Timing of blooms, algal food quality and <i>Calanus glacialis</i> reproduction and growth in a changing Arctic

Abstract The Arctic bloom consists of two distinct categories of primary producers, ice algae growing within and on the underside of the sea ice, and phytoplankton growing in open waters. Long chain omega‐3 fatty acids, a subgroup of polyunsaturated fatty acids (PUFAs) produced exclusively by these algae, are essential to all marine organisms for successful reproduction, growth, and development. During an extensive field study in the Arctic shelf seas, we followed the seasonal biomass development of ice algae and phytoplankton and their food quality in terms of their relative PUFA content. The first PUFA‐peak occurred in late April during solid ice cover at the onset of the ice algal bloom, and the second PUFA‐peak occurred in early July just after the ice break‐up at the onset of the phytoplankton bloom. The reproduction and growth of the key Arctic grazer Calanus glacialis perfectly coincided with these two bloom events. Females of C. glacialis utilized the high‐quality ice algal bloom to fuel early maturation and reproduction, whereas the resulting offspring had access to ample high‐quality food during the phytoplankton bloom 2 months later. Reduction in sea ice thickness and coverage area will alter the current primary production regime due to earlier ice break‐up and onset of the phytoplankton bloom. A potential mismatch between the two primary production peaks of high‐quality food and the reproductive cycle of key Arctic grazers may have negative consequences for the entire lipid‐driven Arctic marine ecosystem.