Russell G. Death

Diversity Patterns in Stream Benthic Invertebrate Communities: The Influence of Habitat Stability

Invertebrate diversity patterns were examined in 11 freshwater habitats (10 streams and a windswept lake shore) of similar physicochemical nature but different thermal and hydrologic stability in the Cass-Craigieburn region, New Zealand. Species richness and density were markedly higher at the more stable sites, but species evenness peaked at sites of intermediate stability. Of the 20 environmental variables examined, a multivariate instability index incorporating temporal variation in depth, temporal variation in current speed, substrate stability, the Pfankuch channel stability index, temperature range, and stream reach tractive force was the single best predictor of the number of species, whereas epilithic pigment concentration was the single best predictor of invertebrate density. The pattern in species richness did not support any of three diversity hypotheses considered. In contrast, the pattern in species evenness suggested competitive exclusion may be occurring patchily and that Hustons dynamic equilibrium model may have some validity, at least at the level of the patch. However, the strong link between productivity and stability apparent in these habitats, and a lack of information on the effects of increased productivity on competition in stream benthic communities makes any firm assessment of the latter model difficult. The observed diversity patterns are, however, consistent with the idea that high diversity is maintained in these habitats by an interaction between low levels of disturbance and habitat patchiness.

A review of the consequences of decreased flow for instream habitat and macroinvertebrates

Abstract The effects of drought on stream invertebrates have been reviewed, but the effects of artificially reduced flows have not. We addressed this knowledge gap by reviewing the literature on the effects of natural low flows and artificially reduced flows (without complete cessation of flow). We considered the effects of low water volume on habitat conditions and on invertebrate community structure, behavior, and biotic interactions. Decreases in discharge usually cause decreased water velocity, water depth, and wetted channel width; increased sedimentation; and changes in thermal regime and water chemistry. Invertebrate abundance increases or decreases in response to decreased flow, whereas invertebrate richness commonly decreases because habitat diversity decreases. Invertebrates differ in their environmental tolerances and requirements, and any loss of habitat area or alteration of food resources from decreased flow can influence organism behavior and biotic interactions. Invertebrate drift often increases immediately after flow reduction, although some taxa are more responsive to changes in flow than others. Natural low flows and artificially reduced flows have similar effects on invertebrates, but the severity (duration and magnitude) of the flow decrease can influence invertebrate responses. Certain invertebrate taxa are especially sensitive to flow decreases and might be useful indicators for reduced flows or flow restoration. The effect of low flow on streams is an important issue, but few empirical studies of the impacts of decreased flow on stream ecosystems have been done, and more manipulative experiments are needed to understand the ecological consequences of decreased flow.

A global experiment suggests climate warming will not accelerate litter decomposition in streams but might reduce carbon sequestration

The decomposition of plant litter is one of the most important ecosystem processes in the biosphere and is particularly sensitive to climate warming. Aquatic ecosystems are well suited to studying warming effects on decomposition because the otherwise confounding influence of moisture is constant. By using a latitudinal temperature gradient in an unprecedented global experiment in streams, we found that climate warming will likely hasten microbial litter decomposition and produce an equivalent decline in detritivore-mediated decomposition rates. As a result, overall decomposition rates should remain unchanged. Nevertheless, the process would be profoundly altered, because the shift in importance from detritivores to microbes in warm climates would likely increase CO(2) production and decrease the generation and sequestration of recalcitrant organic particles. In view of recent estimates showing that inland waters are a significant component of the global carbon cycle, this implies consequences for global biogeochemistry and a possible positive climate feedback.

The effect of patch disturbance on stream invertebrate community structure: the influence of disturbance history

The effect of disturbance history on the recovery of benthic invertebrate communities following disturbance was investigated in four streams in the Southern Alps of New Zealand. Two of the streams had a history of fluctuating discharge and temperature while the others did not. Recovery from disturbance was tested experimentally using baskets of cobbles, a third of which were disturbed every week for 9 weeks, a further third every 3 weeks and the final third left undisturbed. Algal biiomass, number of invertebrate taxa and total number of invertebrates all declined in baskets disturbed more frequently. Although the relative abundance of some taxa declined with time since the last disturbance, no taxa showed a significant decline in absolute abundance. However, several taxa showed marked increases in relative abundance in the less disturbed treatments particularly at the more stable sites. In contrast to the predictions of ecological theory, numbers of taxa and total invertebrates appeared to recover more quickly in the more complex communities at the stable sites. However, if these communities are considered to represent only stable communities, they do support the view that more complex communities will be more resilient. Community structure at the stable sites was also more similar between baskets in the undisturbed treatment than at the unstable sites, suggesting communities had reached a constant state more quickly. The more rapid recovery of communities measured at the stable sites may have been a consequence of experimental scale; disturbed patches were only 0.045 m2 in area and the higher densities of invertebrates at the stable sites meant a larger pool of colonists was available following each experimental disturbance. Nevertheless, ideas of stability in ecological theory and the scale of most spate events suggest this is the appropriate scale for examining community recovery. Furthermore, the larger pool of available colonists could not explain all the differences in community response, as patterns of change in community structure at the stable sites differed considerably more from those expected by purely random colonisation processes than at the unstable sites.

Invertebrate community responses to experimentally reduced discharge in small streams of different water quality

Abstract Water abstraction can alter invertebrate community composition by changing the availability or suitability of habitat for invertebrates. The consequences of water abstraction for the physical habitat and invertebrate communities of small permanent streams are not clear. Therefore, we used whole-channel flow manipulations to imitate real water abstractions. We used weirs and diversions to reduce discharge by >85% in 3 small New Zealand streams (mean discharges: 11–84 L/s), ranging in water quality from pristine to moderately polluted. We sampled benthic invertebrates and periphyton at control and impact sites on each stream before and during 2 mo of artificially reduced discharge during summer 2005, and then we left the diversions in operation throughout the following year and sampled invertebrates again after 9 and 12 mo of flow reduction. During the experiment, wetted width decreased by 24 to 34% at impact sites. Water velocity and depth also decreased at impact sites, but only small changes to conductivity, pH, dissolved O2, and temperature were observed. At the pristine site, density of invertebrates and percentage of Ephemeroptera, Plecoptera, Trichoptera (EPT) individuals decreased in response to reduced flows. Only taxonomic richness decreased at the mildly polluted stream, and reduced discharge had no effect on the invertebrate community at the stream with the poorest water quality. Overall community structure also changed in response to flow reduction at all but the poorest-water-quality stream. Alterations to community composition involved changes in the density of common taxa and collection of fewer rare taxa per sample. Our results indicate that the impacts of water abstraction on invertebrate communities differ between streams that vary in water quality, probably because of the relative sensitivities of invertebrate communities to changes in the physical habitat of these streams.

STREAM ECOSYSTEM FUNCTIONING UNDER REDUCED FLOW CONDITIONS

Assessments of flow reduction in streams often focus on changes to biological communities and in-stream physical characteristics, with little consideration for changes in ecosystem functioning. It is unclear whether functional indicators of ecosystem condition may be useful for assessing the impacts of reduced discharge on small streams. Using weirs and diversions to reduce stream discharge during summer baseflow conditions, we tested the response of leaf breakdown, coarse particulate organic matter (CPOM) retention, and primary production to one month of water abstraction in before-after, control-impact (BACI) designed experiments. Discharge at impact (downstream) reaches decreased by over 85% in each of three small New Zealand streams compared to controls (upstream). There also were decreases in velocity, depth, and wetted width. Sediment cover increased at impact reaches, but there were only small changes to conductivity, pH, and surface water temperature. We installed mesh bags filled with willow leaves in-stream for one month to measure leaf breakdown. Reduced discharge had little influence on leaf breakdown rate in these streams. Travel distances and retention structures for CPOM were evaluated using releases of paper strips and wooden dowelling over a range of discharges. The distance traveled by released CPOM increased with increasing discharge, and the importance of riffles as retention structures increased at lower discharges. We measured the accumulation of chlorophyll a after one month on artificial substrates as an estimate of the relative primary production of control and impact reaches. The differences in chlorophyll a concentrations between control and impact reaches were inconsistent among streams. These ecosystem functions have responded inconsistently to water removal in these streams. However, the strong response of CPOM retention to reduced discharge could complement measures of biological community structure when the influence of reduced discharge is assessed. We recommend further investigation in a wide range of streams to assess the utility of these processes as functional indicators of reduced discharge.

Spatial patterns of macroinvertebrate diversity in New Zealand springbrooks and rhithral streams

Abstract Spatial patterns in macroinvertebrate communities were examined in 4 to 5 rheocrene springbrooks and 4 to 5 nearby rhithral streams in 4 different regions of New Zealand. Physicochemical attributes of springbrooks and rhithral streams were similar, but springbrooks were more stable. Standing crops of periphyton biomass, epilithic C, and organic matter were greater and more variable in springbrooks than in rhithral streams. The number of macroinvertebrate taxa and the total number of individuals were greater at more stable sites. Altitude, habitat stability, and food resource levels were the best predictors of the number of macroinvertebrate taxa, whereas pH, PO4 concentration, habitat stability, and food resource levels were the best predictors of the total number of individuals at a site. Differences in the number of macroinvertebrate taxa between Northern Hemisphere and New Zealand springbrooks may be related to the larger diversity of macroinvertebrate predators in New Zealand springbrooks and to the fact that many New Zealand invertebrates do not use temperature-mediated life-history cues. Altitude, which limits invertebrate dispersal, also may play an important role in determining the maximum number of macroinvertebrate taxa in a given stream. A dispersal–stability framework is proposed to explain the observed patterns of macroinvertebrate diversity in these streams. The general pattern of noninsect dominance in lowland springs may be the result of the interaction between dispersal ability and the effects of the last glaciation.

Habitat heterogeneity drives the geographical distribution of beta diversity: the case of New Zealand stream invertebrates

To define whether the beta diversity of stream invertebrate communities in New Zealand exhibits geographical variation unexplained by variation in gamma diversity and, if so, what mechanisms (productivity, habitat heterogeneity, dispersal limitation, disturbance) best explain the observed broad-scale beta diversity patterns. We sampled 120 streams across eight regions (stream catchments), spanning a north–south gradient of 12° of latitude, and calculated beta diversity (with both species richness and abundance data) for each region. We explored through a null model if beta diversity deviates from the expectation of stochastic assembly processes and whether the magnitude of the deviation varies geographically. We then performed multimodel inference analysis on the key environmental drivers of beta diversity, using Akaikes information criterion and model and predictor weights to select the best model(s) explaining beta diversity. Beta diversity was, unexpectedly, highest in the South Island. The null model analysis revealed that beta diversity was greater than expected by chance in all eight regions, but the magnitude of beta deviation was higher in the South Island, suggesting differences in environmental filtering and/or dispersal limitation between North and South Island. Habitat heterogeneity was the predominant driver of beta diversity of stream macroinvertebrates, with productivity having a secondary, and negative, contribution. This is one of the first studies accounting for stochastic effects while examining the ecological drivers of beta diversity. Our results suggest that local environmental heterogeneity may be the strongest determinant of beta diversity of stream invertebrates, more so than regional- or landscape-scale variables.

Patterns of invertebrate diversity in streams and freshwater springs in Northern Spain

Invertebrate diversity patterns were examined in six rheocrene springs and six nearby, runoff-fed streams in Cantabria, Northern Spain. Periphyton biomass, organic matter and biomass of moss were always higher in springs than streams. Species densities (number of species/area) and rarified species richness (number of species/number of individuals) were lower and invertebrate densities greater in spring habitats. Of 22 variables chlorophyll-a was the best predictor of species richness, whereas total organic matter was the best predictor of invertebrate density, although neither relationship was strong. Spring habitats had invertebrate communities dominated by non-insect taxa (e. g., Echinogammarus, and Hydrobiidae and Neritidae snails), in contrast to the insect dominated communities in runoff-fed streams (e. g., Baetis, Ecdyonurus, Elmis, Prosimulium, Scirtes and Chironomidae). Echinogammarus had the highest densities in springs; an order of magnitude greater than any other taxa. The effects of biotic processes, such as predation from Echinogammarus on community structure may be more marked in springs because predated individuals cannot be as readily replaced by drifting animals from upstream reaches. The reduced diversity in springs compared to streams could be a result of several factors including increased predation from animals such as Echinogammarus or the unusually constant thermal characteristics.

The effect of environmental stability on hyporheic community structure

Interstitial water samples were collected from two well depths (30 and 60 cm) from 8 rivers in the Cass-Craigieburn region, South Island (New Zealand) during January 1998 to assess the hyporheos in streams of differing stability. Hyporheic water chemistry was more similar to surface water chemistry at unstable sites than at stable sites. The greatest diversity of both epigean and hypogean animals occurred at intermediate levels of disturbance; however, invertebrate density declined with increasing bed movement. Abundance of epigean animals also increased with declining environmental stability (i.e., increasing bed movement). Water chemistry was most like the river channel at unstable sites, probably because less stable substrates had increased interstitial flow facilitating a greater supply of water from the surface channel. In contrast, stable sites were spring fed and groundwater had a greater influence. Thus, substrate stability may influence hyporheic water chemistry and porosity by changing the interaction between ground and surface waters. Epigean taxa may be less abundant at stable sites because the chemical nature of the water is least like the river channel (e.g. low dissolved oxygen, higher temperature and higher conductivity), while the abundance of hypogean animals is greater, as these taxa are more tolerant of this water chemistry, and are thus capable of persisting in the stable substrates.