Two calanoids,two lakes,and a decade or two. An updated record and evaluation of occurrence and periodicity of Tropodiaptomus spectabilis and Metadiaptomus meridianus (Copepoda: Calanoida), and alternative stable states in two cascading impoundments

Episodic switching between Tropodiaptomus spectabilis and Metadiaptomus meridianus in L. Midmar (29° 30 S, 30° 12 E), and transitory seasonal alternation of these taxa in L. Albert Falls (29° 26 S, 30° 26 E) recorded in comparative studies of these inter-leading reservoirs between 1989 and 1999, along with historical records for Midmar extending back to 1978, are updated and re-evaluated in light of subsequent and additional findings. In the longer term, an `exclusive' occupancy by one or another species has been maintained in these and other inter-leading impoundments on the quasi-subtropical Mgeni River. Sympatry was invariably transitional. It lasted several months in the upper Midmar system, and where protracted over several years as in the downstream Albert Falls system, was effectively negated by seasonal separation of the taxa. Possible influences of parasitism, changes in water turbidity or pH, dependent or independent of inter-basin water transfers, and such water transfers themselves, are explored and rejected as proximate causes of species replacement. Because evidence for the existence of resting eggs was contradictory (respectively present and absent in preserved and live samples), emergence from diapause is considered an unlikely source of re-colonisation. The guild complement of invertebrate planktivores is comparable in both lakes, seemingly removing predation as a determinant factor. The comparability of food thresholds for various measures of demographic performance suggests remarkable similarity in the competitive potential of these species, which additionally show considerable niche overlap in terms of adult diet, and comparable vulnerability to ellobiopsid parasitism. Ecological separation is thus limited to differentiation of thermal optima, functional feeding biology of juvenile stages – especially nauplii, and sediment tolerance. Evidence now available supports previous arguments that species separations remain attributable largely to marginal species differences in thermal optima, mirrored in the pattern of seasonal separation during an episode of sympatry (1990 to early 1994) in the down-stream Albert Falls system. While no causal explanation can be substantiated from evidence gleaned during this decade of study, the species replacements observed in Midmar appear to reflect alternative stable states, induced by presently unknown factor(s), plausibly temperature, or improbably, human translocation. By contrast, alternative stable states are not manifest in Albert Falls, wherein the longer term, T. spectabilis seemingly represents the only stable state, a feature apparently shared by other warmer reservoirs down the Mgeni cascade. The unpredictability of chaotic dynamics precludes its analysis in this situation, while conversely, the system provides a unique model for meta-population analysis, related to uni-directionality of natural dispersal. However, the cost-effectiveness of continued monitoring for unforeseeable events poses a serious practical dilemma.