Impacts of acid deposition at Plastic Lake: forecasting chemical recovery using a Bayesian calibration and uncertainty propagation approach

Given the importance of models in the development of environmental polices it is necessary to assess the uncertainty introduced by model parameterisation and its impact on predictions. In the current study, an uncertainty framework designed to perform automated calibrations and developed for use with the Model of Acidification of Groundwater in Catchments (MAGIC) was applied to Plastic Lake, a long-term study site in Southern Ontario, Canada. The primary objectives were to investigate the chemical response of soil and surface water at Plastic Lake to proposed acid (sulfur and nitrogen) emissions and assess the use of the framework at a regional level. Despite the relatively high amount of uncertainty associated with many of the model parameters, calibration resulted in relatively narrow parameter convergence. The importance of time-series stream data was clearly evident, with uncertainty decreasing with more observation years. The forecast improvements in stream Acid Neutralizing Capacity at Plastic Lake from–40 μeq/L in 1988 to 14 μeq/L in 2060 had 5 and 95% confidence bounds of–3 and 29 μeq/L, respectively. Despite the limited availability of soil chemical data in Ontario, the approach applied at Plastic Lake is viable on a regional basis given the abundance of water chemistry data.

Drought-induced metal release from a wetland at Plastic Lake, central Ontario

With climate change, droughts may become more frequent in southern Ontario, which could release metals from peat and degrade downstream water quality. Monthly volume-weighted metal (Al, Ba, Be, Cd, Co, Mn, Ni, Pb, Sr, and Zn) concentrations and fluxes in streams and bulk deposition at Plastic Lake were monitored over 20 months in 2002–2003, during which there was a summer drought. Monthly concentrations in the outflow from the wetland (PC1) were variable, with very high concentrations following the drought. With the exception of Pb, statistically significant models of metal concentrations with SO42– and dissolved organic carbon concentrations were developed, and these relationships were used to estimate monthly metal exports between 1980 and 2000. Model predictions for Cd and Zn in PC1 agreed well (p < 0.001) with concentrations measured between 1989 and 1991. Model predictions suggesting peaks in metal concentrations are common in years with pronounced summer droughts. In contrast to ombrotrophic bogs, the PC1 wetland receives the majority of its metal input from the terrestrial catchment, and mass balance approximations indicate no substantial depletion of metal reserves in peat. Drought-induced metal peaks may persist for many decades, potentially contributing to the delayed recovery of surface waters at Plastic Lake, despite declining S deposition.

The impacts of future climate change and sulphur emission reductions on acidification recovery at Plastic Lake, Ontario

Climate-induced drought events have a significant influence on sulphate export from forested catchments in central Ontario, subsequently delaying the recovery of surface waters from acidification. In the current study, a model chain that employed a statistical downscaling model, a hydrological model and two hydrochemical models was used to forecast the chemical recovery of Plastic Lake sub-catchment 1 (PC1) from acidification under proposed deposition reductions and the A2 emission scenario of the Intergovernmental Panel on Climate Change. Any predicted recovery in stream acid neutralising capacity and pH owing to deposition reductions were clearly offset by large acid effluxes from climate-induced drought events. By 2100, ANC is predicted to show large variations ranging between 10 and −30 μmolc L−1. Similarly, predicted pH in 2100 is lower (>0.05 of a pH unit) than the value simulated for 2000 (pH 4.35). Despite emission reductions, the future scenario paints a bleak picture of reacidification at PC1 to levels commensurate with those of the late 1970s. The principal process behind this reacidification is the oxidation of previously stored (reduced) sulphur compounds in wetlands during periods of low-flow (or drought), with subsequent efflux of sulphate upon re-wetting. Simulated catchment runoff under the A2 emissions scenario predictes increased intensity and frequency of low-flow events from approximately 2030 onwards. The Integrated Catchments model for Carbon indicated that stream DOC concentrations at PC1 will also increase under the future climate scenario, with temperature being the principal driver. Despite the predicted (significant) increase in DOC, pH is not predicted to further decline (beyond the climate-induced oxidation scenario), instead pH shows greater variability throughout the simulation. As echoed by many recent studies, hydrochemical models and model frameworks need to incorporate the drivers and mechanisms (at appropriate time-scales) that affect the key biogeochemical processes to reliably predict the impacts of climate change.

The impacts of future climate change and sulphur emission reductions on acidification recovery at Plastic Lake, Ontario

Climate-induced drought events have a significant influence on sulphate export from forested catchments in central Ontario, subsequently delaying the recovery of surface waters from acidification. In the current study, a model chain that employed a statistical downscaling model, a hydrological model and two hydrochemical models was used to forecast the chemical recovery of Plastic Lake sub-catchment 1 (PC1) from acidification under proposed deposition reductions and the A2 emission scenario of the Intergovernmental Panel on Climate Change. Any predicted recovery in stream acid neutralising capacity and pH owing to deposition reductions were clearly offset by large acid effluxes from climate-induced drought events. By 2100, ANC is predicted to show large variations ranging between 10 and −30 μmolc L−1. Similarly, predicted pH in 2100 is lower (>0.05 of a pH unit) than the value simulated for 2000 (pH 4.35). Despite emission reductions, the future scenario paints a bleak picture of reacidification at PC1 to levels commensurate with those of the late 1970s. The principal process behind this reacidification is the oxidation of previously stored (reduced) sulphur compounds in wetlands during periods of low-flow (or drought), with subsequent efflux of sulphate upon re-wetting. Simulated catchment runoff under the A2 emissions scenario predictes increased intensity and frequency of low-flow events from approximately 2030 onwards. The Integrated Catchments model for Carbon indicated that stream DOC concentrations at PC1 will also increase under the future climate scenario, with temperature being the principal driver. Despite the predicted (significant) increase in DOC, pH is not predicted to further decline (beyond the climate-induced oxidation scenario), instead pH shows greater variability throughout the simulation. As echoed by many recent studies, hydrochemical models and model frameworks need to incorporate the drivers and mechanisms (at appropriate time-scales) that affect the key biogeochemical processes to reliably predict the impacts of climate change.

Omnivory of the larval phantom midge (Chaoborus spp.) and its potential significance for freshwater planktonic food webs

Developmental and seasonal changes in the preferred prey and the diet composition of the invertebrate predator Chaoborus punctipennis were determined in Plastic Lake, an acidified (pH 5.6) lake in south-central Ontario, Canada. All instars consumed rotifers (mainly Keratella cochlearis, Ploesoma sp., and Asplanchna priodonta), and instars III and IV fed preferentially on crustaceans (mainly bosminids and copepods). Phytoflagellates (Peridinium sp. and Dinobryon sp.), however, numerically dominated the diet of all instars examined (II–IV), and were consumed by instar II larvae in excess of their relative availability. On 40 and 20% of the sampling dates, instars III and IV, respectively, consumed phytoflagellates in accordance with their relative abundance in the lake. Although the contribution of phytoflagellates to the biomass-based diet of C. punctipennis was low, on one occasion phytoflagellates formed almost half of the diet biomass of instar II larvae. A review of the literature shows that in lakes where phytoflagellate densities are high (≥ 100–200/mL), phytoflagellates contribute ≥ 50% of the diet biomass of all instars of Chaoborus spp. These findings indicate that Chaoborus spp. are omnivores that frequently feed on phytoflagellates even when alternative animal prey are abundant. Consumption of phagotrophic phytoflagellates by Chaoborus spp. and other large invertebrate omnivores, such as Mysis spp., Epischura spp., and cyclopoid copepods, may increase the transfer efficiency of organic carbon from the microbial food web to the upper trophic levels in fresh waters. In acidified lakes, consumption of large dinoflagellates by Chaoborus spp. and other invertebrate omnivores may ameliorate the hypothesized bottleneck impeding the flow of carbon between phytoplankton and zooplankton.