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. It's not hard to see that in the Antarctic, the food web is affected by climate factors. Scientists disagree as to whether the warming trend of the past 50 years is part of a long-term cycle of warming and cooling of the earth or an effect that has been accelerated by human activities. In the current century the burning of fossil fuels has greatly increased the amount of carbon dioxide in the atmosphere. It is well known that carbon dioxide gas is a principal warming agent in the atmosphere.