Michael A. Turner

EFFECTS OF CLIMATIC WARMING ON LAKES OF THE CENTRAL BOREAL FOREST

Twenty years of climatic, hydrologic, and ecological records for the Experimental Lakes Area of northwestern Ontario show that air and lake temperatures have increased by 2�C and the length of the ice-free season has increased by 3 weeks. Higher than normal evaporation and lower than average precipitation have decreased rates of water renewal in lakes. Concentrations of most chemicals have increased in both lakes and streams because of decreased water renewal and forest fires in the catchments. In Lake 239, populations and diversity of phytoplankton also increased, but primary production showed no consistent trend. Increased wind velocities, increased transparency, and increased exposure to wind of lakes in burned catchments caused thermoclines to deepen. As a result, summer habitats for cold stenothermic organisms like lake trout and opposum shrimp decreased. Our observations may provide a preview of the effects of increased greenhouse warming on boreal lakes.

Long-Term Ecosystem Stress: The Effects of Years of Experimental Acidification on a Small Lake

Experimental acidification of a small lake from an original pH value of 6.8 to 5.0 over an 8-year period caused a number of dramatic changes in the lakes food web. Changes in phytoplankton species, cessation of fish reproduction, disappearance of the benthic crustaceans, and appearance of filamentous algae in the littoral zone were consistent with deductions from synoptic surveys of lakes in regions of high acid deposition. Contrary to what had been expected from synoptic surveys, acidification of Lake 223 did not cause decreases in primary production, rates of decomposition, or nutrient concentrations. Key organisms in the food web leading to lake trout, including Mysis relicta and Pimephales promelas, were eliminated from the lake at pH values as high as 5.8, an indication that irreversible stresses on aquatic ecosystems occur earlier in the acidification process than was heretofore believed. These changes are caused by hydrogen ion alone, and not by the secondary effect of aluminum toxicity. Since no species of fish reproduced at pH values below 5.4, the lake would become fishless within about a decade on the basis of the natural mortalities of the most long-lived species.

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.

Natural Sources of Acid Neutralizing Capacity in Low Alkalinity Lakes of the Precambrian Shield

A detailed alkalinity budget was constructed for Lake 239 in the Experimental Lakes Area of northwestern Ontario and for three small watersheds in its terrestrial basin. Alkalinity generation in the lake averaged 118 milliequivalents per square meter per year, 4.5 times as high as the areal rate in the terrestrial basin. Although acid deposition in the area is low, only one of the three terrestrial watersheds was a significant source of alkalinity. A second terrestrial watershed yielded very little alkalinity. The third watershed, which contains a wetland, was a sink for, rather than a source of, alkalinity. An analysis of ion budgets for the lake revealed that more than half of the in situ alkalinity production was by biological rather than geochemical processes. The major processes that generated alkalinity were: biological reduction of SO42-(53%), exchange of H+ for Ca2+ in sediments (39%), and biological reduction of NO3- (26%). Comparison with experimentally acidified Lake 223 revealed that alkalinity production by sulfate reduction increased in response to increased inputs of sulfuric acid.

Assessing the Recovery of Lakes in Southeastern Canada from the Effects of Acidic Deposition

Abstract Reductions in North American sulfur dioxide (SO2) emissions promoted expectations that aquatic ecosystems in southeastern Canada would soon recover from acidification. Only lakes located near smelters that have dramatically reduced emissions approach this expectation. Lakes in the Atlantic provinces, Quebec and Ontario affected only by long-range sources show a general decline in sulfate (SO42−) concentrations, but with a relatively smaller compensating increase in pH or alkalinity. Several factors may contribute to the constrained (or most likely delayed) acidity response: declining base cation concentrations, drought-induced mobilization of SO42−, damaged internal alkalinity generation mechanisms, and perhaps increasing nitrate or organic anion levels. Monitoring to detect biological recovery in southeastern Canada is extremely limited, but where it occurs, there is little evidence of recovery outside of the Sudbury/Killarney area. Both the occurrence of Atlantic salmon in Nova Scotia rivers and the breeding success of Common Loons in Ontario lakes are in fact declining although factors beyond acidification also play a role. Chemical and biological models predict that much greater SO2 emission reductions than those presently required by legislation will be needed to promote widespread chemical and latterly, biological recovery. It may be unrealistic to expect that pre-industrial chemical and biological conditions can ever be reestablished in many lakes of southeastern Canada.

BIOLOGICAL, CHEMICAL AND PHYSICAL RESPONSES OF LAKES TO EXPERIMENTAL ACIDIFICATION

Changes in physical, chemical and biological factors were observed during a 5-yr experimental acidification study in Lake 223 of the Experimental Lakes Area, and compared to a 2 yr pre-acidification period. Significant changes included increased transparency, rates of hypolimnion heating and rates of thermocline deepening; increased concentrations of Mn, Na, Zn, Al, and chlorophyll; decreased concentrations of suspended C, total dissolved N, Fe and chloride; increases in Chlorophyta but decreases in Chrysophyta; the disappearance of the opossum shrimp Mysis relicta and the fathead minnow Pimephales promelas; the appearance of epidemics of the filamentous alga Mougeotea; decreased fitness and decline in numbers of Orconectes virilis; and increased embryonic mortality of the lake trout Salvelinus namaycush.

Disruption of the Nitrogen Cycle in Acidified Lakes

Experimental acidification of two small soft-water lakes caused nitrification to cease at pH values of 5.4 to 5.7. The resulting blockage of the nitrogen cycle caused a progressive accumulation of amnonium. When the epermental acidification of one of the lakes was ended and the pH was raised to 5.4, nitrification resumed after a time lag of 1 year.

Elemental Composition of Littoral Invertebrates from Oligotrophic and Eutrophic Canadian Lakes

The P, N, and C content of littoral macroinvertebrates from 8 lakes located in 3 distinct geographical regions of Canada (central Alberta, northwestern Ontario, and Rocky Mountains near Jasper, Alberta) is described. A wide range of values was found in the body content (all values are % of dry mass) of P (0.4-1.6%), N (5.8-13.7%), and C (32.5-53.5%) in the data set containing invertebrates from all 8 lakes. C:P (63-324), N:P (9.6-60), and C:N (4.2-7.6) (all by atom) also varied widely. This variation was partly related to the different mean body content of P, N, and C among taxonomic groups. However, the mean P, N, and C content of macroinvertebrate assemblages varied little among lakes. The patterns of elemental composition in benthic invertebrates shown here are similar to zoo-plankton and terrestrial insects, and indicate that the strength of stoichiometric constraints acting in littoral food webs will depend on the taxa being considered.

Trophic Dependence of Ecosystem Resistance and Species Compensation in Experimentally Acidified Lake 302S (Canada)

AbstractEcosystem resistance to the impacts of diverse human insults depends on the replacement of sensitive species by ones more tolerant of the stressor. Here we present evidence from a whole-lake acidification experiment (Lake 302S, Experimental Lakes Area, Canada) that resistance and species compensation decline with increasing trophic level. Diverse and fast-growing algal and rotifer assemblages with high dispersal potentials showed significant compensatory species dynamics, resulting in the maintenance of total biomass despite 30%–80% declines in species richness. Canonical correspondence analysis showed that significant compensatory algal and rotifer dynamics were best explained by differential species tolerances of acidified chemical conditions coupled with release from resource limitation and predation. However, less diverse cladoceran, copepod, and fish assemblages showed significant declines in total biomass and weak species compensation with loss of species during acidification. In comparison, algal and zooplankton species dynamics remained relatively synchronized in a nearby unperturbed reference lake (Lake 239) during the experiment. As a result, Lake 302S showed limited ecosystem resistance to anthropogenic acidification. Therefore, we hypothesize that lost species will increase the susceptibility of acidified lakes to the adverse impacts of other environmental stressors (for example, climate warming, stratospheric ozone depletion, invasive species). Consequently, the ecosystem stability of boreal lakes is expected to decline as global change proceeds.

Acidification and alkalinization of lakes by experimental addition of nitrogen compounds

Fertilization of a small lake with ammonium chloride for four years as part of a eutrophication experiment caused it to acidify to pH values as low as 4.6. Implications for acidification of lakes via precipitation polluted with ammonium compounds are discussed.When phosphate was supplied with the ammonium, biological nitrogen uptake, apparently by phytoplankton, was the main mechanism causing acidification. When ammonium was applied without phosphate, it accumulated to high concentrations in solution, after which nitrification caused rapid acidification. In both cases, the whole-lake efficiency of acidification was low, averaging about 13% of the potential acidification of supplied ammonium chloride (Table 2).Subsequent application of phosphate plus sodium nitrate for two years caused the pH of the lake to increase. The efficiency of alkalinization was higher than for acidification, averaging 69% of the potential alkalinization of the supplied sodium nitrate.