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Wally Johnson and Jack Vallentyne played key roles in the establishment of the Experimental Lakes Area (ELA), which comprises a research team, a set of protected lakes, and a field station, with the mandate to quantify anthropogenic impacts to lakes through whole-ecosystem manipulation and monitoring. We begin this collection of papers, celebrating four decades of aquatic research at the ELA, by reflecting on the historical relevance and scientific milestones of the ELA. The remaining papers encompass themes at the core of ELA research: long-term ecological monitoring of unimpacted reference lakes, ecosystem responses to anthropogenic stressors through whole-system experimentation, recovery of manipulated ecosystems from perturbation, and detailed mechanistic studies. Utilizing these approaches, papers in this issue examine a wide variety of anthropogenic impacts on freshwater including the ecosystem effects of climate change, recovery from lake acidification, upland and wetland flooding on methyl mercury levels in biota, endocrine-disrupting chemicals on fish populations, and freshwater aquaculture. These studies emphasize the value of long-term monitoring and experimentation at the ecosystem scale for understanding human impacts on freshwaters. [ABSTRACT FROM AUTHOR]

Changes in climate, in particular significant changes in precipitation and evaporation and thus runoff, as well as changes in regional atmospheric deposition can affect the changes in chemical masses or concentrations in lakes. We have examined the changes in alkalinity, sulfate, and dissolved organic carbon (DOC) during a 20-year period when Lake 302S and Lake 302N at the Experimental Lakes Area (Ontario, Canada) were acidified with sulfuric, nitric, and hydrochloric acids and returned to natural pH. We used the corresponding 20-year history in Lake 239 to build chemical mass balance models for the experimental lakes. With these models, we assessed the production of alkalinity and the loss of sulfate and DOC in the context of atmospheric and watershed inputs and our experimental acid additions. Alkalinity production was greatest during the period of highest acid inputs and not detectable during pH recovery even in the early stages when pH was still low. Sulfate loss was also greatest while sulfate concentrations were increasing but counter to expectations not directly related to sulfate concentrations. Losses of the mass of DOC showed little change in response to acid additions over the 20 years, but first-order losses increased at low pH. [ABSTRACT FROM AUTHOR]

Chemical recovery is hypothesized to promote rapid recovery of benthic algal communities in formerly acidified lakes; this expectation needs modification. We evaluated the hypothesis in a small lake (L302S) in the boreal forest of northwestern Ontario, Canada, during a decade of pH recovery following a prior decade of experimental acidification from pH 6.7 to 4.5. To account for regional changes during the study, we also studied a nearby reference lake (L239). Taxonomic changes in the epilithon (biofilm on rock surfaces) included persistently lower cyanobacterial biomass following its acidification-related decline and increases in both diatoms and greens. Epilithic metabolic recovery was incomplete because the acidification-induced increase in respiration continued, although the prior decline in photosynthesis was reversed. Unexpectedly, blooms of metaphytic filamentous green algae occurred at higher pH during recovery than during acidification. Although several community attributes recovered fully, recovery of many aggregate functional and taxonomic properties lagged improvements in pH. Divergence was greater at the taxonomic than at the functional level. Despite pH recovery, potential causes of incomplete algal recovery include incomplete chemical recovery and the persistent absence of functionally important biota. Our findings counter the assumption that ecological recovery mirrors the pathway of damage caused by a human stressor. [ABSTRACT FROM AUTHOR]

Reintroduction of functionally important species is considered a key strategy for restoring damaged ecosystems. However, the sudden reappearance of an extirpated species may have adverse ecological impacts, degrading ecosystem services. Therefore, we experimentally reintroduced the crayfish Orconectes virilis into a biologically recovering, formerly acidified Boreal Shield lake (Lake 302S, Experimental Lakes Area, Canada) to determine its effect on the littoral food web following a 17-year absence. In June 2004, a single-factor experimental design consisting of two treatment levels (crayfish-less control versus 1.8 introduced crayfish·m–2) was replicated five times for a total of 10 littoral cages (4 m2). Orconectes virilis significantly (P < 0.05) suppressed the total biomass of other benthic invertebrates by 70% primarily because of declines in larval damselflies and midges. In addition, crayfish reduced periphytic biomass by 90% (P < 0.001). Stable isotopic analyses of the mesocosm food webs further indicated that O. virilis likely functioned as an omnivore, exerting direct and possibly indirect effects on other invertebrates and periphyton. Our findings highlight how the reintroduction of O. virilis must be balanced with adequate fish predation to prevent this species from becoming an invader and negatively affecting the productive capacity of boreal lakes. [ABSTRACT FROM AUTHOR]

Anthropogenic stressors are the current drivers of loss of global biodiversity and deterioration of ecosystem function (e.g., primary production). However, it is debatable whether human stressors or associated changes in biodiversity better predict the impairment of ecosystem function. Variation in plankton communities during a whole-lake experiment (Lake 302S, Experimental Lakes Area, Canada) was examined to test whether the stressor treatment effect or subsequent stressor-independent variation in species richness best explained interannual variation in aggregate functional properties, such as productivity or net total biomass. Although significant “biodiversity – ecosystem function” relationships were detected, these correlations were confounded by the negative effect of experimental acidification on species richness. The stressor effect was removed by plotting functional properties against the residuals from the species richness – pH regressions, which generated either negative or nonsignificant relationships. The lack of significant stressor-independent positive relationships between functional properties and species richness highlights the potential greater importance of other mediating factors, such as interactions among multiple stressors, species identity, and altered trophic interactions, at the whole-ecosystem scale. [ABSTRACT FROM AUTHOR]

Lake Store Hovvatn and the adjacent Lake Lille Hovvatn (Norway) are acidified owing to long-term deposition of S and N. By 1974, pH was 4.46 and acid-neutralizing capacity was –42 µequiv.·L–1. Following a lake SO4 reduction from 92 to 33 µequiv.·L–1, pH had increased to 4.8 and acid-neutralizing capacity had increased to –8 µequiv.·L–1 by 2003. The acidification history is well reconstructed using the dynamic model MAGIC. The model predicts that the lakes will not, however, recover to conditions adequate to support a self-reproducing brown trout (Salmo trutta) population. Lake Store Hovvatn was first limed in 1981 and subsequently annually or biannually until 1999, at which time the entire catchment was limed. Liming increased pH to above target levels of 6.0 and reduced inorganic Al to below 5 µequiv.·L–1 in the main water body. Only after the terrestrial liming in 1999, however, was pH potentially adequate for egg survival in the lake during winter, as pH at shallow depths below the ice stayed above 5.5. The results indicate that even the dramatic reduction in acid deposition in Europe will be insufficient to provide water quality adequate for fish populations; such lakes will require some sort of liming for many decades to come. [ABSTRACT FROM AUTHOR]

To investigate potential effects of climate change on lake thermal structure, we examined relationships between the amount of cold-water habitat in late summer (defined as the 10 °C depth), summer weather, and dissolved organic carbon (DOC) concentration over a two-decade period (1981–2002) in a small Boreal Shield lake recovering from acidification. DOC concentration, wind-days (the product of mean daily wind speed and the number of days between ice-out and late-summer stratification), and mean daily temperature were significant predictors of the 10 °C depth in a multiple-regression model. A similar model using simply the number of ice-free days instead of wind-days was almost as effective. The models were quite successful in explaining interannual variations in the 10 °C depth when tested on a chemically and morphometrically similar nearby lake. While factors related to summer weather were important in explaining interannual variations in the amount of late-summer cold-water habitat, increased DOC concentration over the study period largely explained observed long-term decreases in the 10 °C depth (increases in cold-water habitat). DOC concentration was positively correlated with pH. In acidified regions, increases in DOC that accompany the recovery of acidified lakes will need to be considered in assessments of potential climate-change effects on lake thermal structure. [ABSTRACT FROM AUTHOR]