Occurrence of a thermocline in two lakes of the semi-arid region in western Rajasthan,India

Water quality samples were obtained monthly or bimonthly 17 times from May 1974 to May 1975 at three stations in Delaware Bay. In addition, two 12-hour cruises were also conducted at one station in February and April 1975. Surface and bottom water samples were taken. Measurements and analyses included temperature, salinity, dissolved oxygen, silicate, nitrate and nitrite, orthophosphate, ammonia, chlorophylls a, b, and c, phaeopigments, and carotenoids. The annual pattern of temperature was typical of an estuary in the mid-Atlantic Bight. Salinity and dissolved oxygen ranged from 22.9 to 29.7‰ and from 4.53 to 8.53 ml/l, respectively. Nutrient and pigment values showed seasonal peaks. Silicate (30.3 μg-at/1) and orthosphate (1.59 μg-at/1) were highest in September. Highest concentrations of ammonia were commonly measured in July (6.80 μg-at/1) and September (5.13 μg-at/1), and peak concentrations of nitrate and nitrite were recorded in January (24.27 μg-at/l), February (18.2 μg-at/1), and May (16.37 μg-at/1). Peak concentrations of chlorophyll a were measured in August (17.2 μg-at/1), October (15.70 μg-at/1), and March (15.33 μg-at/1). In general, the annual pattern for chlorophylls b and c were similar to chlorophyll a. Comparison with other estuaries and bays (Narrangansett Bay, Long Island Sound, Raritan Bay, and Chesapeake Bay) indicated that concentrations of nutrients and pigments in Delaware Bay were generally similar in magnitude and seasonality, These are the first set of seasonal water quality data for lower Delaware Bay.     

Observations on the superficial sediment temperatures of some lakes in the southeastern United States*

Temperatures of the water column and upper 5 cm of sediment were monitored over a yearly cycle in two South Carolina lakes. Occasional supportive data were also obtained for several lakes in north central Florida. Plans are given for a new type of sediment-interface sampler that is useful in obtaining detailed temperature or chemical profiles extending from the sediment surface upward. The sampler was used in the investigation to demonstrate the thermal microstratigraphy near the mud surface. The deep-water (16 m) temperature for the larger of the two South Carolina lakes changes seasonally from 10·5°C in February to 18·0°C in July. The smaller, shallower (11 m) lake follows an almost identical seasonal cycle but is always 4·0°C cooler because the larger lake receives a heated effluent that has a long-term effect on hypolimnetic temperatures. In both lakes the uppermost sediments are warmer than the overlying water by an average of 0·1 to 1·0°C during the warming period. Heat accretion near the bottom continues but is slower after stratification, probably due to the relatively low temperature (density) differential between water layers in these warm lakes. Cooling in deep water begins long before breakdown of stratification and is apparently caused by cold density currents from the shallows. The coldest water is located in a thin layer just over the sediment. There is evidence from one of the South Carolina lakes and from the Florida lakes that when the density flows begin they at first flow over a warmer water layer that is more dense due to a high electrolyte content derived from the sediment. There is a slight deep water warming in all of the lakes when stratification breaks down. After destratification, the deep water is cooled by turbulence rather than density flows. The surface sediments at this time are consistently warmer than the hypolimnion and remain so through the cooling period. There is strong evidence from one Florida lake that turbulence mixes the upper 3 cm of sediment during the isothermal period. It is concluded that the sediment-water interface of a warm lake will in general experience greater heat flux than that of a comparable cold lake during the periods of temperature maximum and minimum. Conversely, there is likely to be less heat flux during the warming and cooling periods of warm lakes than of cold lakes. Several expected differences in seasonal patterns of temperature and water movement in the deep water of warm and cold lakes are discussed.     

Potential Effects of Climate Warming on Fish Habitats in Temperate Zone Lakes with Special Reference to Lake 239 of the Experimental Lakes Area (ELA), North-Western Ontario

We used simple statistics (e.g. mean temperature, degree days, cumulative volume days) to describe present thermal habitats for cool water (yellow perch, Perca flavescens) and cold water (lake trout, Salvelinus namaycush) fish of a small boreal lake. We then modelled changes in the vertical and temporal extent of these habitats under various scenarios of climatic change that included increases in air temperature of 2°C, 4°C, and 9°C, and positive and negative deviations from present levels of 10% in solar radiation and relative humidity, and 20% in wind speed and the lake water extinction coefficient. Model simulations indicated pronounced changes in the temporal and vertical availability of fish thermal niche space. These changes were mainly driven by the large increases in mean mixed layer temperatures that corresponded to 85% of the increases in air temperature, but, in particular, changes in light attenuation also resulted in some non-linear, unexpected effects in the distribution and seasonal availability of thermal niche space. As expected, classical lake trout thermal habitat (5–15°C) was progressively reduced and almost disappeared in littoral areas in spring and early summer. Perch thermal niche space expanded for air temperature increases of up to 4°C, but largely disappeared for the 9°C increase. We discuss changes in thermal habitat with regard to the life history of lake trout and yellow perch, and include other determinants of fish habitat to evaluate the potential of these species for long-term ecological success under climatic warming.     

Regional and hierarchical perspectives of thermal regimes in subarctic, Alaskan lakes

1. We examined 60 clear, stained and glacial lakes in Alaska to quantify the relative importance of climate setting, morphometry, transparency, and lake typology influences on various thermal characteristics including duration of growing season, water temperature, mixing depth (MD) and heat content. We used analysis of variance (ANOVA ) to test for differences in thermal characteristics in association with lake type and employed simple and multiple regression techniques to determine functional relationships between variables. 2. Latitude accounted for 60% of the total variance in length of growing season. Although the date of maximum heat content was consistent among lake types, stained lakes had longer growing seasons compared with clear and glacially turbid lakes. 3. Maximum water temperatures were approximately 3 °C higher in stained lakes and 3 °C lower in glacial lakes compared with clear lakes. Mean water column temperature was significantly lower in glacial lakes (5.9 °C) compared with clear lakes (7.4 °C), but there was no statistical difference between clear and stained lakes (7.2 °C) or between stained and glacial lakes. Maximum surface temperatures were positively related (r²=0.51) to colour (humic stain), but negatively related (r²=0.40) to inorganic turbidity (glacial silt). 4. Only about half of the lakes in our data set underwent summer stratification. None of the glacial lakes developed a distinct thermocline, but stained lakes had shallower MDs (mean 8 m) than clear lakes (mean 12 m). Thus, the MD to total depth ratio for glacial lakes was unity compared with mean values of 0.66 for clear lakes and 0.34 for stained lakes. Fetch explained a significant fraction (51%) of the total variance in MD. Considering all lakes, MD was inversely related to transparency (Secchi depth). In contrast, considering only stratified clear and stained lakes, MD was positively related to Secchi depth (SD), the fraction of the total variance explained was 23%. The sign of the slope was dependent on the mixture of lake types. 5. Despite significant (ANOVA ) differences in water temperatures, growing season, and MDs among the three lake types, there were no statistical differences in the summer heat budget associated with lake type. In addition, heat budgets were poorly correlated with lake area, depth and volume. In contrast, mean water column temperature was strongly and inversely related (r²=0.77) to mean depth. 6. Potential explanations for the similarity in summer heat budget among lake types and weak correlation with morphometry were attributed to different patterns in vertical heat distribution associated with lake typology (colour and turbidity) differences. 7. Multiple linear regression including climatic (latitude and altitude), morphometric, and lake typology (colour and turbidity) factors demonstrated a hierarchical (climate–morphometry–typology) regulation of growing season characteristics, water temperatures, stratification and heat retention. A regional and hierarchical framework for lake thermal characteristics adds to our understanding of potential responses to climatic change and may be important for regional management objectives for fisheries.     

Convection of waters in Lakes Maninjau and Singkarak,tropical oligomictic lakes

The ecology of a lake is mainly controlled by mixing processes; particularly, in tropical oligomictic lakes, the occurrence frequency and magnitude of convection govern the vertical mixing of chemicals and organisms. In this study, vertical profiles of water temperature, dissolved oxygen, electric conductivity, turbidity, and chlorophyll a were measured in 2014, 2015, 2017, and 2018 in two Sumatran deep lakes, Lakes Maninjau and Singkarak. In Lake Maninjau, intensive surveys on the profiles were also conducted in three different seasons in 2018. The comparison of the profiles between 2015 and 2017 indicated the events of large convection down to the lake bottoms happened in both of the lakes. Similarly, small convection down to around 30 m depth was found in the period between May and Jul, 2018. Air temperature drops up to five degrees centigrade were observed in these periods, confirmed by the changes in lake surface temperature estimated by MODIS imagery for the lakes. The magnitudes of the convective events were discussed with the observed amounts of heat loss and the estimated heat transfer through lake surface. Furthermore, the influences of such events on anoxic hypolimnetic waters were evaluated and considered from the view of climate change.

     

Thermal regimes of Florida lakes

Water column temperatures were determined monthly for 24 lakes and bimonthly for 5 lakes in peninsular Florida during 1979. Three geographical groups (north, central, south) were delineated from mean monthly water column temperatures for individual lakes. On a monthly basis, northern lakes were least similar to southern lakes, while central Florida lakes displayed greater affinity to the southern than to the northern lake group. Temperature differences between lake groups broke down during late summer. Subtropical lakes have been defined tentatively as those Florida lakes south of 28° latitude which possess warm monomictic circulation and a mean annual temperature of 24.2 ± 4.8 °C with minimum water column temperature rarely less than 14 °C and summer maxima rarely exceeding 31 °C. While all lakes in Florida are clearly warm monomictic annual nutrient cycling and productivity patterns may be influenced by inter-group differences in the timing and duration of water column circulation.     

Thermal stratification and annual heat budget of a Florida sinkhole lake

Summary Aerial photographs of the Florida landscape, an area of Karst topography, show the country-side to be pockmarked by small lakes, many of which are nearly circular in surface outline. Lake Mize is quite typical of these sinkhole or doline lakes in its morphometric features, including its morning glory basin shape. The deeper portion of the basin occupies a relatively small fraction of the area and volume of the lake. Lake Mize becomes stably stratified at superficial depths in early spring and remains so until late fall or early winter. Even the violent winds of a hurricane are insufficiently strong to break up the stratification. Stratification disappears by December and the lake circulates through the winter months during which the water temperature drops to the winter low-generally near 11° C. The circulation pattern is typical of subtropical or warm monomictic lakes-a single, extended period of circulation with the minimum temperature always well above 4° C.Lake Mize has an extremely small annual heat budget. Based upon empirical evidence this is due to a number of factors including latitude (ca 30° N latitude), altitude (ca 30 m above sea level), protected location of the lake, small surface area and restricted solar heating. However, the regression equations of GORHAM relating annual heat budgets to various morphometric parameters of larger temperate zone lakes are not useful for predicting the annual heat budget of a lake such as Lake Mize. Year to year variations in the annual heat budget were rather large, ranging from a low of 3767 cal/cm² to a high of 6003 calf cm², so that the highest annual heat budget was roughly 1.6X that for the lowest of the three years. One expects a fair amount of year to year variation in annual heat budgets based upon Hutchinson's discussion (1957). Since we are dealing with an extremely small annual heat budget small changes are magnified when viewed on a percentage basis.     

Daily patterns of mixing and nutrient concentrations during early autumn circulation in a small sheltered lake

1. Autumn circulation in lakes is currently conceived to occur very rapidly, being controlled mainly by wind‐power dynamics, decreasing irradiance and heat flux. In addition, autumn mixing is usually related to nutrient redistribution in the vertical column, resulting in its overall increase. To test these assumptions, mixing and nutrient dynamics in a Spanish small, wind‐sheltered, mesotrophic, seepage lake were studied daily during autumn circulation. 2. The seasonal erosion of the pycnocline in Las Madres Lake was the outcome of vertical and horizontal exchanges of heat and matter. The overall mixing of the water column lasted 3 months, which was an unexpected period for a rather shallow lake. Two periods of mixing could be envisaged until full circulation was attained. First, a slightly faster period of pycnocline deepening than that predicted by the heat flux and wind stress model of Fisher et al. (1979) occurred for 41 days, mixing most of the water column down to within two meters of the bottom. Then a much slower process took place promoting frequent instability of the bottom layer and resulting in entire mixing in a further 52 days. 3. Vertically, the whole mixing process was a response to weak surface cooling, resulting from the mild air temperatures of the semiarid climate of the area, and weak wind stress, because of low wind fetch and high shelter. Horizontally, a gravity current transporting cold, denser water from western shallower areas of the lake and materials produced by the decomposition of organic matter of littoral origin may produce a bottom layer of increased density, thus impinging on vertical stability. Seepage inputs of water of roughly constant temperature might also have increased bottom density. Bottom density enhancement resulted in a double diffusion process. 4. Only in‐lake nitrogen content increased until full circulation was attained, whereas carbon showed no trend and phosphorus declined. External processes, such as seepage exchange and atmospheric deposition, coupled to internal processes, such as nitrification, oxidised phosphorus precipitation and complexation with organic carbon, might have been responsible for the areal nutrient patterns observed. 5. Our study demonstrates that current models of water column mixing and nutrient redistribution in lakes during autumn circulation must be improved to encompass the effects of external inputs, including horizontal heat and matter exchange.     

Spatial and seasonal distribution of phytoplankton in an African Rift Valley lake (L. Albert,Uganda, Zaire)

An account is given of the seasonal succession and spatial distributionof phytoplankton derived from a one-year sampling programme (1961–62)on Lake Albert, a large African rift lake. There is evidence of regulationby both physical and nutrient factors. These are influenced by a markedseasonality of temperature/density stratification (temperature range <3°C) and of water input that induces some polarizationalong the axis of the lake. Prevailing concentrations of soluble reactivephosphorus are high (>100 µg l^-1), but those ofinorganic nitrogen are low and, with large depletions of silicate-silicon,may limit algal production. The diatom and cyanophyte components of thephytoplankton show different relationships to the seasonal cycle of limitedstratification and vertical mixing. There are possible relationships, bygrazing, to the described seasonal and spatial abundance of planktonicCladocera. Comparisons are made with other African lakes, especially LakeTurkana which is of similar shape and size and shows end to end polarizationof the aquatic biota.     

The annual cycle of heat content and mechanical stability of hypersaline Deep Lake,Vestfold Hills,Antarctica

Deep Lake, a hypersaline lake of about ten times seawater concentration, rarely freezes and is characterized by a monomictic thermal cycle, Winter circulation, at c. –17 °C, lasts for two to three months. In summer, epilimnetic temperatures from 7–11 °C result in large vertical thermal gradients (21–26 °C) which combine with the enhanced rate of density change per degree Celsius, accompanying such high salt concentration, to produce a particularly stable density configuration in Deep Lake (Schmidt stability c. 8000 g-cm cm^–2; 0.785 J cm^–2). The Birgean annual heat budget (c. 24500 cal cm^–2; 102.7 10³ J cm^–2) is comparable to that of a temperate lake with a similar mean depth, despite the comparatively high ratio of Birgean wind work to annual heat budget (0.37 g-cm cal^–1). Deep lake retains c. 50% of the incident solar radiation during the short summer heating period; within the range estimated for first class lakes in North America. Extended daylight hours certainly contribute to the high maximum rate of heating in the lake (444 cal cm^–2 day^–1; 1.86 10³ J cm^–2 day^–1). Deep Lake cools at a rate less than half its average heating rate. Partitioning the total stability into thermal and saline components shows that salinity can contribute up to c. 20% of the maximum summer Schmidt stability. In early summer, the effect of small melt-streams is to increase stability by diluting the epilimnion. In autumn, evaporative water loss can overtake this effect, creating small de-stabilizing salinity gradients. The usually short-term stabilizing influence of snowfall and drift is less predictable, but is probably more common in winter when strong winds are most frequent.Hypersalinity has a profound effect on the physical cycle of Deep Lake, through freezing point depression and the increased rate of density change with temperature. These changes affect the lake's biota, both in relation to osmotic stress, and by effectively exposing them to a more thermally extreme environment. A comparison between Deep Lake and a smaller lake of similar salinity (Lake Hunazoko, Skarvs Nes), demonstrates that it is inappropriate to consider the biological effects of salinity in isolation. The smaller lake offers warmer epilimnetic conditions for at least part of the summer, which may explain the much greater limnetic algal production in Lake Hunazoko.     

Seasonality of phytoplankton in relation to silicon cycling and interstitial water circulation in large,shallow lakes of central Canada

The main basins of Lake Winnipeg (52°N 97°E) and Southern Indian Lake (57°N 99°W) had similar phytoplankton cycles during their open water seasons. A brief spring algal maximum was followed by an early summer minimum and, subsequently, an extended autumnal increase when highest biomasses were observed. The maxima were dominated by Melosira spp. The seasonal cycle of Melosira followed closely the seasonal cycle of dissolved Si. These basins exhibited a typical phytoplankton cycle for dimictic lakes even though they did not form a significant thermocline (1°C per meter).The lakes were well-mixed because they were shallow and had large wind fetches. Although thermal stability of the water column was always low, it was positive until maximum heat content was achieved at which time it became nil or negative. These lakes heated and cooled rapidly, and sediment heat storage was a substantial fraction of the total heat budget. Because heating and cooling of water and of sediments were out of phase, heat exchange at the sediment surface could control vertical circulation of interstitial water, nutrient exchange across the sediment-water interface and the seasonality of phytoplankton. Thermal gradients in the sediments during the heating season would be quite pronounced (4°C per meter).It is proposed that positive stability in interstitial waters during the heating season would impose molecular diffusive transport on the sediment column. When the lakes begin to cool, the upper interstitial water column would become thermally unstable and circulation would occur within the sediments. This would result in the observed net flux of dissolved Si, and other nutrients, out of the sediments into the overlying waters. As a consequence, in Lake Winnipeg and Southern Indian Lake the highest phytoplankton biomasses and productivity occurred in the late summer and autumn.     

Morphometry and trophic state modify the thermal response of lakes to meteorological forcing

We hypothesised that (i) a summer heat wave would increase the thermal stability of lakes and (ii) the size and trophic state differences would modify the lakes’ responses to heating. Within 2 years, 2008 and 2009, we studied the thermal and optical regimes of two adjacent stratified lakes in northern Italy, the oligo-mesotrophic Lake Monate (2.5 km², max. depth 34 m) and the eutrophic Lake Varese (14.8 km², 26 m). After the cold winter 2008–2009, a heat wave starting in May turned the whole year 2009 the second hottest after 2003. The particular sequence of meteorological events resulted in extreme vertical temperature gradients and unusually high thermal stability of both lakes. All calculated thermal parameters showed the highest values in 2009 while also the values for 2008 exceeded considerably those published for these lakes in the past. Due to the large wind exposed surface, wind mixing was supposedly the dominating mechanism of heat transfer in the shallower eutrophic Lake Varese where, due to low water transparency, large amount of solar energy trapped in the upper layers markedly increased the thermal stability. In the deeper and more transparent Lake Monate, the deeper penetrating solar irradiance contributed to better energy dissipation within the water column and smaller interannual differences in thermal stability.     

Marine Lake Mogilnoe (Kildin Island,the Barents Sea): one hundred years of solitude

Lake Mogilnoe (Kildin Island, the Barents Sea) is a marine stratified lake, a refuge for landlocked populations of marine organisms. Unlike other known marine lakes from polar areas, which communicate with the sea by water percolation at the surface, Mogilnoe has a subterranean connection with the sea like tropical and subtropical anchialine lakes. Similarly to some other marine lakes, Mogilnoe has traditionally been considered to be biologically isolated from the sea and subject to little change. We review the current status of the physical features, zooplankton and benthos of Mogilnoe and trace changes that have occurred in the lake since the start of observations in 1894. The anaerobic bottom water layer has expanded by 100 %, while the upper freshwater layer has diminished by 40 %. The species diversity of zooplankton and macrobenthos has halved. The occurrence of Atlantic cod likens Mogilnoe to some other Arctic marine lakes while the presence of large flocks of sea anemones, scyphomedusae and suberitid sponges makes it similar to tropical anchialine lakes. Lake Mogilnoe is not entirely biologically isolated; accidental introduction of species from the sea does occur. We argue that the idealised model of an isolated steady-state ecosystem can be applied to a marine lake with caution. A model of fluctuating abiotic environment and partial biological isolation portrays the real situation better.     

Regional coherence of climatic and lake thermal variables of four lake districts in the Upper Great Lakes Region of North America

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.     

Will northern fish populations be in hot water because of climate change?

Predicted increases in water temperature in response to climate change will have large implications for aquatic ecosystems, such as altering thermal habitat and potential range expansion of fish species. Warmwater fish species, such as smallmouth bass, Micropterus dolomieu , may have access to additional favourable thermal habitat under increased surface-water temperatures, thereby shifting the northern limit of the distribution of the species further north in Canada and potentially negatively impacting native fish communities. We assembled a database of summer surface-water temperatures for over 13 000 lakes across Canada. The database consists of lakes with a variety of physical, chemical and biological properties. We used general linear models to develop a nation-wide maximum lake surface-water temperature model. The model was extended to predict surface-water temperatures suitable to smallmouth bass and under climate-change scenarios. Air temperature, latitude, longitude and sampling time were good predictors of present-day maximum surface-water temperature. We predicted lake surface-water temperatures for July 2100 using three climate-change scenarios. Water temperatures were predicted to increase by as much as 18 °C by 2100, with the greatest increase in northern Canada. Lakes with maximum surface-water temperatures suitable for smallmouth bass populations were spatially identified. Under several climate-change scenarios, we were able to identify lakes that will contain suitable thermal habitat and, therefore, are vulnerable to invasion by smallmouth bass in 2100. This included lakes in the Arctic that were predicted to have suitable thermal habitat by 2100.     

Effects of fluctuating temperatures on the population dynamics of Hexarthra bulgarica (Wiszniewski) from high mountain lakes in Sierra Nevada (Spain)

We examine the development of Hexarthra bulgarica (Wisniewski) populations in relation to thermal stability in natural environments. A high frequency sampling program was developed simultaneously in two high mountain lakes: a shallow one, with daily large temperature changes but little surface-bottom temperature difference and a deeper one with more stable temperature but vertical heterogeneity in the water profile. Since the capacity of H. bulgarica to perform vertical migrations in these lakes of Sierra Nevada is already known, we have studied the relationship between egg ratios and chlorophyll-a concentration, mean temperatures and temperature instability (measured as the daily rate of temperature change — TCR — as well as the surface-bottom temperature difference — SBT -) in both lakes. Results show that the intensity of temperature fluctuations has a positive effect on the egg-ratios, as TCR is only correlated with that variable in the shallow lake and SBT is only correlated with egg-ratios in the deeper one.     

Evolution of temperature and salt structure of Lake Bonney, a chemically stratified Antarctic lake

A resurgence of interest in the ecology of perennially ice-covered lakes in the McMurdo dry valleys has necessitated a review of our knowledge of the physical and chemical properties of these unusual lakes. Salinities in the ice-covered lakes cover a range from freshwater to hypersaline brines. Recent measurements of salt composition and concentrations in Lake Bonney reveal little change below the chemocline since extensive measurements made in 1960–1961, although lake level has risen by approximately 5 m since that time. The rise in lake level has resulted in a thickening of the freshwater layer above the chemocline. Temperature structure has adjusted to the effects of increased lake level on heat transfer processes such as transmission and absorption of solar radiation in the water column.Questions about how water-column stability affects biology in Lake Bonney have motivated the formulation of a method to compute density from in situ measurements of temperature, conductivity and pressure. Owing to high salt concentration and unique ion ratios, we modified the UNESCO Equation of State for seawater to predict density at salinities greater than 42. The modifications merge smoothly with the UNESCO equations at a salinity of 42. At salinities below 42 the UNESCO equations give excellent predictions of density.     

Intensity of mineralization processes in mountain lakes in NW Slovenia

The potential and actual intensity of mineralization in sediments of fourteen mountain lakes and one subalpine lake in NW Slovenia have been measured. Potential mineralization was measured as the intensity of the electron transport system (ETS) activity of microzoobenthos and microbial communities and the actual mineralization as the oxygen consumption of respiration processes, both measured at a standard temperature of 20°C. The lakes are of different trophic levels and some exhibit seasonal anoxia. All but one are hardwater lakes. Two layers of sediment cores from the deepest point of the lakes were analysed: a surface layer and one below 15 cm. Significant differences among different lakes in their ETS activity and oxygen consumption in the surface and lower layers of sediment were observed. ETS activities and oxygen consumption rates were higher in the surface layers of all the lakes. From the three investigated deterministic factors (temperature, lake depth and total phosphorus in the water column) on sedimentary metabolism ETS activity in the surface layer correlated significantly with total phosphorus and lake depth, but oxygen consumption rate showed a significant correlation only with total phosphorus. The relationship between oxygen consumption and ETS activity was also investigated. ETS activities correlated with oxygen consumption rates according to the equation of logR = 0.421^* logETS + 0.898 (r=0.82; n=30; p<0.001). The R/ETS ratio was lower at the sediment surface than in the layers deeper than 15 cm. It is concluded that ETS activity and oxygen consumption are good indicators of the intensity of the metabolic activity and mineralization in lake sediments. As the characteristics of lakes and some environmental factors influence the ETS activity and the oxygen consumption differently, the same R/ETS ratio should not be used as conversion factor in calculations for different lakes.     

Quantifying effects of phytoplankton on the heat budgets of two large limnetic enclosures

1. The aestival heat budgets of two large limnetic enclosures within a small lake in the English Lake District were studied. During summer, these enclosures had different nutrient supplies and consequently different phytoplankton populations. 2. As initial temperature profiles were similar and the incoming surface heat and momentum fluxes for the two enclosures were identical, subsequent changes in the heat budget were assumed to be induced by the biological differences between the enclosures. The proposed mechanism is an increased surface absorption of solar radiation leading to extra surface warming and a consequent excess loss of heat to the atmosphere through long‐wave emittance and sensible and latent heat fluxes, conservatively estimated to be of the order of 10–30 W m^?2. 3. Theoretical calculations show that potential effects on a heat budget could be considerably larger than those observed here. The inherent non‐linearity of the heat fluxes implies that such effects will be more important in warmer lakes than in colder ones. 4. Thermocline depth and strength were also altered by the response to differences in phytoplankton. 5. Any changes in climate or in nutrient loading from the catchment which substantially affect abundance or timing of phytoplankton populations in a lake will consequently also change the thermal structure of the lake.     

Influence of elevated CO2 concentrations on thermal tolerance of the edible crab Cancer pagurus

Current trends of global climate change affect marine ectothermal animals not only through the increase in ambient temperature. Synergistic effects of carbon dioxide and temperature changes as well as more frequent hypoxia events must also be considered. As a first attempt, the combined effects of warming and elevated CO₂ concentrations were investigated in the edible crab (Cancer pagurus). Arterial oxygen tension (PaO₂) in the haemolymph was recorded on-line during a progressive warming scenario from 10 to 22 °C and cooling back to 10 °C. Hypercapnia (1% CO₂) caused a significant reduction of oxygen partial pressure in the haemolymph as well as a large, 5 °C downward shift of upper thermal limits of aerobic scope. The present findings are the first to show that hypercapnia causes enhanced sensitivity to heat and thus, a narrowing of the thermal tolerance window of a marine ectotherm. Such interactions of ambient temperature and anthropogenic increases in ambient CO₂ concentrations will need to be considered during future investigations of the effects of climate change on ecosystems.