Comparative study of the reproduction of Calanus helgolandicus in well-mixed and seasonally stratified coastal waters of the western English Channel

Field estimates of the production and viability of eggs of the copepod Calanus helgolandicus were conducted weekly during 1993 and 1994 at two fixed stations in the western English Channel. The stations were located in the coastal zone off Roscoff (NW France) and Plymouth (SW England). These areas are characterized by homogeneous and seasonally stratified waters, respectively. In these two different environments, the egg production rate varied seasonally and was significantly correlated with chlorophyll a concentration, but not with surface temperature. Significant correlations between fecundity and particulate carbon and nitrogen were found only off Plymouth. The seasonal pattern of fecundity differed between the two sites. Off Plymouth, spawning occurred earlier, lasted longer and was significantly higher than in Roscoff waters. In both localities, fecundity was generally below the maximum specific egg production rate (similar to 60 eggs female(-1) day(-1)), implying that adult females were permanently under, or close to, limiting food conditions. Egg viability showed seasonal variations in both localities, ranging between 20 and 100%. However, in 1994, the amplitude of egg viability variation was lower in stratified waters. The results show that the potential fecundity of C.helgolandicus was different between mixed and stratified waters of the western English Channel. Food quality as well as quantity appear to be important for the reproductive success of this copepod species.