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Comparison of abundance, vertical distribution and reproduction of the cousin species, the boreal  Calanus finmarchicus  and temperate  Calanus helgolandicus  was carried out on four cruises in July and August north of the Dogger Bank, North Sea. During this period, the water column was highly stratified with a tidally generated deep chlorophyll maximum at 30 m depth. When co-occurring, a separation of the species was evident, where  C. finmarchicus  preferred colder (9°C) deeper waters, while  C. helgolandicus  stayed in the warmer (16°C) surface waters. Egg production rates (EPRs) were not statistically different between the species, and the population egg production depended primarily on female abundance and was generally higher for  C. finmarchicus . EPRs of the  Calanus  spp. were best explained by the abundance of autotrophic and heterotrophic dinoflagellates, flagellates and ciliates. Hatching success remained over 90% at all times but the estimated naupliar survival (N1–N6) was only 9%. The chlorophyll maximum supported highest faecal pellet production and egg production at the stations close to the bank. This study shows that  C. finmarchicus  can remain reproductively active in the North Sea ecosystem longer than previously thought, and with warmer surface temperatures retreat to cooler, deeper waters utilizing the deep chlorophyll maximum. This implies that  C. finmarchicus  cannot be reliably sampled with the Continuous Plankton Recorder during summer.

Biological processes in the North Sea: comparison of Calanus helgolandicus and Calanus finmarchicus vertical distribution and production