D. Dudley Williams

Buffer Zone versus Whole Catchment Approaches to Studying Land Use Impact on River Water Quality

Secondary databases, GIS and multivariate analysis tools were used to determine whether there was a correlation between water quality and landscape characteristics within three local southern Ontario watersheds. Whole catchment and 100 m buffer zone influences on water quality over three seasons were compared. Chemical fluxes were also calculated and used to compare the loading of pollutants to downstream environments. Urban land use had the greatest influence on water quality. The influence of agricultural land use was variable and did not agree with the results of other studies. The only natural landscape variables that appeared to have an influence on water quality were slope and silt-clay surficial geology deposit. There was a clear trend of increased chemical fluxes with increasing urban land use intensity within a watershed. Forested land use appeared important in mitigating water quality degradation. The catchment landscape characteristics appeared to have slightly greater influence on water quality than the 100 m buffer. The results of this study may have been influenced by the scale and accuracy of databases used. The secondary data were useful in determining major trends in water quality and possible non-point origins of surface water pollution, and in identifying areas that are in need of further investigation.

Environmental constraints in temporary fresh waters and their consequences for the insect fauna

Temporary fresh waters are defined as bodies of fresh water that experience a recurrent dry phase of varying length that is sometimes predictable in both its time of onset and duration. The aquatic insects that live in these habitats are strongly influenced by 2 main groups of constraints, physicochemical and biological factors. Taxa particularly well-represented in temporary running waters are the Ephemeroptera, Hemiptera, Coleoptera, Trichoptera, and tipulid and chironomid Diptera. With the exception of Ephemeroptera, all these groups are common also in temporary lentic waters, but the latter also support many species of Odonata, together with culicid, ceratopogonid and ephydrid Diptera. In general, however, the diversity of aquatic insects is lower in temporary than in permanent waters. Seasonal succession of species is commonplace in temporary waters, affecting food web structure. Adaptations to drought are reviewed, and traits common to the most successful taxa include a highly flexible life cycle, temperature-linked development, possession of a diapausing or otherwise protected egg, and high powers of dispersal. A trend towards terrestrialisation of immature stages is noted amongst at least some members of even strongly aquatic taxa. Several directions for future research are discussed including study of the controlling influence of the biological environment on the fauna and the formers interaction with physicochemical influences, temporal variation in the structure of food webs, and genetic analyses of populations. A recommendation is made for greater quantification in data collection, particularly on seasonal changes in species abundance, their life histories, and trophic relationships, as this will allow greater opportunity for modelling the community dynamics of these ecologically distinct habitats. Finally, the concept that temporary waters constrain their faunas is exposed as being based more on human perception than on fact.

The recolonization mechanisms of stream benthos

The animals recolonizing an area of denuded stream substrate are thought to come from four main sources. These are drift, upstream migration within the water, migration from within the substrate, and aerial sources, e.g., oviposition. An experiment in a Canadian stream showed drift to be the most important source of recolonizing animals, contributing 41.4% of the total number that settled on an area of 1800 cm2. This compares with 28.2% from aerial sources while upstream migration and movement up from within the substrate contributed about equally (18.20% and 19.1 00, respectively). Clearly then, all four directions are important in repopulating bare areas. Further, many groups of organisms were found to have preferred directions from which they recolonize and this, it is argued, may lead to the establishment of separate and distinct faunal assemblages if the other directions are excluded.

The Importance of Temporal Resolution in Food Web Analysis: Evidence from a Detritus‐Based Stream

A series of time-specific food webs for the macroinvertebrate riffle com- munity of Duffin Creek, Ontario was constructed using dietary information obtained from the analysis of gut contents. Trophic links were quantified using a dietary index of relative importance. Precision of the analysis was maintained at a high level by: (1) identifying dietary items as accurately as possible via direct gut analysis; (2) identifying web members to the species level, thus avoiding the taxonomic aggregation and lumping of size classes common in food web analyses; and (3) ensuring temporal resolution of the web by deter- mining ontogenic variation in the diets of dominant members of the community. The Duffin Creek webs are heavily detritus-based with a large proportion of top-to-basal, and intermediate-to-basal links. Top-to-basal links, proportions of top and intermediate species, and lower connectance (0.180-0.219) varied temporally. Trophic connectance ranged from 0.090 to 0.109, consistent with values expected for a web consisting largely of specialist feeders. Weak links made up the largest proportion of total links in the webs, whereas very strong links made up the smallest proportion. Omnivory was more common than indicated in other webs and can be attributed to ontogenic diet switching. Comparison of the statistics for a summary-web with those generated for the time-specific webs indicated that the total number of links per web, total number of species, number of top and intermediate species, and linkage density were much greater for the summary web. In view of these differences, the importance of temporal resolution when assessing food web structure and dynamics is emphasized. The possibility that some of the observed features in our web are common to other detritus-based webs is considered. Future studies of this calibre are justified.

Comparison of ecological communities: The problem of sample representativeness

Obtaining an adequate, representative sample of ecological communities to make taxon richness (TR) or compositional comparisons among sites is a continuing challenge. Although randomization in the collection of sample units is often used to assure that sampling is representative, randomization does not convey the concept of how well samples represent the community or site from which they are drawn. In ecological surveys, how well a sample represents a community or site literally means the similarity in taxon composition and relative abundance between a sample and the community from which it is drawn. Using both field and simulated data, we show that the proportion of the total taxon richness at a site (%TTR) achieved with a fixed sample size varies across sites, which in turn causes changes in site-to-site differences in observed TR with sample size. This means that equal-sized samples may differentially represent the communities from which they are drawn. However, the similarity of a sample to the community from which it is drawn cannot be measured directly because the taxon composition and relative abundance of the community is usually unknown. We propose to estimate it by measuring the average similarity among replicate samples randomly drawn from a community, i.e., autosimilarity, which is measured with Jaccard Coefficient in this study. Using the same data sets, we found that: (1) samples of equal size from different sites or communities achieved different levels of autosimilarity, with lower levels achieved in taxon richer sites, indicating variation in how well samples of equal size represent their respective communities; (2) %TTR was positively and almost linearly correlated with autosimilarity, indicating that autosimilarity might be a good predictor of TTR; and (3) when samples were compared at the same level of autosimilarity, similar %TTRs across different sites were achieved (i.e., the relative differences in taxon richness among sites became independent of sample size). We conclude that standardization on autosimilarity, rather than on sample size, can improve the accuracy of taxon richness comparisons.

Biological, chemical and physical characteristics of downwelling and upwelling zones in the hyporheic zone of a north-temperate stream

Along a single stream riffle, there is a typical flow pattern in which surface water enters the hyporheic zone in a downwelling zone at the head of the riffle and hyporheic water returns to the stream surface in an upwelling zone at the tail of the riffle. Distinct patterns of physical and chemical conditions in the hyporheic zone are likely to determine patterns of microbial activity and occurrence of hyporheic fauna. Interstitial water and core samples were taken at three depths in the downwelling and upwelling zones of a single riffle in the Speed River, Southern Ontario, Canada. Physical and chemical characteristics of the hyporheic water, bacterial density, protein content, detritus content and faunal composition of the hyporheic sediment were analysed. The downwelling and upwelling zones differed significantly in temperature, pH, redox potential, dissolved oxygen and nitrate with significant positive correlations occurring among the latter three. There were no differences in bacterial density or detritus content between the two zones nor between depths in either zone, but protein content, considered to be a measure of biofilm biomass, was significantly higher in the downwelling zone. Total density of hyporheic fauna and the number of taxa decreased with increasing depth in both upwelling and downwelling zones, and were positively correlated with surface water characteristics (oxygen, temperature and nitrate), sediment protein content and detritus; however, only a weak correlation was found with zone. The composition of taxa differed between the two zones, and faunal distribution was correlated with dissolved oxygen, detritus, protein content and depth.

SEASONAL BOUNDARY DYNAMICS OF A GROUNDWATER/ SURFACE-WATER ECOTONE

Interstitial water, faunal samples, and hydrogeological data were collected beneath a riffle on the Speed River, southern Ontario, Canada. The purpose was to identify the location and seasonal fluctuation of the hyporheic/groundwater interface and to examine several aspects of water mass chemical signatures and the dynamics of the interstitial fauna. Concentration discontinuities in several water chemistry parameters delineated the chemical boundary between the true groundwater and hyporheic habitats. The groundwater mass was characterized as having higher levels of ammonium, alkalinity, and conductivity, and lower nitrate levels. Differences in water chemistry between the hyporheic and groundwater zones persisted throughout the year, though no single variable differed quantitatively between these two zones on all occasions. The location of the chemical discontinuity varied seasonally. Whereas hyporheic and groundwater faunal subunits of the interstitial community were identified and the location of the subunits coincided with the chemical breaklines, response to shifts in the position of the hyporheic/groundwater interface was taxon rather than subunit based. Fauna therefore provided poor spatial resolution in terms of pinpointing the location of the interface. Boundary fluctuation coincided with extremes in seasonal discharge patterns and was regulated by the relative strength of the upward force of baseflow and the downward force of advecting surface water. Identifying patterns of fluctuation of the hyporheic/groundwater interface, and consequently hyporheic habitat volume, may have important consequences for the storage, retention, and cycling of nutrients in lotic ecosystems.

Nutrient and flow vector dynamics at the hyporheic / groundwater interface and their effects on the interstitial fauna

Environmental conditions in the interstices beneath streams and rivers with porous beds are unlike those found either on the bed surface or in the true groundwater. For most of the year, in many streams, the bulk of the water in the hyporheic zone is provided by baseflow but, as it passes across the hyporheic/groundwater interface, the physical and chemical nature of this groundwater changes, probably in response to mixing with surface water. Factors promoting the influx of surface water are associated with features of the bed and channel morphology. The upper and lower boundaries of the hyporheic zone are thought to vary in time, but at any instant they can be defined. As a habitat, the hyporheic zone fits the definition of an ecotone, although certain adverse features may result in reduced species diversity. There are limited, correlative, data available on the relationship of the fauna (hyporheos) to interstitial conditions and further study of the general biology of both species and populations is needed. In an attempt to stimulate future research on these systems, some preliminary models of hyporheic dynamics are proposed.

Distribution of the benthos within the substratum of a Welsh mountain stream

Samples collected with a standpipe corer at depths of 10-15 and 15-25 cm below the gravel surface of the Afon Hirnant show that benthic invertebrates occur in considerable numbers down to at least 25 cm, which is only 5 cm above the bedrock. The numbers in these samples, together with estimates of the numbers in the top 5 cm as revealed by kick samples, indicate a faunal density of around a quarter of a million per square metre. This is a far greater number than is usually reported from stream beds, but it is shown to be in line with estimates made in very different streams by methods that collect specimens from deep in the substratum. Clearly the usual types of quantitative sample are quite inadequate.

Evaluation of predator-induced stress on field populations of stoneflies (Plecoptera)

The influence of predaceous fish in affecting components of fitness of immature and adult macroinvertebrates was examined using a predator enclosure/exclosure experiment in a southern Ontario stream. The stonefly Paragnetina media is a semivoltine species with a midsummer emergence period. After exposure of P. media to rainbow trout (Oncorhynchus mykiss) between May and August (repeated twice, 1987 and 1989), we observed significant reductions in nymphal density, size (head width), and conditions (mass/head width). Reductions in density were attributable to nymphal emigration and predation. Selective predation on larger stoneflies (direct effects), together with changes in nymphal feeding rate, activity, and distribution (indirect effects), reduced size and condition of stoneflies exposed to trout. After August—to—October (1986 and 1989) and January—to—April (1988) exposures, we observed no effect of trout on these parameters. Adults stoneflies (P. media and Alloperla mediana) cannot feed, and consequently any energy that they channel into reproductive efforts must be derived from the aquatic stage. Thus, we postulated that the impact of trout on immature P. media and A. mediana might manifest itself negatively on stonefly adults. Although size and emergence pattern of adult P. media and A. mediana did not differ between enclosure/exclosure sections of stream, in the presence of trout both species suffered significant reductions in condition, fecundity, and total number to emerge. These findings demonstrate that direct and indirect effects of a predator can cascade from immature to adult prey and from aquatic to terrestrial ecosystems.