Cladoceran Periodicity Patterns in Relation to Selected Environmental Factors in Two Cascading Warm-Water Reservoirs Over a Decade

Abstract

Temporal changes in occurrence, population density and reproductive intensity (% of breeding individuals, clutch size) of species of the planktonic cladoceran assemblage in two adjacent, cascading, warm, monomictic reservoirs were studied over roughly 10 years. Findings are explored in relation to seasonal changes and inter-annual variability in water temperature (and stratification intensity), water transparency, water level, chlorophyll content and presence and abundance of other species in the assemblage as determinants or modifiers of species periodicity. Most of the numerically significant taxa present (4 in upstream Midmar, 7 in downstream Albert Falls) showed repeated patterns of periodicity. Large daphniids (Daphnia pulex and longispina) were virtually perennial occupants, except during conditions of elevated suspended sediment associated with fresh river inflows during floods, or sediment re-suspension at annual turnover and/or low water levels during drought phases of the hydrological cycle. Most other taxa (D. laevis, Diaphanosoma excisum, Ceriodaphnia reticulata, and Moina micrura) were essentially summer species, although some temporal separation was reflected in intra-seasonal, and between-lake differences in their occurrence. D. laevis occurred during early and/or late summer. C. reticulata was confined to late summer; significant densities arose in Midmar only in the final years of study, when it effectively replaced D. laevis. Diaphanosoma was a protracted summer occupant, with a more extended (and bimodal) presence in the warmer waters of Albert Falls. Moina and Bosmina were restricted to Albert Falls, and contrasted in seasonal occurrence. Moina was restricted to the mid-summer period, while Bosmina showed the greatest temporal variation of all taxa, generally appearing during periods of physical disturbance. Mean instantaneous birth rates for the entire study period contrasted widely among taxa (0.09–0.35 day⁻¹), but ranked concordantly with the seasonal categorisations given above – with highest and lowest rates among seasonally restricted and perennial taxa, respectively. Correlation analysis revealed overall that the abundance of most individual species related to water transparency (≅ suspended sediment), chlorophyll content and temperature (in rank order), although correlations were generally stronger with temperature than with chlorophyll. Several correlates were directionally inconsistent (+ vs −) for given species between lakes. However, these correlates of species densities undoubtedly translate into influences on species composition, and hence 'patterns' of periodicity. Positive pair-wise associations (Jaccard Index) existed for many species, reflecting temporal congruencies related to their periodicity. Collectively, however, the species in each reservoir were negatively associated, indicating separation on some undetermined basis (competition?).