ALBERT

Notes: Multiple calibration targets were used to calibrate a two-dimensional finite-difference model of a ground water lake system. The calibration targets included (1) steady-state head data, (2) transient head data, (3) head gradients, and (4) flowpath information. Because calibration was sensitive to the ratio of horizontal to vertical hydraulic conductivity, four models, each with different assumptions about anisotropy, were developed. All four models produced acceptable calibration to either heads or flowpath, but only one model was well calibrated to all targets. In that model, stratification of the upper aquifer was represented by introducing several dipping layers of low permeability. This allowed the use of a small ratio of horizontal to vertical anisotropy for individual layers but produced a large effective anisotropy for the upper aquifer as a whole.

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: SUMMARY. 1. A network of seventeen long-term ecological research sites funded by the National Science Foundation (NSF) and spanning sites in arctic to tropical climates, low to high altitudes and wet to dry environments, provides evidence for the increasing popularity of sustained ecological research in the U.S.A.2. The sites function as regional or national facilities for long-term research as well as for comparative and process studies by investigators from the operating institutions and by visiting researchers.3. The aquatic habitats include a variety of lakes ponds, wetlands and a playa; montane, woodland, tundra and prairie streams; as well as salt marsh, estuary, ocean beach and inshore oceanic sites.

Notes: Abstract We studied the factors causing spatial and temporal patterning of interstitial water chemistry in Crystal Bog, a 7 ha northern Wisconsin kettle-hole peatland. Over the course of the snow-free season Crystal Bog exhibited spatial and temporal patterns in chemistry, especially hydrogen-ion, dissolved organic carbon, and specific conductance. The peatland contains a 0.5 ha pond that has water more dilute than the interstitial water of the surrounding peatland. The direction of groundwater flow between the lake and the peatland was seasonally dependent. In the spring and early summer, flow was from the lake into the peatland, especially on the eastern side of the lake. This flow resulted in a plume of relatively dilute surface interstitial water in the peatland. In mid and late summer direction of groundwater flow was from the peatland into the lake and the dilute plume was reduced in areal extent. By fall the direction of water flow was again from the lake to the peatland. The spatial and temporal heterogeneity in water chemistry produced by the seasonal variation in the direction of horizontal water flow was substantial. Minimum and maximum observed concentrations of dissolved organic carbon (DOC) in the interstitial water of the peatland, for example, differed by more than a factor of three, and pH ranged between 3.8 and 5.0. Correlations of DOC with anion deficit and hydrogen ion concentration and experiments of photo-oxidation of water samples showed that organic acids were the primary cause of acidity in the peatland. Specific conductance was highly correlated with DOC, probably because of DOC's correlation with the very conductive hydrogen ion. In Crystal Bog it was possible to use the relatively simple measure of specific conductance to estimate the temporal and spatial distribution of the more difficult to determine DOC.