Growth of two species of Southern Ocean copepod in relation to their environment

Shreeve, Rachel S. (2002). Growth of two species of Southern Ocean copepod in relation to their environment. PhD thesis The Open University.



Measurements of egg production rates (EPR), growth and development of the early stages of the Southern Ocean biomass dominant copepods, Calanoides acutus and Rhincalanus gigas, were made over the course of four consecutive summer cruises in the vicinity of South Georgia. For both species EPR was found to be weakly but significantly related to Chlorophyll α although for C. acutus stages CII-CIV decreased from 0.24 to 0.14, and for R. gigas CI-CIII from 0.06-0.04. Overall, values for both species were within the range predicted by recent global models of copepod growth. Neither stage duration or g varied systematically with either temperature (mean 0-60m) or food (Chlorophyll α 0-60m). However carbon mass of nearly all species stages was negatively and significantly related to silicate levels (mol m-20 - 60m) 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. 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 dependant 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 larger-scaled. 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 Chlorophyll α 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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