Temperature-dependent production of marine copepods: a global synthesis.

Estimating the production of marine copepods depends on measuring two key variables: biomass and growth rate. The major difficulty in estimating production of marine copepods and other zooplankton has been the inability to obtain precise, rapid measurements. In practice the variability in measurement of biomass greatly exceeds that in growth rate. It is shown here that individual growth rates of copepods can be accurately estimated from data on generation times and the weights of eggs and adults. Analysis of 181 separately published estimates of generation time for 33 species of copepods at environmental temperatures ranging from -1.7 degrees to 30.7 degrees C shows that temperature alone explains more than 90% of the variance in growth rate. Temperature dependence of growth rate transcends species differences. Weight-specific growth rate appears to be independent of body size. We hypothesize that food may not be limiting to growth in nature; the impression that food is limiting may be due to sampling at the wrong scales. Another possible cause of the apparent maximum growth rates of copepods in nature is predation mortality, which could selectively remove slower-growing individuals from the population. The temperature-dependent model developed here predicts the phenomenon of decreasing body size with increasing environmental temperature, often observed for single species of copepods. A method is suggested for making more accurate estimates of secondary production by using modern instrumentation to make quasi-synoptic measurements of biomass and temperature and using the temperature-dependent model to estimate individual growth rates.