Seasonal dynamics and life histories of three sympatric species of Pseudocalanus in two Svalbard fjords

Abstract

Small copepods are the most diverse and numerous group in high-latitude zooplankton, yet our knowledge of important species remains poor because of the difficulties involved in correct species identification. In this study, we use a molecular method of identification, a species-specific polymerase chain reaction, to provide the first description of the seasonal dynamics and life histories of the important genus Pseudocalanus in two Svalbard fjords with contrasting environments. We conducted monthly investigations in the relatively warm and ice-free Adventfjorden, supplemented with seasonal samples from the colder, seasonally ice-covered Billefjorden. We found three species of Pseudocalanus (the Arctic P. acuspes and P. minutus, and the boreal P. moultoni). Pseudocalanus acuspes had a distinct annual life cycle and dominated during summer, when it actively reproduced. Surprisingly, the boreal P. moultoni was present year-round in both fjords and was the dominant species during winter; the presence of all life stages of this species throughout the year suggests a more continuous reproduction. The Arctic P. minutus was the rarest of the three species and was likely able to complete its life cycle in Billefjorden but not in Adventfjorden. Our study demonstrates that closely related species may have different life strategies and environmental preferences, which presumably make high-latitude zooplankton communities more resilient to climate change impacts on genus but not necessarily on species level.

Seasonal Variability in the Zooplankton Community Structure in a Sub-Arctic Fjord as Revealed by Morphological and Molecular Approaches

“…The pronounced annual cycle in abundance in P. acuspes, peaking during summer and declining in winter, indicates a dependence on the spring bloom for growth and development. This is in agreement with recent observations from Svalbard (Ershova et al, 2021) and previous studies in the Baltic sea (Peters et al, 2006;Renz and Hirche, 2006), Balsfjord (Norrbin, 1991) and Nova Scotia (McLaren et al, 1989). P. acuspes life cycle is described as mostly annual, but the P. acuspes population can also produce a second generation (McLaren et al, 1989;Norrbin, 1991;Peters et al, 2006) and such a second reproductive effort could explain the second peak in BWSR in autumn.…”

“…While P. acuspes dominated in spring/summer and only P. moultoni was found in addition, P. minutus and P. elongatus appeared in autumn when all four species were found in more or less equal proportions. A similar succession has been recently observed in Svalbard (Ershova et al, 2021) indicating species specific differences in life history strategies. The pronounced annual cycle in abundance in P. acuspes, peaking during summer and declining in winter, indicates a dependence on the spring bloom for growth and development.…”

“…Particularly for Calanus and Pseudocalanus, metabarcoding provided a more detailed insight into the species composition than previous studies. Since life cycles and life strategies may differ even in closely related species within the same genus (McLaren et al, 1989;Lischka and Hagen, 2005;Ershova et al, 2021), correct species identification is crucial for describing species specific life history strategies and for documenting changes in population dynamics (see below). Pseudocalanus lacks easily distinguishable morphological features that would aid species identification, particularly when it comes to the early life stages, and they are therefore often reported as Pseudocalanus spp.…”

“…The distribution of P. minutus is generally restricted to ice-covered Arctic shelf seas (Melnikov et al, 2005;Persson et al, 2012;Ershova and Kosobokova, 2019) or the deep Atlantic Ocean (Wiborg, 1955;Aarbakke et al, 2017). In Isfjorden, a Svalbard fjord heavily influenced by Atlantic advection and lacking a seasonal ice cover, P. minutus failed to complete its life cycle and was only advected into the fjord during the summer months (Ershova et al, 2021). Similarly, we observed P. minutus in Ramfjord only between September and March, together with other non-resident species, suggesting an Atlantic origin.…”

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

“…Moreover, they have often concentrated on only one or few dominant species (Lischka and Hagen, 2005;Søreide et al, 2010;Ershova et al, 2021b) and rarely on the whole zooplankton community (Weydmann et al, 2013;Darnis and Fortier, 2014;Bandara et al, 2016).…”