Cold water plankton are bigger

Posted on March 23, 2016

Because large-bodied zooplankton species have higher grazing rates, take a wider size-range of food items, and evade predators more easily than small species, variations in zooplankton body size can have a strong effect on aquatic food webs. An international group of biologists looked at the influence of temperature on the size of planktonic crustaceans by analysing published data from 123 lakes in eight latititudinal regions (Greenland, Alaska, Ontario, British Columbia, Turkey, Florida, Brazil, Ethiopia). For each crustacean group (cladocerans, calanoids, cyclopoids) and region they calculated mean body size (dry weight), weighted by species densities. The latitudinal range was 6-74o, the temperature range 2-30oC. Regression analysis showed that the mean body size of cyclopoids declined significantly with increasing temperature, and the trend was similar, although weaker, for cladocerans. The same tendency has also been reported for various groups of marine crustaceans, and has been explained by the fact that high temperatures lead to shorter generation times and smaller adults. The stronger relationship for cyclopoids than cladocerans could be explained by the fact that cyclopoids have well-developed escape behaviours and are therefore more likely to be controlled by temperature than by fish predation. In support of this interpretation, a separate analysis of the Greenland data found that cladoceran body size was five times higher in fishless lakes than those with fish, but cyclopoids were only slightly smaller in the presence of fish. Unlike the other two groups, calanoid crustaceans showed no body-size – temperature relationship, and the reason for this difference is unknown.

Reference: Havens, K.E. et al. 2015. Temperature effects on body size of freshwater crustacean zooplankton from Greenland to the tropics. Hydrobiologia 743, 27–35.