Consequences of changing sea-ice cover for primary and secondary producers in the European Arctic shelf seas: Timing, quantity, and quality
The Arctic ice cover poses severe limitations on the productive period of marine autotrophs that form the base of the marine food web. Sea-ice algae begin to grow in early spring within and underneath the ice, producing a substantial biomass despite very low light intensities. Pelagic algal blooms, in contrast, normally occur after ice breakup, at high latitudes as late as July–September. The timing of these blooms is crucial for the quantity and quality of primary and secondary production, and therefore for the transfer of energy and matter to higher trophic levels. Recent findings from Rijpfjorden, north-eastern Svalbard indicate that ice algae, rather than pelagic algae, trigger the reproduction of Arctic zooplankton around Svalbard. The key herbivore in Arctic shelf seas, the copepod Calanus glacialis, timed its seasonal migration, foraging, and reproduction to the ice algal bloom, which preceded the pelagic algal bloom by two months. The growth of this secondary producer’s offspring, however, was dependent on the later bloom of phytoplankton and higher sea-water temperatures. In 2007, reproduction and growth of C. glacialis and the primary production regime matched perfectly. The persistent ice cover in summer 2008, however, led to a mismatch between the pelagic algal bloom and the growth of the new copepod generation, resulting in a fivefold lower biomass of C. glacialis in August 2008 compared to 2007. The initiation of the ice algal bloom is mainly determined by the solar angle, whereas the pelagic algal bloom requires more light and is therefore governed to a larger degree by ice thinning and the unpredictable ice breakup. We conclude that both a too early as well as a too late ice breakup can cause a mismatch between primary and secondary producers, with negative consequences for the entire lipid-based Arctic marine food web.