We examined the impact of lateral and longitudinal hydrological connectivity on copepod diversity (local species richness, diapause strategies and population genetics) in ponds of a North American floodplain. Results of a General Linear Model (GLM) which included three environmental pond variables showed that species richness in these ponds was significantly influenced only by pond type (permanent ponds, temporary ponds with connections to other waterbodies during floods, isolated temporary ponds). Local species richness in connected temporary ponds had significantly higher scores of local richness compared to isolated temporary ponds (means 7.4 and 2.7, respectively). Such enhancement of species diversity in connected ponds appears to be facilitated by increased dispersal during floods and also pertains to the diversity of diapause strategies in the ponds studied. Species with and without diapause were present in connected temporary ponds, but absent from isolated temporary ponds, which contained only species capable of diapausing during the dry season. To explore the significance of hydrological connectivity of longitudinal dispersal between upstream and downstream sites, we used RAPD markers in populations of a calanoid copepod. The increase of average heterozygosity H (0.301 to 0.477) in an upstreamdownstream direction and generally low genetic distances (Nei's D < 0.05) suggested maintenance of gene flow by flood-mediated dispersal between connected ponds. Lower heterozygosity (0.289) of the population in a downstream pond protected by levees suggested the interruption of gene flow when dispersal by flooding is blocked.

Zooplankton populations in floodplain habitats such as oxbow lakes are adversely affected through wash out or dilution. We hypothesized that zooplankton may produce diapausing eggs to avoid local extinction caused by fl oods. To test this hypothesis, we determined the type of clutch (diapausing or subitaneous) produced by Diaptomus pallidus (Copepoda: Calanoida) fe males during a 13-month period in eight ponds on the Little Ta llahatchie River, Mississippi, floodplain. At five of eight sites, a maximum of 10-60 % of D. pallidus fe males switched from production of subitane ous to diapausing clutches between mid-November and early December. The timing of the switch occurred before the historical date of first fl ood. Females did not pro duce any diapausing clutches at two of four sites where fl ooding occurs irregularly due to flood control. The size of the egg bank at sites with flood control was smaller than at sites without flood control. The results suggest that fl ooding regime is impor tant in shaping some life history characteristics of this species.