Vol. 102: 217-227, 1993 l MARINE ECOLOGY PROGRESS SERIES Mar. Ecol. Prog. Ser. Published December 30 Reproduction of the planktonic copepod Calanus finmarchicus in the Lower St. Lawrence Estuary: relation to the cycle of phytoplankton production and evidence for a Calanus pump Stephane Plourde, Jeffrey A. Runge Institut Maurice-Lamontagne, Department of Fisheries and Oceans. CP 1000. Mont-Joli. Quebec. Canada G5H 324 ABSTRACT: Due to the dynamic physical environment of the Lower St. Lawrence Estuary, the spring phytoplankton bloom in the Laurentian Channel occurs late in the season, typically in mid-June, but the high phytoplankton biomass is sustained throughout the summer months. In this study, relation- ships between the phytoplankton production cycle, water temperature, and the reproductive cycle of Calanus finmarchicus Gunnerus, a predominant planktonic copepod in the Lower Estuary, were inves- tigated during spring-summer 1991. Field observations showed that the final stages of oocyte matura- tion in C. finmarchicus females did not begin until the onset of the spring phytoplankton bloom in mid- June. High egg production rates, as estimated by the number of eggs released by females incubated immediately after capture, commenced 1 wk later and persisted until late August. Egg production rates were significantly correlated with an index of gonadal maturity in females and were consistent with a rectilinear or curvilinear relationship with chlorophyll a standing stock. Laboratory experiments showed that: (1) in presence of food (the diatom Thalassiosira weissfloggii), maturation of oocytes would proceed and females could spawn eggs at least 2 mo before the spring bloom; (2) without food, the oocytes did not develop past immature stages, except in a small minority of the population; and (3) colder temperatures in early spring would prolong the lag between the onset of the spring bloom and the start of egg production by less than 4 d. Combined with concurrent microscopic measurements of oil sac volume, the results do not rule out the possibhty that lipid reserves were used to support the early stages of oogenesis, but do show that the majority of females did not use lipid reserves for vitello- genesis prior to the spring phytoplankton bloom. It is suggested that the Lower St. Lawrence Estuary is an important region of C. finmarchicus production in summer which, because of the residual surface circulation, may act as a Calanus 'pump' to influence levels of zooplankton biomass in the Gulf of St. Lawrence and on the shelf off Nova Scotia. KEY WORDS: Calanus . Egg production . Primary production cycle . Zooplankton INTRODUCTION The Lower St. Lawrence Estuary is a long (200 km), narrow (50 km) and deep (350 m) body of water con- necting the Upper Estuary, into which the St. Lawrence River flows, with the Gulf of St. Lawrence. The dynamic physical environment of the Lower Estuary has a large influence on the pattern and processes of its marine planktonic inhabitants. Due to the large runoff from the St. Lawrence IZlver, residence tinles in the surface layer in spring are short (Sinclair et al. 1981) and seeding of diatoms into the surface layer may be restricted during the runoff period (Levasseur et al. 1984). As a consequence, the spring phyto- plankton bloom in the Lower Estuary typically does not start until mid-June. However, tidal mixing at the head of the Laurentian Channel supplies nutrients to the surface waters of the Lower Estuary periodically throughout the summer, and the phytoplankton bloom, in which diatoms and dinoflagellates predominate, usually persists into September (Levasseur et al. 1984, Therriault & Levasseur 1985). C3 Inter-Research 1993