Air temperatures are sometimes used as easy substitutes for stream temperatures. To examine the errors associated with this substitution, linear relationships between 39 Minnesota stream water temperature records and associated air temperature records were analyzed. From the lumped data set (38,082 daily data pairs), equations were derived for daily, weekly, monthly, and annual mean temperatures. Standard deviations between all measured and predicted water temperatures were 3.5°C (daily), 2.6°C (weekly), 1.9°C (monthly), and 1.3°C (annual). Separate analyses for each stream gaging station gave substantially lower standard deviations. Weather monitoring stations were, on average, 37.5 km from the stream. The measured water temperatures follow the annual air temperature cycle closely. No time lags were taken into account, and periods of ice cover were excluded from the analysis. If atmospheric CO2 doubles in the future, air temperatures in Minnesota are projected (CCC GCM) to rise by 4.3°C in the warm season (April‐October). This would translate into an average 4.1°C stream temperature rise, provided that stream shading would remain unaltered.

Daily water temperature, dissolved oxygen (DO) profiles, and ice and snow covers (where applicable) were simulated for 27 types of small lakes (up to 10 km 2 surface area) at 209 locations in the contiguous United States under past climate (observed from 1961 to 1979) and for projected doubled atmospheric carbon dioxide (23CO 2 ) climate conditions. A verified, process-oriented, dynamic, and one-dimensional year-round lake water quality model (MINLAKE96) was applied to simulate water temperature and DO profiles continuously in daily time steps over a 19-yr simulation period. This regional lake model has no geographic constraints on the model's physical and chemical processes, but the climate forcing is a function of geographic location. Model calibration parameters and initial conditions are correlated to lake geometry, trophic state, and location. The 23CO 2 climate scenario is projected to increase lake surface temperatures by up to 5.2uC when the climate scenario projects an increase of mean annual air temperature up to 6.7uC. The 23CO 2 climate scenario is projected to increase the duration of seasonal summer stratification by up to 67 d, to shorten ice cover by up to 90 d, and to reduce the maximum ice thickness by up to 0.44 m. Under a 23CO 2 climate scenario, lake anoxia during the period of ice cover is projected to shorten so that fish winterkill can be avoided, but the periods of hypolimnetic summer anoxia are projected to lengthen. These projected changes will have many significant effects on ecological conditions and aquatic habitats in lakes in the contiguous United States.An increase of atmospheric CO 2 and other greenhouse gases causes climate warming, which alters water temperature and dissolved oxygen (DO) in lake waters (Blumberg and Di Toro 1990). These water quality changes have a profound effect on fish habitat (Regier et al. 1990;Magnuson et al. 1990). Water temperatures and DO concentrations in Minnesota lakes under several projected climate scenarios were previously simulated for the open water season . The following is a summary and analysis of simulations of long-term average water temperature, DO conditions, and ice and snow cover characteristics in small lakes (up to 10 km 2 surface area) over the contiguous United States (U.S.) under past and projected future climate scenarios. This study is an expansion of the previous investigations that focused on Minnesota lakes only to small lakes in 209 locations (dots in Fig. 1) in 48 states over the contiguous U.S. Longitude of the locations (or weather stations) ranged from 68u19W (Caribou, Maine) to 124u339W (Quillayute, Washington) and latitude from 25u489N (Miami, Florida) to 48u349N (International Falls, Minnesota). Elevations of the weather stations above mean sea level ranged from 2 m in coastal areas (Miami, Florida) to 2135 m in mountainous areas (Tucson, Arizona). It was assumed that the lakes simulated have the same elevations as the weather stations used. In this study, daily water temperature and DO profiles are simulated...

Water temperatures and dissolved oxygen (DO) concentrations in lakes are related to climate. Temperature and DO in 27 lake classes (3 depth classes x 3 surface area classes x 3 trophic states) were simulated by numerical models with daily weather data input. The weather data used are from the 25-yr period . The lakes and the weather are representative of the north-central U.S. Daily profiles ofwater temperature and DO concentrations were computed and several temperature and DO characteristics extracted from this information base. Temperature and minimum oxygen requirements for good growth of cold, cool, and warm water fish were then applied to determine the length of the good-growth periods and the relative lake volumes available for good growth. All characteristics are presented in graphical form using lake surface area, maximum lake depth, and Secchi depth as independent variables. The surface area and maximum depth were combined in a lake geometry ratio which is a relative measure of the susceptibility of a lake to stratification; Secchi depth was retained as a measure of lake transparency and trophic state. To determine an effect of latitude, we investigated a southern and northern region separately. The effect of climate change due to a projected doubling of atmospheric CO, was investigated by applying the output from the GISS 2 x CO, global circulation model to the lake models.The following is an analysis of recently completed simulations that project long-term average water temperature and dissolved oxygen conditions in lakes of the northcentral U.S. and their suitability for fish habitat. Minnesota was selected for this study because it has many valuable lakes, an extensive lake database, and is located in the center of the continent. It is also at a latitude where climate change may have the greatest impact on aquatic ecosystems. Baseline water-quality simulations were made with historical records of meteorological parameters known to influence the temperature and the dissolved oxygen (DO) of north-temperate lakes. We used processoriented, deterministic numerical modeling which, when sufficiently calibrated and validated, offers the possibility AcknowledgmentsThe investigation described herein was conducted for the U.S. Environmental Protection Agency/OPPE in cooperation with the Environmental Research Laboratory, Duluth, as a part of a project on climate change effects on fisheries. The Minnesota Supercomputer Institute, University of Minnesota, provided a resource grant and access to its CRAY2 supercomputer.Eville Gorham and Joseph Shapiro provided valuable suggestions that considerably improved the manuscript.

A B S T R A C T The African great lakes are of utmost importance for the local economy (fishing), as well as being essential to the survival of the local people. During the past decades, these lakes experienced fast changes in ecosystem structure and functioning, and their future evolution is a major concern. In this study, for the first time a set of onedimensional lake models are evaluated for Lake Kivu (2.288S; 28.988E), East Africa. The unique limnology of this meromictic lake, with the importance of salinity and subsurface springs in a tropical high-altitude climate, presents a worthy challenge to the seven models involved in the Lake Model Intercomparison Project (LakeMIP). Meteorological observations from two automatic weather stations are used to drive the models, whereas a unique dataset, containing over 150 temperature profiles recorded since 2002, is used to assess the model's performance. Simulations are performed over the freshwater layer only (60 m) and over the average lake depth (240 m), since salinity increases with depth below 60 m in Lake Kivu and some lake models do not account for the influence of salinity upon lake stratification. All models are able to reproduce the mixing seasonality in Lake Kivu, as well as the magnitude and seasonal cycle of the lake enthalpy change. Differences between the models can be ascribed to variations in the treatment of the radiative forcing and the computation of the turbulent heat fluxes. Fluctuations in wind velocity and solar radiation explain inter-annual variability of observed water column temperatures. The good agreement between the deep simulations and the observed meromictic stratification also shows that a subset of models is able to account for the salinity-and geothermalinduced effects upon deep-water stratification. Finally, based on the strengths and weaknesses discerned in this study, an informed choice of a one-dimensional lake model for a given research purpose becomes possible.

Results of a lake model intercomparison study conducted within the framework of Lake Model Intercomparison Project are presented. The investigated lake was Großer Kossenblatter See (Germany) as a representative of shallow, (2 m mean depth) turbid midlatitude lakes. Meteorological measurements, including turbulent fluxes and water temperature, were carried out by the Lindenberg Meteorological Observatory of the German Meteorological Service (Deutscher Wetterdienst, DWD). Eight lake models of different complexity were run, forced by identical meteorological variables and model parameters unified as far as possible given different formulations of processes. All models generally captured diurnal and seasonal variability of lake surface temperature reasonably well. However, some models were incapable of realistically reproducing temperature stratification in summer. Total heat turbulent fluxes, computed by the surface flux schemes of the compared lake models, deviated on average from those measured by eddy covariance by 17–28 W m⁻². There are a number of possible reasons for these deviations, and the conclusion is drawn that underestimation of real fluxes by the eddy covariance technique is the most probable reason. It is supported by the fact that the eddy covariance fluxes do not allow to close the heat balance of the water column, the residual for the whole period considered being ≈–28 W m⁻². The effect of heat flux to bottom sediments can become significant for bottom temperatures. It also has profound influence on the surface temperatures in autumn due to convective mixing but not in summer when the lake stratification is stable. Thus, neglecting sediments shifts the summer–autumn temperature difference in models lacking explicit treatment of sediments considerably. As a practical recommendation based on results of the present study, we also infer that in order to realistically represent lakes in numerical weather prediction and climate models, it is advisable to use depth-resolving turbulence models (or equivalent) in favor of models with a completely mixed temperature profile