LAKE SEDIMENT CHRYSOPHYTE SCALES FROM THE NORTHEASTERN U.S.A. AND THEIR RELATIONSHIP TO ENVIRONMENTAL VARIABLES

Chrysophyte scale assemblages were analyzed in the surface sediments (0–1 cm) of 146 lakes sampled in the U.S. Environmental Protection Agency's (EPA) Environmental Monitoring and Assessment Program–Surface Waters (EMAP-SW) in the northeastern U.S.A. Chrysophyte data from the EMAP lakes were combined with a previous study of 71 Adirondack PIRLA (Paleoecological Investigation of Recent Lake Acidification) lakes and collectively analyzed to examine the indicator potential of scaled chrysophytes in the northeastern U.S.A. with respect to several environmental variables. Canonical correspondence analysis (CCA) was used to determine which environmental variables influenced the distributions of species. Forward selection and Monte Carlo permutation tests showed that 51% of the variance in the chrysophyte assemblages was related to pH. The other six significant variables (conductivity, chloride, total phosphorus [TP], elevation, lake depth, and watershed area) contributed an additional 31% of the total (82%) variance explained by the seven forward-selected variables. Similar to previous studies, many taxa showed distinct distribution patterns with respect to pH. Partial and constrained CCAs indicated that, although all seven variables explained significant proportions of variation in the species data, a reliable inference model could be developed only for lake-water pH. The strength of this model ( R ²= 0.78, RMSE_boot= 0.47 of a pH unit) is comparable to a recently constructed diatom-based model for the EMAP lakes. The use of both models in paleolimnological and biomonitoring studies would be advantageous because they would provide two independent lines of evidence of environmental change.     

Chernobyl-derived radiocaesium in fish as dependent on water quality and lake morphometry

¹³⁷Cs concentrations in perch Perca fluviatilis , pike Esox lucius and roach Rutilus rutilus obtained from lakes of different size and water quality in an area which received about 10–67 kBq m⁻²¹³⁷Cs, were compared with environmental data. Radiocaesium concentrations were highest in pike, and were about two to three times higher in the pike and perch than in the roach. The largest perch had about four times more ¹³⁷Cs than the smallest ones, but the activities in the pike and roach were independent of fish size. All of the water quality parameters examined correlated with ¹³⁷Cs concentrations in the fish, but the concentrations of ¹³⁷Cs in the bottom sediment did not. ^l37Cs fallout and water retention time in lakes showed a positive correlation with fish ¹³⁷Cs, while the size of the catchment area, phosphorus content of the water, water colour, pH and electrical conductivity showed a negative one. Thus there was more ¹³⁷Cs in the fish from the oligotrophic lakes than from the eutrophic ones, from the less humic lakes than from the more humic ones, from the more acid than from the less acid ones and from the lakes containing less potassium than from those containing more potassium. The depth, area and volume of the lake did not affect radiocaesium in the fish. Regression equations are presented for predicting ¹³⁷Cs in fish on the basis of environmental data. Fallout, total phosphorus and pH together had the highest influence in multiple regression models but were less significant in the roach than in the perch and pike.     

Inorganic carbon limitation of photosynthesis in lake phytoplankton

1. Inorganic carbon availability influences species composition of phytoplankton in acidic and highly alkaline lakes, whereas the overall influence on community photosynthesis and growth is subject to debate.
2. The influence of total dissolved inorganic carbon (DIC) and free CO₂ on community photosynthesis was studied in six Danish lakes during the summer of 1995. The lakes were selected to ensure a wide range of chlorophyll a concentrations (1–120 μg l^–1), pH (5.6–9.6) and DIC concentration (0.02–2.5 m m ). Photosynthesis experiments were performed using the ¹⁴C technique in CO₂-manipulated water samples, either by changing the pH or by adding/removing CO₂.
3. Lake waters were naturally CO₂ supersaturated during most of the experimental period and inorganic carbon limitation of photosynthetic rates did not occur under ambient conditions. However, photosynthesis by phytoplankton in lakes with low and intermediate DIC concentrations was seriously restricted when CO₂ concentrations declined. Similarly, photosynthesis was limited by low CO₂ concentrations during phytoplankton blooms in the hardwater alkaline lakes.     

The nutrient budget of a small eutrophic loch and the effectiveness of straw bales in controlling algal blooms

1. A 1-year intensive study of nutrient flows in Airthrey Loch, central Scotland, a small eutrophic [mean total phosphorus (TP) = 61.1 μg P l^–1] well-flushed freshwater body (area, 6.9 ha; mean depth, 1.85 m; volume, 1.274 × 10⁵ m³; retention time, 0.44 yr), was undertaken.
2. The nutrient budget was dominated by large allochthonous P inputs, equivalent to an areal load to the loch of 8.56 kg P ha^–1 yr^–1, which occurred predominantly during winter. In the summer, when TP inputs were low, water column levels of TP still increased, as a result of aerobic P release from sediments.
3. Sorption experiments indicated potential for sediment P release at water P concentrations of up to 200 μg P l^–1.
4. Aerobic release rate of P from sediments to the water column of Airthrey Loch was estimated to be of the order of 1 mg P m^–2 day^–1, and occurred during periods of elevated water column pH.
5. Straw bales placed in the loch to retard algal blooms were found not to have any demonstrable impact on algal concentrations observed during the study.     

The nutrient budget of a small eutrophic loch and the effectiveness of straw bales in controlling algal blooms

1. A 1-year intensive study of nutrient flows in Airthrey Loch, central Scotland, a small eutrophic [mean total phosphorus (TP) = 61.1 μg P l^–1] well-flushed freshwater body (area, 6.9 ha; mean depth, 1.85 m; volume, 1.274 × 10⁵ m³; retention time, 0.44 yr), was undertaken.
2. The nutrient budget was dominated by large allochthonous P inputs, equivalent to an areal load to the loch of 8.56 kg P ha^–1 yr^–1, which occurred predominantly during winter. In the summer, when TP inputs were low, water column levels of TP still increased, as a result of aerobic P release from sediments.
3. Sorption experiments indicated potential for sediment P release at water P concentrations of up to 200 μg P l^–1.
4. Aerobic release rate of P from sediments to the water column of Airthrey Loch was estimated to be of the order of 1 mg P m^–2 day^–1, and occurred during periods of elevated water column pH.
5. Straw bales placed in the loch to retard algal blooms were found not to have any demonstrable impact on algal concentrations observed during the study.     

Chironomid stratigraphy in the shallow and eutrophic Lake Søbygaard, Denmark: chironomid–macrophyte co-occurrence

1. A sediment core from the shallow, hypertrophic Lake Søbygaard (mean depth ∼1 m; [TP] 310 μg P L⁻¹) was analysed for subfossil remains to reconstruct chironomid community changes in relation to the succession and disappearance of aquatic macrophytes. 2. Species composition in the 1.10 m core indicates a succession from a 'naturally' eutrophic state to a hypertrophic state during recent centuries. Radiometric dating (²¹⁰Pb) of the uppermost 20 cm of the sediment core (∼1932–93) indicates that sediment accumulation rate had doubled in recent decades. 3. Changes in chironomid assemblages were in close agreement with changes in both diatoms and macrophyte remains in the same core. Distinct changes in chironomid communities reflect the eutrophication process and macrophyte succession through Chara , Ceratophyllum and Potamogeton dominance to the present state, with complete loss of submerged vegetation and dominance by phytoplankton. 4. The co-occurrence and relationship between aquatic macrophyte diversity and recent subfossil chironomid assemblages were assessed from an additional 25 Danish lakes. There was good agreement between the macrophyte and chironomid-based lake groupings. Overall, a significant difference ( P <0.001) was found in chironomid assemblages among lakes in different macrophyte classes. In a pair-wise comparison, the poorly buffered mesotrophic lakes and the alkaline eutrophic lakes had significantly different chironomid assemblages. 5. Chironomid taxa commonly reported to be associated with macrophytes ( Cricotopus , Endochironomus and Glyptotendipes ) were shown also to be indicators of highly productive lakes lacking abundant submerged vegetation.     

The occurrence and ecological importance of dissolved ATP in fresh water

SUMMARY. Dissolved ATP, defined as ATP which passes through 0.2 μm filters, was found in fresh water. During the spring diatom bloom in two eutrophic Danish lakes, concentrations of dissolved ATP varied between 0.1 and 3.8 μgl⁻¹, constituting 14–76% of the total ATP (particulate plus dissolved ATP). The kinetics of the light emission obtained from mixing firefly enzyme with dissolved ATP demonstrated that the major proportion of the dissolved ATP was in fact ATP. Despite some variations, the seasonal changes in dissolved ATP paralleled the changes in the increasing phytoplankton population during the rise of the diatom blooms. The dissolved ATP increased after the diatom peak, indicating that release of ATP from the phytoplankton due to mortality may be a major source of dissolved ATP.
Consumption of dissolved ATP was evaluated in uptake experiments using ³H-ATP. Rates of uptake of ³H-ATP by micro-organisms (diameter 0.2–0.6 μm) proved to be close to the rates for ³H-D-glucose uptake. The variations in ³H-ATP uptake during the diatom blooms showed non-systematic changes and ranged between 1.0 and 15.8% h⁻¹ (mean = 4.9% h⁻¹) of the quantity added. Turnover rates for dissolved ATP varied between 12 and 730 ng l⁻¹ h⁻¹ (mean = 175 ng l⁻¹). These rather high rates of turnover suggest that dissolved ATP is an important compound in the metabolism of freshwater bacteria.     

Covariation among Alaskan chrysophyte stomatocyst assemblages and environmental gradients: A comparison with diatom assemblages

Northern lakes, for which we have little long-term data, are predicted to experience profound impacts with greenhouse warming. In order to assess the usefulness of chrysophytes as paleoenvironmental indicators, stomatocysts were enumerated from the surface sediments of 51 Alaskan lakes distributed along a strong climatic gradient. A total of 142 cyst morphotypes were described using light microscopy, of which 13 are believed to be new forms.
Principal components analysis of the stomatocyst assemblages showed a limited amount of variation (Λ₁= 0.11, Λ₂= 0.09). However, redundancy analysis identified total phosphorus (TP), sodium (Na), and altitude (ALT) as significant variables in terms of their ability to describe the distribution of stomatocysts. Major ion and nutrient concentrations were also found to be strong predictors of diatom variation from the same lake set. In comparison to the diatom assemblages from the same lakes, the cysts were less abundant and showed a lower amount of species variation. Reasons for the lack of variation may be due to the predominance of unornamented and collective category cysts (i. e. cysts which cannot be differentiated under the light microscope).     

Phytoplankton structure in different lake types in central Finland

Phyloplankton structure and its relation to physical and chemical properties of the water was studied in 58 central Finnish lakes. The biomass ranged from 0.2 to 14.2 g m⁻³ and the number of taxa per sample ranged from 33 to 152. The lakes were grouped into 5 types according to their trophic state: eutrophic, dyseutrophic, mesotrophic, oligotrophic, and acid oligotrophic lakes. The average biomass in eutrophic lakes was 5.57 g m⁻³, in dyseutrophic 3.54 g m⁻³, 1.23 g m⁻³ in mesotrophic, 0.52 g m⁻³ in oligotrophic and 0.39 g ⁻³ in acid oligotrophic lakes. The average number of taxa per sample in the corresponding lake types were 109. 1, 79.3, 97.9, 90.9 and 43.8, respectively. The phytoplankton communities in eutrophic lakes were characterized by blue-green algae (21.2% of total biomass) and green algae (18.7% of total biomass). In dyseutrophic lakes the proportion of green algae was much smaller (7.2% of total biomass) than in eutrophic lakes, whereas the proportion of diatoms and cryptophytes was higher (28.2 and 20.4% of total biomass, respectively). Chrysophytes dominated in the oligotrophic and mesotrophic lakes (27.3–39.9% of total biomass). The contribution of dinoflagellates to the total biomass was highest in the most oligotrophic acidified lakes and in those lakes the relative proportions of blue-green and green algae were much higher than in the typical oligotrophic lakes. The lakes were also grouped into 8 community types according to the dominating algal group. Cyanophyceae- and Chlorophyceae-types characterized the eutrophic lakes, whereas Chrysophyceae-Dinopheceae-type was typical for most oligotrophic lakes. The other 5 types occurred in mesotrophic and oligotrophic lakes but the physical and chemical properties of these lakes did not differ much.     

The relative abundance of brown trout in acidic softwater lakes in relation to water quality in tributary streams

The influence of the water quality of tributary streams on the relative abundance in benthic gillnet catches (catch per unit effort, cpue) of allopatric brown Salmo trutta was assessed in associated acidic, softwater lakes. The study was carried out over 6 years (1989–1994) in 15 lakes located at altitudes between 230–715 m a.s.l. in two Norwegian catchments. The water quality of the main inlets and outlets varied little, as indicated by their of pH range (4·93–5·51) and calcium concentrations (0·19–0·44 mg 1⁻¹), but varied more with respect to concentrations of inorganic, monomeric aluminium (7·0–41·0 μg l⁻¹). Most of the lakes were also fed by secondary streams with better water quality: a maximum pH of 6·56, calcium levels of up to 0·74mg 1⁻¹, and inorganic aluminium levels as low as l·0 μg 1⁻¹. The cpue was inversely correlated with lake altitude ( r ²=0·50), and thus was adjusted to a mean altitude. The calcium concentration in the richest secondary stream to each lake, its richness judged on the basis of its acid-neutralizing capacity, had the highest predictive power of the variability in cpue ( r ² = 0·49).The calcium content in the other secondary streams or in the main inlets and outlets did not correlate with cpue. Alkalinity in the main outlets correlated to some extent with cpue ( r ² = 0·27). It is suggested that secondary streams with good water quality provide important refuges for the recruitment of brown trout in acidic softwater lakes.     

The distribution and abundance of young (I +–II +) perch, Perca fluviatilis L., in a deep eutrophic lake, England

Seasonal changes in the vertical and horizontal distribution of young (I +–II +) perch in Rostherne Mere, a deep (max. depth 31.0 m) eutrophic lake are described using sonar. During the summer (June–August), perch were present within the thermocline (9–14° C) between 7 and 10 m. There was no significant difference between samples during this period, nor was there any evidence of shoaling. From September onwards perch moved into deeper waters. By late winter fish had become patchy in their distribution. Total numbers, during the summer, were estimated to be 988 ha ¹ with 95% confidence limits of ±115 (biomass 98.8 kg ha ⁻¹). Densities (no. m⁻³) fell by over 90% after the autumn overturn. The implications for predators are discussed. Acoustic sampling provides a useful, non-destructive method of sampling eutrophic lakes on a routine basis. Information obtained is of ecological, behavioural and physiological interest.     

ENVIRONMENTAL FACTORS CORRELATED WITH CHRYSOPHYTE CYST ASSEMBLAGES IN LOW ARCTIC LAKES OF SOUTHWEST GREENLAND1

We analyzed the relationship between chrysophyte cyst assemblages in surface sediment samples and limnological and geographical variables for 70 lakes located along Søndre Strømfjord in southwest Greenland. Over 247 stomatocysts were identified and of these, 153 were sufficiently abundant for use in statistical analyses. Eight stomatocysts were considered to be new and are described formally. Canonical correspondence analysis indicated that conductivity was the dominant variable explaining cyst distribution, reflecting the large conductivity gradient in lake water chemistry in this area. High conductivity lakes had distinctive cyst assemblages with lower diversity than low alkalinity lakes, where assemblages were similar to alpine soft‐water lakes elsewhere. The high conductivity lakes, however, had similar cysts to other saline lakes elsewhere in the arctic. Additionally, pH, calcium, maximum depth, longitude, sulfate, total phosphorus, and altitude all explained significant amounts of variability of cyst assemblages. Longitude was the only geographical variable that explained cyst variability independently of other variables (i.e. had a unique effect), which suggests that the climatic gradient from the coast to the head of the fjord has a structuring effect on cyst assemblages. Conductivity (weighted‐averaging partial least squares, r²=0.917; root mean square error=0.142; r²_jack=0.861, root mean square error of prediction=0.191) and pH inference models (weighted averaging, r²=0.924; root mean square error=0.158; r²_jack=0.826, root mean square error of prediction=0.240) were developed. For the pH model, high conductivity lakes (>800 μ S₂₀·cm^?1) were removed. Both models are statistically robust and could be applied to lakes in west Greenland to reconstruct conductivity and/or pH. Such paleolimnological reconstructions provide the means of acquiring long‐term data for use in the evaluation of, for example, regional paleoclimatic models.     

Random mutagenesis of Legionella pneumophila with mini-Tn10

Abstract The degradation of sheets of poly(3-hydroxybutyrate- co -3-hydroxyvalerate) (BIOPOL^®) by aerobic sewage sludge was analyzed. Degradation of the polymer was highly dependent on the pH of the culture medium and was maximal between pH 7 and pH 8.5. Below pH 6 and above pH 9 the degradation rate was very low. Agitation of the culture fluid had relatively little influence on the rates of degradation. 1.2×10⁵ aerobic polymer-degrading bacteria per ml sewage sludge were identified by halo formation on solid poly(3-hydroxybutyrate) (PHB)-containing media. The number of PHB-degrading bacteria in other ecosystems amounted to 3.8×10³ per ml sludge of a fresh-water lake, 9.2×10⁵ per g garden-soil, 1.3×10⁶ per g field-soil and 4.3×10⁶ per g compost.     

Sodium Transport from Blood to Brain: Inhibition by Furosemide and Amiloride

Abstract: Brain sodium uptake in vivo was studied using a modified intracarotid bolus injection technique in which the uptake of ²²Na ⁺ was compared with that of the relatively impermeable molecule, [³H]l-glucose. At a Na ⁺ concentration of 1.4 m M , Na ⁺ uptake was 1.74 ± 0.07 times greater than l -glucose uptake. This decreased to 1.34 ± 0.04 at 140 m M Na ⁺, indicating saturable Na ⁺ uptake. Relative Na ⁺ extraction was not affected by pH but was inhibited by amiloride ( K _i= 3 ± 10⁻⁷ M ) and by 1 m M furosemide. The effects of these two inhibitors were additive. Brain uptake of ⁸⁶Rb ⁺, a K ⁺ analogue, was measured to study interaction of K ⁺ with Na ⁺ transport systems. Relative ⁸⁶Rb ⁺ extraction was also inhibited by amiloride; however, it was not inhibited by furosemide. The results suggest the presence of two distinct transport systems that allow Na ⁺ to cross the luminal membrane of the brain capillary endothelial cell. These transport systems could play an important role in the movement of Na ⁺ from blood to brain.     

RETENTION AND EXCHANGE OF POTASSIUM IN A SYNAPTOSOMAL FRACTION OF MOUSE BRAIN

Abstract— Myelin, synaptosomal and mitochondrial fractions obtained from homogenates of whole mouse brain contain K⁺ which can exchange with ⁴²K⁺ at 2º in 0·32 m -sucrose. The content and rates of exchange of K⁺ were greater at pH 8·2 than at 6·1. In the synaptosomal preparations, the rates of exchange and content of ⁴²K⁺ and K⁺ declined progressively with decreasing pH.
Of the total synaptosomal K⁺, 95 per cent could exchange with external ⁴²K⁺. At pH 7·5, 20 per cent of the K⁺ and 78 per cent of the Na⁺ appeared to reside in osmotically insensitive pools. Synaptosomal K⁺ at 2º was slowly displaced by NaCl (0·18 m ) and the rate of exchange between ⁴²K⁺ and K⁺ was retarded. KCI (0·18 m ) did not readily displace endogenous Na⁺. Synaptosomal K⁺ exchanged with exogenous K⁺ more rapidly than with exogenous Na⁺.
These observations have been discussed in terms of possible roles for ion exchange as the principal means by which K⁺ traverses the plasma membrane at 2º.     

Environmental factors influencing chlorophyll v. nutrient relationships in lakes

SUMMARY. 1. A model relating log chlorophyll a concentration to log epilimnetic total phosphorus (TP) concentration was re-examined based on: (a) comparative and temporal studies of four stratifying Wisconsin and other highly eutrophic temperate lakes; (b) comparative summer lake surveys from Iowa and Alberta.
2. Although P-limited, deeper lakes with long hydraulic residence times and low external and internal nutrient loading in summer had summer chlorophyll a yields below model predictions based on spring and summer epilimnetic TP concentrations.
3. For lakes with summer epilimnetic TP between 30 and 80 mg m⁻³, chlorophyll a concentrations exceeded model predictions based on summer TP. This relationship held even for Lake Delavan, Wisconsin, where the ratio of available N to P was unfavourably low during spring turnover, and where the trans-thermocline N:P flux ratio was sub-optimal for algal needs in early summer.
4. With increasing summer TP concentrations and/or increasing epilimnetic circulation depth (>5m), chlorophyll a concentrations fell below model predictions—independent of the potential for N-limitation. This plateauing in chlorophyll a response occurred at lower epilimnetic TP content (−2) in lakes with elevated non-algal light extinction coefficients. Using Tailing's algorithm for the'column compensation point' (algal photosynthesis = algal respiration over diel cycle), light limitation best explains this fall-off in chlorophyll a yield.
5. The failure of the Dillon & Rigler (1974) spring TP v . summer chlorophyll a model for these Wisconsin lakes is unrelated to N-limitation. Instead, it reflects internal adjustment in take TP in response to stratification and seasonal external P loading.     

Tracking Recovery Patterns in Acidified Lakes: A Paleolimnological Perspective

Long-term data are often lacking to effectively assess patterns of lake acidification and recovery. Fortunately, paleolimnological techniques can be used to infer past changes in lakewater acidity and related variables by means of biological indicators, such as diatom valves and chrysophyte scales, preserved in ²¹⁰Pb-dated sediment cores. We summarize paleolimnological data that we have gathered from 36 Sudbury (Ontario) and 20 Adirondack Park (New York) lakes to estimate the magnitude of lake acidification and any subsequent recovery in these lake systems. In both regions, many lakes were shown to have acidified considerably, some over two pH units, since the 1850s. Although some recovery was noted in both lake regions, Sudbury lakes generally showed larger increases in inferred lakewater pH with recent declines in sulfur emissions. Possible explanations of these differences include the greater decrease in sulfate deposition in the Sudbury area, as well as generally longer residence times of lakes in Sudbury, perhaps allowing for more in-lake alkalinity generation. In addition, Sudbury lakes generally had higher pre-industrial pH levels, suggesting that lakes with higher natural buffering capacities are more likely to recover more quickly with declines in deposition, even if they had been acidified to a great extent.     

Isolation of Yersinia enterocolitica-resembling Organisms and Alteromonas putrefaciens from Vacuum-Packed Chilled Beef Cuts

Yersinia enterocolitica -resembling organisms were found at levels of 10⁷/g on a high pH (pH ≧ 6·0) vacuum-packaged beef striploin held for 6 weeks at 0·2°C, but did not exceed 10⁵/g on normal pH (pH < 6·0) striploins held for 10 weeks. Gram negative bacteria that produced H₂S on peptone iron agar were isolated from high pH vacuum packed striploins. These organisms were identified as Alteromonas putrefaciens . They attained levels of about 10⁷/g in 6 weeks at 0–2°C, at which time greening of the fat surface and 'drip'had occurred. On meat of normal pH, counts of A. putrefaciens were less than 10⁴/g after 6 weeks and no greening was evident.     

Mechanisms of cobalt uptake in plants: 60CO uptake and distribution in Chara

The mechanism of cobalt uptake was investigated using cells of the giant alga Chara corallina in which it is possible to resolve separately uptake by the cell wall and actual influx across the cell membrane. The absorption of ⁶⁰Co by Chara cells appeared to saturate within 2 h, but this was mainly due to rapid uptake into the cell wall which accounted for 87–92% of the total activity. Even after prolonged desorption most of the cell‐associated ⁶⁰Co was found on the cell wall. The intracellular distribution of absorbed ⁶⁰Co was investigated by fractionating the cell into cytoplasm and vacuole. It was shown that ⁶⁰Co influx to the vacuole occurs simultaneously with influx to the cytoplasm. The transported species appears to be Co²⁺ rather than the less charged Co(OH)⁺ or Co(OH)₂. ⁶⁰Co influx is pH dependent (optimum pH 7–9), and is sensitive to some other divalent metals. Influx from solutions containing 1 µ M ⁶⁰Co was inhibited by 5 µ M Cd²⁺, Cu²⁺, and Zn²⁺, but Mn²⁺ and Ni²⁺ had no significant effect. The sensitivity of Co uptake to N ‐ethyl maleimide (NEM) and cysteine suggests that the transport system involves direct binding of CO²⁺ to ‐SH groups.     

Three species of fishes from an eutrophic, seasonally alkaline lake are not more tolerant to acute exposure to high pH in the laboratory

A number of different freshwater fish species (perch Perca fluviatilis , roach Rutilus rutilus and rudd Scardinius erythrophthalamus ) from either eutrophic (Slapton Ley, a seasonally alkaline lake) or non-eutrophic waters were compared with respect to their sodium uptake kinetics and tolerance to acute (1 h) exposure to pH 9·5. Further comparisons were made with rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta . The influence of fish size was also investigated in rainbow trout. Exposure to pH 9·5 was found to disrupt sodium balance and inhibit ammonia excretion in all species and sizes of fishes. The origin of fishes did not have a significant effect on the sodium uptake kinetics or the physiological responses to high pH water. The fishes from the eutrophic lake therefore did not appear to have any increased tolerance to acute exposure to alkaline water. In contrast to previous studies there was no inhibition of Na⁺ uptake during exposure to high pH. Indeed in some groups of fish Na⁺ uptake was actually stimulated, as was Na⁺ efflux. These differences are attributed to experimental water composition and interspecific differences in physiology. It was not always possible to size-match fishes of the different species, so rainbow trout were used to assess the effect of body mass (from 2 to 40 g), on Na⁺ uptake kinetics and Na⁺ or ammonia fluxes during alkaline water exposure in rainbow trout. Size had no significant effect on these measurements within this narrow range, which helps validate the comparison between species in this study.