Phenology of invertebrates living in a sahelian temporary pond

In order to study and classify successional stages in sahelian temporary pond ecosystems, the hydrological dynamics and the invertebrate composition of such a pond in central Senegal were investigated during a complete annual cycle, from the first rains until desiccation. The biotic part of the study focussed on zooplankton and on the pelagic and epibenthic macrofauna in the central region of the pond. Physical and chemical variables of the water were measured and compared with meteorological data. The pond was extant from late June 1994 to the end of January 1995. Changes in water levels correlated well with precipitation and evaporation. Dissolved oxygen levels, pH and Secchi depths fluctuated strongly. Conductivity was related to the depth of the pond and increased considerably when the water in the pond evaporated at the end of the wet season. Zooplankton species showed distinct peak densities at different times for different species. Cladocera were numerically the most important during the early season. Later, zooplankton was dominated by various species of copepod. Ostracods were less numerous, but this may be due to the fact that benthic microcrustaceans were not sampled separately. Macroinvertebrates consisted mainly of phyllopods and aquatic insects. There was a clear trend in the succession of taxa. Fairy shrimps (Branchiopoda; Anostraca) and tadpole shrimps (Branchiopoda; Notostraca) dominated the early community of the pond. The difference in development between two fairy shrimps, the fast growing Branchinella chudeaui and the more slowly developing Streptocephalus sudanicus, is discussed in detail. Densities of aquatic insects, mainly backswimmers of the genus Anisops (Hemiptera; Notonectidae) and diving beetles (Coleoptera; Dytiscidae), increased towards the end of the season. Four stages in the faunal succession in the pond were distinguished: (i) colonization and dominance by early crustaceans, (ii) dominance by more slowly establishing crustaceans, (iii) a shift towards insects and (iv) desiccation. It is concluded that this succession reflects differences in life-history strategies. These strategies, in turn, seem to be largely restricted to specific taxa because of phylogenetic constraints. As such, crustaceans seem to exhibit a wider variety of life-history tactics than insects.