Freshwater clams (Anodonta grandis simpsoniana) exposed to 51-55 µg · L-1 of dissolved microcystin-LR (MC-LR) in the laboratory for 3 days did not accumulate MC-LR equivalents (MC-LReq). However, clams placed in three eutrophic lakes with phytoplankton containing MC-LR (concentrations from below detection to 8.3 µg · L-1 cellular toxin) for 12-28 days accumulated the toxin (24 ± 7 to 527 ± 330 ng · g-1 MC-LReq; mean ± SE). The relative MC-LReq concentrations in clams reflected MC-LR concentrations in lake phytoplankton, but individual variation was high. In individual clams exposed for 24 days, the average MC-LReq concentration was usually greater in the visceral mass than in gills and muscle, but average toxin concentrations in the three tissues were similar (587, 310, and 364 ng · g dry weight-1). In clams removed from the lake and placed in toxin-free water, MC-LReq concentrations in tissues declined rapidly for 6 days (by 69-88%) but remained relatively stable for the remaining 15 days. Analysis of clam tissues appears to be a more sensitive MC-LR indicator than analysis of phytoplankton. Accumulation of potent cyanobacterial toxins by this clam warrants further study as many are consumed by muskrats (Ondatra zibethicus), which in turn are consumed by terrestrial predators.

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This paper presents a review of two models (i.e., arching and lateral squeezing) developed for predicting earth pressures in soil-bentonite (SB) cutoff walls. The assumptions of these existing models are discussed, a modified lateral squeezing (MLS) model is presented, and all three models are compared based on predicted horizontal stresses for representative field conditions. Each model predicts that the stress distribution within a SB cutoff wall may be considerably lower than a geostatic distribution, particularly at depth. The arching model yields the lowest stress distribution but may underestimate the true distribution due to the assumption of rigid trench sidewalls. The MLS model (1) allows sidewall deformation and (2) accounts for the stress-dependent nature of SB backfill compressibility. The study also finds that additional model development is needed to characterize the stress state of a SB cutoff wall in three dimensions.

Eleven prairie saline (conductivity 1.8-58.8 mS cm-') lakes were examined over the 1994 growing season to determine what salinity-related factor or factors were responsible for controlling phytoplankton standing crops. The study lakes were characterized by high total P (0.15-24.2 mg liter-'), total N (3.75-12.35 mg liter-'), total Fe (55-2,800 p,g liter-'), dissolved organic C (40-195 mg liter-l), pH and alkalinity, but comparatively low (usually cl00 kg liter-') dissolved inorganic N. Chlorophyll a (Chl a) concentrations in all but the two least saline lakes were relatively low (20 pg liter-'), up to three orders of magnitude below those predicted by freshwater P-based models. High alkaline phosphatase activities (APA) and rapid '2P0, (orthophosphate) uptake indicated that the two least saline lakes were P limited; these lakes had seston deficient in P, N, and protein. APA and "2P0, uptake were below detection in the more saline lakes (conductivity >3 mS cm-'), indicating P sufficiency; seston from these lakes was deficient in N but not protein. Nitrogen-fixing cyanophytes were important only in one of the lakes examined. Nutrient addition bioassays indicated that phytoplankton biomass was not limited exclusively by inorganic N availability, nor by a combination of MO and N. For water from all but one of the P-sufficient lakes, addition of Fe to bioassays resulted in a remarkable increase in Chl a concentrations. Addition of Fe and MO had the same effect as that of Fe alone, while the most saline lake appeared to be limited by one or more additional trace elements (but not MO). Reducing the alkalinity of the bioassay water stimulated growth in the same manner as the Fe additions, suggesting that the bioavailability of the (largely particulate) Fe already present was severely restricted by lakewater alkalinity. Some component of lake-water alkalinity (which increased with conductivity in these lakes) appears to be the key factor limiting Fe bioavailability and restricting phytoplankton standing crops in the higher salinity lakes.

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Shallow, closed-basin saline lakes found in semiarid areas of Canada tend to be sensitive to changes in precipitation : evaporation ratios. Historical climatic information indicates that the area is becoming increasingly arid and this trend is expected to continue under current climate-change scenarios. The water chemistries and phytoplankton of six Alberta saline (> 1 g liter-' total dissolved solids) lakes were studied over a 12-yr arid period to evaluate potential chemical and biological effects of climate-induced increases in brine conductivity. Major ion concentrations, relative proportions of major ions, chlorophyll a concentrations, and the relative importance of different phytoplankton phyla were evaluated with respect to the increasing conductivity profiles of the lakes and with respect to the range of conductivities represented in the lake series. When the data were combined, concentrations of Na+, K+, Mg 2+, SOd2-, Cl-, and alkalinity were positively correlated with conductivity and Ca2+ was negatively correlated with conductivity. The relative proportions of Na+, SOd2-, and Cl-increased significantly with brine conductivity (2,900-30,900 @), and the proportions of other ions decreased. The variability of these relationships suggests that the brine compositions in some of these lakes are strongly influenced by local surface water and groundwater chemistry. Increases in conductivity were usually accompanied by decreases in Chl a concentration and a shift away from cyanophyte species in favor of chlorophytes, cryptophytes, and chrysophytes. This change in the importance of these phytoplankton phyla, which occurs at relatively low salinities (-3,500 PS cm-*), acts to limit phytoplankton biomass in these P-sufficient systems by restricting the growth of nitrogen-fixing species. Salinity-linked shortages of nutrients other than P may account for these changes,