Copepod feeding patterns before and during a spring bloom in Bedford Basin, Nova Scotia

Abstract

Samples of zooplankton were collected at frequent ~ntervals in the top 30 m of Bedford Basin over one 48 h and two 24 h sampling periods in February, early March and late March, 1986. Measurements of gut pigment content showed that Pseudocalanus sp. and Acartia sp. in early March had bimodal diurnal feeding rhythms, with peaks in activity at around dawn and dusk, while in February and in late March they appeared to be feeding more-or-less continuously. The sampling series covered the period of the algal spring bloom and maximal ambient chlorophyll concentrations measured in the top 30 m increased from 3.5 pg I-' in February to 21.0 pg I-' in early March, decreasing to 12.8 pg I-' in late March. We suggest that the diurnal feeding rhythms found in early March resulted from an imbalance between the ability of copepods to ingest food, which would have incrt,ased rapidly during the early spring bloom as phytoplankton levels increased, and their capacity to digest it. Later on in the bloom period, it seems that ingestion and digestion rates became bdlanced again so that diurnal rhythms were not apparent in late March. Such an effect may have been due to adaptive processes occurring in the digestive physiology of the copepods a s the result of exposure to high food concentrations, or perhaps to changes in the composition of the food itself. Related laboratory experiments showed that hunger and light intensity both affected feechng behavior in vitro in a way consistent with that observed In situ