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.     

Eutrophication of Lake Geneva indicated by the oligochaete communities of the profundal

Primary production rates and total phosphorus concentrations indicated that Lake Geneva (Switzerland) was meso-eutrophic from 1970 to 1983. Worm communities of the profundal (50–309 m deep) were very similar in 1978 and 1983. Species numerically dominant in eutrophic lakes — such as Potamothrix hammoniensis, P. heuscheri and Tubifex tubifex — constituted the bulk (75%) of the communities. Species numerically dominant in mesotrophic lakes (mostly Potamothrix vejdovskyi) or in oligotrophic lakes (mostly Stylodrilus heringianus) constituted respectively 18% and 7% of the worm communities. The dominance of eutrophic species increased with depth in the whole lake; it increased also in the eastern region of the lake which is directly exposed to the heavy organic inputs of the Rhône River. Oligotrophic and mesotrophic species decreased along the same gradients. Species dominant in oligotrophic lakes were absent in 1978 and 1983 from the deepest area of Lake Geneva (300–309 m) whereas they constituted therein 25% of worm communities in 1967. Data based on worm species groups — i.e. species with similar resistance to eutrophication pooled together — were more easy to analyse statistically than those based on the isolated species. Thus, the relative abundance of three species groups, expressed in several ways, can indicate precisely the trophic state of a lake.     

Chemical limnology of soft water lakes in the Upper Midwest

Water samples from 36 lakes in northern Minnesota, Wisconsin, and Michigan were collected and analyzed during 1983–1984. All study lakes were dilute and had total alkalinities of less than 150 eq · L^–1. Minnesota lakes have hydrologic inputs from the watershed and inputs of base cations derived from the watershed. Study lakes in Minnesota had higher total alkalinities, dissolved organic carbon, and noncarbonate alkalinity as a result of watershed inputs. Lakes in Michigan and Wisconsin were precipitation-dominated seepage lakes that have lower concentrations of base cations than lakes in Minnesota. All of the study lakes have lower sulfate concentrations than expected, based on atmospheric wet deposition and evapotranspiration.Pore water samples collected from one of the study lakes—Little Rock Lake—in Wisconsin were used to calculate diffusive fluxes between the sediment and water column. According to these calculations, the sediments were a source of total alkalinity and Ca²⁺ and a sink for SO₄ ^2–. The sediment-water exchange of total alkalinity, Ca²⁺, and SO₄ ^2– appears to be important in the whole-lake budgets of these ions for Little Rock Lake.     

Interactions between phytoplankton,zooplankton and fish in a shallow,hypertrophic lake: a study of phytoplankton collapses in Lake Søbygård,Denmark

Since 1983 severe phytoplankton collapses have occurred 1–4 times every summer in the shallow and hypertrophic Lake Søbygård, which is recovering after a ten-fold decrease of the external phosphorus loading in 1982. In July 1985, for example, chlorophyll a changed from 650 µg l^–1 to about 12 µg 1^–1 within 3–5 days. Simultaneously, oxygen concentration dropped from 20–25 mg O₂l^–1 to less than 1 mg O₂l^–1, and pH decreased from 10.7 to 8.9. Less than 10 days later the phytoplankton biomass had fully recovered. During all phytoplankton collapses the density of filter-feeding zooplankton increased markedly, and a clear-water period followed. Due to marked changes in age structure of the fish stock, different zooplankton species were responsible for the density increase in different years, and consequently different collapse patterns and frequencies were observed.The sudden increase in density of filter-feeding zooplankton from a generally low summer level to extremely high levels during algae collapses, which occurred three times from July 1984 to June 1986, could neither be explained by changes in regulation from below (food) nor from above (predation). The density increase was found after a period with high N/P ratios in phytoplankton or nitrate depletion in the lake. During that period phytoplankton biomass, primary production and thus pH decreased, the latter from 10.8–11.0 to 10.5. We hypothesize that direct or indirect effects of high pH are important in controlling the filter-feeding zooplankton in this hypertrophic lake. Secondarily, this situation affects the trophic interactions in the lake water and the net internal loading of nutrients. Consequently, not only a high content of planktivorous fish but also a high pH may promote uncoupling of the grazing food-web in highly eutrophic shallow lakes, and thereby enhance eutrophication.A tentative model is presented for the occurrence of collapses, and their pattern in hypertrophic lakes with various fish densities.     

Phosphorus dynamics following restoration measures in the Loosdrecht Lakes (The Netherlands)

External phosphorus loads to three shallow lakes in the Netherlands were reduced by eliminating waste-water discharge and by dephosphorization of the supply water, with which water level is controlled. Concentrations of total-phosphorus and chlorophyll a were significantly reduced during 1980–1986 in L. Breukeleveen, but not in L. Vuntus and L. Loosdrecht. In 1983–1986 the phosphorus flow through several trophic levels was determined. Changes over these years were not significant. External input to the lakes still contributes substantially to the phosphorus input. Release from the sediments also contributed to the cycling of the phosphorus. Excretion by large crustacean zooplankters was important in phosphorus recycling, and delivered 20–30% of the daily phytoplankton phosphorus demand. A similar contribution is expected from fish. If one wants recovery of the lakes to be accelerated, additional measures are needed.     

2000-2014年我国西北地区湖泊面积的时空变化

以2000-2014年Landsat TM、ETM+以及OLI影像为基础,通过人工目视解译方法提取我国西北地区111个面积10.0 km²以上湖泊矢量数据,并对湖泊面积变化总体趋势、空间分异特征以及成因进行分析。结果表明：（1）2000-2014年期间,西北地区湖泊总面积呈整体增加的态势,由1.58×10⁴ km²增加为1.74×10⁴ km²。（2）在研究时段内,我国西北地区湖泊面积在整体增加的同时也表现出明显的空间差异。根据湖泊面积变化不同,研究区可以被划分为稳定、扩张、萎缩3种类型和5个区域：新疆维吾尔自治区北部与青海省南部为湖泊面积稳定区;塔里木盆地、昆仑山北麓、阿尔金山、柴达木盆地、可可西里地区、祁连山南麓至三江源北部大片区域为湖泊面积扩张区;由准噶尔盆地和吐鲁番盆地组成的北疆中部地区和喀喇昆仑山北坡山区为湖泊萎缩区。（3）受研究区复杂的自然人文环境影响,在不同区域引起湖泊面积变化的原因不尽相同：总体而言,降水增加、气温上升导致的冰川融水增加和冻土水分释放是湖泊面积呈现扩张趋势地区的最主要原因;在湖泊面积稳定区,湖泊所处的地形条件以及湖泊与河流间强烈的水量交换起着关键作用;日益增强的人类活动是湖泊萎缩区的主要影响要素,但在喀喇昆仑山北坡地区,冰川物质平衡状态的变化也可能发挥着重要作用。     

Lake restoration with and without dredging of phosphorus-enriched upper sediment layers

Human activity has been the cause of continuing decline of water quality in most Dutch lakes. Development of lake restoration programmes must take into account the lake functions. Major reduction of the nutrient and pollutant loading is the primary step in lake restoration. Still, the recovery of eutrophic lakes is retarded frequently because of internal phosphorus loading by the lakes' sediments. Sediment dredging, as an additional tool for water quality management to accelerate accomplishing the desired water quality, is studied. In this paper we evaluate the preliminary results of eight lake restoration projects in the Netherlands. The lakes are compared in order to estimate the magnitude of the internal phosphorus loading. Dredging as an additional measure was carried out twice in the peatlake Geerplas. In the Nieuwkoop Lakes only the external phosphorus loading was substantially reduced from 0.9 to 0.2 g P m^–2 y^–1. Provisional results of these two shallow peatlake restoration projects focussed on eutrophication abatement with and without dredging, are presented. Both show a decrease in phosphorus concentration in the lakes. The necessity to dredge the lakes is discussed.     

Palaeolimnology in the English Lakes — some questions and answers over fifty years

In a review of work on 10 of the very numerous lakes of the English Lake District — work which involved co-operation between palaeolimnologists from different disciplines and between palaeo- and neolimnologists — the questions asked by the researchers are listed and it is shown which kind of lake has provided constructive answers. Complete and conformable sediment profiles covering the last ca 14 000 years are found in these lakes, but no single profile is optimal for study of the whole period. Knowledge of the pattern of sediment accumulation is necessary before the record in microfossils and changing sediment composition can be interpreted with confidence. The demonstration that the sediment-source material of these open lakes must have been largely allochthonous established the importance of the sediment column as a record of changes through time in the composition and stability of the soils of each catchment. These changes illustrate the influence of:

Phosphorus transformations in the epilimnia of small humic forest lakes

Research on epilimnetic phosphorus transformations in lakes had led to the establishment of a generalised picture which is promoted in reviews and textbooks. However, it would be wrong to believe that this established view can be applied to all lakes. Certainly the transformations of phosphorus in the summer epilimnia of small, coloured, humic forest lakes in southern Finland show features which deviate markedly from the generally accepted paradigm. (1) The plankton appears to be limited simultaneously by P and N rather than being strongly P limited. (2) Particulate P is typically a rather small fraction of the total P. (3) Molybdate reactive P may be a large part of the total P, although much of this MRP is not free orthophosphate, especially in highly humic lakes. (4) Turnover times for PO₄-P are normally long (50–1000 minutes); turnover times appear to be longer in highly humic lakes. (5) Added 32 PO₄ passes rapidly to macrozooplankton, apparently with rapid turnover of zooplankton phosphorus. (6) Sedimentation of phosphorus from the typically shallow epilimnion appears high, but may be partially compensated by biological retrieval of phosphorus from the phosphorus-rich hypolimnion.     

Paleolimnological reconstruction of the effects of atmospheric deposition of acids and heavy metals on the chemistry and biology of lakes in New England and Norway

Sediment cores from nine lakes in southern Norway (N) and six in northern New England (NE) were dated by ¹³⁷Cs, ²¹⁰Pb and in NE also by pollen, and were analyzed geochemically and for diatoms. Cores from two N and three NE lakes were analyzed for cladocerans. ¹³⁷Cs dating is unreliable in these lakes, probably due to mobility of Cs in the sediment. In Holmvatn sediment, an up-core increase in Fe, starting ca. 1900, correlates with geochemical indications of decreasing mechanical erosion of soils. Diatoms indicate a lake acidification starting in the 1920's. We propose that soil Fe was mobilized and runoff acidified by acidic precipitation and/or by soil acidification resulting from vegetational succession following reduced grazing. Even minor land use changes or disturbances in lake watersheds introduce ambiguity to the sedimentary evidence relating to atmospheric influences. Diatom counts from surface sediments in 36 N and 31 NE lakes were regressed against contemporary water pH to obtain coefficients for computing past pH from subsurface counts. Computed decreases of 0.3–0.8 pH units start between I890 and I930 in N lakes already acidic (pH 5.0–5.5) before the decrease. These and lesser decreases in other lakes start decades to over a century after the first sedimentary indications of atmospheric heavy metal pollution. It is proposed that the acidification of precipitation accompanied the metal pollution. The delays in lake acidification may be due to buffering by the lakes and watersheds. The magnitude of acidification and heavy metal loading of the lakes parallels air pollution gradients. Shift in cladoceran remains are contemporary with acidification, preceding elimination of fishes.     

Rapid adaptive responses of rosette‐type macrophyte Vallisneria natans juveniles to varying water depths: The role of leaf trait plasticity

Rosette‐type submerged macrophytes are widely distributed across a range of water depths in shallow lakes and play a key role in maintaining ecosystem structures and functions. However, little is known about the rapid adaptive responses of such macrophytes to variations in water depth, especially at the juvenile stage. Here, we conducted a short‐term in situ mesocosm experiment, in which the juveniles of Vallisneria natans were exposed to a water depth gradient ranging from 20 to 360 cm. Twenty‐two leaf‐related traits were examined after 4 weeks of growth in a shallow lake. Most (18) traits of V. natans generally showed high plasticity in relation to water depth. Specifically, juveniles allocated more biomass to leaves and had higher specific leaf area, leaf length‐to‐width ratio, chlorophyll content, and carotenoids content in deep waters, displaying trait syndrome associated with high resource acquisition. In contrast, V. natans juveniles in shallow waters had higher leaf dry matter content, leaf soluble carbohydrate content, carotenoids per unit chlorophyll, and peroxidase activity, pertaining to resource conservation. Notably, underwater light intensity was found to be the key factor explaining the trait plasticity along the water depth gradient, and 1.30 mol photons m⁻² d⁻¹ (at 270 cm) could be the optimal irradiance level based on the total biomass of V. natans juveniles. The present study highlights the significance of leaf trait plasticity for rosette‐type macrophytes in response to variations in water depth and sheds new light on the differences between trade‐offs in deep‐ and shallow‐water areas.     

Carbon burial by shallow lakes on the Yangtze floodplain and its relevance to regional carbon sequestration

Floodplain lakes may play an important role in the cycling of organic matter at the landscape scale. For those lakes on the middle and lower reaches of the Yangtze (MLY) floodplain which are subjected to intense anthropogenic disturbance, carbon burial rates should, theoretically, be substantial due to the high nutrient input, increased primary production and high sediment accumulation rates. There are more than 600 lakes >1 km² on the Yangtze floodplain including 18 lakes >100 km² and most are shallow and eutrophic. ²¹⁰Pb‐dated cores were combined with total organic carbon (TOC) analyses to determine annual C accumulation rates (C AR; g C m^?2 yr^?1) and the total C stock (since ~1850). The sediment TOC content is relatively low with an average <2% in most lakes. C AR ranged from ~5 to 373 g C m^?2 yr^?1, resulting in C standing stocks of 0.60–15.3 kg C m^?2 (mean: ~5 kg C m^?2) since ~1850. A multicore study of Chaohu lake (770 km²) indicated that spatial variability of C burial was not a significant problem for regional upscaling. The possible effect of changes in lake size and catchment land use on C burial was examined at Taibai lake and indicated that lake shrinkage and declining arable agriculture had limited effects on C AR. The organic C standing stock in individual lakes is, however, significantly dependent on lake size, allowing a simple linear scaling for all the MLY lakes. Total regional C sequestration was ~80 Tg C since ~1850, equivalent to ~11% of C sequestration by soils, but in ~3% of the land area. Shallow lakes from MLY are a substantial regional C sink, although strong mineralization occurs due to their shallow nature and their role as C sinks is threatened due to lake drainage.     

Protist diversity in a permanently ice-covered Antarctic Lake during the polar night transition

The McMurdo Dry Valleys of Antarctica harbor numerous permanently ice-covered lakes, which provide a year-round oasis for microbial life. Microbial eukaryotes in these lakes occupy a variety of trophic levels within the simple aquatic food web ranging from primary producers to tertiary predators. Here, we report the first molecular study to describe the vertical distribution of the eukaryotic community residing in the photic zone of the east lobe (ELB) and west lobe (WLB) of the chemically stratified Lake Bonney. The 18S ribosomal RNA (rRNA) libraries revealed vertically stratified populations dominated by photosynthetic protists, with a cryptophyte dominating shallow populations (ELB–6 m; WLB–10 m), a haptophyte occupying mid-depths (both lobes 13 m) and chlorophytes residing in the deepest layers (ELB–18 and 20 m; WLB–15 and 20 m) of the photic zone. A previously undetected stramenopile occurred throughout the water column of both lobes. Temporal variation in the eukaryotic populations was examined during the transition from Antarctic summer (24-h sunlight) to polar night (complete dark). Protist diversity was similar between the two lobes of Lake Bonney due to exchange between the photic zones of the two basins via a narrow bedrock sill. However, vertical and temporal variation in protist distribution occurred, indicating the influence of the unique water chemistry on the biology of the two dry valley watersheds.     

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.     

Eutrophication of Lakes Leman and Neuchâtel (Switzerland) indicated by oligochaete communities

In 1978–80, oligochaete communities of meso-eutrophic Lake Léman (Lake of Geneva) were compared to those of mesotrophic Lake Neuchâtel. Worm species were classified into three groups corresponding to their increasing tolerance to eutrophication: (1) oligotrophic species, mostly Peloscolex velutinus, Stylodrilus heringianus; (2) mesotrophic species, mostly Potamothrix vejdovskyi, P. bedoti; (3) eutrophic species, mostly Potamothrix hammoniensis, P. heuscheri, Tubifex tubifex. In both lakes, eutrophic species constituted the bulk of the communities in terms of absolute abundance. However, relative abundance of mesotrophic and eutrophic species was higher in Lake Léman; oligotrophic species were more important in Lake Neuchâtel. These data confirmed the trophic classification of lakes based on chemical parameters. The number of zero values, which perturbated statistical analysis, was reduced by using species groupings instead of isolated species. Thus, making the lakes more comparable even if different species were present in each one. Relative density values based on all samples were distributed among 4 density classes for the 3 species groupings. The 12 resulting frequencies described the community structure expressed in terms of eutrophication. Furthermore, these frequencies may be used for comparison of eutrophication levels in several lakes.     

Geology and sedimentary history of Lake Traunsee (Salzkammergut,Austria)

Traunsee was formed by glacial overdeepening of a pre-existing fault system. Present-day morphology is characterized by a deep (189 m) narrow trough with steep slopes (>50°) in the southern part surrounded by the Northern Calcareous Alps. The northern part of the lake is bordered by flysch and glacial deposits with gentle slopes (<30°) and exhibits several ridges, basins and troughs.During the late and postglacial period, more than 45 m of sediment has accumulated in the central basin. Sedimentation in the southern part of Traunsee is mainly controlled by the river Traun forming a prograding delta in the south and — within the past 50 years — by industrial tailings consisting mainly of calcite. Sediments are distributed by undercurrents and by turbidites. Cores from the central basin thus show an intercalation of Traun-derived dolomite-rich sediments with anthropogenic muds from the tailings deposited up to more than 6 km from its source.Within the northern basin, land slides from the flysch region played an important role leading to drastic changes in the morphology of slopes and adjacent basins. These slumps have persisted until historic times. Sedimentation in the shallow sublittoral regions is dominated by benthic biogenic decalcification.The frequency of turbidite sedimentation within the profundal basin decreased during the last 200 years probably due to man's activities in the drainage area such as regulation of rivers and torrents. Sedimentation rates during the past decades range from 2–3 cm/a in the southern basin to 0.4 cm/a in the northern part as shown by ¹³⁷Cs-dating.     

Response of a shallow Mediterranean lake to nutrient diversion: does it follow similar patterns as in northern shallow lakes?

1. In view of the paucity of data on the response of warm shallow lakes to reductions in nutrient loading, this paper presents a long‐term limnological data set to document changes in the food‐web of a shallow Mediterranean lake (Lake Albufera, Valencia, Spain) that has experienced reductions in phosphorus (P) (77%) and nitrogen (N) (24%) loading following sewage diversion. 2. Nine years after sewage diversion, P concentration in the lake was reduced by 30% but remained high (TP = 0.34 mg L^?1), although the mean water retention time in the lake was only 0.1 years. Nitrate concentrations did not significantly change, probably because the lake continued to receive untreated effluents from ricefields. 3. Chlorophyll a concentration was reduced by half (annual mean of 180 μg L^?1). Cyanobacteria abundance remained high but its composition changed towards smaller species, both filamentous and chroococcal forms. 4. Cladocera abundance increased and reached peaks twice a year (December to March and July to September). After nutrient reduction, short‐term clear‐water phases (up to 5 weeks) occurred during February to March in several years, concomitant with annual flushing of the lake and lower fish densities. The abundance of Cladocera in winter contrasted with the spring peaks observed in northern restored shallow lakes. The zooplankton to phytoplankton biomass ratio remained lower than in northern temperate shallow lakes, probably because of fish predation on zooplankton. 5. Improvement of the water quality of Lake Albufera remained insufficient to counteract littoral reed regression or improve underwater light allowing submerged plants re‐colonise the lake. 6. Sewage diversion from Lake Albufera impacted the food web through the plankton, but higher trophic levels, such as fish and waterfowl, were affected to a lesser degree. Although the fish species present in the lake are mainly omnivorous, long‐term data on commercial fish captures indicated that fish communities changed in response to nutrient level and trophic structure as has been observed in restored shallow lakes at northern latitudes. 7. Phosphorus concentrations produced similar phytoplankton biomass in Lake Albufera as in more northern shallow lakes with abundant planktivorous fish and small zooplankton. However, in Lake Albufera, high average concentrations were maintained throughout the year. Overall, results suggest that nutrient control may be a greater priority in eutrophicated warm shallow lakes than in similar lakes at higher latitudes.     

Production and chlorophyll concentration of epipelic and epilithic algae in fertilized and nonfertilized subarctic lakes

The production and chlorophyll concentration of epipelic and epilithic algae was measured during four years (1972–1975) in two shallow, Swedish subarctic lakes. One lake (Lake Hymenjaure) was fertilized with phosphorus or a combination of phosphorus and nitrogen while the other (Lake Stugsjön) served as a reference. The benthic algae in both lakes were dominated by Cyanophyceae of the same species during the whole investigation. The chlorophyll concentration of epipelic and epilithic algae was 100 and 20 mg·m^–2 respectively and fairly constant during the season. In 1974–1975 there was a significant increase in chlorophyll concentration of the benthic algae in Lake Hymenjaure, probably as a response to the poorer light climate in the lake due to a large phytoplankton development. The annual benthic production was 3.4–7.2 gC·m^–2 and it was not enhanced by the fertilization. Compared to the other primary producers (phytoplankton and macrophytes) the benthic algae constituted 70–83% of the total production in Lake Stugsjön. In Lake Hymenjaure, however, the importance of the benthic algae decreased from 50 to 22% of the total due to the great increase in phytoplankton production induced by the lake fertilization.     

Oligochaetes and water pollution in two deep Norwegian lakes

Analyses of the oligochaete fauna of two of the deepest lakes in Scandinavia — the Norwegian lakes Mjösa (450 m) and Tyrifjorden (295 m), revealed a totally different species composition in the deep profundal compared with the upper profundal - in contact with the nutrient-enriched epilimnion. In both lakes a pronounced thermal stratification develops in the summer, thus the epilimnion receiving gross organic pollution behaves differently from the profundal. The lakes are each effectively divided into two bodies of water with limited water exchange between them, i.e. one major oligotrophic body and one minor more nutrient-rich. Since the 1950s both lakes have been exposed to heavy pollution of various kinds. In Lake Mjösa in 1975 and 1976 unpleasant algal blooms of the blue-green alga Oscillatoria bornetii fa. tenuis occurred. Bottom samples obtained at the same time revealed that the deep central bottoms of the lake were totally dominated by oligotrophic oligochaete indicators, i.e. by Stylodrilus heringianus and Spirosperma ferox, while the fauna of the upper profundal in the vicinity of domestic and agricultural sewage outfalls, wood processing industries, etc. was dominated by Limnodrilus hoffmeisteri and Tubifex tubifex in great abundance, indicating enriched conditions. Several other species indicative of eutrophy, were absent, most of them belonging to the genus Potamothrix. A fairly similar situation exists in Lake Tyrifjorden, where, for instance, in the shallow bay of Steinsfjorden — heavily eutrophied by agricultural wastes — blooms of blue-green algae have caused problems from time to time. The same oligochaete communities as in Lake Mjösa distinguish the central oligotrophic bottoms from the regionally more enriched upper profundal. The likely reasons for an intact profundal oligochaete fauna are great volumes of oxygen-rich hypolimnic water of low temperature and a high bottom/lake surface area ratio.     

Silica-scaled chrysophytes (Synurophyceae) from some rivers and shallow lakes in Hungary

Silica-scaled chrysophytes are a special group in the phytoplankton of rivers and shallow lakes. They are present the whole year at low density, but sometimes — mainly during autumn and spring — some species can become frequent and very numerous to form real water blooms.In this study, seven species of silica-scaled chrysophytes have been recorded and identified by electron microscopy of phytoplankton samples from some rivers and shallow lakes in Hungary. Most of these silica-scaled chrysophytes have previously been found in eutrophic and hypertrophic localities, and almost all of them are common and widely distributed, even cosmopolitan. Only Synura echinulata Korshikov has not previously been recorded from Hungary. Some of them as Synura curtispina (Petersen & Hansen) Asmund, S. petersenii Korshikov, Mallomonas acaroides Perty emend. Ivanov, M. tonsurata Teiling were frequently found in the phytoplankton samples. Sometimes in autumn and winter Synura petersenii was the most abundant species in the phytoplankton and formed blooms.