Peter J. Dillon

Regional trends in aquatic recovery from acidification in North America and Europe

Rates of acidic deposition from the atmosphere (‘acid rain’) have decreased throughout the 1980s and 1990s across large portions of North America and Europe. Many recent studies have attributed observed reversals in surface-water acidification at national and regional scales to the declining deposition. To test whether emissions regulations have led to widespread recovery in surface-water chemistry, we analysed regional trends between 1980 and 1995 in indicators of acidification (sulphate, nitrate and base-cation concentrations, and measured (Gran) alkalinity) for 205 lakes and streams in eight regions of North America and Europe. Dramatic differences in trend direction and strength for the two decades are apparent. In concordance with general temporal trends in acidic deposition, lake and stream sulphate concentrations decreased in all regions with the exception of Great Britain; all but one of these regions exhibited stronger downward trends in the 1990s than in the 1980s. In contrast, regional declines in lake and stream nitrate concentrations were rare and, when detected, were very small. Recovery in alkalinity, expected wherever strong regional declines in sulphate concentrations have occurred, was observed in all regions of Europe, especially in the 1990s, but in only one region (of five) in North America. We attribute the lack of recovery in three regions (south/central Ontario, the Adirondack/Catskill mountains and midwestern North America) to strong regional declines in base-cation concentrations that exceed the decreases in sulphate concentrations.

POTENTIAL EFFECTS OF CLIMATE CHANGES ON AQUATIC SYSTEMS: LAURENTIAN GREAT LAKES AND PRECAMBRIAN SHIELD REGION

The region studied includes the Laurentian Great Lakes and a diversity of smaller glacial lakes, streams and wetlands south of permanent permafrost and towards the southern extent of Wisconsin glaciation. We emphasize lakes and quantitative implications. The region is warmer and wetter than it has been over most of the last 12000 years. Since 1911 observed air temperatures have increased by about 0.118C per decade in spring and 0.068C in winter; annual precipitation has increased by about 2.1% per decade. Ice thaw phenologies since the 1850s indicate a late winter warming of about 2.58C. In future scenarios for a doubled CO2 climate, air temperature increases in summer and winter and precipitation decreases (summer) in western Ontario but increases (winter) in western Ontario, northern Minnesota, Wisconsin and Michigan. Such changes in climate have altered and would further alter hydrological and other physical features of lakes. Warmer climates, i.e. 2 CO2 climates, would lower net basin water supplies, stream flows and water levels owing to increased evaporation in excess of precipitation. Water levels have been responsive to drought and future scenarios for the Great Lakes simulate levels 0. 2t o 2 .5 m lower. Human adaptation to such changes is expensive. Warmer climates would decrease the spatial extent of ice cover on the Great Lakes; small lakes, especially to the south, would no longer freeze over every year. Temperature simulations for stratified lakes are 1‐78C warmer for surface waters, and 68C cooler to 88C warmer for deep waters. Thermocline depth would change (4 m shallower to 3.5 m deeper) with warmer climates alone; deepening owing to increases in light penetration would occur with reduced input of dissolved organic carbon (DOC) from dryer catchments. Dissolved oxygen would decrease below the thermocline. These physical changes would in turn aAect the phytoplankton, zooplankton, benthos and fishes. Annual phytoplankton production may increase but many complex reactions of the phytoplankton community to altered temperatures, thermocline depths, light penetrations and nutrient inputs would be expected. Zooplankton biomass would increase, but, again, many complex interactions are expected. Generally, the thermal habitat for warm-, cool- and even cold-water fishes would increase in size in deep stratified lakes, but would decrease in shallow unstratified lakes and in streams. Less dissolved oxygen below the thermocline of lakes would further degrade stratified lakes for cold water fishes. Growth and production would increase for fishes that are now in thermal environments cooler than their optimum but decrease for those that are at or above their optimum, provided they cannot move to a deeper or headwater thermal refuge. The zoogeographical boundary for fish species could move north by 500‐600 km; invasions of warmer water fishes and extirpations of colder water fishes should increase. Aquatic ecosystems across the region do not necessarily exhibit coherent responses to climate changes and variability, even if they are in close proximity. Lakes, wetlands and streams respond diAerently, as do lakes of diAerent depth or productivity. DiAerences in hydrology and the position in the hydrological flow system, in terrestrial vegetation and land use, in base climates and in the aquatic biota can all cause diAerent responses. Climate change eAects interact strongly with eAects of other human-caused stresses such as eutrophication, acid precipitation, toxic chemicals and the spread of exotic organisms. Aquatic ecological systems in the region are sensitive to climate change and variation.

Effect of landscape form on export of dissolved organic carbon, iron, and phosphorus from forested stream catchments

Predicting the effects of climate change and atmospheric deposition on water quality requires predicting the effects of landscape form on export of substances downstream. In this paper, we present dissolved organic carbon (DOC), total phosphorus (TP), and iron (Fe) export data (1980–1992) for 20 relatively undisturbed, forested catchments draining into seven lakes in central Ontario and develop regression models of chemical export as functions of landscape composition. The extent of wetlands was correlated with export of DOC and TP; the proportion of the catchment covered by peatlands accounted for 78% of the variance in a regression model of long-term average DOC export and 76% of the variance in a model of color “export.” Peatland coverage and Fe export together explained 76% of the variance in a long-term average TP export model, which is consistent with published experimental evidence that Fe facilitates P complexation with DOC in surface waters. The long-term average Fe export model was not significant when all 20 catchments were included. However, Fe export from the 14 catchments with thin tills was a function only of peatland coverage (R2 = 0.71), suggesting that Fe export is dependent to a large extent upon either the export of organic material or the reducing conditions. The long-term export models worked well when export of a substance was dominated by peatlands but was not very sensitive to the influence of mineral soils. The long-term average TP/DOC ratio was remarkably constant among most whole-lake catchments, ranging from 1.4 to 2.0 mg P/g C, the exception being the catchment of anthropogenically acidified Plastic Lake with a ratio of 0.8. Fe export to Plastic lake was also enriched relative to TP export compared with the other lakes. Therefore TP export to Plastic Lake may be limited by some mechanism related to acidification.

Dissolved organic and inorganic carbon mass balances in central Ontario lakes

Mass balances of dissolved organic carbon (DOC) and dissolvedinorganic carbon (DIC) based on stream and precipitation inputs andoutflows were measured for seven unproductive lakes in central Ontariobetween 1981 and 1989. Net annual CO2 evasion occurred in sixof the seven study lakes with minor net invasion in the seventh. Atmosphericinvasion might have been significant at certain times of the year, particularlyduring the growing season. Net evasion rates were greater than DIC loadingrates, indicating partial mineralization of the terrestrially-derived DOC in thelakes. A steady state mass balance model adequately described the variationin DOC retention between lakes. Net annual carbon accumulation of forestcommunities based on estimates of net ecosystem production may beoverestimated because of significant export of carbon to lakes via streamsand groundwater, particularly in catchments with extensive peatlands.

Export of DOC from forested catchments on the Precambrian Shield of Central Ontario: Clues from 13C and 14C

Export of dissolved organic carbon (DOC) from forested catchmentsis governed by competing processes of production, decomposition, sorptionand flushing. To examine the sources of DOC, carbon isotopes (14Cand 13C) were analyzed in DOC from surface waters, groundwatersand soils in a small forested catchment on the Canadian Shield in centralOntario. A significant fraction (greater than 50%) of DOCin major inflows to the lake is composed of carbon incorporated into organicmatter, solubilized and flushed into the stream within the last 40 years. Incontrast, 14C in groundwater DOC was old indicating extensiverecycling of forest floor derived organic carbon in the soil column beforeelution to groundwater in the lower B and C soil horizons. A small uplandbasin had a wide range in 14C from old groundwater values atbaseflow under dry basin conditions to relatively modern values during highflow or wetter antecedent conditions. Wetlands export mainly recently fixedcarbon with little seasonal range. DOC in streams entering the small lakemay be composed of two pools; an older recalcitrant pool delivered bygroundwater and a young labile pool derived from recent organic matter.The relative proportion of these two pools changes seasonally due thechanges in the water flowpaths and organic carbon dynamics. Althoughchanges in local climate (temperature and/or precipitation) may alterthe relative proportions of the old and young pools, the older pool islikely to be more refractory to sedimentation and decomposition in thelake setting. Delivery of older pool DOC from the catchment andsusceptibility of this older pool to photochemical decomposition mayconsequently be important in governing the minimum DOC concentrationlimit in lakes.

Precambrian Shield Wetlands: Hydrologic Control of the Sources and Export of Dissolved Organic Matter

Most Precambrian Shield forested catchments have some wetland component. Even small riparian wetlands are important modifiers of stream chemistry. Dissolved organic matter (DOM) is one of the most important products exported by wetlands in streams. Stratigraphic control of hydraulic conductivity generally leads to decreasing conductivity with depth. Thus important flowpaths occur in the uppermost organic rich layers and are reflected in chemical profiles of dissolved organic carbon (DOC). Accumulation of DOC in peat porewaters is the net effect of production, consumption and transport. DOC profiles vary with degree of interaction with the surrounding upland catchment and distance from the edge of the wetland as well as internal processes within the wetland. In wetlands, DOM production is offset by flushing resulting in decreasing DOC concentrations with increasing flows. Despite old carbon (2,000 to 3,000 years) at relatively shallow depths, 14C activity in DOC exported from wetlands is mostly modern (recent carbon), consistent with shallow flowpaths and export of DOM from shallow organic rich horizons. In contrast, the source area for DOM in upland catchments with developed B horizon soils increases with antecedent soil moisture conditions resulting in increasing DOC concentrations with higher stream flows. Activity of 14C in stream DOC from upland catchments span a range from low activities (older carbon) similar to B horizon soil water during dry moisture conditions to values slightly less than modern (more recent carbon) during high moisture conditions. The more modern carbon activities reflect the increased contribution of the organic rich litter and A horizon soil layers in the area immediately bordering the stream under wet antecedent moisture conditions. Reduced hydrologic export or loss of wetlands under drier climatic conditions may result in in larger fluctuations in stream DOC concentrations and reduced DOM loads to lakes.

Photolytic regulation of dissolved organic carbon in northern lakes

We examined the extent to which photolytic and nonphotolytic decomposition rates of dissolved organic carbon (DOC) could account for the annual retention or loss of DOC inputs in lakes (retention is equal to stream inputs plus atmospheric inputs minus stream discharge which is equivalent to storage in sediments plus degassed to atmosphere). Losses of DOC inputs to sediments and the atmosphere were large, averaging 38 to 70% of total inputs in seven study lakes between 1980 and 1992. Up to 50% of stream DOC was lost as inorganic C when exposed to solar radiation during 6 to 11 day surface exposures in bottles whereas lake DOC concentration was unaffected by solar radiation. Stream DOC loss was significantly less in the dark suggesting a low microbial consumption rate. Photodecay constants, extrapolated to each of the study lakes after correction for in situ mixing conditions and extinction of UVA and UVB, were similar to corresponding mass balance rate constants representing sediment storage and losses to the atmosphere. This suggests that photodecay is potentially large enough in situ to account for all of the DOC losses to the atmosphere and sediments in the low DOC lakes ( 4 mg L−1). The mass balance and photodecay approaches employed in the study of carbon budgets show that UV degradation is probably an important mechanism in transfer of stream DOC to the sediment particulate C pool and to the atmosphere.

THE EFFECT OF EL NIÑO-RELATED DROUGHT ON THE RECOVERY OF ACIDIFIED LAKES

AbstractAlthough SO2 emissions and deposition rates havedeclined substantially since the implementation of sulphuremission control programmes in North America [1], recovery(measured as decreases in

The role of ammonium and nitrate retention in the acidification of lakes and forested catchments

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