Copepod growth and development around South Georgia: relationships with temperature, food and krill

Measurements of egg production rates (EPR) and growth of the early stages of 2 of the Southern Ocean biomass dominant copepods, Calanoides acutus and Rhincalanus gigas, were made over the course of 4 consecutive summer cruises which were carried out in the vicinity of South Georgia. For both species, EPR was found to be weakly but significantly related to chlorophyll a (chl a) although for C. acutus it was below maximum levels recorded in spring. Juvenile mass specificgrowth rates (g) were found to be body mass, stage and species dependent. Mean g for C. acutus stages CII to CIV decreased from 0.24 to 0.14, and for R. gigas stages CI to CIII from 0.06 to 0.04. Overall, values for both species were within the range predicted by recent global models of copepod growth. Neither stage duration nor g varied systematically with either temperature (mean, 0 to 60 m) or food (chl a, 0 to 60 m). However, carbon mass of nearly all species stages was negatively and significantly related to silicate levels (mol m–2, 0 to 60 m), suggesting the positive effect of past production levels. Ordination of zooplankton species occurrence by station across the survey area indicated that changes in abundance were more pronounced than changes in species composition, and that variation in total copepod abundance was also well explained by silicate levels. Our study indicated that changes in EPR, carbon mass and abundance of copepod populations at South Georgia were all strongly regulated by local primary production. Variation of chlorophyll biomass appeared largely dependent on temperature, rather than grazing pressure exerted by either copepods or krill. Krill at South Georgia were more abundant in colder, silicate replete waters and their presence is presumed to be governed by factors operating at the large scale. In contrast, copepod abundance appeared to differ in response to smaller scale variation in the environment and was linked through silicate to factors determining phytoplankton growth. In turn, chl a concentration was strongly and positively related to habitat temperature. This suggests the importance of the physical environment rather than grazing as ultimate factors controlling phytoplankton biomass in this productive ecosystem.

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