Towards a global model of in situ weight-specific growth in marine planktonic copepods

Abstract

The dependency of in situ weight-specific fecundity of adult females (as egg production) and growth of juveniles (as somatic production) upon individual body weight in marine planktonic copepods was examined. A compilation was made of results where wild-caught individuals were incubated in natural seawater (often pre-screened to remove large organisms), at near in situ temperatures, over short periods of the order of 24 h. The results demonstrate that for the adult broadcast-spawning group weight-specific fecundity rates are dependent upon body weight, but independent of temperature. We postulate this may be the result of global patterns in available phytoplankton. Weight-specific growth rates are dependent upon individual temperature and body weight in juvenile broadcast-spawners, with rates declining as body weight increases. Sac-spawners have growth/fecundity rates that are independent of body weight in adults, juveniles, and both combined, but which are temperature-dependent. Globally applicable equations are derived which may be used to predict growth and production of marine copepods using easily quantifiable parameters, namely size-distributed biomass and temperature. Some of the variability in growth which remained unaccounted for is the result of variations in food quantity and quality in the natural environment. Comparisons of the rates compiled here over the temperature range 10 to 20 °C with previously compiled food-saturated rates over the same temperature interval, revealed that in situ rates are typically sub-optimal. Adults appear to be more food-limited than juveniles, adult rates in situ being 32 and 40% of those under food saturation in broadcasters and sac-spawners, respectively, while juvenile in situ rates are on average ∼70% of those at food saturation in both broadcasters and sac-spawners.