In situ twilight grazing rhythm during diel vertical migrations of a scattering layer of Calanus finmarchicus 1

• Published in: Limnology and Oceanography
• Main Authors: Simard, Yvan, Lacroix, Guy, Legendre, Louis
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1985
• Subjects: Aquatic Science; Oceanography; Calanus finmarchicus; Copepods
• Online Access: http://dx.doi.org/10.4319/lo.1985.30.3.0598; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.4319%2Flo.1985.30.3.0598; https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1985.30.3.0598

A scattering layer of Calanus finmarchicus was sampled every 90 min during 48 h in the lower St. Lawrence estuary. Grazing activity (phytoplankton pigments in the gut) and the percentage of recently and nonrecently fed copepods (from observation of food in the gut) were monitored in two strata (0–30 m and 30–100 m). Phytoplankton was restricted to the upper stratum. A bimodal twilight grazing rhythm was observed: the first feeding period, in the upper stratum, begins after sunset; it is followed by the “midnight sinking” in the deeper stratum (2–3 h), during which the gut content is evacuated; after this, the animals return to the upper stratum for a second meal, before the dawn descent. In both migrations, there was a dynamic interchange of individuals between the two strata, which masked the fact that all the copepods migrated to the upper stratum and that a dawn rise actually happened. The feeding time in the warmer surface water was very short. Feeding ceased rapidly even if phytoplankton concentration was low. A large increase in the number of migrants did not change the feeding patterns. Results support the hypothesis that the behavior of C. finmarchicus during the central phase of vertical migrations in late summer is closely linked to an in situ grazing rhythm.