Population structure of the copepods Centropages typicus and Temora stylifera in different environmental conditions

The population structure (NI to adults) of the copepods Centropages typicus and Temora stylifera was investigated in parallel at two sites in the eutrophic and oligotrophic waters of the Gulf of Naples (Tyrrhenian Sea) with the aim of identifying the environmental factors that modulate the occurrence of these calanoids that are abundant in the neritic waters of the Mediterranean Sea. Monthly zooplankton sampling was conducted from May 1999 to June 2000 in 5–7 discrete depth layers in the upper 200 m with a 70-μm mesh net. Temperature, salinity, and fluorescence profiles were recorded by CTD casts. The remarkable differences observed in the temporal and spatial patterns of the two populations throughout their developmental cycles seem to result more from different responses to environmental factors than from different life-history traits. C. typicus was abundant in early to mid-summer and was distributed in the whole water column; T. stylifera followed in late summer–autumn and occurred preferentially in the upper 30 m. Temperature was the main regulating factor for T. stylifera. The availability of autotrophic food appeared relatively more important for C. typicus, although unequivocal signals were not observed, suggesting that more complex trophic interactions likely affect the dynamics of the two populations.