In an article about herring in Alaska, it was mentioned that the zooplankton in colder waters were much higher in lipids than the zooplankton in warmer waters. Is that because the zooplankton store up lipids for the winters when the days are much shorter and the phytoplankton growth is limited?
Yes — that is the main reason.
In high-latitude waters like those around Alaska, primary production is highly seasonal. Phytoplankton blooms are intense but short (spring/summer), then light and ice conditions sharply limit growth for many months. Large calanoid copepods that dominate cold-water zooplankton (especially *Calanus* species) convert that brief pulse of food into very large lipid stores — often wax esters that can exceed 50% of dry body weight. They use those stores to survive winter, fuel metabolism during diapause at depth, and support reproduction before the next bloom. (LINK)
Warmer waters have more continuous (or less extreme) production, so the same strategy is less necessary. The zooplankton community also shifts: cold years favor large, lipid-rich copepods; warm years favor smaller species with lower lipid content. That difference in prey quality is well-documented for Pacific herring, pollock, and other forage fish in the Bering Sea. (LINK)
A secondary factor is membrane adaptation: organisms in colder water tend to have more unsaturated fatty acids to keep cell membranes fluid. That affects lipid *composition*, but the large difference in *total* lipids is primarily the energy-storage strategy for winter food scarcity. (LINK)
So the article’s observation matches a well-established pattern: cold-water zooplankton are fatter because they must bank energy against a long, dark, low-production winter.