Effect of flow variability modification and hydraulics on invertebrate communities in the great fish river (Eastern Cape province, South Africa), with particular reference to critical hydraulic thresholds limiting larval densities of Simulium chutteri lewis (Diptera, Simuliidae)

Abstract

Flow patterns in the Great Fish River in the Eastern Cape province of South Africa have changed from being seasonal, with predictable no-flow periods during winter, to perennial following the completion of an inter-basin transfer scheme in 1977 to provide a regular supply of irrigation water. Simulium chutteri (Diptera: Simuliidae) consequently became a problem species of pestilential proportions, due to increased flow volumes and current velocities favouring this species. In this study, aquatic invertebrates were sampled from the stones-in-current biotope with a range of current velocities at three sites on the Great Fish River, with a particular focus on pool/rapid areas favouring S. chutteri. The main aim of this paper was to determine whether critical hydraulic thresholds, including current velocity, could be derived for this species. Knowledge of the hydraulic preferences of S. chutteri is a prerequisite for any integrated control programme which combines larvicidal control with flow manipulation. S. chutteri was found to favour the rapids biotope with current velocities in excess of 90 cms À1, with preferences for higher current velocities increasing with life cycle stage. Additional hydraulic variables, at the scale measured in this study, did not correlate with larval densities. There is potential for more effective long-term control of problem populations of larval blackfly in the Great Fish River through further research on the potential for using constructed in-stream vanes to reduce current velocities in rapids of the river at critical periods of the year (July-October), based on flow duration/current velocity relationships.

Invertebrate hydraulic preferences and predicted impacts of changes in discharge in a large river

“…In our study, filtering taxa such as Simuliidae and the net-spinning Hydropsyche had rheophilic preferences similar to those observed in smaller German streams and elsewhere (e.g. Rivers-Moore et al, 2007). Moreover, our results are in agreement with those of Dolédec et al (2007), who showed that differences in hydraulic habitat preferences across surveys were stronger for generalists than for specialists.…”

“…In our study, filtering taxa such as Simuliidae and the net‐spinning Hydropsyche had rheophilic preferences similar to those observed in smaller German streams and elsewhere (e.g. Rivers‐Moore et al. , 2007).…”

A model to predict outbreak periods of the pest blackfly Simulium chutteri Lewis (simuliidae, Diptera) in the Great Fish River, Eastern Cape province, South Africa

“…It is thus during the late winter to early spring (August to September) that the blackfly larvae develop with minimal biotic interference (de Moor, 1989), and this then favours exponential population increase at this time of the year. Assuming that some minimal flows are maintained it is assumed that the proposed flow controls will have minimal negative effects on hydropsychid populations, since this group was shown to prefer lower flow velocities (<0.8 m s À1 ) than S. chutteri (Rivers-Moore et al, 2007). If flow reductions could target the period when the hydropsychid population levels are at their lowest then maximum efficiency in keeping simuliid population levels in check can be achieved.…”

“…Curve 9 was included in the analyses to illustrate the relationship between velocity and width in the upstream pool. While a limited hydraulic habitat is theoretically available to larval blackfly in the pool biotope (15% where velocities are in excess of 1 m s À1 ), the Froude numbers of these flows would be too low in practice for this biotope to be suitable as habitat, and blackfly larvae are typically absent (Rivers-Moore et al, 2007). The percentage of available hydraulic habitat was found to range between 0% and 90%, with a sharp inflection point for flows in excess of 2 m 3 s À1 , based on the integrals of curves 1-8 (Table II).…”

“…It is assumed that availability of food is not a limiting factor, with hydrological influences overriding potential biotic effects (Statzner et al, 1988). Rivers-Moore et al (2007) showed that a flow velocity threshold of 1 m s À1 exists for S. chutteri larvae, below which the probability of an outbreak, defined as more than 22 500 larvae m À2 after Palmer (1994), was minimal; The duration of the aquatic stage of S. chutteri is 12-45 days and is inversely related to mean daily water temperature (de Moor, 1982b(de Moor, , 1989. In this model, it was assumed that a linear relationship exists between water temperature and duration of the aquatic stage, with this duration being estimated from a 7-day moving average of mean daily water temperatures.…”