Ice-related seasonality in zooplankton community composition in a high Arctic fjord

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Seasonal mesozooplankton community composition was studied monthly from March to October 2007 in the high Arctic, Rijpfjorden (Svalbard), and related to abiotic (hydrography, sea ice) and biotic (ice algae and phytoplankton biomass) environmental conditions. The community was numerically dominated by the cosmopolitan Oithona similis, whereas Arctic Calanus glacialis was the dominant taxon in terms of biomass. Seasonal mesozooplankton community development was largely influenced by the sea ice and hydrographic conditions, which impacted the primary production regime in the fjord. Three distinct periods could be identified based on species and life stages composition: (i) winter–spring transition (March–June), with high ice algal biomass in April–June, characterized by peak abundances of Pseudocalanus minutus, Calanus glacialis females and Clione limacina; (ii) sea ice break-up and phytoplankton bloom (July), with high numbers of Calanus nauplii and young copepodids, as well as larvae of benthic crustaceans such as Cirripedia and Decapoda; and (iii) ice-free post-bloom period (August–October), when the pulse of warm waters into the fjord resulted in development of a community with Atlantic characteristics and peak abundances of Oithona similis, Oithona atlantica, Limacina helicina and Echinodermata larvae within the upper 50 m. At the same time, older copepodids of Calanus glacialis and Calanus finmarchicus had already descended to overwintering depths (>100 m). Despite the 2–3 months delay in the phytoplankton bloom compared with ice-free Svalbard fjords, the Rijpfjorden mesozooplankton managed to fulfill their life cycles in a similar time, likely due to the utilization of the ice algal bloom, and warmer water enhancing species growth and development in late summer.

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