The relative importance of food and temperature to copepod egg production and somatic growth in the southern Benguela upwelling system

The fecundity and somatic growth rates of Calanus agulhensis and Calanoides carinatus, the dominant large calanoid copepods in the southern Benguela upwelling system, as well as the fecundity of several other common copepods, were measured between September and March of 1993/94 and 1994/95. Mean egg production of most copepods was low at >30 eggs female-1 day-1 {Calanoides carinatus 23.7, Calanus agulhensis 19.0, Neocalanus tonsus 16.1 and Rhincalanus nasutus 26.1), whereas the mean fecundity of Centropages brachiatus was significantly greater (83.6 eggs female-1 day-1). This study also presents the first comprehensive field estimates of the fecundity of Nanno-calanus minor (mean: 26.1 eggs female-1 day-1, range: 0.0–96.2 eggs female-1 day-1) and of somatic growth of N6 and all copepodite stages of Calanoides carinatus (decreasing from 0.58 day-1 for N6 to 0.04 day-1 for C5). Somatic growth rates of Calanus agulhensis also declined with age: from 0.57 day1 for N6 to 0.09 day1 for C5. Data on growth rates were used to assess the relative importance of food [as measured by total chlorophyll (Chi) a concentration], phytoplankton cell size (proportion of cells >10 µm) and temperature to the growth of copepods. Multiple regression results suggested that fecundity and somatic growth rates were positively related to both Chi a concentration and phytoplankton cell size, but not to temperature. Although it was not possible to separate the effects of Chi a concentration and phytoplankton cell size, data from previous laboratory experiments suggest that copepod growth is not limited by small cells per se, but by the low Chi a concentrations that are associated with these particles in the field. Despite growth not being directly related to temperature, a dome-shaped relationship was evident in some species, with slower growth rates at cool (<13°C) and warm (>18°C) temperatures. The shape of this relationship mirrors that of Chi a versus temperature, where poor Chi a concentrations are associated with cool and warm temperatures. It is concluded that the effect of food limitation on growth of copepods outweighs that of temperature in the southern Benguela region. Sources of variability in relationships between growth and Chi a concentration are discussed

Growth rates of copepods in the southern Benguela upwelling system: The interplay between body size and food

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-mu m 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 (r(2) = 0.93, P < 0.001), indicating that small copepods were always sowing 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