagic tunicates entered marine ecosystems during the Neogene, 20-30 million years ago (Rigby & Milsom 2000). Subsequently, such gelatinous phyla and subphyla have, impressively, survived periods of abrupt environmental change that wiped out
2 Result includes peer-reviewed journal articles published from 2008-2018, with
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148 marine reptiles, ammonites and trilobites (Condon et al. 2012). Still, the knowledge that gelatinous organisms have “been here the whole time” is of little comfort to the
scientific community and the public. In fact, this apparent resilience may be abetting the notion that the global oceans are undergoing a gelatinous zooplankton takeover. Furthermore, compared to marine mammals and fish, gelatinous zooplankton are grossly understudied (Richardson et al. 2009), with this longstanding lack of information no doubt fostering the sense of urgency in understanding how these jellylike organisms are responding to physical and chemical change.
There are also several complications associated with studying gelatinous zooplankton in the field, and these can make drawing concrete conclusions about the temporal and spatial patterns of gelatinous species difficult. To elaborate on this, discrete net hauls are common practice for gaining information on gelatinous blooms (e.g. bloom density), however, this method can provide only a snapshot of what are often spatially inconsistent aggregations with rapidly changing community structure. Consequently, net sampling does not always provide most accurate estimates of the size, biomass, distribution and movement of blooms (Schaub et al. 2018). Until recently, the trophic role of gelatinous species has also been difficult to estimate. Gelatinous organisms tend to rapidly deteriorate once consumed, and they often appear absent (or are in such poor condition they are unidentifiable) when the gut contents of a higher trophic level species are analysed (Arai et al. 2003). This, in turn, has led to biased diet assessments, with the gelatinous component of diets of potential predators, in particular, underestimated (Berry et al. 2015). Further, reducing our knowledge of gelatinous zooplankton is the global focus on jellyfish (Condon et al. 2013, Sanz-Martín et al. 2016). This focus has allowed other gelatinous phyla and subphyla (including
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149 tunicate salps) to fly under the research radar. Undeniably, the long-term neglect of Thaliacean biology and ecology can partly be credited to genera such as Salpa not posing a direct threat to human health. Furthermore, tunicates are, generally, less abundant than cnidarians and ctenophores, and constitute only 2.5% of total global zooplankton biomass (Lucas et al. 2014). Still, by clogging fishing nets, and even power plant components (Sneed 2012, Boero et al. 2013), salp blooms can exert a major effect on economic returns and human quality of life.
Based on current knowledge, one of the major poorly-understood consequences of salp blooms is their overall ecosystem effect, particularly that in the Southern Ocean. Like jellyfish, Southern Ocean salps also receive media attention, with their potentially increasing abundances “spelling big trouble” for the Antarctic and Southern Ocean ecosystem (Carr 2017, Engelmann 2018). In some respects, the trophic structure of the Southern Ocean and Antarctic ecosystem is more fragile than many temperate regions, owing to a reliance on Euphausia superba: the primary food source for numerous higher trophic level species. What is crucial to remember, however, is that behind any labels bestowed by public media, published data (much of it sporadic) on the most abundant Southern Ocean salp, Salpa thompsoni, has only been available for the last fifty years. It is also imperative to approach Southern Ocean salp research with an open mind; not to solely seek out apparent increases in salp numbers, but to appreciate what abundance information might also reveal about population cycles and environmental drivers. Considering the evolutionary success of salps, and the potential for localized summer blooms to numerically exceed that of all other Southern Ocean zooplanktors in a region (Steinberg et al. 2015), our incomplete knowledge of S. thompsoni biology and ecology presents itself as a major hurdle in understanding fundamental trophodynamics in polar
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150 regions. In turn, grey areas surrounding the knowledge of these transparent organisms may eventually hinder advancements in pro-active ecosystem-based policy.
Baseline information against which to quantify change in gelatinous zooplankton populations is necessary for eliminating the “perceptionalised bias” (where an observer establishes patterns based on occurrences from the immediate past only) that
surrounds research into the long-term trends of species such as S. thompsoni (Condon et al. 2012). This multi-faceted thesis presents baseline informationon S. thompsoni
biology, trophodynamics and environmental responses in the ecologically significant, but understudied Kerguelen Plateau Region. The data and findings from this research will have applications for subsequent studies further deciphering S. thompsoni
trophodynamics, as well as whole-of-ecosystem models. Along with providing new information on Southern Ocean salps, findings from this study will also add perspective to existing S. thompsoni knowledge. The impact of improved perspective, or sorting “fact from fiction” should not be underestimated, as common perceptions significantly influence (whether it be consciously or sub-consciously) how scientific research is conducted and communicated.
In summary, this thesis found that, within the Kerguelen Plateau Region:
• Discrete S. thompsoni blooms can exceed 2, 500 individuals 1,000 m-3, which, compared to 1993-2009 West Antarctic density trends, is larger than most summer maxima (Loeb & Santora 2012).
• During summer 2016, the maximum S. thompsoni density (2,560 ind. 1,000 m-3) was approximately 6x greater than the historical (1982-2008) maximum (603
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151 ind. 1,000 m-3) for this region. Maximum 2016 densities also occurred further south (63.29°S) than historical maxima (circa 60°S) in this region.
• Summer 2016 S. thompsoni bloom production was associated with lower chlorophyll-a (highest abundances occurred at stations where depth-integrated chlorophyll-a was <65 mg m-3), late afternoon to twilight light conditions, and sea-ice retreat.
• January-February signifies a period of high S. thompsoni aggregate production, and preliminary solitary production, which is consistent with West Antarctic Peninsula accounts of life cycle (Loeb & Santora 2012).
• Salpa thompsoni aggregate carbon and nitrogen content is more variable, and lower, than Southern Ocean euphausiids (Euphausia frigida, E. superba,
Euphausia triacantha and Thysanoessa macrura).
• Based on δ13C, δ15N ratios, S. thompsoni is more similar to E. superba than to other Southern Ocean euphausiids (E. frigida, E. triacantha and T. macrura) in terms of trophic position.
• The diets of Kerguelen Plateau S. thompsoni and E. superba were shown to overlap through the shared consumption of diatoms in the genus Fragilariopsis.
• As a food source for higher trophic level species, S. thompsoni aggregates contain, on average, less than 100x the energy content (~27 Joules ind.-3 vs. 2,900 Joules ind.-3) and one sixth of the protein content (7% dry weight vs. 42% weight) of Euphausia superba.
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152
• Based on 1981-2016 abundance data, the southern Kerguelen Plateau region is a potential hotspot for horizontal (latitudinal and longitudinal distribution), and vertical (water-column distribution) habitat overlap between S. thompsoni and
E. superba.
• Based on historical (prior to 2016)density patterns, S. thompsoni has only been sampled during summers when latitudinal sea ice extent has not extended further north than 58.24°S.
• Over the last 35 years of sampling, the rate of trawls retrieving S. thompsoni
that were aimed at an acoustic E. superba target has not decreased, highlighting the need for further investigation into salp “bycatch” in euphausiid targeted trawls.