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.
Highlights
Climate change is likely to have major impacts on global ecosystems, and this is especially the case in the rapidly warming Arctic regions[1,2]
Salinity was highest during February-April (34.7–34.8) while the lowest values were observed during July–September (33.6—34.1)
Among the mechanisms that govern Oithona population dynamics is its omnivorous diet, low mortality, thermal plasticity, and the frequent reproduction and the ability to take over the scene at the right moment
Summary
Climate change is likely to have major impacts on global ecosystems, and this is especially the case in the rapidly warming Arctic regions[1,2]. There have only been a few seasonal studies on zooplankton including O. similis carried out in Spitsbergen fjords using nets: in 80 s25, and 90 s, when the fjords on the west coast of Svalbard still experienced cold Arctic conditions and seasonal sea ice cover[12,26,27]; and more recently from data from sediment t raps[28]. We expect that due to being broadly o mnivorous[16], and because of the known ability of O. similis to reproduce year-round[43], O. similis would be capable of continuous reproduction in such an high Arctic region, due to the exceptional seasonality of the environment, a pattern of increased reproductive activity and abundance, driven by biotic and abiotic factors will be distinguished, differently to more stable populations from temperate climates[32]
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