MEPS 254:253-268 (2003)  -  doi:10.3354/meps254253

ABSTRACT: Field evidence is presented showing that hatching success in the copepods Acartia clausi and Calanus helgolandicus was greatly modified during 2 major late-winter diatom blooms in the North Adriatic (Mediterranean) Sea in 1997 and 1998 compared to post-bloom conditions in early summer of 1997. In February of both years, diatoms represented >90% of the microplankton at all depths of inshore stations, and from 40 to 95% at different depths of offshore stations. The diatom blooms differed between the 2 yr in that Skeletonema costatum (66.3%) and Pseudo-nitzschia delicatissima (23.2%) dominated in 1997, whereas the following February the bloom was almost entirely composed of P. delicatissima (80.6%). In June 1997, diatoms were much less abundant, even though they still comprised about 50% of the algal population at some of the more coastal stations; the diatom species composition in this period had shifted to a dominance of Chaetoceros decipiens and Dactlyliosolen fragilissimus. The population structure and vertical distribution of the copepods A. clausi and C. helgolandicus were examined to determine the effects of these diatom blooms on population recruitment rates. A. clausi was the most abundant copepod sampled in June, constituting 47.7% of total copepod numbers, whereas low abundances were recorded in February 1997 (1.6%) and 1998 (1.8%). C. helgolandicus numbers were also low in winter 1998 (0.82% of total copepod numbers). Egg production rates (EPR) for A. clausi were highest during the blooms, with values of 10.6 and 18.4 eggs female^-1 d^-1 in 1997 and 1998 respectively, compared to 6.2 eggs female^-1 d^-1 in June 1997. However the most dramatic differences between bloom and post-bloom conditions were in terms of egg hatching viability, with mean values of 15.4 and 16.1% in February 1997 and 1998 respectively, as opposed to 89.1% in June. Fecal pellet numbers for females were highest in winter, indicating active feeding on diatoms. Total egg production rate (TEP, the product of EPR and total number of females m^-3) was much greater in June (863 eggs m^-3 d^-1) than in February 1997 (17.6 eggs m^-3 d^-1) and 1998 (140 eggs m^-3 d^-1) due to the higher number of females in early summer. In turn, naupliar recruitment rates (TNR) were extremely high in June (790.2 nauplii m^-3 d^-1) as opposed to February 1997 (2.8 nauplii m^-3 d^-1) and February 1998 (15.6 nauplii m^-3 d^-1). EPR for C. helgolandicus in February 1998 was low (9.7), with only 6.5% hatching viability. Values for TEP (54 eggs m^-3 d^-1) and TNR (4.7 nauplii m^-3 d^-1) were lower than for A. clausi. We conclude that the main factor responsible for reduced population recruitment rates of both species in winter was low hatching viability and that the bulk of annual recruitment in the North Adriatic Sea occurred in post-bloom conditions, since the mixed protistan community that succeeded the bloom was probably a better food source for copepod embryogenesis. The results are discussed in relation to laboratory findings on the toxic effects of diatoms on copepod reproduction.

KEY WORDS: Diatoms · Copepods · Distribution · Egg production · Hatching success