Feeding of Acartia clausi and Pseudodiaptomus hessei (Copepoda: Calanoida) on natural particles in a tropical lagoon (Ebrié, Côte dʹIvoire)

Grazing of the copepods Acartia clausi and Pseudodiaptomus hessei on natural particles was studied from on board experiments during several 24 h time series performed between 1993 and 1997 in four sites of the Ebrie´ Lagoon (Coˆ te d’Ivoire). Ingestion rates of both species increased linearly with food concentration until a concentration threshold (5.5 109 lm3 l 1 for A. clausi and 5.2 109mm3 l 1 for P. hessei) beyond which the relation presented a plateau. Both species poorly selected the peak of available particles (range 3–6mm equivalent spherical diameter, ESD) but A. clausi seek preferentially smaller particles (6–21 lm ESD) than P. hessei (9–33 lm ESD). When the proportion of the preferred particles in the food offered decreased, A. clausi extended its selectivity towards both smaller and larger particles whereas P. hessei extended its selectivity towards larger particles only. As a consequence of these patterns, the useful particle concentration (UPC) was higher for A. clausi than for P. hessei. In addition, the ratio of the UPC for the two species showed a positive relationship with the ratio of their respective biomass. The significance of these results for the adaptation capacities of the two species and for the ecosystem functioning are discussed. A. clausi which is more suited than P. hessei to exploit smaller particles (3–6 lm) which dominate the seston, has a food competitive advantage. The inadequacy between the seston food-size composition and the selective patterns of the two main zooplankton species of the Ebrie´ Lagoon explains that they could be food limited despite the high trophic level of the lagoon. It could also partly explain the low transfer efficiency between phytoplankton and zooplankton in this ecosystem.