Temporal changes in DOC concentrations seem more consistent with the evolution of climate parameters rather than SO₄²⁻ concentrations despite the fact that most lakes were in the process of recovery, showing increases in pH.

No 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 (SO4(2-)) 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 SO4(2-), 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.

The acidification of forest soils and surface waters and their relatively poor recovery record following reductions in atmospheric sulphur emissions is a major ongoing environmental problem, particularly in northeastern North America. The slow recovery of surface water is widely hypothesized to result from depletion of reservoirs of base cations in soil. This is concordant with the theory that the acid-neutralizing capacity (ANC) of lakes is likely proportional to the size of the exchangeable base cation reservoirs present in surrounding watershed soils. However, data describing these linkages are still nonexistent in the literature. Here we show that lake ANC is highly predictable (r 2 = 0.75) based on the size of the exchangeable Ca 2+ reservoir in soil in 21 catchments representative of soil and lake conditions encountered in northeastern North America. This finding indirectly supports the hypothesis that the poor recovery of surface water from acidification is governed by the size of base cation reservoirs present in catchment soils. The size of the base cation reservoir in soil is thus a strong indicator of the acid-base status of both soils and surface waters.Résumé : L'acidification des sols forestiers et des eaux de surface ainsi que leur récupération relativement mitigée malgré une diminution des dépôts atmosphériques de soufre représentent toujours un important problème environnemental, particulièrement dans le nord-est de l'Amérique du Nord. La faible récupération des eaux de surface a été principalement attribuée à la diminution des réservoirs de cations basiques dans les sols. Cela suppose que la capacité des eaux de surface à neutraliser les apports acides (CNA) est proportionnelle à la taille des réservoirs de cations basiques échangeables dans les sols des bassins versants. Bien que cette hypothèse paraisse plausible, elle n'a pas encore été démontrée. Dans cette étude, nous démontrons que la CNA des lacs peut être prédite avec succès (r 2 = 0,75) à partir de la taille du réservoir échangeable de Ca 2+ pour 21 bassins versants forestiers représentatifs des conditions lacustres et édaphiques rencontrées dans le nord-est de l'Amérique du Nord. Cette découverte supporte indirectement l'hypothèse selon laquelle la faible amélioration de la CNA des eaux de surface est contrôlée par la taille des réser-voirs échangeables de cations basiques présents dans le sols. Ces derniers sont donc des indicateurs fiables du statut acido-basique non seulement des sols mais aussi des eaux de surface.Houle et al. 474

Surface water browning, the result of increasing concentrations of dissolved organic matter (DOM), has been widespread in northern ecosystems in recent decades. Here, we assess a database of 426 undisturbed headwater lakes and streams in Europe and North America for evidence of trends in DOM between 1990 and 2016. We describe contrasting changes in DOM trends in Europe (decelerating) and North America (accelerating), which are consistent with organic matter solubility responses to declines in sulfate deposition. While earlier trends (1990–2004) were almost entirely related to changes in atmospheric chemistry, climatic and chemical drivers were equally important in explaining recent DOM trends (2002–2016). We estimate that riverine DOM export from northern ecosystems increased by 27% during the study period. Increased summer precipitation strengthened upward dissolved organic carbon trends while warming apparently damped browning. Our results suggest strong but changing influences of air quality and climate on the terrestrial carbon cycle, and on the magnitude of carbon export from land to water.