The outcome of this competition affects competition between zooplankton species, which are primary consumers. Colder winters mean greater ice cover than warmer winters. Diatoms are the preferred food of krill , which are in turn eaten by many other animals in this food web. So when winters are very cold and there is a lot of ice, almost every group in the food web does better because more food energy is transferred from lower to higher feeding or trophic levels . When conditions are reversed, and milder winters occur, more of the ice melts and different species of phytoplankton have an advantage for growth and reproduction.
Warmer winters favor smaller phytoplankton such as cryptophytes.
Unfortunately, these are not consumed as well by krill , so less of the food energy from cryptophytes gets into the food web. A big population of cryptophytes means a smaller population of diatoms since the two species compete for nutrients. Smaller diatom populations mean less food for krill . Less krill means less food for vertebrate predators. In the past 50 years, winters that have extensive sea ice development have been less frequent, leading to reduced krill populations.