A manipulative study of macroinvertebrate grazers in Hong Kong streams: do snails compete with insects?

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Effects of channelisation on stream habitat in relation to a spate and flow refugia for macroinvertebrates in northern Japan

1. The effects of channelisation on macroinvertebrates were examined in relation to a spate and flow refugia. Habitat components that can function as flow refugia were identified in a small, low‐gradient stream in northern Hokkaido, Japan. 2. Macroinvertebrates and their habitat characteristics (depth, current velocity and substratum) were sampled and measured in natural and channelised sections on three occasions: before, during and immediately after a spate. For macroinvertebrate sampling and habitat measurements, five (riffle, glide, pool, backwater and inundated habitats) and three (channelised‐mid, channelised‐edge and inundated habitats) habitat types were classified in the natural and channelised section, respectively. 3. The rate of velocity increase with discharge was compared among habitat types to determine which habitat types were less affected by increased discharge. The rate was the highest in riffles followed by glides and channelised‐mids. Backwaters maintained low current velocity even at high flow. In addition, current velocity in both natural and channelised inundated habitats was low relative to other habitat types during the spate. 4. Through the spate, total density of macroinvertebrates in channelised‐mids and taxon richness in both channelised‐mids and edges decreased. In the natural section, however, such a significant decrease was not found except for taxon richness in pools. This indicated that the spate had a greater impact on assemblages in the channelised section. Riffle assemblages exhibited a rapid recovery immediately after the spate, suggesting the existence of flow refugia in the natural section. Among the habitat types we examined, backwaters and inundated habitats appeared to have acted as flow refugia, because these habitats accumulated macroinvertebrates during the spate. 5. The lower persistence of the macroinvertebrate assemblage in the channelised section was attributable to the lower availability of flow refugia such as backwaters and inundated habitats. Our results emphasised the importance of considering flow fluctuations and refugia in assessing the effects of channelisation. In addition, the lateral heterogeneity of stream channels should be considered in stream restoration and management.     

Seasonal dynamics of benthic macroinvertebrate communities in the Lower Ardèche River (France)

Seasonal sampling at six locations on the Lower Ardèche River was effected irregularly during 1982, 1983 and 1984. The seasonal structure of the taxa-sample matrix, much more important than sample location, is demonstrated using graphical interpretation.The Mediterranean aspect of the lower reaches of the Ardèche River is pointed out through its community structure which includes species often collected in other typical Mediterranean streams, e.g. Oligoneuriella rhenana, Ephoron virgo, Ecdyonurus dispar, E. insignis, Choroterpes picteti.Water temperature and day length were the determinant influences on seasonal variations in population structure of benthic macroinvertebrates. Three periods were distinguished: water (November to April), spring (May, June and beginning July) and summer (end of July to October). Spring and autumnal spates marked the limits of the summer and winter periods. Hence, annual fluctuations of this seasonal typology may occur in response to the hydrological regime.     

Resistance and resilience of ecosystem metabolism in a flood-prone river system

1. Gross primary production (GPP) and ecosystem respiration (ER) were analysed for 18 months in two reaches of the River Thur, a prealpine river in Switzerland. The upper reach at 655 m above sea level (a.s.l.) is bedrock constrained, has a high slope (0.60%) and a catchment area of 126 km2. The lower reach at 370 m a.s.l. has a more extensive hyporheic zone, a lower slope (0.17%) and a catchment of 1696 km2.
2. In both reaches, temporal patterns of stream metabolism reflected the occurrence of bed-moving spates. Average reductions of GPP and ER by spates were 53 and 24% in the upper reach, and 37 and 14% in the lower reach, respectively. The greater resistance of ER than GPP in both reaches shifted the ecosystem metabolism towards heterotrophy (decrease of the ratio of GPP to ER (P/R)) following spates.
3. Recovery of GPP was significantly faster in the lower reach and exhibited distinct seasonal variation (positive correlation with incident light). The differences in stability (both resistance and resilience) between reaches reflected differences in geomorphic settings and disturbance regime.
4. Stepwise regression analysis was used to explore the potential influence of season, disturbance and prevailing environmental conditions on stream metabolism in each reach. Time since spate plus temperature explained 73 and 86% of variation in ER and GPP, respectively, in the upper reach and 55% of variation in ER in the lower reach. Season plus prevailing environmental conditions explained 67% of variation in GPP in the lower reach.
5. To test how the perception of stability may change with increasing scale of observation, the disturbance regimes of 12 sites were compared with the disturbance regime of the entire Thur catchment. The analysis suggests that stream metabolism at the catchment scale is far more resistant to high flow events than at the reach scale.     

The influence of within-stream disturbance on dissolved nutrient levels during spates

In rivers, variations in concentrations of many dissolved nutrients occur during spates. Increases are usually attributed to concentrated point or non-point inputs, and decreases to dilution associated with rainfall. Increased discharge disturbs sediments and benthic communities, but the effects of such disturbance on nutrient levels are difficult to isolate. Measurements of nutrient levels over three artificial spates revealed that substantial variations in dissolved organic carbon, dissolved phosphate, silicate, nitrate, and potassium levels could result from increased discharge in the absence of allochthonous inputs. Variations were closely related to peaks in suspended solids concentration or water height. Increases in biochemical oxygen demand and suspended bacteria also occurred. Variations in phosphate and silicate could be accounted for by a balance between release of ‘sediment interstitial water’ and exchange processes involving suspended and freshly exposed sediment. An increase in nitrate, during one spate, was probably due to a reduction in the effect of benthic denitrification. Small peaks in dissolved organic matter concentration were detected over each spate. We propose that within-stream disturbance is a factor which may contribute to variations in dissolved nutrient concentration during the rising hydrograph in natural spates.     

The influence of hydrology on the structure of invertebrate communities in two streams flowing into Lake Nakuru,Kenya

Stream fauna communities were studied in the Baharini Springbrook and the Njoro River, which flow into Lake Nakuru. The streams have significantly different flow regimes. Consequently, discharge-related factors influence their faunal composition and abundance. The Baharini Springbrook is perennial, slow-flowing with low steady discharge, a higher conductivity and a low conductivity gradient along its course. The Njoro River is intermittent, fast-flowing with a higher and variable discharge, lower conductivity and a steeper conductivity gradient. Due to its permanent and steady surface stream flow, the Baharini Springbrook has a higher number of invertebrate species and higher abundances than the Njoro River. However, in spite of the intermittent nature of the Njoro River, during periods of relatively steady discharge the composition of its fauna is similar to that of the Baharini Springbrook. Similarity in the substratum characteristics of the two streams and their close proximity is a possible explanation for the close relationship in their faunal composition. At the onset of flow after a drought, rapid colonisation of the lowland intermittent stretch of the Njoro River occurred probably by drift from upstream perennial sections and/or from the hyporheos. Initial small spates at the onset of surface flow seem not to affect the invertebrate abundance, but later bigger spates during and after much heavier rains reduced the faunal abundance significantly. The recovery of faunal abundance to pre-flood levels in the Njoro River was completed in 27 days. Taxon richness was, however, less affected by the spates.     

Response of grazing impacts of an algivorous fish (Pseudogastromyzon myersi: Balitoridae) to seasonal disturbance in Hong Kong streams

1. Manipulative experiments were carried out in four Hong Kong streams (two shaded, two unshaded) to investigate the impact of grazing by an algivorous fish, Pseudogastromyzon myersi, on benthic algal biomass and assemblage composition. Experiments were conducted and repeated during both the dry and wet seasons to determine whether spate‐induced disturbance modified any grazing effect. Treatments comprised fish exclusion and inclusion via closed and open cages, with a no‐cage treatment used as a control for the cage effect. Treatments were maintained for 4 weeks in each experimental run. 2. Grazing by P. myersi reduced benthic algal biomass and the organic matter content of periphyton in open cages and the no‐cage treatment relative to closed cages. The similarity between open‐cage and no‐cage treatments was evidence that the overall difference among treatments was caused by limiting fish access to closed cages and not merely an artifact of caging. Grazing effects were broadly similar in all streams, but there was a significant statistical interaction between treatments and seasons. 3. Analysis of dry‐season data matched the overall trend in inter‐treatment differences, confirming the effects of grazing by P. myersi on algal biomass and periphyton organic matter. Significant differences in algal assemblage composition between closed‐cage and no‐cage treatments during the dry season reflected reductions in the abundance of erect, stalked diatoms (Gomphonema) and filamentous cyanobacteria (Homeothrix). Removal of these vulnerable overstorey algae by P. myersi resulted in greater abundance of understorey diatoms (Achnanthes and Cocconeis) in the no‐cage treatment in all streams during the dry season. The composition of algal assemblages in open cages was intermediate between the other two treatments. 4. Although fish densities were greater in all streams during the wet season, spate‐induced disturbance obscured grazing effects and there were no significant differences among treatments attributable to fish grazing. Seasonal variation in impacts of P. myersi grazing provides support for the harsh‐benign hypothesis, and confirms that biotic factors are less important controls of stream algal biomass and assemblage structure during periods (i.e. the wet season in Hong Kong) when abiotic disturbances are frequent or intense.