The hydrology of Lake Victoria

Summary Evidence is presented to show that Lake Victoria is triply stratified the whole year round. The conclusions of FISH (1957) that a uninodal internal seiche operates in the lake is shown to be invalid, and the oscillations of isotherms at E.A.F.R.O. Open Lake Station observed over some years are explained by the day to day wind changes.It is calculated that an internal seiche could operate between the middle and bottom layers, but not between the middle and surface layers, since any such seiche would break down the stratification. Surface waves of the type which normally induce internal seiches are commonly observed on the lake, so it is concluded that the extreme shallowness and great length of Lake Victoria cause a rapid frictional damping of any incipient seiche.Lake Victoria has an annual heat budget of about 9,000 gram/cals/cm². This amount of heat is lost between March and August and regained between August and March. The loss of heat is occasioned mainly by the increase in evaporation from the lake in the period May to October, and partly by an increase in conduction and backradiation. The increase in evaporation rate between May and October assists in disposing of the surplus water derived from the rains in April.The lowest layer in the lake is believed to derive from the inflow of rivers. This layer mixes with the middle layer at the south end of the lake. An apparent area of mixing is suggested north of the Sesse Islands, also.Current measurements indicate a general northward flow of surface water in response to the wind. This is accompanied by a compensating flow of the two lower layers southwards. This latter movement is mainly phasic but probably has a small residual component.Lake level readings from gauges round the lake indicate that the west, north and north-east portions of the lake rise more rapidly in level than the south portion, from April to July.

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Résumé Cette étude montre une stratification permanente du Lac Victoria en trois couches. Elle tend à montrer que la conclusion à l'existence d'une seiche interne uninodale (Fish, 1957) n'est pas valable, et que les oscillations des isotherms observés sont dues au changement du régime des vents d'un jour à l'autre.Le calcul prouve qu'il est possible qu'une seiche interne agisse entre la couche profonde et la couche moyenne, mais certainement pas entre la couche moyenne et la surface, puis-qu' alors elle detruivrait la stratification.Des vagues du type qui normallement provoque les seiches sont couramment observées sur le Lac Victoria. L'Auteur en conclut que la tres faible profondeur et la forme allongée de ce lac causerait un rapide amortissement par friction d'une éventuelle seiche.Le Lac Victoria a un bilan calorifique annuel de 9,000 g/cal/cm². Cette quantité de chaleur est perdue entre mars et août. La perte de chaleur est essentiellement due à l'augmentation d'évaporation de mai à octobre et partiellement à une augmentation de conduction et de radiation. L'accroissement du taux d'évaporation de mai à octobre contribue à eliminer le surplus d eau provenant des précipitations d'avril.Les causes les plus profondes proviendraient de l'apport des rivières. Les couches profondes et moyennes se mêlent a l'extrémité Sud du lac. Il semble aussi que le mélange se fasse au Nord des îles Sesse.Le vent provoque en surface un courant dirigé vers le Nord, compensé par un deplacement vers le Sud des deux couches inférieurs. Ce deplacement est surtout periodique mais à aussi une l égère composante résiduelle.Des relevés de niveau tout autour du lac montrent que d'avril à juillet celui-ci s'élève plus rapidement à l'Ouest, au Nord et au Nord-Est qu'au Sud.

     

Effects of the discharge of thermal effluent from a power station on Lake Wabamun,Alberta, Canada — Limnological features

The physical and chemical aspects of Lake Wabamum have been described. Modifications to the thermal and dissolved oxygen regimes through the discharge of thermal effluent into the eastern region of the lake are discussed. This discharge has, therefore, changed the environmental conditions under which the biota exists in the eastern portion of the lake. It was also shown that this discharge of heated water had no effect upon the water chemistry while the power station itself contributed silica, in the form of fly ash, to the system. Tables of the phytoplankton, zooplankton, aquatic macrophytes, and fish are provided. From a biological and limnological stand point the lake can be classified as a moderately eutrophic lake, especially in the eastern portion.     

Perennially ice-covered Lake Hoare,Antarctica: physical environment,biology and sedimentation

Lake Hoare (77° 38 S, 162° 53 E) is a perennially ice-covered lake at the eastern end of Taylor Valley in southern Victoria Land, Antarctica. The environment of this lake is controlled by the relatively thick ice cover (3–5 m) which eliminates wind generated currents, restricts gas exchange and sediment deposition, and reduces light penetration. The ice cover is in turn largely controlled by the extreme seasonality of Antarctica and local climate. Lake Hoare and other dry valley lakes may be sensitive indicators of short term (< 100 yr) climatic and/or anthropogenic changes in the dry valleys since the onset of intensive exploration over 30 years ago. The time constants for turnover of the water column and lake ice are 50 and 10 years, respectively. The turnover time for atmospheric gases in the lake is 30–60 years. Therefore, the lake environment responds to changes on a 10–100 year timescale. Because the ice cover has a controlling influence on the lake (e.g. light penetration, gas content of water, and sediment deposition), it is probable that small changes in ice ablation, sediment loading on the ice cover, or glacial meltwater (or groundwater) inflow will affect ice cover dynamics and will have a major impact on the lake environment and biota.     

Climate and climatic variability at Lake Myvatn

The climate of the Lake Myvatn region is examined through the use of weather station data, using the years from 1971 to 2000 as a reference period. Variations in mean monthly temperature and precipitation at Reykjahlid (Myvatn) are compared with variations at seven other weather stations in north east Iceland. The area is drier and colder than coastal stations and exhibits a seasonal cycle in temperature that is larger than found at the coast. The temperature is significantly influenced by the number of sunlight hours only during the summer months. During summer, the influence of a sea breeze circulation can be clearly identified.The variability of climate since 1936 is also examined in comparison with the seven other weather stations. It is found that temperature variations at the different stations are highly correlated, but for precipitation the correlation is significant but much weaker.The influence of the large scale atmospheric circulation on temperature and precipitation at Lake Myvatn is also examined. It is found that the air temperature at Lake Myvatn is most sensitive to an east-west dipole in the large scale sea surface pressure field, i.e. to a pattern that is very different from the spatial pattern associated with the North Atlantic Oscillation (NAO). Precipitation at Lake Myvatn is to some degree influenced by the NAO, but generally precipitation is associated with northerly winds and cold temperatures whereas southerly winds at Lake Myvatn are likely to be drier.     

Effects of thermal effluents from the Bergum power station on the phytoplankton in the Bergumermeer

Summary The Bergum power station (600 MW) of the Friesian Provincial Electricity Board is situated at the northern shore of the Lake Bergum. The lake has a mean depth of 1.3 m and a surface area of 4.4 km². Its northern half is separated by a break-water into an intake area in the north-west and a discharge area in the north-east.The Lake Bergum is connected with other water bodies in the northern provinces of the Netherlands by four canals. The whole yaer various amounts of water enter Lake Bergum mainly from the western canal (Prinses Margrietkanaal) and to a lesser extent from the southern canal (De Lits). In wet seasons lake water flows off, mainly after passing the power station, to the northern canal (De Zwemmer); then the heated water (22 m³.sec_–1) does not enter the discharge area of the lake. When evapo-transpiration exceeds precipitation lake water flows off mainly to the eastern canal (Kolonelsdiep). In these relatively dry periods most of the heated water returns to the lake in the discharge area.We found that the mean increase in water temperature effected by the condensors of the power station was ca. 5°C; the maximum increase was 7.5°C. On average about 25% of the whole lake had a noticable higher (1°C) temperature than the intake water, only 6.5% was about 2°C above ambient temperatures.For about 3.5 years (1974–Sept. 1978) water samples for analysis of the chlorophyll concentrations of the different areas within the lake and the surrounding canals were taken every week during the growing season, and fortnightly during the winter period. The chlorophyll concentrations of the intake water were about 5% higher than those of the discharge water leaving the power station. Near the mouth of the northern canal in the discharge area still small, but significant lower chlorophyll concentrations were found. The southern half of the lake, in which practically no elevated water temperatures were found, had significant higher chlorophyll concentrations (10–15%) than the intake area. Water entering the lake from the western canal had significant (10–15%) lower chlorophyll concentrations than the intake area of the lake. Probably, relatively chlorophyll-poor canal water and chlorophyll-rich water from the southern lake area mix in the intake area. While the water passes the power station the chlorophyll concentrations decrease. In the discharge area of the lake the chlorophyll concentrations of the discharge water gradually increase again to values equal to those of the intake area.During the last 2 years of the research period oxygen production and consumption experiments were conducted almost every month. In each experiment light and dark botties containing intake and discharge water were suspended in water with both water temperatures. The light intensities during the incubation periods (2–3 hours) were chosen according to maximum production values. The incubations were started within one hour and/or one day after sampling. Directly after sampling gross productivity of the intake water incubated at discharge temperatures was about 1.5 times as high as at intake temperatures. The gross productivity of the discharge water was always somewhat lower than the gross productivity of the intake water incubated at corresponding temperatures. After one day this inhibiting effect of passage through the power station had increased, even when the discharge water had been cooled down to intake temperatures immediately after sampling.The oxygen consumption of the discharge water incubated at discharge temperatures as well as at intake temperatures was about 1.3 times the oxygen consumption of the intake water at intake temperatures. After one day the discharge water, which had stayed at discharge temperatures, consumed 1.6–1.7 times as much as the intake water incubated at intake temperatures. The oxygen consumption of the discharge water which had been cooled down to intake temperatures directly after sampling, was after one day still 1.3 times the oxygen consumption of the intake water at intake temperatures.This research was financially supported by the Ministerie van Volksgezondheld en Millieuhygiëne (Ministry of Public Health and the Environment). An extensive report (in Dutch) will be published this year.     

Basin-scale control on the phytoplankton biomass in Lake Victoria, Africa

The relative bio-optical variability within Lake Victoria was analyzed through the spatio-temporal decomposition of a 1997–2004 dataset of remotely-sensed reflectance ratios in the visible spectral range. Results show a regular seasonal pattern with a phase shift (around 2 months) between the south and north parts of the lake. Interannual trends suggested a teleconnection between the lake dynamics and El-Niño phenomena. Both seasonal and interannual patterns were associated to conditions of light limitation for phytoplankton growth and basin-scale hydrodynamics on phytoplankton access to light. Phytoplankton blooms developed during the periods of lake surface warming and water column stability. The temporal shift apparent in the bio-optical seasonal cycles was related to the differential cooling of the lake surface by southeastern monsoon winds. North-south differences in the exposure to trade winds are supported by the orography of the Eastern Great Rift Valley. The result is that surface layer warming begins in the northern part of the lake while the formation of cool and dense water continues in the southern part. The resulting buoyancy field is sufficient to induce a lake-wide convective circulation and the tilting of the isotherms along the north-south axis. Once surface warming spreads over the whole lake, the phytoplankton bloom dynamics are subjected to the internal seiche derived from the relaxation of thermocline tilting. In 1997–98, El-Niño phenomenon weakened the monsoon wind flow which led to an increase in water column stability and a higher phytoplankton optical signal throughout the lake. This suggests that phytoplankton response to expected climate scenarios will be opposite to that proposed for nutrient-limited great lakes. The present analysis of remotely-sensed bio-optical properties in combination with environmental data provides a novel basin-scale framework for research and management strategies in Lake Victoria.     

Density structure of the fine surface water layer of large lakes and remote measurement of the temperature

Remote sensing of surface temperatures in large lakes differs in some critical respects from that in marine conditions. Firstly, the atmospheric structure over large lakes has some continental features, which makes the calculation of its influence more complicated. Secondly, the vertical temperature structure in the fine surface layer of a lake is more complex than in marine waters. A prominent feature of the thermal structure of spring heating of large lakes (e.g. in Lake Ladoga from May to July) is the existence of a thermal front, which is manifested on the lake surface as a fine strip with steep horizontal temperature gradient. The existence of a thermal front makes it possible to calibrate infra-red space survey data without using in situ surface temperature measurements. The calibration of arbitrary heat brightness values for +4 °C can be obtained by plotting a frequency distribution histogram of the heat brightness values obtained for the IR-image pixels. While the thermal front is in existence, the heat brightness frequency distribution appears bimodal, and the brightness value at the minimum between the peaks is assigned the temperature of +4 °C. Laboratory and field experiments have enabled us to separate two types of near-surface density structures. Under conditions of density instability in the fine surface layer, microconvection limits the magnitude of vertical temperature difference in this layer to a constant range of 0.6–0.8 °C. However, when a stable density structure prevails during calm conditions, much greater vertical temperature differences may exist.     

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.     

On polynuclear aromatic hydrocarbons contamination levels in the ecosystem of Lake Peipsi in the 1970s–1980s

The data on benzo(a)pyrene (BaP) and other polynuclear aromatic hydrocarbons (PAH) pollution of Lake Peipsi and its tributaries in the 1970s–1980s have been summarized. Systematic data on the pollution of Lake Peipsi by BaP are available. The BaP content in lake water varied from <0.1 to 237 ng 1^–1. The content and qualitative composition of PAH varied in different observations. PAH were accumulated mainly in the bottom sediments; in their surface layer BaP concentrations were 10³–10⁶ times higher than in water. The concentration of the PAH in bottom sediments varied from 16 to 580 µg kg^–1 in the lake and from 40 to 2800 µg kg^–1 in its tributaries. The content of BaP in fish and biota was not high. Among the pollution sources the atmospheric pollution played quite a significant role. The lake was also polluted by oil spillage and fuel exhausts from boats and ships. The PAH concentration in the water of the tributaries was somewhat higher than in the area of Lake Peipsi.     

Resuspension in a shallow eutrophic lake

The frequency and the importance of wind-induced resuspension were studied in the shallow, eutrophic Lake Arresø, Denmark (41 km², mean depth 3 m). During storm events in autumn 1988 lake water samples were collected every 2–8 hours by an automatic sampler at a mid-lake station. The concentration of suspended solids and Tot-P was found to increase markedly. During storms up to 2 cm of the superficial sediment was resuspended, and the concentration of resuspended solids in the water column rose to 140 mg l^–1. The resuspended particles had a relatively high settling velocity and on average, a relatively short residence time in the water column of 7 hours.A model which describes the concentration of resuspended solids as a function of wind velocity and of settling velocity of the resuspended particles is presented. Using additional wind velocity data from a nearby meteorological station, the model has been used to calculate the frequency of resuspension events and concentration of resuspended solids for the period from May to November 1988.These calculations show that resuspension occurred about 50% of the time. Average flux of suspended solids from the sediment to the water was 300 g m^–2 d^–1 and during 50% of the time lake water concentration of suspended solids was more than 32 mg l^–1. A relationship between concentration of suspended solids and Secchi-depth is presented. Because of resuspension, Secchi-depth in Lake Arresø is reduced to 0.5 m.Resuspension also had a marked effect on Tot-P concentration in the lake water, and P input to the lake water being totally dominated by resuspension events.     

Phytoplankton ecology of the Lake of Menteith,Scotland

The results discussed in this paper represent the first seasonal ecological study carried out on the phytoplankton of the Lake of Menteith. All measured nutrients reached maximum levels during the winter, with silicate showing particularly high concentrations (up to 85 µg at Si l^–1). During the summer period phosphate, nitrate and silicate showed almost complete exhaustion in surface waters. The lake water was consistently alkaline, never falling below pH 7, while the alkalinity ranged from 20 to 24 mg CaCO₃ l^–1. Generally, the nutrient status of the main inflow had a rapid effect on the water quality of the lake.The region of the lake under investigation showed no thermal stratification at any period of the year, although continuous thermal gradients were recorded in the winter. The continual circulation of the water mass probably prevented oxygen saturation from falling below 77% even following a large phytoplankton bloom and subsequent decomposition.From an examination of net phytoplankton samples the Lake of Menteith could be described as blue-green or blue-green/diatom in nature. From the quantitative study, large pulses of Melosira, Asterionella and Fragilaria were recorded in the spring. The disappearance of the species appears to be related to silicate limitation. The summer growth of Asterionella may have been promoted by a nitrogen source other than nitrate and nitrite, both of which were reduced to critical levels. This alternative source of combined nitrogen may have been contributed by nitrogen-fixing algae in the lake. Three species of Anabaena were recorded, all of which produced large populations during the year.Department of Botany, The University of GlasgowPresent Address: Department of Biology, College of Science, University of Sulaimaniyah, Sulaimaniyah, Iraq     

A survey of toxicity of cyanobacterial blooms in Lake Ladoga and adjacent water bodies

Twentyfive cyanobacterial blooms in Lake Ladoga and adjacent water bodies were studied in the summer of 1990–1992. Toxicity of the water bloom material for mice was detected in 9 cases. The maximal tolerable doses (MTD) of the material extracted from biomass varied within 3–30 mg kg^–1 mouse body weight; 50% lethal doses (LD₅₀) were within 45–125 mg kg^–1. Toxic water blooms were registered in Karelian lakes and in the Neva Bay, Gulf of Finland. Cyanobacterial samples collected on the eastern coast of Lake Ladoga proved to be non-toxic. The species identified in toxic bloom material included Anabaena circinalis, A. flos-aquae, A. lemmermannii, Anabaena sp., Aphanizomenonflos-aquae, Gloeotrichia echinulata, G. pisum, Microcystis aeruginosa and Oscillatoria sp. These data suggest that toxic forms of cyanobacteria are widespread in Karelian lakes belonging to the drainage basin of Lake Ladoga.     

Palaeolimnological analyses as information source for large lake biomonitoring

A routinely applicable palaeolimnological sampling and analysis programme for large lake monitoring was developed as part of the Lake Saimaa Biomonitoring Project (1990–1993). The scope of palaeolimnological analyses is to gain background information of the recent past (ca. 0–200 yr) of the lake ecosystem that is being monitored. We analyzed short sediment cores from three contrasting basins of the complex lake Saimaa in eastern Finland (nutrient-poor clearwater Lake Puruvesi; oligotrophic mesohumic Lake Paasivesi, and eutrophied and polluted mesohumic Lake Haukivesi). Each basin shows a unique developmental history under the various forms of human influence on the ecosystem. The following research approaches are evaluated in the study: echosounding, dating (²¹⁰Pb varves, soot stratigraphy), element stratigraphies, biological remains (diatoms, cladocera, chironomids, sedimentary pigments).     

The temporal and spatial distribution, composition and abundance of Protozoa in Chaohu Lake, China: Relationship with eutrophication

Systematic investigations into the temporal and spatial distribution, composition and abundance of protozoa in two regions with different trophic levels in Chaohu Lake, a large, shallow and highly eutrophic freshwater lake in China, were conducted during 2002–2003. A total of 114 species of protozoa, including phytomastigophorans, zoomastigophorans, amoebae and ciliates, were identified from 120 polyurethane foam unit (PFU) samples exposed at four stations and from various types of natural substrates. Of the 114 taxa, 36 core species were found on PFU substrates and 23 of these were found on natural ones. Protozoan abundance and chemical–physical parameters at nine sampling stations, four in the western lake and five in the eastern part, indicate trophic gradient changes along the lake. Seasonal variations in the species composition of major groups at littoral PFU sampling stations illustrate the effect of a severe algal bloom on the protozoan community structure. Temporal and spatial distributions of individual abundance as functions of water temperature and trophic status were revealed. This study demonstrates again that the PFU artificial substrate method samples protozoan communities more effectively than routine natural substrate methods.     

Interaction of water quantity with water quality: the Lake Chapala example

Water quality may be significantly determined by water quantity. Lake Chapala, México is a large lake beset with numerous water quality problems. The decline in water volume over the past 20 years, a serious problem itself, is associated with causing or enhancing several problems of quality. Five such problems are explored herein. These are: extensive infestations of water hyacinth (Eichhornia crassipes), a declining native fishery, light limitation of phytoplankton production at the base of the food chain, shallow-water algal blooms resulting in water supply treatment problems, and the presence of toxic metals in the harvested and sold fishes.     

Picoplankton photosynthesis and diurnal variations in photosynthesis-irradiance relationship in a eutrophic and meso-oligotrophic lake

The abundance and relative importance of autotrophic picoplankton were investigated in two lakes of different trophic status. In the eutrophic lake, measurements of primary production were performed on water samples in situ and in a light incubator three times during the day whereas for the oligotrophic lake, only one measurement of primary production was performed on water samples in the incubator. Dark-carbon losses of phytoplankton from Lake Loosdrecht were investigated in time series. Cell numbers of autotrophic picoplankton in eutrophic Lake Loosdrecht (3.2 × 10⁴ cells ml^–1) were lower than in meso-oligotrophic Lake Maarsseveen (9.8 and 11.4 × 10⁴ cells ml^–1 at the surface and bottom respectively). In the phytoplankton of both lakes the ratio of picoplankton production increased with decreasing light intensity. In Lake Loosdrecht depth-integrated contribution of picoplankton to total photosynthesis was less than 4%. The P-I-relationship showed diurnal variations in light saturated photosynthesis, while light limited carbon uptake remained constant during the day. Dark carbon losses from short-term labelled phytoplankton during the first 12 hours of the night period accounted for 10–25% of material fixed during the preceeding light period.     

Spatial distribution and seasonal changes of pesticides in Lake Biwa,Japan

The spatial distribution and seasonal variation in the concentrations in Lake Biwa of pesticides used in paddy fields were studied. Lake Biwa is the largest lake in Japan and is a recognized water resource for 14 million people in the Kinki district. Samples were collected nine times from April to December 2001 at ten sites within the lake and at the mouths of six influent rivers. Weekly sampling was also carried out at a single site on an effluent river. Among the 20 pesticides analyzed, the detection frequencies in surface water were almost 100% for simetryn, bromobutide, and isoprothiolane; around 75% for molinate and pyroquilon; around 30% for three herbicides and one fungicide; and almost zero for the remaining substances. The maximum concentrations of pesticides detected frequently in the lake were in the range 0.1–0.4µgl^–1. The occurrence of a few pesticides below the thermocline may be explained by thermal stratification and vertical circulation. Although the thermocline suppressed vertical diffusion in spring and summer during pesticide application periods, a few pesticides remaining at the surface of the lake in winter were transported to the hypolimnion by vertical circulation and remained there even after the reestablishment of the thermocline. The half-lives of pesticides in the lake were estimated to be more than a year for simetryn, half a year for bromobutide, 1.5 months for molinate, and 1 month for dimepiperate. The main cause of elimination for molinate and dimepiperate was estimated to be degradation, that for simetryn was outflow, and for bromobutide both degradation and outflow were significant.     

Precursors of carcinogenic N-nitroso compounds in Lake Peipsi

The concentration of precursors of carcinogenic N-nitroso compounds, nitrates and nitrites as well as ammonia, in the surface water of Lake Peipsi and its tributaries has been determined during the period 1985–1988. The nitrate and nitrite content was also analysed in bottom sediments and fish from the lake.The nitrate concentration in the water of Lake Peipsi varied from 0.01 to 2.33 mg NO₃&z.sbnd;N l^–1, the average value from 0.27 to 1.60 mg NO₃&z.sbnd;N l^–1, with the lowest concentrations in summer. The variations may be caused by different pollution loads, meteorological conditions, and assimilation of nitrates by plants and algae.The nitrate content in the water of rivers was on an average somewhat higher in comparison with its concentrations in the lake. The concentrations of nitrites were, as a rule, about an order of magnitude lower than those of nitrates. The amount of ammonia varied from 0.15 to 0.36 mg NH₄&z.sbnd;N l^–1.At present the concentrations of the studied nitrogen compounds are not essential and do not prevent from using the lake for recreation and drinking water supply.     

The change of sediment composition during recovery of two Finnish lakes induced by waste water purification and lake oxygenation

Total oxygen deficit occurred regularly during stagnation periods in the deepest part of Lake Kallavesi in the period 1973–1986. The sediment was black and anaerobic during the first sampling in 1987. After beginning of artificial lake oxygenation and efficient purification of waste waters of a paper board mill in 1986 the oxygen deficit decreased gradually and a light brown oxidized uppermost sediment layer appeared and began to thicken. The following changes in the sediment composition were observed during 1987–1996: loss on ignition, total nitrogen and BOD₇ concentrations decreased in the uppermost sediment layer (0–2 cm) and BOD₇ concentration increased in the next sediment layer beneath (2–10 cm). There were no significant change in phosphorus and iron concentrations.Lake oxygen, total phosphorus and suspended solids concentrations fluctuated in a noticeable degree in Lake Huruslahti depending on waste water input and artificial oxygenation during the years 1980–1993. Oxygen condition was good at times of successful waste water elimination and lake oxygenation while deterioration of either resulted oxygen deficiency as well as increase of total phosphorus and suspended solids concentration. Most of the internal load entered with suspended solids during periods of total oxygen deficiency.An explanation for the findings in Lake Huruslahti could be microbiological. Gas formation inside sediment lift organic material towards top of the sediment and into the water, but after the lake recovery the material retain in the sediment. Also in Lake Kallavesi microbiological gas formation resuspended sediment particles with phosphorus into the overlaying water prior to oxygenation. During oxygenation microbiological processes in uppermost sediment utilize the anaerobic metabolic products, organic acids and methane, and block gas formation. Organic substances remain in the top sediment decomposing gradually in the uppermost layer.     

Chemical composition of interstitial water and diffusive fluxes within the diatomaceous sediment in Lake Myvatn, Iceland

The sub-arctic Lake Myvatn is one of the most productive lakes in the Northern Hemisphere, despite an ice-cover of 190 days per year. This is due to relatively high solar radiation, nutrient rich inflow waters, N₂ fixation and internal nutrient loading. In order to define direction and magnitude of diffusive fluxes, soil water samplers were used to collect interstitial water from 25–150 cm depth, from within the diatomaceous sediment at the bottom of Lake Myvatn. Water depth at the sampling site was 225 cm. The pH of the interstitial water ranged from 7.16 to 7.30, while the pH of the lake water was 9.80–10.00. The concentrations of most solutes were similar 16 cm above the bottom of the lake at the sampling site and at the lake outlet. The concentrations of NO₃, S, F, O₂, Al, Cr, Mo, V, U, Sn and Sb were higher in the lake water than in the interstitial water. They will therefore diffuse from the lake water into the interstitial water. The concentrations of orthophosphates, PO₄, and total dissolved P were highest at 25 cm depth, but Co and NH₄ concentrations were highest at 50 to 100 cm depth. Thus they diffuse both up towards the lake bottom and down deeper into the sediments. The concentrations of Na, K, Ca, Mg, Sr, Mn, Li and alkalinity were greater within the sediments than in the lake water and increased continuously with depth. The Si concentration of the interstitial water was higher than in the lake water, it was highest at 25 cm depth and decreased slightly down into the sediments. The concentration gradient was greatest for bicarbonate, HCO₃ ^–, 1.5×10^–7 mol cm^–3 cm^–1, and then in declining order for the solutes with the highest gradient; NH₄, Si, Na, Ca, Mg, -S (diffusion into the sediments), K, PO₄, Cl, Fe and Mn. The estimated annual diffusive flux of PO₄ for Lake Myvatn was 0.1 g P m^–2 yr^–1, about 10% of the total PO₄ input to Lake Myvatn. The H₄SiO₄° flux was 1.3 g Si m^–2 yr^–1, <1% of both the input and the annual net Si fixation by diatoms within the lake and the diffusive flux of dissolved inorganic carbon was 1% of the annual net C fixation by diatoms. Annual diffusive flux of NH₄ ⁺ was 1.9 g N m^–2 yr^–1 similar to the input of fixed N to the lake and 24% of the net N fixation within Lake Myvatn. Thus it is important for the nitrogen budget of Lake Myvatn and the primary production in the lake since fixed nitrogen is the rate determining nutrient for primary production.