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 0·11°C per decade in spring and 0·06°C 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 2·5°C. 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 0·2 to 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 1–7°C warmer for surface waters, and 6°C cooler to 8°C 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 affect 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 differently, as do lakes of different depth or productivity. Differences 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 different responses. Climate change effects interact strongly with effects 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. Assessments of these potential effects are in an early stage and contain many uncertainties in the models and properties of aquatic ecological systems and of the climate system. © 1997 John Wiley & Sons, Ltd.     

EFFECTS OF CLIMATE CHANGE ON FRESHWATER ECOSYSTEMS OF THE SOUTH-EASTERN UNITED STATES AND THE GULF COAST OF MEXICO

The south-eastern United States and Gulf Coast of Mexico is physiographically diverse, although dominated by a broad coastal plain. Much of the region has a humid, warm temperate climate with little seasonality in precipitation but strong seasonality in runoff owing to high rates of summer evapotranspiration. The climate of southern Florida and eastern Mexico is subtropical with a distinct summer wet season and winter dry season. Regional climate models suggest that climate change resulting from a doubling of the pre-industrial levels of atmospheric CO₂ may increase annual air temperatures by 3–4°C. Changes in precipitation are highly uncertain, but the most probable scenario shows higher levels over all but the northern, interior portions of the region, with increases primarily occurring in summer and occurring as more intense or clustered storms. Despite the increases in precipitation, runoff is likely to decline over much of the region owing to increases in evapotranspiration exceeding increases in precipitation. Only in Florida and the Gulf Coast areas of the US and Mexico are precipitation increases likely to exceed evapotranspiration increases, producing an increase in runoff. However, increases in storm intensity and clustering are likely to result in more extreme hydrographs, with larger peaks in flow but lower baseflows and longer periods of drought. The ecological effects of climate change on freshwaters of the region include: (1) a general increase in rates of primary production, organic matter decomposition and nutrient cycling as a result of higher temperatures and longer growing seasons: (2) reduction in habitat for cool water species, particularly fish and macroinvertebrates in Appalachian streams; (3) reduction in water quality and in suitable habitat in summer owing to lower baseflows and intensification of the temperature–dissolved oxygen squeeze in many rivers and reservoirs; (4) reduction in organic matter storage and loss of organisms during more intense flushing events in some streams and wetlands; (5) shorter periods of inundation of riparian wetlands and greater drying of wetland soils, particularly in northern and inland areas; (6) expansion of subtropical species northwards, including several non-native nuisance species currently confined to southern Florida; (7) expansion of wetlands in Florida and coastal Mexico, but increase in eutrophication of Florida lakes as a result of greater runoff from urban and agricultural areas; and (8) changes in the flushing rate of estuaries that would alter their salinity regimes, stratification and water quality as well as influence productivity in the Gulf of Mexico. Many of the expected climate change effects will exacerbate current anthropogenic stresses on the region's freshwater systems, including increasing demands for water, increasing waste heat loadings and land use changes that alter the quantity and quality of runoff to streams and reservoirs. Research is needed especially in several critical areas: long-term monitoring of key hydrological, chemical and biological properties (particularly water balances in small, forested catchments and temperature-sensitive species); experimental studies of the effects of warming on organisms and ecosystem processes under realistic conditions (e.g. in situ heating experiments); studies of the effects of natural hydrological variation on biological communities; and assessment of the effects of water management activities on organisms and ecosystem processes, including development and testing of management and restoration strategies designed to counteract changes in climate. © 1997 John Wiley & Sons, Ltd.     

POTENTIAL EFFECTS OF CLIMATE CHANGE ON AQUATIC ECOSYSTEMS OF THE GREAT PLAINS OF NORTH AMERICA

The Great Plains landscape is less topographically complex than most other regions within North America, but diverse aquatic ecosystems, such as playas, pothole lakes, ox-bow lakes, springs, groundwater aquifers, intermittent and ephemeral streams, as well as large rivers and wetlands, are highly dynamic and responsive to extreme climatic fluctuations. We review the evidence for climatic change that demonstrates the historical importance of extremes in north–south differences in summer temperatures and east–west differences in aridity across four large subregions. These physical driving forces alter density stratification, deoxygenation, decomposition and salinity. Biotic community composition and associated ecosystem processes of productivity and nutrient cycling respond rapidly to these climatically driven dynamics. Ecosystem processes also respond to cultural effects such as dams and diversions of water for irrigation, waste dilution and urban demands for drinking water and industrial uses. Distinguishing climatic from cultural effects in future models of aquatic ecosystem functioning will require more refinement in both climatic and economic forecasting. There is a need, for example, to predict how long-term climatic forecasts (based on both ENSO and global warming simulations) relate to the permanence and productivity of shallow water ecosystems. Aquatic ecologists, hydrologists, climatologists and geographers have much to discuss regarding the synthesis of available data and the design of future interdisciplinary research. © 1997 John Wiley & Sons, Ltd.     

Effects of climate change on thermal properties of lakes and reservoirs, and possible implications

Meteorologic-driven processes exert large and diverse impacts on lakes’ internal heating, cooling, and mixing. Thus, continued global warming and climate change will affect lakes’ thermal properties, dynamics, and ecosystem. The impact of climate change on Lake Tahoe (in the states of California and Nevada in the United States) is investigated here, as a case study of climate change effects on the physical processes occurring within a lake. In the Tahoe basin, air temperature data show upward trends and streamflow trends indicate earlier snowmelt. Precipitation in the basin is shifting from snow to rain, and the frequency of intense rainfall events is increasing. In-lake water temperature records of the past 38 years (1970–2007) show that Lake Tahoe is warming at an average rate of 0.013°C/year. The future trends of weather variables, such as air temperature, precipitation, longwave radiation, downward shortwave radiation, and wind speed are estimated from predictions of three General Circulation Models (GCMs) for the period 2001–2100. Future trends of weather variables of each GCM are found to be different to those of the other GCMs. A series of simulation years into the future (2000–2040) is established using streamflows and associated loadings, and meteorologic data sets for the period 1994–2004. Future simulation years and trends of weather variables are selected so that: (1) future simulated warming trend would be consistent with the observed warming trend (0.013°C/year); and (2) future mixing pattern frequency would closely match with the historical mixing pattern frequency. Results of 40-year simulations show that the lake continues to become warmer and more stable, and mixing is reduced. Continued warming in the Tahoe has important implications for efforts towards managing biodiversity and maintaining clarity of the lake.     

SENSITIVITY OF AQUATIC ECOSYSTEMS TO CLIMATIC AND ANTHROPOGENIC CHANGES: THE BASIN AND RANGE,AMERICAN SOUTHWEST AND MEXICO

Variability and unpredictability are characteristics of the aquatic ecosystems, hydrological patterns and climate of the largely dryland region that encompasses the Basin and Range, American Southwest and western Mexico. Neither hydrological nor climatological models for the region are sufficiently developed to describe the magnitude or direction of change in response to increased carbon dioxide; thus, an attempt to predict specific responses of aquatic ecosystems is premature. Instead, we focus on the sensitivity of rivers, streams, springs, wetlands, reservoirs, and lakes of the region to potential changes in climate, especially those inducing a change in hydrological patterns such as amount, timing and predictability of stream flow. The major sensitivities of aquatic ecosystems are their permanence and even existence in the face of potential reduced net basin supply of water, stability of geomorphological structure and riparian ecotones with alterations in disturbance regimes, and water quality changes resulting from a modified water balance. In all of these respects, aquatic ecosystems of the region are also sensitive to the extensive modifications imposed by human use of water resources, which underscores the difficulty of separating this type of anthropogenic change from climate change. We advocate a focus in future research on reconstruction and analysis of past climates and associated ecosystem characteristics, long-term studies to discriminate directional change vs. year to year variability (including evidence of aquatic ecosystem responses or sensitivity to extremes), and studies of ecosystems affected by human activity. © 1997 John Wiley & Sons, Ltd.     

Effect of climatic change on snowmelt runoffs in mountainous regions of inland rivers in Northwestern China

Snow is one of the most active natural elements of snow cover through its high albedo, variation of the the cryosphere on the earth surface. Its unique proper- snow cover distribution and frozen soils in regional ties, such as areal extent, surface albedo, and snow scales not only affect local climate and environments, depth are important parameters in global energy bal- but also feedback to large-scale, or even global cli- ance models. On global and terrestrial scales, a large matic change th…     

Water temperature dynamics in High Arctic river basins

Despite the high sensitivity of polar regions to climate change and the strong influence of temperature upon ecosystem processes, contemporary understanding of water temperature dynamics in Arctic river systems is limited. This research gap was addressed by exploring high‐resolution water column thermal regimes for glacier‐fed and non‐glacial rivers at eight sites across Svalbard during the 2010 melt season. Mean water column temperatures in glacier‐fed rivers (0.3–3.2 °C) were lowest and least variable near the glacier terminus but increased downstream (0.7–2.3 °C km^–1). Non‐glacial rivers, where discharge was sourced primarily from snowmelt runoff, were warmer (mean: 2.9–5.7 °C) and more variable, indicating increased water residence times in shallow alluvial zones and increased potential for atmospheric influence. Mean summer water temperature and the magnitude of daily thermal variation were similar to those of some Alaskan Arctic rivers but low at all sites when compared with alpine glacierized environments at lower latitudes. Thermal regimes were correlated strongly (p < 0.01) with incoming short‐wave radiation, air temperature, and river discharge. Principal drivers of thermal variability were inferred to be (i) water source (i.e. glacier melt, snowmelt, groundwater); (ii) exposure time to the atmosphere; (iii) prevailing meteorological conditions; (iv) river discharge; (v) runoff interaction with permafrost and buried ice; and (vi) basin‐specific geomorphological features (e.g. channel morphology). These results provide insight into the potential changes in high‐latitude river systems in the context of projected warming in polar regions. We hypothesize that warmer and more variable temperature regimes may prevail in the future as the proportion of bulk discharge sourced from glacial meltwater declines and rivers undergo a progressive shift towards snow water and groundwater sources. Importantly, such changes could have implications for aquatic species diversity and abundance and influence rates of ecosystem functioning in high‐latitude river systems. Copyright © 2012 John Wiley & Sons, Ltd.     

Landscape influence on small‐scale water temperature variations in a moorland catchment

We monitored temperatures in stream water, groundwater and riparian wetland surface water over 18 months in a 3.2‐km² moorland catchment in the Scottish Highlands. The stream occupies a glaciated valley, aligned east–west. It has three main headwater tributaries with a large north facing catchment, a south facing catchment and the smallest east facing headwater. The lower catchment sampling locations begin after the convalescence of all three headwaters. Much of the stream network is fringed by riparian peatlands. Stream temperatures are mainly regulated by energy exchanges at the air–water interface. However, they are also influenced by inflows from the saturated riparian zone, where surface water source areas are strongly connected with the stream network. Consequently, the spatial distribution of stream temperatures exhibits limited variability. Nevertheless, there are significant summer differences between the headwaters, despite their close proximity to each other. This is consistent with aspect (and incident radiation), given the south and east facing headwaters having higher temperatures. The largest, north‐facing sub‐catchment shows lower summer diurnal temperature variability, suggesting that lower radiation inputs dampen temperature extremes. Whilst stream water temperature regimes in the lower catchment exhibit little change along a 1‐km reach, they are similar to those in the largest headwater; probably reflecting size and comparable catchment aspect and hydrological flow paths. Our results suggest that different parts of the channel network and its connected wetlands have contrasting sensitivity to higher summer temperatures. This may be important in land management strategies designed to mitigate the impacts of projected climatic warming. Copyright © 2015 John Wiley & Sons, Ltd.     

Temperature in lowland Danish streams: contemporary patterns,empirical models and future scenarios

Continuous temperature measurements at 11 stream sites in small lowland streams of North Zealand, Denmark over a year showed much higher summer temperatures and lower winter temperatures along the course of the stream with artificial lakes than in the stream without lakes. The influence of lakes was even more prominent in the comparisons of colder lake inlets and warmer outlets and led to the decline of cold‐water and oxygen‐demanding brown trout. Seasonal and daily temperature variations were, as anticipated, dampened by forest cover, groundwater input, input from sewage plants and high downstream discharges. Seasonal variations in daily water temperature could be predicted with high accuracy at all sites by a linear air‐water regression model (r²: 0·903–0·947). The predictions improved in all instances (r²: 0·927–0·964) by a non‐linear logistic regression according to which water temperatures do not fall below freezing and they increase less steeply than air temperatures at high temperatures because of enhanced heat loss from the stream by evaporation and back radiation. The predictions improved slightly (r²: 0·933–0·969) by a multiple regression model which, in addition to air temperature as the main predictor, included solar radiation at un‐shaded sites, relative humidity, precipitation and discharge. Application of the non‐linear logistic model for a warming scenario of 4–5 °C higher air temperatures in Denmark in 2070‐2100 yielded predictions of temperatures rising 1·6–3·0 °C during winter and summer and 4·4–6·0 °C during spring in un‐shaded streams with low groundwater input. Groundwater‐fed springs are expected to follow the increase of mean air temperatures for the region. Great caution should be exercised in these temperature projections because global and regional climate scenarios remain open to discussion. Copyright © 2006 John Wiley & Sons, Ltd.     

Changes in the thermal structure of moderate to large sized lakes in response to changes in air temperature

Interannual variability in the thermal structure of lakes is driven by interannual differences in meteorological conditions. Dynamic or mechanistic models and empirical or statistical methods have been used to integrate the physical processes in lakes enabling the response of the thermal structure to changes in air temperature to be determined. Water temperature records for Lake Mendota, WI., are possibly the most extensive for any dimictic lake in the world and allowed both approaches to be used. Results from both techniques suggest the mixed layer temperature increases with increasing air temperature. Results from the empirical approach suggested epilimnion temperatures increase 0.5 to 1.0°C per 1.0°C increase in air temperature compared to 0.4 to 0.85°C estimated from a dynamical model (DYRESM). Increased air temperatures are related to significant warming in deep water temperatures in the absence of stratification; however, mid summer hypolimnion temperatures are expected to change very little or increase only slightly in response to climatic warming. Both approaches suggest increases in air temperatures increase the length of summer stratification; results from the dynamic model suggest an increase of approximately 5 days per 1°C increase in air temperature. Longer stratification is reflected in shallower late summer thermocline depths. With these quantitative relationships and forecast increases in air temperature for the 2 × CO₂ climatic scenario (Greenhouse Effect) from three General Circulation Models, projections are made describing the changes in the future mean thermal structure of moderate to large sized lakes.     

Changing river temperatures in northern Germany: trends and drivers of change

Climate change is one of the main drivers of river warming worldwide. However, the response of river temperature to climate change differs with the hydrology and landscape properties, making it difficult to generalize the strength and the direction, of river temperature trends across large spatial scales and various river types. Additionally, there is a lack of long‐term and large‐scale trend studies in Europe as well as globally. In this study, we investigated the long‐term (25 years; 132 sites) and the short‐term (10 years; 475 sites) river temperature trends, patterns and underlying drivers within the period 1985–2010 in seven river basins of Germany. The majority of the sites underwent significant river warming during 1985–2010 (mean warming trend: 0.03 °C year^?1, SE = 0.003), with a faster warming observed during individual decades (1985–1995 and 2000–2010) within this period. Seasonal analyses showed that, while rivers warmed in all seasons, the fastest warming had occurred during summer. Among all the considered hydro‐climatological variables, air temperature change, which is a response to climate forcing, was the main driver of river temperature change because it had the strongest correlation with river temperature, irrespective of the period. Hydrological variables, such as average flow and baseflow, had a considerable influence on river temperature variability rather than on the overall trend direction. However, decreasing flow probably assisted in a faster river temperature increase in summer and in rivers in NE basins (such as the Elbe basin). The North Atlantic Oscillation Index had a greater significant influence on the winter river temperature variability than on the overall variability. Landscape and basin variables, such as altitude, ecoregion and catchment area, induced spatially variable river temperature trends via affecting the thermal sensitivity of rivers, with the rivers in large catchments and in lowland areas being most sensitive. Copyright © 2016 John Wiley & Sons, Ltd.     

近40年洞庭湖区内湖水面面积变化及其驱动因素

选择1979-2016年间多时期、多类型、多光谱遥感数据,分析评价洞庭湖区内湖近40年的面积变化.结果表明,最近40年洞庭湖区内湖面积保持相对稳定,丰水期间呈上升趋势,枯水期间波动下降,2016年内湖总面积比1980s初减少3.94%.随着湖泊面积增加,湖泊水面面积变化的比例和幅度逐渐减小,大型湖泊（>10 km²）和中型湖泊（5~10 km²）面积相对稳定,小型内湖（<5 km²）面积变化尤为剧烈.内湖水面面积主要受降雨、蒸发等气候因素和生产生活取水、防洪排涝和退田还湖等人为活动调控.1980-2000年和2001-2015年两个时期,洞庭湖区多年平均降雨量呈现不同程度的下降趋势,多年平均蒸发量明显上升.三峡工程运行后,三口分流衰减,但水资源需求量不断增长,退田还湖和留蓄雨洪作为水资源使得丰水期间内湖水面面积增长,气候变化和水资源开发利用导致枯水期水面面积趋于减少.有必要加强洞庭湖区内湖的研究和保护,适当退田还湖提高湖泊率,优化三口水系格局,实施河湖水系连通工程,缓解洞庭湖区季节性水资源紧张问题.     

EFFECTS OF CLIMATE CHANGE ON INLAND WATERS OF THE PACIFIC COASTAL MOUNTAINS AND WESTERN GREAT BASIN OF NORTH AMERICA

The region designated as the Pacific Coastal Mountains and Western Great Basin extends from southern Alaska (64°N) to southern California (34°N) and ranges in altitude from sea level to 6200 m. Orographic effects combine with moisture-laden frontal systems originating in the Pacific Ocean to produce areas of very high precipitation on western slopes and dry basins of internal drainage on eastern flanks of the mountains. In the southern half of the region most of the runoff occurs during winter or spring, while in the northern part most occurs in summer, especially in glaciated basins. Analyses of long-term climatic and hydrological records, combined with palaeoclimatic reconstructions and simulations of future climates, are used as the basis for likely scenarios of climatic variations. The predicted hydrological response in northern California to a climate with doubled CO₂ and higher temperatures is a decrease in the amount of precipitation falling as snow, and substantially increased runoff during winter and less in late spring and summer. One consequence of the predicted earlier runoff is higher salinity in summer and autumn in San Francisco Bay. In saline lakes, the incidence of meromixis and the associated reduction in nutrient supply and algal abundance is expected to vary significantly as runoff fluctuates. In subalpine lakes, global warming will probably will lead to increased productivity. Lacustrine productivity can also be altered by changes in wind regimes, drought-enhanced forest fires and maximal or minimal snowpacks associated with atmospheric anomalies such as El Niño–Southern Oscillation (ENSO) events. Reduced stream temperature from increased contributions of glacial meltwater and decreased channel stability from changed runoff patterns and altered sediment loads has the potential to reduce the diversity of zoobenthic communities in predominately glacier-fed rivers. Climatic warming is likely to result in reduced growth and survival of sockeye salmon in freshwater, which would, in turn, increase marine mortality. Further research activities should include expanded studies at high elevations and of glacier mass balances and glacial runoff, applications of remote sensing to monitor changes, further refinement of regional climatic models to improve forecasts of future conditions and continued analyses of long-term physical, chemical and biological data to help understand responses to future climates. © 1997 John Wiley & Sons, Ltd.     

Longitudinal and temporal thermal patterns of the French Rh?ne River using Landsat ETM+ thermal infrared images

Understanding the thermal regime of rivers is a key issue for predicting ecosystem change in the context of global warming. However, water temperature is not only influenced by air temperature. To better highlight relative contribution of factors controlling water temperature, we used satellite thermal infrared (TIR) images from Landsat ETM+ to investigate longitudinal and temporal variations in thermal patterns of the French Rh?ne River. Because satellite TIR remote sensing is limited to large rivers, we used an automated water extraction technique to remove pixels contaminated by terrestrial surfaces. We calculated water surface temperatures of the 500?km long reach for 83 dates between 1999 and 2009. The average accuracy and uncertainty of our data, ±1.1 and ±0.4°C for reaches with more than 3?pixels across and ±1.4 and ±0.5°C for reaches with one to 3?pixels across, are comparable to other satellite TIR studies of rivers. Our results confirmed previous studies on the thermal impacts of tributaries and nuclear power plants on the Rh?ne, providing an understanding of their seasonal pattern and their longitudinal impact. We showed temperature differences of 0?C2°C within the largest hydroelectric bypass facilities between the bypass section and the canal, with Montélimar and Caderousse showing the most pronounced differences. Discussion points concern the potential impacts of tributaries and nuclear power plants on the spatio-temporal thermal patterns, as well as the factors responsible for thermal differences in the bypass facilities: length and minimum flow of the bypass section, and tributaries coming into this reach.     

The use of mass balance investigations in the study of the biogeochemical cycle of sulfur

The use of mass balances in the investigation of the biogeochemical cycle of sulfur is reviewed for three systems: 1) upland catchments, 2) wetlands, and 3) lakes. In upland catchments, the major inputs of sulfur are via wet and dry atmospheric deposition, whereas outputs or losses occur primarily through volatilization and/or runoff. In addition, sulfur may be stored in vegetation and in the forest floor. In wetlands (particularly peatlands), a large proportion of the sulfur inputs are derived from surface and groundwater originating in the upland system. Because of the fluctuating water table in wetlands, they can act as a source or sink for sulfate, depending on the redox conditions. Wetlands, therefore, can significantly affect input-output budgets for lakes. In most lakes, only a small portion of the sulfate input is retained, (i.e. not lost from the lake via outflow), indicating that there is an excess of sulfate relative to biological needs. Seepage lakes are exceptions to this generalization. Although the reactivity of the sulfate input to many lakes is low, sulfate levels, especially in regions receiving substantial atmospheric sulfur deposition, are high enough that the portion reduced results in substantial in-lake alkalinity production; in fact, in many cases, alkalinity production from sulfate reduction is greater than that resulting from not only other in-lake processes but from external sources (the catchment) as well. The importance of mass balance investigations in elucidating the biogeochemical cycling of sulfur is stressed and the need for additional studies on a whole-system basis stressed. © 1997 John Wiley & Sons, Ltd.     

Delineating extent and magnitude of river flooding to lakes across a northern delta using water isotope tracers

Hydrological monitoring in complex, dynamic northern floodplain landscapes is challenging, but increasingly important as a consequence of multiple stressors. The Peace-Athabasca Delta in northern Alberta, Canada, is a Ramsar Wetland of International Importance reliant on episodic river ice-jam flood events to recharge abundant perched lakes and wetlands. Improved and systematic monitoring of landscape-scale hydrological connectivity among freshwater ecosystems (rivers, channels, wetlands, and lakes) is needed to guide stewardship decisions in the face of climate change and upstream industrial development. Here, we use water isotope compositions, supplemented by measurements of specific conductivity and field observations, from 68 lakes and 9 river sites in May 2018 to delineate the extent and magnitude of spring ice-jam induced flooding along the Peace and Athabasca rivers. Lake-specific estimates of input water isotope composition (δ_I) were modelled after accounting for influence of evaporative isotopic enrichment. Then, using the distinct isotopic signature of input water sources, we develop a set of binary mixing models and estimate the proportion of input to flooded lakes attributable to river floodwater and precipitation (snow or rain). This approach allowed identification of areas and magnitude of flooding that were not captured by other methods, including direct observations from flyovers, and to demarcate flow pathways in the delta. We demonstrate water isotope tracers as an efficient and effective monitoring tool for delineating spatial extent and magnitude of an important hydrological process and elucidating connectivity in the Peace-Athabasca Delta, an approach that can be readily adopted at other floodplain landscapes.     

ASSESSMENT OF CLIMATE CHANGE AND FRESHWATER ECOSYSTEMS OF THE ROCKY MOUNTAINS,USA AND CANADA

The Rocky Mountains in the USA and Canada encompass the interior cordillera of western North America, from the southern Yukon to northern New Mexico. Annual weather patterns are cold in winter and mild in summer. Precipitation has high seasonal and interannual variation and may differ by an order of magnitude between geographically close locales, depending on slope, aspect and local climatic and orographic conditions. The region's hydrology is characterized by the accumulation of winter snow, spring snowmelt and autumnal baseflows. During the 2–3-month ‘spring runoff’ period, rivers frequently discharge > 70% of their annual water budget and have instantaneous discharges 10–100 times mean low flow. Complex weather patterns characterized by high spatial and temporal variability make predictions of future conditions tenuous. However, general patterns are identifiable; northern and western portions of the region are dominated by maritime weather patterns from the North Pacific, central areas and eastern slopes are dominated by continental air masses and southern portions receive seasonally variable atmospheric circulation from the Pacific and the Gulf of Mexico. Significant interannual variations occur in these general patterns, possibly related to ENSO (El Niño–Southern Oscillation) forcing. Changes in precipitation and temperature regimes or patterns have significant potential effects on the distribution and abundance of plants and animals. For example, elevation of the timber-line is principally a function of temperature. Palaeolimnological investigations have shown significant shifts in phyto- and zoo-plankton populations as alpine lakes shift between being above or below the timber-line. Likewise, streamside vegetation has a significant effect on stream ecosystem structure and function. Changes in stream temperature regimes result in significant changes in community composition as a consequence of bioenergetic factors. Stenothermic species could be extirpated as appropriate thermal criteria disappear. Warming temperatures may geographically isolate cold water stream fishes in increasingly confined headwaters. The heat budgets of large lakes may be affected resulting in a change of state between dimictic and warm monomictic character. Uncertainties associated with prediction are increased by the planting of fish in historically fishless, high mountain lakes and the introduction of non-native species of fishes and invertebrates into often previously simple food-webs of large valley bottom lakes and streams. Many of the streams and rivers suffer from the anthropogenic effects of abstraction and regulation. Likewise, many of the large lakes receive nutrient loads from a growing human population. We concluded that: (1) regional climate models are required to resolve adequately the complexities of the high gradient landscapes; (2) extensive wilderness preserves and national park lands, so prevalent in the Rocky Mountain Region, provide sensitive areas for differentiation of anthropogenic effects from climate effects; and (3) future research should encompass both short-term intensive studies and long-term monitoring studies developed within comprehensive experimental arrays of streams and lakes specifically designed to address the issue of anthropogenic versus climatic effects. © 1997 John Wiley & Sons, Ltd.     

全球变化下青藏高原湖泊在地表水循环中的作用

青藏高原是地球上最重要的高海拔地区之一,对全球变化具有敏感响应.青藏高原作为"亚洲水塔",其地表水资源及其变化对高原本身及周边地区的经济社会发展具有重要的影响.然而,在气候变暖的情况下,构成高原地表水资源的各个组分,如冰川、湖泊、河流、降水等水体的相变及其转化却鲜为人知.湖泊是青藏高原地表水体相变和水循环的关键环节.湖泊面积、水位和水量对西风和印度季风的降水变化非常敏感,但湖泊面积和水量变化在不同区域和时段的响应也不尽相同.湖泊水温对气候变暖具有明显响应,湖泊水温和水下温跃层深度的变化能够对水—气的热量交换具有明显影响,从而影响了区域蒸发和降水等水循环过程.由于湖泊水量增加,高原中部色林错地区湖泊盐度自1970s以来普遍下降.根据60多个湖泊实地监测建立的遥感反演模型研究发现,2000—2019年湖泊透明度普遍升高.对不同补给类型的大湖水量平衡监测发现,影响湖泊变化的气象和水文要素具有较大差异.在目前的暖湿气候条件下,青藏高原的湖泊将会持续扩张.为了深入认识湖泊变化在青藏高原区域水循环和气候变化中的作用,需要全面了解湖泊水量赋存及连续的时间序列变化,需要深入了解湖泊理化参数变化及对湖泊大气之间热量交换的影响,需要更多来自大湖流域的综合连续观测数据.     

A note on the spatial structure of the covariability of observed Northern Hemisphere surface air temperatures

The spatial structure of covariability in the surface air temperature records of the last 109 years is examined using factor analysis. From the results it is indicated that the Northern Hemisphere can be objectively divided into 15 regions with each region having a similar covariance structure of surface air temperatures. From such a division it is noted that warming is evident over 11 out of the 15 regions. The strongest warming is not, however, confined to specific latitudinal belts, for example, the tropics or the arctic, nor is the strongest warming confined to a specific continent. As a consequence of the coarse spatial dsitribution of available data, results should be considered preliminary.     

Landscape and climate change threats to wetlands of North and Central America

North and Central America has a combined total of 2.5 million km² of wetlands, with 51 % in Canada, 46 % in the USA, and the remainder in subtropical and tropical Mexico and Central America. Loss rates are well known for the conterminous USA and for parts of Canada but poorly understood for Mexico and Central America. Wetlands of North America continue to be threatened due to drainage for agriculture and urban development, extreme coastal and river management, water pollution from upstream watersheds, peat mining, waterfowl management, and more recently climate change. Human use of wetlands in this region are many, including receiving ecosystem services such as water purification, flood regulation, climate regulation, and direct provisioning benefits for many cultures living in and among wetlands, especially in the Louisiana Delta and in Mexico and Central America. Climate change affects will cause wetland impacts on coastal wetlands due to sea level rise and on inland wetlands due to changes in precipitation, air temperature, and river discharges. Wetlands, in turn, have a major role in the storage of carbon in boreal regions of Canada and with carbon sequestration in temperate and tropical wetlands of the Americas.