Life cycle of Calanus hyperboreus in the lower St. Lawrence Estuary and its relationship to local environmental conditions

Abstract

We studied the life cycle of Calanus hyperboreus in the lower St. Lawrence Estuary (LSLE) using (1) C. hyperboreus population-stage abundance and body size data, and time series of chl a biomass, collected between 1991 and 1998 at a monitoring station, (2) observations of the sea- sonal pattern in day/night vertical distribution of C. hyperboreus in the LSLE and the adjacent NW Gulf of St. Lawrence (GSL) and (3) observations of long-term C. hyperboreus egg production con- ducted during winter. Total abundance (Stages C1 to C6f) was similar among years, and stage com- position (Stages C4, C5, and C6f) was constant in summer and autumn of each year. However, the abundance of Stages C1 to C3 in May and the stage structure during the summer-autumn period showed significant interannual variation. The seasonal pattern in vertical distribution and body weight suggests that the C. hyperboreus population overwinters in the 200 to 300 m water layer from July to April with the ontogenetic ascent and descent mainly occurring in late April/early May and July, respectively. We propose a 3 yr life cycle for adult females (Stage C6f), with the main reproduc- tive event occurring during the second year of life. C. hyperboreus females initiate gonad maturation in early December and reproduce until late March, 3 to 6 mo prior to the onset of the phytoplankton bloom in the LSLE. Total fecundity of Stage C6f was inversely related to respiration but not to female body size. The interaction between the timing of reproduction and the peak in freshwater runoff likely promote the export of the new cohort in spring and the upstream advection of overwintering animals from the adjacent NW GSL in early summer. This interaction apparently precludes the evo- lution of a population synchronized with the seasonality in biological and physical conditions in the LSLE. We hypothesize that the surface circulation pattern in the LSLE and biological and physical conditions in the NW GSL in spring likely control the interannual variations in abundance of Stages C1 to C3 in spring, and in overwintering stage structure.