Recovery from eutrophication: experiences of reduced phosphorus input to the four largest lakes of Sweden

In-lake concentration changes of phosphorus (P) and nitrogen (N) in lakes V?ttern, V?nern, M?laren, and Hj?lmaren in response to diminished input has been examined from the mid-1960s onwards. In the former two deep and oligotrophic lakes with slow water renewal, drastic reductions in P-input from the middle of 1970s caused just minor reductions in P-concentration over a very long time. At the same time accumulation occurred in the water mass of inorganic N and possible reasons are discussed. In the latter two mesotrophic to hypertrophic lakes, two shallow basins in L. Hj?lmaren showed slow recovery due to release of P from sediments. The same basins and two basins in L. M?laren have suffered from N-deficiency, particularly during the pre-phosphorus reduction years, and nitrogen fixation was indicated. In two L. M?laren basins recovery of in-lake P concentrations was better than expected in comparison to the so-called IMSA-model for lake recovery from nutrient pollution. In the other five lakes/basins chlorophyll concentrations after 20 years were similar compared to those modeled.     

Phytoplankton and water quality characterization: experiences from the Swedish large lakes Mälaren, Hjälmaren, Vättern and Vänern

Phytoplankton and environmental variables have been monitored in the large Swedish lakes M?laren, Hj?lmaren, V?ttern and V?nern since the 1960s. Measures to reduce phosphorus input and industrial waste products were taken during the 1970s. The phosphorus loading was then reduced by 90-95% resulting in a halving of the phosphorus concentrations in the most affected basins. The phytoplankton community reacted rapidly with decreased biomasses of cyanobacteria in summer as well as decreased biomasses of spring diatoms and cryptophycean flagellates. Other reactions were a contracted period of water-bloom, an increased taxon richness, an increased evenness in the biomass over the growth season, and a change in the species size structure within the phytoplankton community. Furthermore, the species richness in the large lakes is compared in relation to lake characteristics. A presentation of the occurrence of toxic cyanobacteria in the lakes is also given. Maximum-minimum values of 13-0.1 micrograms microcystin L-1 are established in connection with water-blooms in Hj?lmaren and M?laren. The use of phytoplankton as a monitoring variable to detect water-quality changes is outlined and assessment criteria are presented.     

Morphometry and hydrology of the four large lakes of Sweden

The four largest Swedish lakes V?nern, V?ttern, M?laren and Hj?lmaren all have a tectonic origin with multidirectional fault fissures, which influence the topography of the bottoms and the creation of subbasins. All the four lakes have been subject to several glacials, the latest of which started to recede from central Sweden c. 10,000 years ago. The many differences between the four lakes can be explained by their shapes, sizes and the relation between the sizes of the drainage basins and the size of the lakes. This paper focuses on processes considered relevant to aquatic organisms of the lakes: water balance, water residence time, water temperature, water exchange between subbasins within the lakes, and other internal water movements. Links between pollution, water exchange and retention of nutrients are discussed with an example from Lake V?nern.     

The environmental consequences of alien species in the Swedish lakes Mälaren, Hjälmaren, Vänern and Vättern

Twenty alien species have become established in the lakes M?laren, V?nern, V?ttern and Hj?lmaren. Intentional introductions include fish and the signal crayfish from North America, ornamental plants, and the Canada goose. Unintentional introductions include the crayfish plague introduced with infected crayfish, the zebra mussel, and Chinese mitten crab introduced with ballast water. The introduction of pathogens and parasites, in particular the crayfish plague, to the lakes has had the greatest environmental and socioeconomic effects and has contributed to the decimation of the indigenous noble crayfish. The stocking of brown trout and salmon with origins from different biogeographical regions has contributed to the extinction of relict indigenous fish species in L. V?nern. Although major ecosystem damage caused by the introduction of alien species, with the exception of the crayfish plague, has not occurred in the four large Swedish lakes, local problems of considerable dignity occur occasionally.     

Four decades of research on the Swedish large lakes Mälaren, Hjälmaren, Vättern and Vänern: the significance of monitoring and remedial measures for a sustainable society

The large lakes of Sweden, M?laren, Hj?lmaren, V?ttern and V?nern, have been subjected to water-quality monitoring for almost four decades. Physicochemical variables, plankton and benthic invertebrates have been regularly assessed. Hydrological and sediment conditions, macrophytes, fish, primary production, bacteria and attached algae have been periodically investigated. The human impact, including industrial activities, was reflected in excessive amounts of organic matter, nutrients, metals and persistent organic compounds. From the late 1960s all municipal sewage works in the catchments of the lakes were upgraded to the highest technical standard, including chemical precipitation of phosphorus, and phosphorus discharge from the sewage works was thereby reduced by 90-95%. In addition, industries were obliged to restrict discharge of harmful substances. The reactions of the lakes to the remedial measures are discussed as well as the value of various indicators. The studies were instrumental in designing a national lake monitoring program. Additionally, results from large lake monitoring have contributed to the establishment of national water-quality criteria including, physical, chemical, and biological indicators.     

Recruitment of pelagic fish in an unstable climate: studies in Sweden's four largest lakes

Pelagic fish population biology was studied in the large Swedish lakes V?nern, V?ttern, M?laren and Hj?lmaren. It is crucial for fish fry in temperate regions to hatch early in the growth season to survive, and achieve large size before winter, and it is suggested that the key factors are to match the spring development of phyto- and zooplankton, but to avoid predation. This is more easily accomplished by the studied spring spawners smelt (Osmerus eperlanus) and pike-perch (Stizostedion lucioperca) than autumn spawners, such as vendace (Coregonus albula). It is shown that hatching of vendace fry shortly after ice-break-up is beneficial for year-class strength. In oligotrophic large lakes with few predatory species a rapid increase in water temperature after ice-break is also promoting recruitment, whereas this is not the case in eutrophic lakes where predation pressure from other species may become too high. The results indicate that autumn spawners will have difficulties in adapting to global warming and it is also suggested that the life history can explain the large variations observed in year-class strength between years.     

Factors affecting population fluctuations of the glacial relict amphipod Monoporeia affinis (Lindström) in Sweden's largest lakes

Factors affecting long-term (1982-2000) population densities of the glacial relict amphipod Monoporeia affinis were studied in Sweden's three largest lakes. Monoporeia showed large population fluctuations in all three lakes, with conspicuous peaks in density occurring in Lakes V?nern and M?laren. In Lake V?nern, amphipod densities showed highly significant relationships with spring maximum diatom biovolume at a 1-yr lag. The lack of relationship between diatom biovolumes and Monoporeia densities in L. V?ttern is likely due to the larger depth and the lower nutrient content of this lake. In eutrophic L. M?laren, summer hypoxia (< 4 mg O2 L-1) is likely an important regulating factor. Hypolimnetic temperature showed a clear periodicity with relatively warm deep water occurring between 1989 and 1994. Hypolimnetic temperatures in V?nern and V?ttern were correlated with total solar irradiance. However, neither hypolimnetic water temperature nor diatom biovolumes correlated with the North Atlantic Oscillation winter index. We speculate that variations in temperature and near-bottom oxygen concentrations negatively affect population densities by acting on recruitment success (reproduction) and juvenile (young-of-the-year) survival.     

Human impact on the fish diversity in the four largest lakes of Sweden

The four largest Swedish lakes, V?nern, V?ttern, M?laren, Hj?lmaren, host important commercial fisheries for char, salmon, trout, whitefish, vendace (cisco), perch, pike-perch, pike and eel, i.e. highly diverse biological resources. Case studies illustrate physical, chemical and biological impacts on some of these commercial species caused by constructions of dams and ship canals, eutrophication, and overexploitation. Although some original species have been lost and a few new species have been added, the recent human interference has basically caused major shifts in dominance of the fish community structures because of eutrophication, alterations in the abundance of eel or crayfish, and due to overfishing. The latter is in some cases caused by the Great Lake Fishery Paradox--in an environment with several predators and competitors, but with ample food resources, especially salmonid fish but also species like pike-perch may adapt a life history favoring growth over sexual maturation. If harvested at a conventional size these populations will decline rapidly due to too small spawning stocks.     

Monitoring of persistent organic pollutants (POPs): examples from Lake Vättern, Sweden

Lake V?ttern in southern Sweden is a large oligotrophic lake with high surface to catchment area ratio (ca. 0.4) and a water residence time of 60 years. The lake combines sensitivity to atmospheric POP-pollution with general susceptibility for effects of POPs and slow concentration decline. Time series, from the 1960s until 1996, of PCBs and DDT in fish and data on TCDD in sediment of L. V?ttern are presented and compared to other large lakes. The long time dataset of POPs in Arctic char (Salvelinus salvelinus) shows a significant annual decline of about 5% for PCBs and 13% for DDT. These rates are in agreement with other studies. Nutrients as well as biological effects and factors affecting the fate of POPs in oligotrophic lakes with long residence times are discussed. We propose that oligotrophic clear lakes are important ecosystems for monitoring POPs in biota.     

Mercury in sediment and fish communities of Lake Vänern, Sweden: recovery from contamination

The past effluents of mercury (Hg) into Lake V?nern were considerable. The consequences of, and recovery from these have been monitored through continuous measurements of mercury in sediment and fish. Mercury levels in lake sediments in the vicinity of the main source of mercury, a chloralkali plant on the northern shore, have only decreased by slightly more than a half since the mid-1970s, despite a radical decrease in effluents from the source, already during the 1960s. The mercury levels in pike (Esox lucius) have decreased to a similar extent during this time period. They are now about 30% higher in the worst affected parts of the lake compared to the least affected parts. Lower levels have been measured in perch (Perca fluviatilis) and salmonoid fish in the lake. Despite the increased presence of mercury in the sediment of Lake V?nern, the mercury levels in the fish of the lake are relatively low compared to fish in lakes situated in the same region, but not affected by any local mercury effluents. As calculated, the total fish biomass of L. V?nern holds less than 1000th of the amount of mercury contained in the upper, biologically active layers of the bottom sediment of the lake. This demonstrates the potential influence of various environmental factors and motivates continued monitoring of mercury levels in the lake in the future.     

Sediment evidence of early eutrophication and heavy metal pollution of Lake Mälaren, central Sweden

Lake M?laren is the water supply and recreation area for more than 1 million people in central Sweden and subject to considerable environmental concern. To establish background data for assessments of contemporary levels of trophy and heavy metal pollution, sediment cores from the lake were analyzed. Diatom-inferred lake-water phosphorus concentrations suggest that pre-20th century nutrient levels in S?dra Bj?rkfj?rden, a basin in the eastern part of M?laren, were higher (c. 10-20 micrograms TP L-1) than previously assumed (c. 6 micrograms TP L-1). Stable lead isotope and lead concentration analyses from 3 basins (S. Bj?rkfj?rden, Gisselfj?rden and Ask?fj?rden) show that the lake was polluted in the 19th century and earlier from extensive metal production and processing in the catchment, particularly in the Bergslagen region. The lake has experienced a substantial improvement of the lead pollution situation in the 20th century following closure of the mining and metal industry. The lead pollution from the old mining industry was large compared to late-20th century pollution from car emissions, burning of fossil fuels and modern industries.     

Effects of suspended solids on cyanobacterial bloom formation under different wind fields

Wind waves and suspended solids (SS) generated by the resuspension of sediments are ubiquitous characteristics of lake ecosystems. However, their effects on phytoplankton remain poorly elucidated in shallow eutrophic lakes. Laboratory experiments were carried out to investigate the responses of Microcystis aeruginosa to SS under static (wind speed of 0 m/s) and breeze (wind speed of 3 m/s) conditions. Results showed that 50 mg/L SS can promote the growth of M. aeruginosa, accelerate the formation of colonies, and increase the floating rate under no-wind conditions. Comparing with static environment, breeze can significantly increase the growth rate of M. aeruginosa and benefit the formation of larger colonies of algae cells. Driven by wind and SS, the buoyancy of the cyanobacteria community in different experimental groups was obviously different. The specific performance was that low SS concentration and breeze were in favor of the floating of cyanobacteria, while high SS concentration went against the floating of algal cells. As a conclusion, wind speed of 3 m/s and 20–50 mg/L SS have a synergistic effect on the formation of cyanobacterial blooms. This study can provide an improved current understanding of bloom formation and turbidity management strategies in shallow eutrophic lakes.

     

Phosphorus in tributaries to Lake Mälaren, Sweden: analytical fractions, anthropogenic contribution and bioavailability

Riverine phosphorus (P) concentration and P-transport to Lake M?laren, the third largest lake in Sweden, has been monitored for 35 years in 12 major tributaries. During a period of 15 months, complementary assessments of particulate P, suspended matter and dissolved reactive P were made. Particulate P comprised 64%, dissolved unreactive P 23% and dissolved reactive P 13% as flow-weighted means, with high seasonal variability. "Background" or "reference" P-transports in the streams were estimated by different methods and the anthropogenic contribution to P-transport was shown to be reduced over time. Potential algal availability of particulate P showed a mean availability of c. 45% for water draining arable and forested land, while the availability was higher for sewage discharge particulate P and algal P. Calculated total bioavailable P in tributaries was shown to equal reactive P measured on coarsely filtered water and it was indicated that the proportion of bioavailable P was higher during the period with high anthropogenic P-contribution than with a lower contribution.     

基于景区湖泊污染的多功能水域清理船设计

针对我国城市景区湖泊富营养化和垃圾污染严重,传统景区水域清理存在各种不足等问题,设计研发多功能水域清理船灭杀蓝藻和清理垃圾,恢复景区湖泊生态环境。结合景区湖泊污染实际状况,提出把臭氧灭杀蓝藻应用于景区湖泊清理的新概念,研发一艘可无线遥控、集打捞垃圾、过滤小型悬浮微粒和灭杀蓝藻功能于一体的多功能水域清理船。通过无线遥控多功能水域清理船,能全面清理湖面垃圾,过滤小型悬浮微粒,臭氧灭杀率能达到90%以上。利用多功能水域清理船能维护景区小湖泊生态环境,在各类景观水体和小型湖泊的污染治理中具有很好的应用前景。     

Mercury content in roach (Rutilus rutilus L.) in circumneutral lakes--effects of catchment area and water chemistry

The environmental influence on the mercury content in roach (Rutilus rutilus L.) is investigated using partial least square regression on 46 environmental variables describing the land use in the catchment area, various catchment area and lake characteristics, lake water chemistry, and fish stock. The Hg content in the fish from the 78 investigated circumneutral lakes is heavily influenced by the land use in their surroundings. The boreal forest lakes possessed the highest Hg levels in roach, whereas fish from lakes surrounded by arable land had lower levels. The Hg levels also showed a negative relationship to the amount of dissolved ions and the total amount of nutrients in lake water. Lake pH did not have any significant influence on the Hg content in roach in these non-acidified lakes. The Hg levels in lakes influenced by large amounts of wetland were less well explained by the presently investigated environmental variables, which implies that the Hg burden in fish from this kind of lake is governed by other factors.     

Reproductive status, blood chemistry, gill histology and growth of perch (Perca fluviatilis) in three acidic lakes

Perch (Perca fluviatilis L.) were sampled soon after spawning in three small acidic lakes (pH 4.3-6.1, Al(lab) 5-106 microg litre(-1), Ca2+ 0.01-0.08 mmol litre(-1)) and in one circumneutral lake (pH 5.9-6.4, Al(lab) 4-12 microg litre(-1), Ca2+ 0.06-0.07 mmol litre(-1)) in southern Finland. Due to the delayed spawning of perch in the acidic lakes, sampling in those lakes was performed later than in the reference lake. In spite of that, the gonadosomatic index (GSI) of males in all the acidic lakes was significantly greater than in the reference lake. Of the two lakes with similar low water pH, the effects on reproduction were more prominent in the lake with higher water Al content. The plasma Ca2+ concentrations of females in the acidic lakes were significantly smaller than in the females of the reference lake. The low female:male plasma Ca2+ ratio (1.0-1.32) depicted delay of spawning. Stress in perch in acidic water was also seen in elevated blood haematocrit values, especially in females. On the other hand, a low plasma Cl- level, a common response to acidic water in salmonids, was not detected in perch in the most acidic lakes. The amount of Al accumulated in the gill epithelium was highest in the most acidified lake with high Al concentration, but was also pronounced in a lake with low pH and low Al concentration.     

Variation in Organic Matter and Water Color in Lake Mälaren during the Past 70 Years

Interest in long time series of organic matter data has recently increased due to concerns about the effects of global climate change on aquatic ecosystems. This study presents and evaluates unique time series of chemical oxygen demand (COD) and water color from Lake Mälaren, Sweden, stretching almost seven decades (1935–2004). A negative linear trend was found in COD, but not in water color. The decrease was mainly due to installation of sewage works around 1970. Time series of COD and water color had cyclic pattern. It was strongest for COD, with 23 years periodicity. Similar periodicity observed in air temperature and precipitation in Sweden has been attributed to the North Atlantic Oscillation index and solar system orbit, suggesting that COD in Lake Mälaren is partly derived from algae. Discharge influenced water color more than COD, possibly because water color consists of colored substances brought into the lake from surrounding soils.     

Spectral attenuation of solar ultraviolet radiation in humic lakes in Central Finland

The attenuation of solar ultraviolet (UV) radiation in five lakes in Central Finland was evaluated through field measurements and/or by determining the optical properties of the lake water during summer 1999. Spectral UV irradiance in the air and at several depths underwater was measured in three lakes (Lake Palosj?rvi, Konnevesi, and Jyv?sj?rvi) with dissolved organic carbon (DOC) ranging from 4.9 to 8.7 mg l(-1) and chlorophyll a ranging from 1.6 to 16 g l(-1). According to the field measurements, 99% of the UV-B radiation was attenuated in approximately a half meter water column in the clearest lake. In the UV-A region at 380 nm, the corresponding attenuation occurred in the upper one meter. In a small humic lake (DOC 13.2-14.9 mg l(-1)) UV-B radiation was attenuated to 1% of the subsurface irradiance within the top 10 cm water column, whereas UV-A radiation (at 380 nm) penetrated more than twice as deeply (maximum 25 cm), as predicted from the absorption coefficients. These results suggest the importance of the dissolved fraction of lake water in governing the UV attenuation in lakes. This was seen from the significant relationship between the vertical attenuation coefficients (Kd) based on field measurements and the absorption coefficients (ad) derived from spectrophotometric laboratory scannings, as well as between Kd and DOC.     

Modeling recovery of Swedish ecosystems from acidification

Dynamic models complement existing time series of observations and static critical load calculations by simulating past and future development of chemistry in forest and lake ecosystems. They are used for dynamic assessment of the acidification and to produce target load functions, that describe what combinations of nitrogen and sulfur emission reductions are needed to achieve a chemical or biological criterion in a given target year. The Swedish approach has been to apply the dynamic acidification models MAGIC, to 133 lakes unaffected by agriculture and SAFE, to 645 productive forest sites. While the long-term goal is to protect 95% of the area, implementation of the Gothenburg protocol will protect approximately 75% of forest soils in the long term. After 2030, recovery will be very slow and involve only a limited geographical area. If there had been no emission reductions after 1980, 87% of the forest area would have unwanted soil status in the long term. In 1990, approximately 17% of all Swedish lakes unaffected by agriculture received an acidifying deposition above critical load. This fraction will decrease to 10% in 2010 after implementation of the Gothenburg protocol. The acidified lakes of Sweden will recover faster than the soils. According to the MAGIC model the median pre-industrial ANC of 107 microeq L(-1) in acid sensitive lakes decreased to about 60 microeq L(-1) at the peak of the acidification (1975-1990) and increases to 80 microeq L(-1) by 2010. Further increases were small, only 2 microeq L(-1) between 2010 and 2040. Protecting 95% of the lakes will require further emission reductions below the Gothenburg protocol levels. More than 7000 lakes are limed regularly in Sweden and it is unlikely that this practice can be discontinued in the near future without adverse effects on lake chemistry and biology.     

Impact of metal pollution,food availability,and excessive fishing on <Emphasis Type="Italic">Rhabdosargus haffara</Emphasis> stock (family: Sparidae) in Timsah lake

The lakes’ fisheries play an important role in Egyptian economy. In 1980s, they provided more than 50 % of harvested fish in Egypt but now their contribution to the Egypt fish production decreased to only 12.5 % in 2012. Lake Timsah, one of the Suez Canal lakes, faced many challenges that lead to serious changes in its water and fish quality, fish production, as well as the catch composition. The present work investigated the impact of pollution, food availability, and excessive fishing mortality on the haffara production in lake Timsah. The distribution of four heavy metals (Pb, Zn, Ni, and Fe) was detected seasonally in water and in muscles, gills, and livers of Rhabdosargus haffara, during 2012 through 2013. Fe and Zn were presented by high values in liver, while Pb and Ni in gills. Generally, the lowest concentrations of all metals were found in muscles. The recorded crustacean organisms (the main food of haffara) decreased from 12 species and 32,079 organisms?m² in 2012 to only 7 species and 7290 organisms?m² in 2013 while the amphipods completely disappeared. This serious change was due to the severe pollution in the lake. A logistic surplus production model was fitted to the catch per unit effort indices, to estimate the maximum sustainable yield and the optimum level of fishing effort. The results revealed that haffara stock at lake Timsah is overfished, and the estimated precautionary target reference points advised the reduction of fishing effort by about 30–50 %.