The variability in dynamics and magnitude of phytoplankton growth seems to depend on local environmental conditions in the ecoregion rather than large-scale events such as the North Atlantic Oscillation (NAO). These changes affect zooplankton grazing pressure and the carbon flux through the food chain. Primary production on the Iceland shelf is high (150–300 g C m−2 year−1) and the productivity is highest in the southwest.
The onset of phytoplankton spring bloom varies between mid-April and mid-May. A trend of later onset of blooms south of Iceland has been observed since 2006. High inflow of Atlantic water to the northern shelf area of Iceland leads to increased primary production. Diatoms dominate the phytoplankton spring bloom over the Icelandic shelf. Dinoflagellates increase in abundance after the spring bloom, while diatoms continue to be relatively abundant. In the autumn there is usually a second bloom of diatoms and dinoflagellates. In some springs the prymnesiophyte Phaeocystis pouchetti becomes predominant in the waters to the north of Iceland.
Mesozooplankton abundance and biomass is generally dominated by Calanus finmarchicus. Mesozooplankton community structure differs south and north of Iceland, being mainly dictated by temperature and salinity differences. Macroplankton is dominated by euphausiids over the shelf edges in the south and west, and in the oceanic areas all around the island. In the oceanic areas north of Iceland, amphipods are also abundant.
The spring mesozooplankton biomass in the upper layers (0–50 m) generally ranges from ca. 1 to 10 g dry weight m−2, with an average of 2–4 g dry weight m−2. Relatively high biomass is usually observed in shelf waters off the southern and western coasts, in the oceanic waters to the north and northeast of Iceland where the Arctic influence is the greatest and large Arctic species dominate, and in offshore waters of the Irminger and Norwegian seas.