On the ecology of Calanus finmarchicus in the Subarctic North Atlantic: A comparison of population dynamics and environmental conditions in areas of the Labrador Sea-Labrador/Newfoundland Shelf and Norwegian Sea Atlantic and Coastal Waters

Abstract

The Norwegian Sea is generally warmer than the Labrador Sea because it is influenced more by Atlantic Water inflows from the south, whereas the latter receives relatively larger inputs of Arctic Water from the north. Despite its more northerly location, the spring bloom generally starts earlier in the Norwegian Sea. Within each of the two seas, however, there are regional and interannual differences in temperature and the timing of the spring bloom. The responses of Calanus finmarchicus populations to these differences in environmental conditions include differences in physical characteristics (e.g. female size), physiological rates (egg production rates) and seasonal cycles of abundance. Females are generally larger in the Labrador Sea and have higher egg production rates for a given chlorophyll concentration than do those in the Norwegian Sea. Within and among areas in both seas, as temperatures increase and spring blooms tend to occur earlier, C. finmarchicus start to reproduce earlier, the new generation develops faster, and in some areas a second generation ensues. In areas where near surface temperatures are relatively high in summer and/or where phytoplankton growth rates are relatively low in summer or autumn, reproduction and development cease, and C. finmarchicus desert the surface layers for their overwintering depths. This occurs in the Norwegian Sea in summer and in the central Labrador Sea in autumn. By contrast, in areas where near surface temperatures remain cool in summer and where phytoplankton growth persists through the autumn, reproduction and development can continue through summer and autumn, probably until winter vertical mixing prevents phytoplankton growth. This occurs on the southern Newfoundland Shelf. Even in areas where the growth season is prolonged, however, a proportion of the first generation, and probably subsequent generations, descends to overwinter. If the size of the overwintering population is used as an index of net productivity, then for equivalent regions in the Norwegian Sea and Labrador Sea (the areas of each most affected by Atlantic inflow), the differences in ambient temperatures and bloom dynamics apparently have little impact. With global warming, as temperatures in the Norwegian and Labrador Seas increase up to a certain threshold, the timing of life history events for C. finmarchicus will likely be advanced and the number of generations produced per year could increase. The time spent in the near surface layers will probably decrease, however, while the overall effect on population size may not be large. Once the temperature threshold for unfavourable survival of C. finmarchicus has been exceeded, the distribution range for C. finmarchicus will likely contract northwards, with important consequences for dependent species in the affected regions.