ALBERT

Notes: 1. Within a region with common climatic conditions, lake thermal variables should exhibit coherent variability patterns to the extent to which they are not influenced by lake specific features such as morphometry and water clarity. We tested the degree of temporal coherence in interannual variability for climatic variables (air temperature and solar radiation) among four lake districts in the Upper Great Lakes Region. We also tested the degree of coherence of lake thermal variables (near-surface temperature, eplimnetic temperature, hypolimnetic temperature and thermocline depth) for lakes within these districts.2. Our four lake districts included the Experimental Lakes Area in north-western Ontario, the Dorset Research Centre area north of Toronto, Ontario, the Northern Highland Lake District in northern Wisconsin, and the Yahara Lakes near Madison in southern Wisconsin. Seventeen lakes were analyzed for lake thermal variables dependent on stratification. Another five lakes were added for the analysis of near-surface temperature.3. The analysis tested whether for monthly and summer means, the climate (air temperature and solar radiation) across the four lake districts was coherent interannually and whether variables which measure the thermal structure of the lakes were coherent interannually among lakes within each lake district and across the four lake districts.4. Temporal coherence was estimated by the correlation between lake districts for meteorological variables and between lake pairs for lake thermal variables. Mean coherence and the percentage of correlations exceeding the 5% significance level were derived both within and between lake districts for lake thermal variables.5. Across the four lake districts, summer mean air temperature was highly coherent while summer solar radiation was less coherent. Approximately 60–80% of the interannual variation in mean summer air temperature at a site occurred across the entire region. Less than 45% of the variation in solar radiation occurred across sites.6. Epilimnetic temperature and the near-surface temperature were highly coherent both within and between lake districts. The coherence of thermocline depth within and between lake districts was weaker. Hypolimnetic temperature was not coherent between lake districts for most lake pairs. It was coherent among lakes within some lake districts.7. The influences of local weather and differences among lakes in water clarity are discussed in the context of differences in levels of coherence among lake thermal variables and among lake pairs for a given variable.

Notes: 1. Within a lake district of relatively homogeneous geomorphology, the responses of lakes to climate are influenced by the complexity of the hydrogeologic setting, position in the landscape, and lake-specific biological and physical features. We examined lake chemical responses to drought in surface water- and groundwater-dominated districts to address two general questions. (1) Are spatial patterns in chemical dynamics among lakes uniform and synchronous within a lake district, suggesting broad geomorphic controls; variable in a spatially explicit pattern, with synchrony related to landscape position, suggesting hydrologic flowpath controls; or spatially unstructured and asynchronous, suggesting overriding control by lake-specific factors? (2) Are lake responses to drought a simple function of precipitation quantity or are they dictated by more complex interactions among climate, unique lake features, and hydrologic setting?2. Annual open-water means for epilimnetic concentrations of chloride, calcium, sulfate, ANC, DOC, total nitrogen, silica, total phosphorus, and chlorophyll a measured between 1982 and 1995 were assembled for lakes in the Red Lake and ELA districts of north-western Ontario, the Muskoka – Dorset district in south-central Ontario, and the Northern Highland district of Wisconsin. Within each district, we compared responses of lakes classified by landscape position into highland or lowland, depending on relative location within the local to regional hydrologic flow system. Synchrony, defined as a measure of the similarity in inter-annual dynamics among lakes within a district, was quantified as the Pearson product-moment correlation (r) between two lakes with observations paired by year. To determine if solute concentrations were directly related to interannual variations in precipitation quantity, we used regression analysis to fit district-wide slopes describing the relationship between each chemical variable and annual (June to May) and October to May (Oct–May) precipitation.3. Among lakes in each of the three Ontario districts, the pattern of chemical response to interannual shifts in precipitation was spatially uniform. In these surface water- dominated districts, solute concentrations were generally a simple function of precipitation. Conservative solutes, like calcium and chloride, tended to be more synchronous and were negatively related to precipitation. Solutes such as silica, total phosphorus, and chlorophyll a, which are influenced by in-lake processes, were less synchronous and relationships with precipitation tended to be positive or absent.4. In the groundwater-dominated Northern Highland lakes of Wisconsin, we observed spatial structure in drought response, with lowland lakes more synchronous than highland lakes. However, there was no evidence for a direct relationship between any solute and precipitation. Instead, increases in the concentration of the conservative ion calcium during drought were not followed by a symmetrical return to pre-drought conditions when precipitation returned to normal or above-average values.5. For calcium, time lags in recovery from drought appeared related to hydrologic features in a complex way. In the highland Crystal Lake, calcium concentrations tracked lake stage inversely, with a return to pre-drought concentrations and lake stage five years after the drought. This pattern suggests strong evaporative controls. In contrast, after five years of normal precipitation, calcium in the lowland Sparkling Lake had not returned to pre-drought conditions despite a rebound in lake stage. This result suggests that calcium concentrations in lowland lakes were controlled more by regional groundwater flowpaths, which track climatic signals more slowly.6. Temporal dynamics driven by climate were most similar among lakes in districts that have a relatively simple hydrology, such as ELA. Where hydrologic setting was more complex, as in the groundwater-dominated Northern Highland of Wisconsin, the expression of climate signals in lakes showed lags and spatial patterns related to landscape position. In general, we expect that landscape and lake-specific factors become increasingly important in lake districts with more heterogeneous hydrogeology, topography or land use. These strong chemical responses to climate need to be considered when interpreting the responses of lakes to other regional disturbances.

Notes: Abstract The African Great Lakes are important sources of fishes and water for domestic use, are used as avenues of transport, and receive agricultural, domestic and industrial effluents and atmospheric residues. Some of these lakes have speciose fish faunas of great interest to science. The catchment areas of some of the lakes are highly populated and user conflicts have increased the demands on the lakes' resources. There have been drastic reductions in fish stocks in most of the lakes due to overfishing. Introductions of new fish species, though followed by increases in fish catches, have been accompanied by a decline and in some cases extinction of native fish species. Some of the lakes have been invaded by the water hyacinth, Eichhornia crassipes. Agricultural activities, deforestation and devegetation of the catchment areas have increased siltation, and led to loss of suitable habitats and biodiversity. There are increased nutrient inputs from agriculture, sewage and industrial discharges and combustion processes which can cause eutrophication. There are also increased threats of toxic pollution from industrial waste discharge, mining, pesticides, and oil residues and spills. Climatic changes may also affect thermal stability of the lakes. These factors threaten availability of dietary protein, clean water and biodiversity. National and international efforts are required to manage the fisheries, guide the introduction of exotics, conserve biodiversity, control the water hyacinth, control eutrophication, reduce input of contaminants and manage climate change.

Notes: Abstract A simple heuristic theory based on conservation of matter and describing the fate of autotrophic production in pelagic ecosystems was developed to assist in conceptualizing how zooplankton affect sedimentation processes in lakes and oceans. The theory predicted that effects of zooplankton on the fraction of autotrophic incorporation that sediments (the “export ratio”) will be a function of factors related both to zooplankton digestion and egestion and to tendencies of particulate matter to sediment directly prior to mineralization. As a result, effects of zooplankton grazing on the export ratio were predicted to be site-dependent, a function of physical conditions and zooplankton communities characterizing an ecosystem. The theory was tested by monitoring autotrophic production of C, N, and P, sedimentation of C, N, and P, and zooplankton biomass in two lakes of the Canadian shield characterized by contrasting morphometry and food-web structure. In Lake 110, a small elongate lake protected from wind exposure, export ratios of C, N, and P declined strongly with zooplankton biomass. In contrast, in L240, a larger lake with considerably greater wind exposure, export ratios increased with zooplankton biomass. These results were consistent with predictions of our theory that effects of zooplankton on sedimentation processes will depend on the tendency of particulate matter to directly sediment relative to the tendency of egested materials to sediment. However, no significant differences in relationships between export ratios for C, N, and P and zooplankton biomass were found.