Vertical migrations and feeding rhythms of Acartia clausi and Pseudodiaptomus hessei (Copepoda : Calanoida) in a tropical lagoon (Ebrie, Cote d'Ivoire)

Diel changes in vertical distribution and gut pigment contents of Acartia c lausi and Pseudodiaptomus hessei were studied during several 24-h time seri es performed between 1993 and 1997 in four sites of the Ebrie Lagoon (Cote d'Ivoire). The sites differed by their morphology and their hydrological st ructure and by the vertical distribution of chlorophyll biomass. Both speci es showed classical diel vertical migrations (DVM). Copepodites and adult s tages of P. hessei were almost benthic during the day and evenly distribute d through the water column at night. The amplitude of DVM of A. clausi incr eased from copepodites I-III to adults. Copepodites and adults of A. clausi increased significantly their gut fluorescence at night, whereas those of P. hessei showed no clear diel feeding rhythm (DFR). These results suggest that A. clausi feed mostly at night on phytoplanktonic particles and P. hes sei feed mostly on benthic algal particles during the day and on sestonic p articles at night. No relationship was observed between DFR and DVM because both patterns occur-red when food was either vertically homogeneous or ver tically stratified. The daily average gut fluorescence of A. clausi increas ed with ambient chlorophyll concentration until around 12-15 mug l(-1), whe reas no relationship was found for P. hessei. The implication of these patt erns on the adaptation capacities and the behaviour of the two species are discussed. The DVM of P. hessei should explain its rarity in the estuarine area, The comparison of our results with previous ones suggests an evolutio n of A. clausi DFR between 1981-1982 and 1996-1997, in relation to an inten sification of eutrophication. (C) 2001 Academic Press.