Norman D. Yan

The Widespread Threat of Calcium Decline in Fresh Waters

Calcium concentrations are now commonly declining in softwater boreal lakes. Although the mechanisms leading to these declines are generally well known, the consequences for the aquatic biota have not yet been reported. By examining crustacean zooplankton remains preserved in lake sediment cores, we document near extirpations of calcium-rich Daphnia species, which are keystone herbivores in pelagic food webs, concurrent with declining lake-water calcium. A large proportion (62%, 47 to 81% by region) of the Canadian Shield lakes we examined has a calcium concentration approaching or below the threshold at which laboratory Daphnia populations suffer reduced survival and fecundity. The ecological impacts of environmental calcium loss are likely to be both widespread and pronounced.

Communities contain closely related species during ecosystem disturbance

Predicting community and species responses to disturbance is complicated by incomplete knowledge about species traits. A phylogenetic framework should partially solve this problem, as trait similarity is generally correlated with species relatedness, closely related species should have similar sensitivities to disturbance. Disturbance should thus result in community assemblages of closely related species. We tested this hypothesis with 18 disturbed and 16 reference whole-lake, long-term zooplankton data sets. Regardless of disturbance type, communities generally contained more closely related species when disturbed. This effect was independent of species richness, evenness, and abundance. Communities already under stress (i.e., those in acidic lakes) changed most when disturbed. Species sensitivities to specific disturbances were phylogenetically conserved, were independent of body size, and could be predicted by the sensitivities of close relatives within the same community. Phylogenetic relatedness can effectively act as a proxy for missing trait information when predicting community and species responses to disturbance.

Acidic deposition: effects on aquatic ecosystems

The literature pertaining to the influence of the atmospheric deposition of acidifying substances on aquatic ecosystems is examined in this article. Important chemical species in atmospheric deposition are first identified, and their influence on stream and ultimately on lake water chemistry discussed. Changes that have accompanied acidification among biological communities of decomposers, primary and higher order producers are discussed. Where they exist, hypotheses relating to causes of community changes are examined.

Regulation of Zooplankton Community Structure of an Acidified Lake by Chaoborus

To test the hypothesis that the structure of zooplankton communities of acidified, fish-free lakes can be regulated by predation from larval Chaoborus, the rates of production of the principal prey species of Chaoborus in Swan Lake, a small, fish-less, acid lake near Sudbury, Canada, were compared with their estimated rates of consumption by Chaoborus. The production of Bosmina longirostris, the major crustacean zooplankter in the lake, rarely exceeded its apparent rate of loss to Chaoborus. In contrast, the production rate of Keratella taurocephala, the dominant rotifer in Chaoborus diets, virtually always exceeded the rate at which it was consumed. The unusually small contribution of Crustacea to total zooplankton biomass in the lake could be attributed to predation by Chaoborus. While Chaoborus can regulate zooplankton community structure in acidified, fish-free lakes, the frequency of occurrence of such control remains uncertain.

Developing Conceptual Frameworks for the Recovery of Aquatic Biota from Acidification

Abstract Surface water acidity is decreasing in large areas of Europe and North America in response to reductions in atmospheric S deposition, but the ecological responses to these water-quality improvements are uncertain. Biota are recovering in some lakes and rivers, as water quality improves, but they are not yet recovering in others. To make sense of these different responses, and to foster effective management of the acid rain problem, we need to understand 2 things: i) the sequence of ecological steps needed for biotic communities to recover; and ii) where and how to intervene in this process should recovery stall. Here our purpose is to develop conceptual frameworks to serve these 2 needs. In the first framework, the primarily ecological one, a decision tree highlights the sequence of processes necessary for ecological recovery, linking them with management tools and responses to bottlenecks in the process. These bottlenecks are inadequate water quality, an inadequate supply of colonists to permit establishment, and community-level impediments to recovery dynamics. A second, more management-oriented framework identifies where we can intervene to overcome these bottlenecks, and what research is needed to build the models to operationalize the framework. Our ability to assess the benefits of S emission reduction would be simplified if we had models to predict the rate and extent of ecological recovery from acidification. To build such models we must identify the ecological steps in the recovery process. The frameworks we present will advance us towards this goal.

The Temporal Coherence of Zooplankton Population Abundances in Neighboring North‐Temperate Lakes

We investigated the temporal coherence (i.e., the correlation or synchrony between time series) of annual abundances among populations of freshwater zooplankton in eight lakes in Ontario, Canada, from 1980 to 1992. We estimated temporal coherence using the intraclass correlation coefficient (ri). While values of ri were relatively low among comparisons of all eight lakes, they were statistically significant for three of the seven common cladoceran and copepod taxa (Bosmina longirostris, Leptodiaptomus minutus, and Mesocyclops edax). These significant positive correlations imply that a portion of the interannual variation in abundance was produced by factors operating on a scale larger than the individual lake catchments. Because the eight‐lake analysis might obscure strong, but conflicting, patterns among lakes in the region, we identified homogeneous and temporally coherent subsets of lakes for each species using an exploratory stepwise deletion procedure. The resultant homogeneous subsets exhibited much greater temporal coherence, accounting for 47% (Eubosmina) to 84% (Leptodiaptomus) of the interannual variation in abundance. Our results suggest that the factors affecting annual variation in zooplankton abundance must be sought both within lakes and beyond their watersheds.

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.

Evidence of biological recovery in acid-stressed lakes near Sudbury, Canada

Reductions in the emissions of SO2 and trace metals from the Sudbury smelters have resulted in substantial improvements in water quality in many surrounding lakes. Significant biological changes have accompanied the chemical improvements. Evidence of relatively rapid recovery was found for benthic filamentous algae, phytoplankton, zooplankton, mobile species of benthic invertebrates, and some fish populations. Organisms with low dispersal ability (e.g. Hyalella azteca) have not yet recolonized these lakes. The partial recovery observed to date shows movement toward re-establishment of biological communities typical of natural Precambrian Shield lakes in this area. These findings offer strong support for further efforts to reduce industrial emissions of pollutants to the atmosphere.

Recovery of Crustacean Zooplankton Communities from Acidification in Killarney Park, Ontario, 1971-2000: pH 6 As a Recovery Goal

Abstract Despite reductions in atmospheric SO42− deposition and resultant decreases in surface water acidity, widespread biological recovery from acidification has not yet been documented. Temporal trends in crustacean zooplankton species richness (number of species) and composition were examined between 1971–2000 in 46 Killarney Park lakes, Ontario, Canada, to assess the degree of biological recovery in lakes with significant water quality improvements, i.e. pH now > 6, compared to 2 other groups: i) lakes which never acidified; and ii) lakes which are still acidified (pH < 6). Time trends in species richness could not be distinguished among the 3 groups of lakes, nor did changes in species richness indicate recovery. In contrast, the zooplankton community composition of lakes in which the pH increased to above 6, as measured by a multivariate index of species abundances, changed from a “damaged” state to one typical of neutral lakes. Some recovery in composition was also documented for the acidic lakes. While still acidic, the pH levels of these lakes have risen. The extent and pace of recovery in Killarney Provincial Park bodes well for the future of other acidified regions in North America and Europe.

An Integrated Multi-Disciplinary Approach for Studying Multiple Stressors in Freshwater Ecosystems: Daphnia as a Model Organism

The increased overexploitation of freshwater ecosystems and their extended watersheds often generates a cascade of anthropogenic stressors (e.g., acidification, eutrophication, metal contamination, Ca decline, changes in the physical environment, introduction of invasive species, over-harvesting of resources). The combined effect of these stressors is particularly difficult to study, requiring a coordinated multi-disciplinary effort and insights from various sub-disciplines of biology, including ecology, evolution, toxicology, and genetics. It also would benefit from a well-developed and broadly accepted model systems. The freshwater crustacean Daphnia is an excellent model organism for studying multiple stressors because it has been a chosen focus of study in all four of these fields. Daphnia is a widespread keystone species in most freshwater ecosystems, where it is routinely exposed to a multitude of anthropogenic and natural stressors. It has a fully sequenced genome, a well-understood life history and ecology, and a huge library of responses to toxicity. To make the case for its value as a model species, we consider the joint and separate effects of natural and three anthropogenic stressors-climatic change, calcium decline, and metal contaminants on daphniids. We propose that integrative approaches marrying various subfields of biology can advance our understanding of the combined effects of stressors. Such approaches can involve the measuring of multiple responses at several levels of biological organization from molecules to natural populations. For example, novel interdisciplinary approaches such as transcriptome profiling and mutation accumulation experiments can offer insights into how multiple stressors influence gene transcription and mutation rates across genomes, and, thus, help determine the causal mechanism between environmental stressors and population/community effects as well as long-term evolutionary patterns.