Bernhard Statzner

Biotic and Abiotic Controls in River and Stream Communities

Lotic ecologists share a major goal of explaining the distribution and abundance of biota in the worlds rivers and streams, and of predicting how this biota will respond to change in fluvial ecosystems. We discuss five areas of research that would contribute to our pursuit of this goal. For mechanistic understanding of lotic community dynamics, we need more information on: 1. Physical conditions impinging on lotic biota, measured on temporal and spatial scales relevant to the organisms. 2. Responses of lotic biota to discharge fluctuations, including the processes that mediate community recovery following resets caused by spates or droughts. 3. Movements of lotic organisms that mediate gene flow, resource tracking, and multilevel species interactions. 4. Life history patterns, with special emphasis on ontogenetic bottlenecks that determine the vulnerability of populations confronting environmental perturbation. 5. Consequences of species interactions for community- and ecosystem-level processes in rivers and streams. Without attempting to be comprehensive in our review, we discuss limits and limitations of our knowledge in these areas. We also suggest types of data and technological development that would advance our understanding. While we appreciate the value and need for empirical and comparative information, we advocate search for key mechanisms underlying community interactions as the crucial step toward developing general predictions of responses to environmental change. These mechanisms are likely to be complex, and elucidation of interacting bilateral, or multilateral, biotic and abiotic controls will progress only with the continuing synthesis of community- and ecosystem-level approaches in lotic ecology.

Responses of freshwater biota to human disturbances: contribution of J-NABS to developments in ecological integrity assessments.

Abstract Effective ecosystem management in the face of human alterations depends on our ability to quantify ecologically significant changes and to discriminate among impact levels and types. We reviewed the literature on biological responses of freshwater biota to human disturbances over the last century. Many of the main methods for assessing ecological integrity originated in Europe [e.g., Saprobien Index, Trent Biotic Index, Biological Monitoring Working Party, River InVertebrate Prediction And Classification System, multiple traits] or in North America [e.g., Index of Biotic Integrity, leaf-litter breakdown, functional feeding groups]. We used bibliometric data to detail the contribution of J-NABS to developments in assessment of ecological integrity. A total of 225 bioassessment articles were published in J-NABS from 1986 to 2007. This total was ∼½ of the number of bioassessment articles published over the same period in Freshwater Biology (FWB; another leading freshwater journal used for comparison), but the proportion of bioassessment articles in the 2 journals was similar (∼20%). The proportion of bioassessment articles in J-NABS has increased over the last 25 y. This trend indicates the growing interest in the topic and the delay (∼30 y since the Clean Water Act) of scientific action in response to water legislation. Taxonomic composition and multivariate techniques, multimetric indices, lotic systems, macroinvertebrates, land use, and overall human impacts were major themes in J-NABS bioassessment articles. Progress in bioassessment is needed in 3 main areas. First, bioassessments done at large spatial scales are needed to meet requirements of new environmental policies. Second, bioassessment should shift from consideration of taxon losses to losses of ecological functions. Third, statistical techniques are needed for predictive assessment of deviation in ecological integrity between expected (natural) and observed (natural or impacted) conditions. Assessment of functional integrity based on multiple traits is a promising area because biological traits represent universal biological characteristics that are connected to ecosystem functions. However, the relationships between ecophysiological traits of individuals and ecosystem-scale responses must be properly defined for specific types of human impairment, and utility of the approach for routine bioassessments applied by managers must be tested.

Morphological adaptation of shape to flow: Microcurrents around lotic macroinvertebrates with known Reynolds numbers at quasi-natural flow conditions

SummaryUsing Laser Doppler Anemometry we measured current velocities in the median plane around dead lotic macroinvertebrates in a flume which reproduced natural near bottom hydraulics. We investigated specimens of the gastropods Ancylus, Acroloxus, and Potamopyrgus, the amphipod Gammarus, and the larval caddisflies Anabolia, Micrasema, and Silo of various size, various alignment to the flow or which were otherwise manipulated in order to clarify certain questions of adaptation of shape or case building style to flow, or the effects of flow on field distribution patterns. The steepest velocity gradients close to the animals were found near areas of their bodies protruding furthest into the flow. In such regions the rates of potential diffusive exchange processes, the potential corrasion (abrasion through suspended solids), and, for larger specimens, the lift forces (directed towards the water surface) must be highest. Posterior of these areas growing boundary layers formed above those species whose upper contour was approximately parallel to the upstream-downstream direction of the flow. All specimens removed momentum from the flow and thus experience a drag force (directed downstream). From the complete data set we derived the following general conclusions about the physical effects of potential morphological adaptations, taking into consideration diffusion through boundary layers, corrasion, lift forces, friction and pressure drag forces: The physical significance of these five factors generally depends on the Reynolds number of an animal and is largely affected by flow separation, which was significantly related to the ratio of body length to height and the slope of the posterior contour. A simultaneous effective morphological adaptation to all five factors is physically impossible and, in addition, would have to change from life at low (e.g. a young, small specimen of a species) to life at high (e.g. a fully grown specimen of the same species) Reynolds number.

Growth and Reynolds number of lotic macroinvertebrates: a problem for adaptation of shape to drag

Streamlining or dorsoventral flattening of the body of lotic macroinvertebrates is traditionally considered to be a morphological adaptation to reduce drag. This view is rather speculative because the Reynolds numbers (Re) at which these animals live and thus the type of drag that predominantly affects these animals are unknown. Using Laser Doppler Anemometry to measure water velocity such Re were determined from flume experiments. Young, small individuals live at rather low Re. Therefore they experience mainly friction drag. Fully grown individuals live at higher Re, at which pressure drag predominates. To minimize drag differently sized animals must differ in shape. Small specimens should be hemispheric while large ones should be streamlined. Within a species, young and old instars of freely moving aquatic stages are usually of the same shape, which is frequently neither hemispheric nor streamlined. Therefore I conclude that evolution compromises between life at low and high Re.

A Method to Estimate the Population Size of Benthic Macroinvertebrates in Streams

SummaryA method is described by which samples of stream macroinvertebrates can be linked to the roughness of the substratum, the water depth, the velocity, and to a combination of these factors, i.e., Froude number and thickness of the laminar sublayer, at the exact point of sampling. The abundance of Odagmia ornata (Diptera: Simuliidae), the species considered in this study, was not related to roughness, but depended with increasing significance on depth, velocity, Froude number, and laminar sublayer. Therefore, it is suggested that the population size of stream macroinvertebrates can be estimated using the relationships between abundance and the most suitable hydrodynamic factor. For this purpose it is necessary only to measure the physical factors and then to calculate the abundance, using both these measurements and the abundance-hydrodynamic factor relationships formerly established.A longer section of a stream with a relatively wide range of habitats was sampled for 1 week, during which the new method yielded results that reached the level of accuracy per sampling effort obtained to date only when a narrower range of habitats was sampled, usually only one stream riffle. Under the latter conditions, the new method is suggested to yield results of increased accuracy, particularly if samples are taken within a short period.

Field Experiments on the Relationship between Drift and Benthic Densities of Aquatic Insects in Tropical Streams (Ivory Coast). II. Cheumatopsyche falcifera (Trichoptera: Hydropsychidae)

(1) Gutters containing natural substrata colonized by benthic macroinvertebrates were exposed in streams in the Ivory Coast (West Africa) and drifting organisms were captured in 790 samples over a period of more than 600 h. (2) Drift was related to density and to abiotic factors for instars of the hydropsychid caddis Cheumatopsychefalcifera (Ulmer), of which about 55 000 were caught. (3) Drift of larvae peaked at different times at night. Emerging adults and pupal exuviae appeared briefly after sunset. Exuviae of larval instars I-IV were present throughout 24 h. (4) The relationship of drift (y: drift out of 0.1 m2 per unit time or individuals per 1 m3) of a larval instar to its own benthic density (x) differed in its significance at various times and between larval instars. (5) No consistent relationship was found in instar I. In later instars it was best described by y = a + blxb2x4 (instar II); y = axb (instar III); y = aebx (instars IV and V). Since instars IV and V predominated in our trials, an exponential relationship was also found for total larvae of the species. (6) Self-regulation of an upper benthic density by emigration through drift was statistically not evident. (7) Models based on benthic densities explained up to 99% of the variation in the drift; the drift of a larval instar was often defined more precisely by the benthic density of another instar than its own. (8) We conclude that a net of behavioural interactions in the benthic population had a large effect on the loss of individuals through drift. (9) Drift increased with increasing water temperature and velocity in the gutter; moonlight had no depressant effect on the drift. (10) Minor manipulations of the gutter, less drastic than in laboratory experiments, resulted in artifacts in the drift. (11) The average percentage decrease of density through drift over 24 h was 5.6, 9.9, 17-0, 14-3. 5.1, and 9.2 in instars I, II, III, IV, V, and total larvae. (12) Moulting rates, estimated from larval exuvial drift, indicated that the development period for the instars I-IV took more than a month. (13) Drift was reduced in some cases in streams previously treated with insecticides. Drift as determined in the West African Onchocerciasis Control Programme to monitor

Monte Carlo Simulations of Benthic Macroinvertebrate Populations: Estimates Using Random, Stratified, and Gradient Sampling

Accuracy, precision, and cost are major concerns in optimizing a sampling strategy to estimate population sizes of benthic macroinvertebrates. We used Monte Carlo simulations to compare precision and accuracy for various efforts (= cost) to predict density of cohorts and total populations of the water but Aphelocheirus aestivalis (Fabr.) from 3 German rivers. We compared density predictions of this hemipteran made from simultaneously collected physical variables to density predictions made from random sampling. The joint preference factor (JPF) method predicted Aphelocheirus densities from combined preference gradients for depth, velocity, and substratum size. It clearly yielded the most powerful predictions of all physical approaches tested. However, the JPF method sometimes predicted bug density better or worse than random sampling, depending on 1) the mean density of a cohort or total population, 2) the degree of spatial patchiness (contagion), 3) the affordable sample number to predict from, and 4) whether all or only dominant size components of the substratum were considered. We concluded that benthic macroinvertebrate sampling should be optimized using a flexible strategy, depending on the study objectives and the budget. This approach can save considerable time for the given error level required in predictions, and/or can yield a considerable reduction of the error of predictions (up to 50%) with a comparable effort. Optimization of sampling strategies in studies of lotic benthos is appropriate, logical, and timely, because large expenditures for stream management may be based on data provided by lotic ecologists.

Why roads and rivers can be straight or winding: how physical and sociohistorical factors produce contemporary regional landscape patterns

Efforts to understand and predict socioecological systems in an historical context have increased. We investigated sinuosity patterns of roads and rivers to understand why the geometric design of roads and rivers corresponds to natural physical constraints in some regions but not in others. We hypothesized that, for physical reasons, the ratio of road-to-river sinuosity should be ∼1 in canyons, >1 in mountains, and <1 in plains and that sociohistorical influences may have modified these physically based sinuosity patterns, particularly in contemporary plains. We assessed these hypotheses with data from nonurban, intermediate-size roads and small- to intermediate-size rivers from 20 regions in France, Germany, the UK, and eastern and western USA. Distinct sociohistorical differences among regions affected the evolution of road-network geometry, but modern engineering approaches homogenized these legacies. Contemporary road sinuosity reflects former demands for scenic roads in US mountains and different historical inheritance practices in northern and southern Germany. Contemporary river sinuosity reflects physical constraints in canyons and mountains, but major historical, socioeconomic, and cultural differences among regions and countries in plains. We conclude that: 1) understanding the physical context that complicates sociohistorical analyses is essential for formulating hypotheses; 2) tests of such hypotheses may help define region-specific priorities and targets for ecological restoration; 3) linking land change with physical and sociohistorical drivers requires multiple conceptual models; and 4) changes in social, economic, cultural, religious, or political systems occur at different rates, more stochastically, and with fewer predictable consequences, than changes in ecological systems.