Characteristics of egg production of the planktonic copepod, Calanus finmarchicus, on Georges Bank: 1994–1999

We present here a synthesis of observations of egg production rates (EP) of the planktonic copepod, Calanus finmarchicus, carried out during process cruises of the US GLOBEC Northwest Atlantic/Georges Bank program between January and June 1995, 1997 and 1999. Female C. finmarchicus produced eggs at relatively high rates in at least some regions of Georges Bank during all months between January and June. Median, monthly EP varied between 24 eggs female−1 d−1 in January to 50 eggs female−1 d−1 in April–June; the highest mean EP was 86 eggs female−1 d−1. Mean egg diameter was negatively related to ambient mean water-column temperature (0–100 m or bottom), decreasing from 149 to 142 μm between January and June. Direct measurements of body C or N or prosome length–mass relationships were used to determine mass-specific egg production rates. The relationships between estimates of chlorophyll a standing stock (mg chl. a m−2) and both C- and N-specific rates (% d−1) are reasonably well ( r 2 = 0.42 ) described by Ivlev curves. It is likely that chlorophyll standing stock serves as a proxy of both phytoplankton and microzooplankton food concentrations available to adult females. Chlorophyll standing stocks were below the critical concentration (at which EP is 95% of the calculated maximum) at approximately 55% of stations occupied over the study period, indicating frequent food limitation to varying extent. There were periods (e.g., over at least 6 d in April, 1997 on the southern flank) during which food limitation was severe. There was no detectable influence of mean water-column temperature on mass-specific EP. Hatching success varied between 50% and 95% without any seasonal trend. Our qualitative observations suggest the possibility that a significant proportion of hatching nauplii incubations were non-viable, meriting further study.