Growth rates of copepods in the southern Benguela upwelling system: The interplay between body size and food
Limnol. Oceanogr., 44(2), 1999, 382-392 | DOI: 10.4319/lo.1999.44.2.0382
ABSTRACT: Copepod growth rates were estimated from shipboard measurements of egg production of adult female Calanus agulhensis, Calanoides carinatus, Nannocalanus minor, and Centropages brachiatus and molting rates of juvenile stages (N6-C5) of C. agulhensis. Data were obtained during austral spring and summer of 1989-1995 in the southern Benguela upwelling system. While maximum growth rates showed less than a threefold decline over the body-size range examined (525-2,763-um total length), probably owing to allometric considerations, mean growth rate decreased by one order of magnitude, suggesting limitation of growth rate by an environmental factor. Most of this decline in mean growth rate was attributable to food limitation of large copepods. Frequency distributions of growth rate under low food densities were severely skewed toward slow growth rate for large copepods, whereas they were more symmetric for smaller copepods. In contrast, at high food concentrations, the frequency distributions had a high degree of symmetry for all copepods. These frequency distributions were interpreted in terms of a probabilistic model describing the encounter rate of copepods with suitably sized food particles. The effect of food limitation on growth rate was evaluated by regressing the coefficient of variation of growth rate against body size. A strong positive relationship was found (r2 = 0.94, P < 0.001), indicating that small copepods were always growing well, whereas the growth rate of large copepods was more variable. It is suggested that this difference is a consequence of the ability of small copepods to consume small particles, which are present at a relatively constant background density.