Phenology of Oithona similis demonstrates that ecological flexibility may be a winning trait in the warming Arctic

Abstract

Rapidly warming Arctic is facing significant shifts in the zooplankton size-spectra manifested as increasing numbers of the small-sized copepod Oithona similis. Here we present a unique continuous data set covering 22 months, on its copepodite structure along with environmental drivers in the Atlantic-influenced high Arctic fjord Isfjorden (Spitsbergen). Abundance maxima of O. similis were observed in September when the highest seawater temperature was recorded. A high concentration of the indicator species of Atlantification Oithona atlantica was also observed at that time. The clear dominance of O. similis in the zooplankton community during the dark, theoretically unproductive season emphasizes its substantial role in sustaining a continuous carbon flow, when most of the large herbivorous copepods fall into sleeping state. The high sex ratio observed twice in both years during periods of high primary production suggests two main reproductive events per year. O. similis reproduced even in very low temperatures (< 0 °C) previously thought to limit their fecundity, which proves its unique thermal tolerance. Our study provides a new insight on ecology of this key copepod of marine ecosystems across the globe, and thus confirm the Climatic Variability Hypothesis assuming that natural selection favour species with such flexible adaptive traits as O. similis.

Seasonal mesozooplankton patterns and timing of life history events in high-arctic fjord environments

“…The timing of decent and ascent was slightly shifted in O. smilis compared to C. glacialis and Pseudocalanus spp. Oithona similis became abundant in the upper 50 m first in August and peaked in abundance during autumn similarly to observations by Balazy et al (2021) in the nearby Adventfjorden. At this time, C. glacialis and Pseudocalanus spp.…”

“…At this time, C. glacialis and Pseudocalanus spp. had already descended to greater depths, leaving the surface waters to O. similis to exploit and reproduce with little competition (Balazy et al, 2021). Oithona similis is omnivorous, with predilection for motile prey and is feeding actively during summer, autumn, and winter (Lischka and Hagen, 2016), even though it is also able to accumulate energy stores in the form of lipids (Narcy et al, 2009).…”

“…In Svalbard, life histories of dominant copepod species and the pteropod Limacina helicina have been the subject of detailed yearround investigations in Kongsfjorden (Lischka and Hagen, 2005;Lischka and Hagen, 2016), Isfjorden (Arnkvaern et al, 2005;Balazy et al, 2021;Boissonnot et al, 2021;Ershova et al, 2021b;Hatlebakk et al, 2022), and Rijpfjorden (Søreide et al, 2010). Seasonal research on the whole mesozooplankton community has been conducted in Kongsfjorden (Hop et al, 2002;Kwasniewski et al, 2003;Hop et al, 2006;Walkusz et al, 2009;Hop et al, 2019) and Rijpfjorden (Weydmann et al, 2013), but these studies are based on data collected with rather coarse resolution in time.…”

Annual variations of abundance, egg production rate, and community structure of Oithona similis in Jiaozhou Bay, the Yellow Sea

“…O.similis is one of the most common, often dominating copepods in artic and subartic regions. Dvoretsky, 2009b, 2011;Balazy et al, 2021).The annual cycle investigation of O. similis population dynamics were mainly conducted in northwestern Svalbard waters (Lischka and Hagen, 2005), Isfjorden (Balazy et al, 2021), and Barents sea (Dvoretsky and Dvoretsky, 2009a). In north-western Svalbard, there were two peaks of O. similis abundance in the whole year,the maximum abundance was in November (704,633 ind/m -2 ), the minor one was in February (about 300000ind/m -2 ), the other researchers found that the two peaks of O. similis were in July and September in Barents Sea(560 ind/m³) and Arctic Isfjorden (about 4500 ind/m³).…”

Cells of matter and life – towards understanding the structuring of particles and plankton patchiness in the Arctic fjords

“…For example, the spatial switch in relative roles between Pico and Nano fractions in KONG in 2017 was explained by the preference of the Pico fraction species for regenerated nitrogen, while a higher proportion of the species representing Nano fraction was observed in the relatively cold and turbid surface water in the inner basin (Kim et al, 2020). Observed differences in size structure could also reflect differences in plankton phenology, as a result of e.g., the seasonal succession from larger diatoms to smaller dinoflagellates, meroplankton blooms, or different developmental stage composition of the dominating copepods (White et al, 2015;Balazy et al, 2019;Szeligowska et al, 2020;Balazy et al, 2021).…”