The Arctic does not sleep over the winter!

The polar environments possess extreme light conditions, with midnight sun periods and polar nights[1]. Zooplankton[2] species have adapted different strategies to survive the polar night, reducing their activity in comparison to the rest of the year, which has led to the popular belief that the Arctic sleeps during the winter time. However, recent studies have detected a high biologic activity (including feeding, growing and reproduction) and have been raising doubts of this dormant views about the polar night.

Copepos are one of the most known groups of zooplankton in the Arctic. This image ilustrates the species Calanus finmarchicus. Image from Russ Hopcroft

Studies focused on zooplankton communities during the polar night typically use equipment that only detects large individuals, leaving larval stages and species with a small size unnoticed. As a result, the knowledge on the impact of small species and young life stages in the arctic ecosystems is limited. Copepods[3] contribute for the carbon retention in superficial water layers and are an important food source to heterotrophic predators during winter time. In this context, the researchers in this study aimed to describe the zooplankton community structure in the Barents Sea and around the archipelago of Svalbard (70º to 81ºN) during the polar night in january, with the goal to obtain more knowledge on the life-history traits of arctic copepods.

They identified a total of 75 species. Copepods dominated the zooplankton community in both abundance and biomass, with a significant dominance of small sized individuals (graphic below). Based on the life stages’ composition they were able to detect three main wintering strategies: (1) Calanusspp. spends winter in a late immature stage, using the autumn resources for larvae development and strengthening, waiting for the spring to continue their cycle; (2) Microsetellanorvegicaspends the winter in an adult stage, which can serve as an advantage while preparing for the eggs production over the next spring season; (3) lastly, Pseudocalanusspp. reproduce during the winter season (although with a low egg production rate), leading to the believe that they take advantage of the winter for the growth and development of larvae without the risk of predators, which are mostly absent during this time.

Reproduction during winter season can be a strategy to assure the survival of younger life stages, in a period where both predation and competition with other species and larvae is reduced. Even in the case of lower prey availability, which can contribute to a higher starving risk. Although the Arctic winter may be a low activity season for many taxa, some zooplankton species have shown to adapt through these different strategies. These observations contribute for a view of the Polar Night as a season for copepods to rest but with great abundance rates.

The Arctic may be calm during the winter season, but it doesn’t fall asleep, waiting for the sun to come back!

[1] Polar nights: The polar night takes place when the night lasts the total 24 hours. It is a phenomenon that occurs in the polar regions.
[2] Zooplankton: Group of aquatic organisms without the ability to generate their own energy. They have low locomotion and live along the water column.
[3] Copepods: Crustacean group inside the zooplankton community, with a reduced size of 1 to 5 mm in length.

Source: High abundances of small copepods early developmental stages and nauplii strengthen the perception of a non-dormant Arctic winter. Barth-Jensen, C., Daase, M., Ormańczyk, M. R., Varpe, Ø., Kwaśniewski, S., & Svensen, C. Polar Biology, 45(4), 675–690 (2022).

Author: Débora Carmo


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