Bernadette Pinel-Alloul

LOCAL AND REGIONAL ZOOPLANKTON SPECIES RICHNESS: A SCALE-INDEPENDENT TEST FOR SATURATION

Assemblages of coexisting species are formed by immigration from a re- gional pool of colonists and local interactions among species and with the physical envi- ronment. Theory suggests that the shape of the relationship between regional and local species richness may indicate the relative roles of dispersal and local interactions in limiting local diversity. Here we examine patterns of regional and local species richness in freshwater crustacean zooplankton to test whether linear (suggesting dispersal limitation) or curvilinear (suggesting saturation, via strong local control) functions best fit the data. Local richness appeared saturated when regions of different spatial extents were included on the same graph. However, this pattern was influenced by differences in scale among surveys. We corrected for the effects of regional scale by plotting mean local richness against the residuals of the species-landscape area relations. Controlling for the extent of the regional scale produced much more linear patterns, suggesting strong dispersal limitation. We present a simple graphical model to explain how variation among surveys in the geographic size of regions can produce apparent saturation of local diversity even if the underlying pattern of local and regional richness is linear. We also compare the predictive power of residual regional richness on local richness with that of several local features in a multiple regression model. Local richness exhibits strong relationships with both residual regional richness and pH. We argue that the relative strengths of local and regional processes depend on the definition of the regional scale. A variety of evidence suggests that local processes play a major role in generating differences in zooplankton diversity among lakes within a bio- geographic region. Evidence for the importance of dispersal limitation comes largely from comparisons of lakes across very large scales. Our analysis suggests that linear patterns of local and regional diversity are not incompatible with strong local interactions.

Spatial heterogeneity as a multiscale characteristic of zooplankton community

Zooplankton spatial heterogeneity has profound effects on understanding and modelling of zooplankton population dynamics and interactions with other planktonic compartments, and consequently, on the structure and function of planktonic ecosystems. On the one hand, zooplankton heterogeneity at spatial and temporal scales of ecological interest is an important focus of aquatic ecology research because of its implications in models of productivity, herbivory, nutrient cycling and trophic interactions in planktonic ecosystems. On the other hand, estimating zooplankton spatial variation at the scale of an ecosystem, is a powerful tool to achieve accurate sampling design. This review concentrates on the spatial heterogeneity of marine and freshwater zooplankton with respect to scale. First to be examined are the concept of spatial heterogeneity, the sampling and statistical methods used to estimate zooplankton heterogeneity, and the scales at which marine and freshwater zooplankton heterogeneity occurs. Then, the most important abiotic and biotic processes driving zooplankton heterogeneity over a range of spatial scales are presented and illustrated by studies conducted over large and fine scales in both oceans and lakes. Coupling between abiotic and biotic processes is finally discussed in the context of the ‘multiple driving forces hypothesis’.

Spatial and environmental components of freshwater zooplankton structure

Abstract:Paradigms based upon physical and geochemical bottom-up processes or trophic cascade/top down theory have been developed to explain heterogeneity of freshwater zooplankton, but these paradigms taken alone seem unable to encompass the full range of zooplankton variability and have led to controversy. The goal of the study is to test the hypothesis that both abiotic and biotic environmental factors, and spatial structuring, explain simultaneously the large-scale spatial heterogeneity of freshwater zooplankton in Quebec lakes. Partial Canonical Correspondence Analysis (Partial CCA) was used to partition the variance of zooplankton species abundances into independent components: a) non-spatial environmental factors (physics, chemistry, morphometry, phytoplankton and fish communities), b) spatial component of environmental influence, c) pure spatial factors (geographic coordinates), and d) undetermined. Our study shows that pure and spatially-structured abiotic and biotic environmental factors (a+b) e...

Steady-state distribution of metals among metallothionein and other cytosolic ligands and links to cytotoxicity in bivalves living along a polymetallic gradient

The present study was designed to assess the environmental effects of metals in a field setting. We explored exposure-->bioaccumulation-->effects relationships in freshwater molluscs exposed to metals in their natural habitat. Indigenous floater mussels (Pyganodon grandis) were collected from ten limnologically similar lakes located along a Cd, Cu and Zn gradient. Ambient free-metal ion concentrations were estimated as a measure of metal exposure. Metallothionein (MT) was measured in mussel gills and metal partitioning among the various cytosolic protein pools was determined by size exclusion chromatography. Various biomarkers were also measured, including malondialdehyde (MDA) concentrations in the gills and in the digestive gland, glutathione-peroxidase and glutathione-reductase activities in the digestive gland, and lipid concentrations in the gonad. Cadmium and MT concentrations in the gill cytosol increased along the contamination gradient, but Cu and Zn levels were independent of the ambient free-metal ion concentrations. The distribution of Cd among the various cytosolic complexes remained quite constant: 80% in the MT-like pool, 7% in the low molecular weight pool (LMW<1.8 kDa) and 13% in the high molecular weight pool (HMW>18 kDa). For these chronically exposed molluscs there was thus no threshold exposure concentration above which spillover of Cd occurred from the MT pool to other cytosolic ligands. However, the presence of Cd in the LMW and HMW fractions suggests that metal detoxification was imperfect, i.e. that P. grandis was subject to some Cd-related stress at low chronic exposure concentrations. Consistent with this suggestion, MDA concentrations, an indicator of oxidative stress, increased with gill cytosolic Cd. In the digestive gland, MDA concentrations were unrelated to any of the measured metals, but glutathione-peroxidase and glutathione-reductase activities increased with gill cytosolic copper. We speculate that cytosolic Cu catalyses the production of reactive oxygen species, to which the organism reacts by increasing activities of the two enzymes, thus preventing the accumulation of reactive oxygen species. Lipid concentrations in the gonad did not decrease with any of the measured toxicological parameters, suggesting that energy reserves for reproduction were not compromised in the metal-contaminated mussels. The results of the present study, where chronically exposed bivalves were collected from their natural habitat along a metal contamination gradient, contrast markedly with what would have been predicted on the basis of experimental metal exposures, and clearly demonstrate the need to study metal exposure-->bioaccumulation-->effects relationships in natural populations.

Environmental stability and lake zooplankton diversity – contrasting effects of chemical and thermal variability

Environmental variability in space and time is a primary mechanism allowing species that share resources to coexist. Fluctuating conditions are a double edged sword for diversity, either promoting coexistence through temporal niche partitioning or excluding species by stochastic extinctions. The net effect of environmental variation on diversity is largely unknown. We examined the association between zooplankton species richness in lakes and environmental variability on interannual, seasonal and shorter time scales, as well as long-term average conditions. We analyzed data on physical, chemical and biological limnology in 53 temperate zone lakes in North America and Europe sampled over a combined 1042 years. Large fluctuations in pH, phosphorus and dissolved organic carbon concentration on different time scales were associated with reduced zooplankton species richness. More species were found in lakes that showed greater temperature variation on all time scales. Environmental variability on different time scales showed similar or, in some cases, stronger associations with zooplankton species richness compared with long-term average conditions. Our results suggest that temporal fluctuations in the chemical environment tend to exclude zooplankton species while temperature variability promotes greater richness. The results indicate that anthropogenic increases in temporal variability of future climates may have profound effects on biodiversity.

Spatial Heterogeneity in Freshwater Zooplankton: Variation with Body Size, Depth, and Scale

The effects of body size, depth, and sampling scale on spatial heterogeneity were examined in the zooplankton community of a small lake. Analyses were performed by regression analysis of 27 sets (3 scales ° 3 depths ° 3 dates) of replicate (n = 4) samples of the natural zooplankton (cladocerans, copepods, rotifers) community of Lake Cromwell, Quebec, Canada. Spatial heterogeneity was measured as the variance among the four randomly arranged replicate samples of taxa taken at each scale—depth—date combination. The spatial distribution of populations of zooplankton in this community was found to be typical of the spatial heterogeneity encountered in other freshwater and marine ecosystems. The effect of population density on spatial heterogeneity was comparable to that found for other flora and fauna. Small animals were more heterogeneous than large ones, and populations sampled on large spatial scales or at greater depths showed greatest spatial variability. These effects were stable over the season. Few taxa or life stages diverged significantly from these trends.

Macroinvertebrate community as a biological indicator of ecological and toxicological factors in Lake Saint-François (Québec).

To assess the potential of the macroinvertebrate community for monitoring variation in the environmental quality of large rivers, the response of littoral macrobenthos in Lake Saint-François, a fluvial lake of the St Lawrence River (Québec) are described. First, the composition of total macroinvertebrate communities and important taxonomic groups as well as the biotic ICI-SL index in 16 littoral stations varying in sedimentology, water chemistry and contamination are described to define indicator species groups and environmental quality ranks. Thereafter, the relative contribution of ecological and toxicological factors in explaining the variation observed in macroinvertebrate assemblages and biotic index were quantified using partial canonical correspondence analysis. Cluster analyses based on taxonomic composition separated five groups of stations where macroinvertebrate assemblages varied in density, composition and tolerance to pollution. The ICI-SL biotic index varied from 7.2 to 27.2 with a mean value of 19 +/- 6. The ICI-SL values determined for the macroinvertebrate communities in Lake Saint-François did not reflect an important deterioration in environmental quality, and there was some agreement between the environmental quality ranking of the stations expressed either by the ICI-SL index or the community cluster analysis. Water conductivity and phosphorus concentration, followed by macrophyte types (Chara, Ceratophyllum) and sediment grain size, were the most significant ecological variables to explain variation in macroinvertebrate communities and derived ICI-SL index in Lake Saint-François. Among the toxicological factors, metals in water (Fe, Cr, Pb, Mn, Zn) and sediment (Mn, Pb, Se), as well as the composite indices of metal and organic contamination (water CI, sediment CI, sediment total PAHs) were the most important factors. The contamination factors selected in our models represented contaminant sorption processes rather than direct toxicological effects. The lack of clear relationships between contaminants and macroinvertebrate variables reflected the relative low level of contamination in the stations sampled in Lake Saint-François. There were some interactions between toxicological and ecological variables that should be considered in the planning of sampling and interpretation of biomonitoring studies. However, the large amount of unexplained variance (49.2-86.6%) in the CCA models underlined the limitations of the use of the indices of macroinvertebrate community structure that were assessed in this study for biomonitoring purposes in the absence of a contrasting pollution gradient.

Periphyton as an indicator of water quality in the St Lawrence River (Québec, Canada)

The performance of various algal indices to document improvements in water quality across a low nutrient concentration gradient was assessed during 2 years in the St Lawrence River (Quebec, Canada). Water-quality variables and periphyton samples were collected on navigational buoys near Montreal during the spring, summer and fall of 1994 and 1995. Exposure to urban wastewater varied widely within the sector surrounding the island of Montreal, with some areas upstream receiving no direct effluents and areas further downstream receiving treated and untreated wastewater. Faecal coliform concentrations provided a good tracer of effluents and were significantly correlated to nutrient concentrations (r = 0.33-0.72, p < 0.001) and water transparency (r = 0.70, p < 0.001). Despite a strong gradient in faecal coliform concentration (< 2 to > 20 000 UFC/100 ml), algal biomass and diversity did not reflect differences between sites with varying levels of urban wastewater. Taxonomic composition of periphyton communities, particularly the presence of the cyanophyte Plectonema notatum Schmidle, was related (r = 0.48, p = 0.004) to exposure to urban effluents. Variables describing seasonal changes (temperature, Julian day, river discharge, conductivity, NO2-NO3) explained a large fraction of total variance (38-52% of total variance) and thus exerted the predominant influence on algal biomass and species composition in the St Lawrence River. Variables describing the presence of effluents explained 1-22% of the variance in compositional data. Subtle changes in periphyton species composition were the only response to different levels of exposure to urban wastewater in the Montreal area, which represented relatively small differences in comparison to natural seasonal variability.

Short-term variations in specific biovolumes of different bacterial forms in aquatic ecosystems.

Short-term and spatial fluctuations in specific biovolumes (volume x cell−1) of different morphological categories of planktonic bacteria were estimated microscopically. Samples were taken from two lakes occurring in two different climatic systems: Lake Aydat (France) and Lake Cromwell (Canada). The study was done in summer, using 24-hour cycles of sampling.Due to their large size, the specific volume of filamentous bacteria constituted, on average, the major part (>70%) of the total specific volume of all bacterial forms considered. Greatest variations in specific biovolumes were recorded for filamentous bacteria (coefficients of variation ranged from 16 to 109%). These variations were more pronounced in the oxygenated and microaerophilic strata (DOC ≈1.5 mg liter−1). Fluctuations in cell volume were high (coefficients of variation =12–80%) for coccal bacteria, whereas no marked fluctuations were found for the rod and vibrio bacteria (coefficients of variation =4–10%).Evidence of diel patterns of cell volume of filamentous bacteria is provided. These cells displayed their maximum size during the day until early night, indicating cell division was occurring at night. Homogeneous circadian patterns were not provided by specific volume variations of coccal, rod, and vibrio bacteria.Statistical relationships between bacterial specific biovolumes and the biotic and abiotic parameters considered are discussed.

A doubly multivariate model for statistical analysis of spatio-temporal environmental data

A doubly multivariate statistical model, called the matrix normal model, is presented for its ability to take spatial and temporal autocorrelation, heteroscedasticity and non-stationarity in the mean into account. Its only underlying assumptions are sp ace-time separability of the theoretical autocovariance function and normality. An algorithm for maximum likelihood estimation of the spatial and temporal autocovariance matrices under the matrix normal model, including criteria of convergence and existen ce of solutions, is given. The resulting maximum likelihood estimates are used in modified ANOVA and correlation analysis of spatio-temporal repeated measures. In particular, the matrix normal model allows (i) the computation of distinct Boxs epsilo n estimates in adjusting the significance probability of the modified ANOVA F-tests for space, time and space-time effects, and (ii) the development of a spatio-temporal version of Dutilleuls modified t-test in correlation analysis. Sp atio-temporal repeated measures data from a case study in limnology, with phytoplankton biomass as random variable of interest, illustrate the model and derived framework.