A spatial and temporal study of Pseudocalanus acuspes, P. minutus and P. moultoni in the Svalbard – Barents Sea region and their potential as environmental indicator species in a changing Arctic

Many species of small copepods are morphologically similar and thus often grouped together at genus level rather than studied as individual species. However, species within the same genus may differ quite a lot in life history strategies and traits. In this study, I used a species-specific polymerase chain reaction to explore the abundance, distribution and size range of Pseudocalanus sibling species in the Svalbard – Barents Sea region. The main aim was to investigate if they have distinct environmental preferences and the potential to be used as valid environmental indicator species. Temporal patterns in species composition were investigated monthly year-round in a high Arctic fjord (Billefjorden, Svalbard) over an entire year, while spatial patterns in species composition were investigated during a three-week cruise in the Barents Sea across the Polar Front from 73º S to 78º N. P. acuspes was the most abundant species in Billefjorden and it dominated year-round except in June when the overall Pseudocalanus abundance was low and P. minutus were dominating. P. minutus was also present year-round in Billefjorden comprising 20 - 42 % of the total Pseudocalanus abundance from May to August 2021, while in the other months they had a lower total abundance between 4 and 15 %. P. moultoni was only present in 7 out of 12 months in Billefjorden, with the highest abundance in June and July (16-20 % of the Pseudocalanus community). P. minutus had the overall largest average CIV and CVs, and while P. acuspes had the overall largest adults, P. moultoni had the overall largest single individuals. The gap and inconsistency between body size in CV and adult females, could be explained by differences in feeding strategy and timing of reproduction. P. minutus overwintered as CIV and CV in Billefjorden and started to reproduce in February and peaked in April when most CV had moulded into adults and hence were using their lipids reserves on maturation and reproduction rather than growing larger during the spring showing a capital breeding strategy. P. acuspes on the other hand overwintered as CIII and CIV, and adults had a late peak in July after CV were grazing during the spring and becoming adults during the summer displaying an income breeding strategy. In the Barents Sea, P. acuspes was the most abundant species at the northernmost (T1, P2) and at the Polar Front stations (T3, T4, and T5). P. moultoni on the other hand, was the most abundant species at the southernmost stations (P1, C and the reference station), but barely present at the stations furthest North (1.3 – 3.2 % of the total Pseudocalanus abundance). My study demonstrates the importance of sampling the entire populations (CI-adults) and not only parts of it when investigating species distributions, and preferable throughout the entire year to avoid biases due to seasonality in population structures and advective events (i.e., Billefjorden)