Utilisation of small-sized food algae byCalanus finmarchicus(Copepoda, Calanoida) and the significance of feeding history

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Abstract
Temporal trends and effects of changes in the food environment on egg and faecal-pellet production of Calanus finmarchicus were studied over five weeks using four different food regimes, including monocultures of the cryptophyte Rhodomonas baltica (ca. 8 μm diameter), the haptophyte Emiliania huxleyi (ca. 4 μm diameter), and natural E. huxleyi dominated water from a mesocosm enclosure. The functional responses of egg and faecal-pellet production and clearance/ingestion on algal concentration showed that the copepod could utilise R. baltica and E. huxleyi. High concentrations (> 800 μg C 1-1) of both species were needed to reach maximum rates, but significant egg production occurred at low food concentrations ( 4–7 eggs female-1 day-1 at 50–100 μg C 1-1). The life-time egg production was 1000–1800 eggs female-1 in a constant surplus food environment (900 μg C 1-1 of R. baltica), whereas variable food environments generated lower feeding and egg production. Switching to a reduced food environment gave a gradual (2–4 days) reduction in feeding and egg production whereas switching from reduced to surplus food caused a slower response with significant effects of feeding prehistory still after several weeks. The directly measured gross growth efficiency of C. finmarchicus with E. huxleyi as food was on average 31 %whereas indirect estimates from the time series, using ingestion/ egestion relationships, gave average values between 30 and 40 % for the four groups. Peak values of 50–70% were close to the theoretical limit for C.finmarchicus, calculated on the basis of their metabolism and bioenergetic costs of egg production. Our results show that C. finmarchicus is capable of efficiently converting nanoplankton carbon to secondary production, that it can achieve an optimal production during nanoplankton blooms and that the low summer concentrations of nanoplankton algae in temperate waters can sustain low but continuous reproduction. The significant effects from trophic history and changes in the trophic environment put constraints on the egg-production method to estimate copepod feeding and production. We discuss the significance of small-sized algae for the life cycle of C. finmarchicus and we show that the production rate of faecal pellets can be used as a quick and robust method to estimate grazing rate in this copepod.