Hydrological cycle and interannual variability of the aquatic community in a temporary saline lake (Fuente de Piedra,Southern Spain)

Fuente de Piedra is a shallow, temporary saline lake whoseseasonal behavior is strongly dependent on the annual hydrologicalbudget. In this study, we outline the characteristics of Fuente dePiedra Lake for two years that had different hydrological budgets.The high precipitations in 1989–90 caused the lake not to dry asusual, and decreased both salinity and the amplitude of changes.There were also differences in nutrient dynamics, with generallylower concentrations of soluble reactive phosphorus and ammonium,whereas in the more humid year nitrate showed a distinct maximum inwinter. Winter bloom chlorophyll a concentrations were alsomuch higher in 1989–90 (>600 g l-1) but there wasalso a winter productive phase that was presumably poorly coupledwith consumption processes that predominate in spring. Planktonicassemblages were different between years. Highly halotolerantphytoplankton species (Dunaliella salina and D. viridis) became scarcer, and especially two previouslyunrecorded diatoms (Cyclotella sp. and Chaetoceros sp.)became dominant in the bloom time in the wet year. The speciesrichness of the zooplankton increased in the wet year, with newspecies appearing that were not collected during 1987–88(Branchinectella media, Daphnia mediterranea, Macrothrix sp.,Arctodiaptomus salinus, Cyclops sp., Sigara sp...).There was also a much higher development of macrophytes (Ruppiadrepanensis, Althenia orientalis, Lamprothamnium papulosum)and bird populations, especially flamingoes (Phoenicopterusruber).Important interannual variations in this sort of system pointto the need for long term studies to observe the whole range ofstates that define the lake as an entity.     

Species composition and phytoplankton biomass in a tropical African lake (Lake Awassa,Ethiopia)

The species composition and phytoplankton biomass of Lake Awassa, Ethiopia were studied from September 1985 to July 1986 in relation to some limnological features of the lake. During the study period, three phases of thermal stratification were recognized: a period of unstable stratification and near-complete mixing was followed by a stable stratification period and another period of complete mixing. Complete mixing was associated with cooling of air temperature with an influx of cool rain and high rainfall. The underwater light penetration showed a similar pattern over the whole period with the highest in the red, and the lowest in the blue spectral region. Euphotic depth varied between 1.6 and 3.0 meters with the highest measurements corresponding to the stable stratification period. PO₄-P concentrations ranged between 23 and 45 µg l^–1 and NO₃-N concentrations varied between 7 and 14 µg l^–1 during the study period. Both nutrients showed increasing values associated with mixing periods and/or the rainy season.A total of 100 phytoplankton species were identified with 48% of the taxa represented by green algae, 30% by blue-green algae, 11% by diatoms, and the rest by chrysophytes, dinoflagellates, cryptomonads and euglenoids. The dominant phytoplankton species were Lyngbya nyassae, Botryococcus braunii and Microcystis species. Seasonal biomass variation was pronounced in the first two species but not in Mycrocystis. Phytoplankton biomass increased following the mixing period in December, and thermal destratification during May to July which was also a period with high rainfall and relatively high nutrient concentration. While the seasonal variation of the total phytoplankton community in Lake Awassa was relatively low (coefficient of variation < 20%), it was higher in some of the individual component species.     

Burrowing beetles of the genus Bledius (Staphylinidae) as agents of bioturbation in the emergent areas and shores of an athalassic inland lake (Fuente de Piedra,southern of Spain)

Bledius (Elbidus) bicornis (Germ.) and B. (Eucerotobledius) furcatus (Oliv.) (Coleoptera: Staphylinidae) are the most important burrowing species in the emergent areas and shores in the athalassic lake of Fuente de Piedra (Málaga, S. of Spain). A first estimate of the importance of these organisms in this system is presented. These insects kick out sediment during their burrowing activity, which accumulates on the surface near the burrows as tumuli which can be easily eroded. The lake perimeter (17 km) is densely colonized (usual densities from 1700 to 2500 ind m⁻²). The amount of granulated material that can be potentially kicked out was 46.22 g dry wt m⁻² day⁻¹. At the same time, the material that constitutes the tumuli shows different characteristics from the compact ground below the surface. Thus, it is relatively enriched with organic matter (6.15 g per square meter), soluble phosphate (406.5 μg m⁻²) and ammonium (4856 μg m⁻²), whereas it lacks nitrate. Results of a transect from uninhabited areas to zones of maximum population density also show a similarity between the higher ground level of ammonium and phosphate concentrations and population density.     

Seasonality in Nutrients and Phytoplankton Production in Two Shallow Lakes: Waigani Lake,Papua New Guinea,and Barton Broad,Norfolk, England

Waigani Lake, near Port Moresby, Papua New Guinea and Barton Broad, Norfolk, England are both shallow lakes nutrient-enriched from sewage effluent disposal. In Waigani Lake phytoplankton biomass varied seasonally with lower levels (100-200 mg chlorophyll α m⁻³) during the wet season increasing to over 400 mg chlorophyll α m⁻³ at the end of the dry season. Secchi disc depths varied between 0. 11 and 0. 34 m. Phytoplankton productivity in Waigani Lake was very high throughout the year (range: A_max 4,370-21,000 mg O₂ m⁻³ h⁻¹) but production was lower during the wet season (range: A_max 4,370-12,700 mg O₂ m⁻³ h⁻¹). High surface productivity was recorded from August to December except on sampling days when the weather was overcast. Productivity throughout the year declined rapidly with depth. Algal biomass in Barton Broad varied from 3-10 mg chlorophyll α m⁻³ in winter but increased in spring and was very high in summer (200-500 mg chlorophyll α m⁻³). Secchi disc depth varied from 0.21 m in August 1976 to 1.76 m in December. Phytoplankton production in Barton Broad was low in winter (range: A_max 247-1,250 mg O₂ m⁻³ h⁻¹) but increased markedly in spring and summer with the highest rate (A_max 6,850 mg O₂ m⁻³ h⁻¹) being recorded in August. Surface inhibition was observed during summer except when the weather was overcast. Seasonality in nutrients and phytoplankton in Waigani Lake appear to be related to rainfall. Nutrient concentrations in Barton Broad are more closely related to phytoplankton activity which, in turn, correlates with seasonality in solar radiation.     

Zooplankton Seasonal Abundance of South AmericanSaline Shallow Lakes

The central provinces of Argentina are characterized by the presence of a high number of shallow lakes, located in endorheic basins, many of which have elevated salinities as well as eutrophic or hypereutrophic condition. The zooplankton of four saline shallow lakes of the province of La Pampa was studied on a monthly basis during a 2‐year period to determine its temporal and spatial variation. The surface of these shallow lakes (<2.5 m depth) varied between 56.8 and 215.9 ha, and some have from 8.4 to 20.8 g · l^–1. The more saline lakes have “clear” water and the less saline lakes “turbid” water. Fishes, Jenynsia multidentata , were present in only two lakes during the last two months of the studied period. The zooplankton was composed of 17 taxa of Rotifera, 5 taxa of Cladocera and 4 taxa of Copepoda. The low diversity and the faunistic composition are characteristic of saline environments. Although the studied lakes share 38% of the species, the faunistic similarity was higher between the two least saline lakes. The lowest diversity was found in the two most saline lakes. All four shallow lakes were characterized by their very high zooplankton density, especially in the least saline lakes (<80000 ind · l^–1). The abundance is significantly correlated with the water transparency but not with salinity. The zooplankton temporal variation was characterized by the alternation of macro‐ and microzooplankton, probably regulated by competition and intrazooplanktonic predation. In each lake, the spatial abundance distribution of the macro‐ and microzooplankton was homogeneous. It was related to the shallow depht of the lakes and their polymictic condition. The Scheffer model on alternative states in shallow lakes acknowledges that it cannot be applied to saline lakes because Daphnia , the main responsible for the clear water state, is not tolerant to high salinity. Our study shows that the most saline lakes, where the halophylic Daphnia menucoensis is abundant, have also the most clear waters. Another difference that we found with regards to the mentioned model is that, in turbid lakes, it could not have had a top‐down control on macrozooplankton exerted by fishes because in these lakes fishes were practically absent. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Plankton dynamics in a high mountain lake (Las Yeguas,Sierra Nevada,Spain). Indirect evidence of ciliates as food source for zooplankton

A detailed sampling programme during the ice-free season (July–September) in the oligotrophic lake Las Yeguas (Southern Spain) has shown a well-defined time lag between phytoplankton and zooplankton maximum standing stocks, the former displaying a peak (23 μgC l^-1) just after the ice-melting, and the latter by the end of September (80 μgC l^-1). A ratio of autotrophs to heterotrophs lower than 1 which lasted more than two thirds of the study period may suggest a high algal productivity per unit of biomass. The estimated strong top-down regulation of phytoplankton by zooplankton indicates an efficient utilization of resources. A comparative analysis between the available food supply and the critical food concentration that is necessary to maintain the population of Daphnia pulicaria (which constitutes up to 98% of the heterotrophic biomass) proves this species to be food-limited in the lake under study. To explain the dominance (and development) of such large-bodied cladoceran population, we discuss the possibility of the utilization of naked protozoan ciliates (Oligotrichidae) as a complementary high quality food source, and the exploitation of benthic resources through a coupled daily migration behaviour.     

Interactions between Phytoplankton,Zooplankton, and Fish in the Nutrient Rich Shallow Lake Hjarbæk Fjord,Denmark

In the shallow, nutrient-rich dammed-up lake Hjarbæk Fjord the balance between phytoplankton, zooplankton, and fish was completely disturbed during the summer period. Extensive growth of small blue-green algae (Oscillatoria cfr. trichoides, Pseudanabaena galeata) and chlorococcal green algae (Scenedesmus spp., Monoraphidium contortum, Actinastrum hantzschii) induced a pH rise to 10.5 in July. Fish were driven into the rivers, and many that did not escape died. After the fish had disappeared daphniae multiplied explosively and grazed away the major part of the phytoplankton. Ammonia concentrations rose and oxygen concentrations dropped to noxious levels for highly sensitive fish in August–September. Larger phytoplankton species (Aphanizomenon flos-aquae, Pediastrum boryanum, Pandorina morum, a. o.) were not grazed by the daphniae and gave rise to a maximum of copepods in September. Fish returned to the lake in October, when pH, ammonia, and oxygen levels had normalized.     

Population dynamics of Limnothrix redekei,Oscillatoria lanceaeformis,Planktothrix agardhii and Pseudanabaena limnetica (cyanobacteria) in a shallow hypertrophic lake (Spain)

Populations of Limnothrix redekei, Oscillatoria lanceaeformis, Planktothrix agardhii and Pseudanabaena limnetica were found in a hypertrophic, gravel-pit lake near Madrid (Spain), throughout a one year sampling at weekly intervals. Physico-chemical factors, phytoplankton biomass and net growth rates were measured. Oscillatoria lanceaeformis was only observed a few weeks, probably being related to phosphorus limitation. Planktothrix agardhii biomass was related to PhAR irradiances, light attenuation coefficient in the water, Brunt-Väisäla frequency and decrease of soluble reactive phosphorus. Limnothrix redekei and Pseudanabaena limnetica biomass values were related to a decrease of inorganic nitrogen and temperature. A different lag response of populations was observed in relation to the environmental features.     

Distribution and population dynamics of ostracodes (Crustacea,Ostracoda) in a karstic lake: Lake Banyoles (Catalonia,Spain)

Qualitative and quantitative benthic samples were collected monthly between December 1986 and December 1987 from several stations at various depths and basins in the karstic Lake Banyoles (Catalonia, Spain), to study the spatial distribution and seasonality of ostracodes. A total of eight ostracode species were found in the lake. Of these, Candona neglecta Sars, Isocypris beauchampi Paris and Cypria ophtalmica Jurine were the most common ostracodes at all depth stations. Darwinula stevensoni Brady & Robertson, Ilyocypris species (I. bradyi Sars and I. gibba (Ramdohr)), Cyprideis torosa (Jones) and Cyclocypris ovum Jurine, on the other hand, were confined to sublittoral and littoral areas of the lake.Substrate and organic matter content appeared to be relevant factors determining the distribution of ostracode species at the different stations. The highest abundance of ostracodes were in the sublittoral (Basin IV–7 m and Basin I-5 m) where the substrate was coarse and sediment had a high organic matter content (>35% LOI). Low oxygen levels in the hypolimnetic waters (i.e. < 1 mg l^–1) may explain the absence of ostracodes at deeper zones in stations where the anoxic period lasts more than four months. On the other hand, low temperatures seem to favour the development of C. neglecta and D. stevensoni, while with higher temperatures I. beauchampi, C. torosa and C. ovum increased their abundances. However, no clear pattern can be observed, so ostracode species in Lake Banyoles, apparently, have non-seasonal life histories.     

Top-down control of phytoplankton in a shallow hypertrophic lake: Little Mere (England)

Top-down control of phytoplankton by zooplankton is possible through reductions in density of zooplanktivorous fish. Little Mere is a shallow lake where the effects of sewage effluent caused such a reduction. This allowed the large-bodied cladoceran, Daphnia magna Straus, to develop huge populations, preventing potentially large algal crops from developing.Subsequent diversion of the effluent is anticipated to lead to recovery of the fish community, reduced numbers of large-bodied grazers, and increased phytoplankton biomass. Whether the aquatic plant community, present in Little Mere, is resilient to such changes may depend upon whether cyanophytes are favoured, or not.     

Succession of phytoplankton in a deep stratifying lake: Mondsee,Austria

Phytoplankton numbers, biovolume, chlorophyll-a and various physico-chemical characteristics were followed at weekly intervals in Mondsee, Austria during the year 1982. Secchi-disk transparency varied from 10 m in winter to 2 m in September. Prior to the onset of stratification phosphate-phosphorus concentration was 4 µg 1^–1 decreasing to undetectable values thereafter. Nitrate-nitrogen dropped from 590 µg 1^–1 to about 100 µg 1^–1 during the same time. The vernal bloom was dominated by Asterionella formosa Hass. which abruptly declined after silicon depletion. Spring growth ceased in early June, when Tabellaria flocculosa (Lyngb.) Kütz var. asterionelloides Grun. dominated. Oscillatoria rubescens D.C. and Microcystis aeruginosa Kütz. dominated summer and early autumn followed by the chrysophyte Dinobryon divergens Imh. and D. sociale Ehr. which formed up to 69% of total biovolume in October. Thereafter diatoms and Cryptophyceae (Rhodomonas lacustris Pascher and Ruttner, Cryptomonas pusilla Bach.) became abundant again.Maximum chlorophyll-a concentration in the epilimnion (16 µg 1^–1) was reached during spring growth of the diatoms. During summer higher chlorophyll-a levels were always associated with the metalimnetic layer of Oscillatoria.Compared with earlier studies, both the total biovolume and the share of Oscillatoria rubescens significantly decreased because of reduced nutrient loading of the lake and wash-out of Oscillatoria (theor. renewal time of the lake: 1.7 years).     

Seasonal changes in the chemistry and biology of a meromictic lake (Big Soda Lake,Nevada, U.S.A.)

Big Soda Lake is an alkaline, saline lake with a permanent chemocline at 34.5 m and a mixolimnion that undergoes seasonal changes in temperature structure. During the period of thermal stratification, from summer through fall, the epilimnion has low concentrations of dissolved inorganic nutrients (N, Si) and CH₄, and low biomass of phytoplankton (chlorophyll a ca. 1 mgm ^-3). Dissolved oxygen disappears near the compensation depth for algal photosynthesis (ca. 20 m). Surface water is transparent so that light is present in the anoxic hypolimnion, and a dense plate of purple sulfur photosynthetic bacteria (Ectothiorhodospira vacuolata) is present just below 20 m (Bchl a ca. 200 mgm^-3). Concentrations of N H₄ ⁺, Si, and CH₄ are higher in the hypolimnion than in the epilimnion. As the mixolimnion becomes isothermal in winter, oxygen is mixed down to 28 m. Nutrients (NH₄ ⁺, Si) and CH₄ are released from the hypolimnion and mix to the surface, and a diatom bloom develops in the upper 20 m (chlorophyll a > 40 mgm^-3). The deeper mixing of oxygen and enhanced light attenuation by phytoplankton uncouple the anoxic zone and photic zone, and the plate of photosynthetic bacteria disappears (Bchl a ca.10mgm^-3). Hence, seasonal changes in temperature distribution and mixing create conditions such that the primary producer community is alternately dominated by phytoplankton and photosynthetic bacteria: the phytoplankton may be nutrient-limited during periods of stratification and the photosynthetic bacteria are light-limited during periods of mixing.     

The importance of nitrogen in Pyramid Lake (Nevada,USA), a saline,desert lake

The increase in human development in the downstream portion of the Pyramid Lake drainage basin has resulted in increased nutrient loading to the lake. Since this is a deep, terminal lake, concern over nutrient build up and change in trophic status exists. On the basis of lake chemistry which shows consistently high concentrations of total reactive-P (mean = 55 µg P l^–1) relative to dissolved inorganic-N (DIN) (mean = 15 µg N 1^–1), it has been hypothesized that Pyramid is N-limited. However, no systematic study of nutrient limitation had been undertaken. Nutrient enrichment bioassays conducted throughout an entire year clearly showed that additions of DIN resulted in a 350–600% stimulation of chlorophyll production. Phosphate, when added singly or in combination with DIN, had no effect. This positive response to N-addition was significant at all times of the year except, (1) immediately after complete lake mixing in February when a large pool of hypolimnetic nitrate was injected into the euphotic zone, and (2) during a fall bloom of the nitrogen fixing species Nodularia spumigena. The positive response to N-addition in the bioassay experiments was strong between March and November. However, the seston exhibited only a gradual depletion of nitrogen relative to carbon over this same period. PN:PC ratios suggested no N-deficiency in phytoplankton biomass in February, March and April, moderate N-deficiency in May, June and July and, severe N-deficiency from August until winter turnover. The appearance of nitrogen fixing blue-green algae in September supports the hypothesis of N-limitation in the summer-autumn. In evaluating the nutrient status of a lake, the concepts of nutrient stimulation versus nutrient deficiency versus nutrient limitation must clearly be defined.This paper is dedicated to G. Evelyn Hutchinson who first visited Pyramid Lake in 1933.     

The development and persistence of alternative ecosystem states in a large,shallow lake

1. Palaeolimnological data were used to investigate drivers of the community of primary producers in Lake Mattamuskeet, North Carolina, U.S.A. This is a large, shallow lake with two basins currently dominated by phytoplankton and macrophytes. The two basins were divided in 1940 by the building of a roadway across the lake, which also corresponded with the divergence in their ecosystem state. 2. Photosynthetic pigments, organic matter and nutrients (P, N, C, S) were analysed in sediment cores from each basin to reconstruct the primary producer community over the past c. 100 years. We sought to answer two questions. First, what changes to the ecosystem resulting from the building of the roadway caused the development of different primary producer communities in the two basins? Second, why have the alternative ecosystem states persisted despite a variety of human perturbations since 1940? 3. K‐means cluster analysis and principal component analysis were applied to identify three sediment types based on photosynthetic pigment data: sediments indicating low productivity (low pigment concentrations), sediments associated with macrophytes (chlorophyll a and b) and with phytoplankton (alloxanthin and aphanizophyll). In addition, other palaeolimnological proxies measured, such as loss on ignition, total phosphorus, total organic carbon/total nitrogen and other nutrients, were different in post‐1940 sediments within the two basins. 4. These differences suggest characteristics, such as nutrient cycling, water depth and other physical changes resulting from roadway construction, combined to establish and maintain the differing communities of primary producers in the two basins. Furthermore, Fe/S dynamics and waterfowl herbivory probably contributed to the development of the two ecosystem states.     

Occurrence,Dynamics and Production of Picoplankton in Hungarian Shallow Lakes

Phototrophic picoplankton were detected in 10⁵-10⁶ cells/ml concentrations in seven water bodies of differing chemistry and trophic level. Dominant picoplankters were, at all sites, coccoid cyanobacteria of 0.8-1.2 μm dimensions, exhibiting red or yellow autofluorescence. Apart from the effects of water temperature their quantitative dynamics were significantly influenced by the nitrogen supply and the herbivorous zooplankton (Cladocera). Their maximum contribution to the total planktonic primary production was about 50%, both in mesotrophic and hypereutrophic environments. However during the bloom of filamentous nitrogen-fixing blue-greens their role became negligible. In phytoplankton communities the significance of picoplankton is overestimated several times, when based on the cell count, while it is underestimated on the basis of biomass. The most useful characteristics of phytoplankton size groups was the total surface area of their cells.     

Dynamics of phytoplankton detritus in a shallow,eutrophic lake (Lake Loosdrecht,The Netherlands)

A study was made of the mortality and aerobic decomposition of light- and phosphorus-limited cultures of Oscillatoria limnetica, a dominant phytoplankton species in shallow, eutrophic Lake Loosdrecht (The Netherlands). When placed in the dark at 20 °C, most cells died and lysed within twelve days. The labile organic matter was completely decomposed within three weeks. Absorbance spectra indicated that blue green algae may contributed significantly to the refractory dissolved substances in the lake. Refractory particulate matter constituted from 7 to 24% of the biomass of O. limnetica, depending on the growth rate before incubation in the dark. The decomposition rate of this fraction was 0.005 d^–1. On a basis of a steady-state model of the dynamics of phytoplankton detritus, the areal organic dry weight concentration of the detritus in the lake is ca. 60 g m^–2. This means the quantities of detritus in the seston and epipelon are about equal.     

Long-term phytoplankton changes in a shallow hypertrophic lake,Albufera of Valencia (Spain)

A long-term phytoplankton study was carried out in the Albufera of Valencia, a shallow hypertrophic lake (surface area 21 km², mean depth 1 m, total inorganic nitrogen load 155 g m^-2 y^-1, total inorganic phosphate load 15 g m^-2 y^-1) from 1980 to 1988. The lake functions as a reservoir for the surrounding rice cultivation. From 1940's to 1988, its phytoplankton assemblage has been altered from a mesotrophic to a hypertrophic character, as consequence of the increasing pollution. For 1980–88, annual variations in the phytoplankton were less pronounced than seasonal changes. The hypertrophic and morphometric features of the lake favoured the stability of the phytoplankton assemblage and chlorophyll a levels during the study period. Seasonal and horizontal distribution of the total phytoplankton abundance and biomass were highly influenced by the hydrological cycle of the lagoon. Compared with other shallow nutrient rich lakes, the Albufera of Valencia is similar to the shallow hypertrophic lakes of the Netherlands.     

The influence of a predatory ostracod,Australocypris insularis,on zooplankton abundace and species composition in a saline lake

The influence of the predatory ostracod Australocypris insularis on zooplankton abundances and species composition in an Australian saline lake was monitored over 8 months in aquaria and tank microcosms. Control, Predator (double A. insularis Control densities) and No Predator (all A. insularis removed) treatments were set up in small 11.5l aquaria and large 883.5l tanks. Microcosm salinities were manipulated to accord with seasonal lake salinities and ranged from 35 to 110 gl^-1. Densities of the calanoid copepods Calamoecia clitetellata and C. salina, the small ostracods Diacypris compacta and D. dietzi, and juvenile ostracods (excluding A. insularis) were significantly reduced by A. insularis predation in the aquaria experiments. Daphniopsis sp. was only present (in low numbers) in the No Predator aquaria. Results from the large tank experiments were not as clear-cut, due to difficulties in the manipulation of predator numbers. Differing salinity tolerances of predator and prey may confound the predation impact of A. insularis.     

Assessment of Components Controlling Phytoplankton Photosynthesis and Bacterioplankton Production in a Shallow,Alkaline, Turbid Lake (Neusiedlersee,Austria)

Various aspects of phytoplankton photosynthesis were investigated between 1968 and 1980 in Neusiedlersee, Austria, a shallow (z̄ = 1.3 m), well mixed lake with much turbidity resulting from frequent resuspension of inorganic sediment particles. The influences of the great environmental variability on biomass, photosynthesis and phytoplankton production were determined and discussed with respect to the increasing nutrient concentrations in the lake. Investigations of bacterioplankton biomass and production revealed their importance for nutrient regeneration and as food for large zooplankton populations. The results are compared with those from several other shallow lakes, and an attempt is made to identify characteristics to distinguish the different types.     

Seasonal and Interannual changes of chemical characteristics and phytoplankton in a mountain lake of the Eastern Italian Alps (Lake Calaita,Trentino)

Measurements at Lake, Calaita, a small mountain lake located at 1605 m. a.s .l. in a metamorphic catchment area in the south of the Trentino Region (Northern Italy), have revealed great seasonal variations in the chemical characteristics and phytoplankton community during the ice free period in 1992 and 1993. The acidity present in wet precipitations (H⁺, NH₄⁺) was neutralised within the drainage basin by mineral dissolution which led to an increase of basic cations and alkalinity in the runoff. The dilution during periods of higher discharge, e.g. in spring and autumn, resulted in low values of alkalinity (up to 60 μeq 1⁻¹), pH (mostly <6.7) and conductivity (<8 μS cm⁻¹, 20°C). In summer, the decrease in runoff caused higher alkalinity (>10 μeq 1⁻¹), pH (6.9–7.4) and conductivity values (up to 30 μS cm⁻¹). The phytoplankton showed a major development in summer (with biovolume values of up to 7000 mm³ m⁻³), two different taxa being dominant in 1992 (Oocystis cf. lacustris) and 1993 (Synedra sp.). Unpredictable climatic conditions have a strong influence on the physical stability of the lake, which makes it more difficult to explain the evolution of the phytoplankton community as opposed to deeper lakes.