Effect of food quality on carbon and nitrogen growth efficiency in the copepod Acartia tonsa

Populations of the copepod Acartia tonsa were fed a mixture of algal prey (diatom Thalassiosira weissflogii, prymnesiophyte Emiliania huxleyi, dinoflagellate Aureodinium [Gymnodium] pigmentosum) supplied at saturating concentrations, grown under either nitrogen-sufficient or nitrogen-deplete conditions, in order to study the impact of food quality on production and development throughout the life cycle of the copepod. Changes in predator population structure and biomass were recorded, along with consumption of each of the algal groups, permitting C and N growth efficiencies to be estimated. There was a clear difference in the Acartia tonsa population structure when fed N-sufficient or N-deplete prey, with those fed N-deplete prey slower to develop and reproduce and laying fewer eggs. Algal nutrient status affected selectivity between the diatom and dinoflagellate, the latter being favoured under nutrient-deplete conditions, perhaps in part because their C:N ratio was less susceptible to altered nutrient status. There was no clear difference in the N growth efficiency (N-GE, typically 5%) between N-sufficient and N-deplete prey, but certainly efficiency did not increase with N-deplete prey. C growth efficiency (C-GE) declined from 5 to 2% with N-deplete prey. However, while the ratio of N-GE:C-GE was clearly different between N-sufficient (1) and N-deplete (2.5) treatments, actual growth efficiencies increased with time during the progression to later life history stages, culminating in highest efficiencies during active egg production. Caution should be exercised in assigning GE and predation rates in models incorporating zooplankton feeding on prey of variable nutrient status; these parameters are not constants and GE estimates from egg production experiments are likely to significantly overestimate efficiencies over the whole copepod life cycle.