Major efforts have been made worldwide to improve the ecological quality of shallow lakes by reducing external nutrient loading. These have often resulted in lower in-lake total phosphorus (TP) and decreased chlorophyll a levels in surface water, reduced phytoplankton biomass and higher Secchi depth. Internal loading delays recovery, but in north temperate lakes a new equilibrium with respect to TP often is reached after <10-15 years. In comparison, the response time to reduced nitrogen (N) loading is typically <5 years. Also increased top-down control may be important. Fish biomass often declines, and the percentage of piscivores, the zooplankton:phytoplankton biomass ratio, the contribution of Daphnia to zooplankton biomass and the cladoceran size all tend to increase. This holds for both small and relatively large lakes, for example, the largest lake in Denmark (40 km 2 ), shallow Lake Arresø, has responded relatively rapidly to a ca. 76% loading reduction arising from nutrient reduction and top-down control. Some lakes, however, have proven resistant to loading reductions. To accelerate recovery several physico-chemical and biological restoration methods have been developed for north temperate lakes and used with varying degrees of success. Biological measures, such as selective removal of planktivorous fish, stocking of piscivorous fish and implantation or protection of submerged plants, often are cheap versus traditional physico-chemical methods and are therefore attractive. However, their long-term effectiveness is uncertain. It is argued that additional measures beyond loading reduction are less cost-efficient and often not needed in very large lakes. J. P. Jensen was deceased.

Although reservoirs are similar to natural lakes in many respects, such driving forces as water retention time and watershed features can play important roles in the limnology of manmade lakes. With the goal of investigating how these factors influence the limnology of tropical reservoirs, physical and chemical variables were measured at four sampling sites in two reservoirs in southern Brazil, from June 2002 to June 2003. Funil Reservoir is located in one of the most-populated areas in the country, in the Paraíba do Sul river basin, which drains and drastically influences the water quality of the reservoir. In contrast, Lajes Reservoir is located in a well-preserved area, with its water retention time varying from six to 30 times longer than for Funil Reservoir. Funil Reservoir is a turbid (median euphotic zone = 4.3 m), eutrophic reservoir (median total phosphorus (TP) = 3.1 μM), with a high phytoplankton biomass (median chlorophyll-a concentration = 10.0 μg L -1 ). In contrast, Lajes Reservoir is a transparent (median euphotic zone = 9.2 m), mesotrophic water system (median TP = 1.0 μM), with a low phytoplankton biomass (median chlorophyll-a = 1.9 μg L -1 ). Both reservoirs were stratified during the summer months, but isothermy was only observed in Funil Reservoir. Because of its short water retention time, Funil Reservoir is a much more dynamic system than Lajes Reservoir, with a pronounced temporal pattern related to changes in its water column and its phytoplankton biomass. Spatial heterogeneity is more evident in Lajes Reservoir, mainly as a consequence of its location in a preserved area, long water retention time and the presence of net cages for fish culture in the waterbody. The typical spatial zonation found in reservoirs, related to nutrient sedimentation and light availability, however, is more evident in Funil Reservoir than in Lajes Reservoir. Despite the similarities between these two water systems, which are in the same geographical region with similar climate, and are comparable in size, the distinct watershed features and water retention time are responsible for marked differences between these reservoirs.

Trends in recent years have indicated that cyanobacterial blooms in tropical reservoirs are increasing in frequency, magnitude and geographical distribution. Funil Reservoir in southeastern Brazil has experienced eutrophication in the recent decades, resulting in lasting and intense toxic cyanobacterial blooms. As input of nutrients is high during the year, the aim of the present study was to evaluate the role of other variables related to changes in cyanobacterial biomass and composition. The dominant group found over the entire study period was Cyanobacteria (97% of total biomass), which contributed to low diversity. A shift of nitrogen-fixing (Anabaena circinalis and Cylindrospermopsis raciborskii) and non nitrogen-fixing (Microcystis aeruginosa) cyanobacteria was observed. Redundancy analysis indicated that physical factors such as temperature, changes in the mixing zone and light intensity were the main driving factors of the seasonal succession. Nitrogenfixing cyanobacteria dominated in periods of low light in the deepest mixing zone, and also seemed to have experienced stronger grazing effects as the density of the large zooplankton group was related to cyanobacteria biomass. M. aeruginosa bloomed in warm stratified waters, high water levels and during months with more daylight, when the zooplankton density was drastically reduced. Although the long-standing dominance of cyanobacteria may be related to high nutrient availability, the present study showed that under high and constant input of nutrients, other factors, especially physical variables, present a more plausible explanation to promote changes in species composition.KEY WORDS: Cyanobacterial bloom · Cylindrospermopsis · Microcystis · Anabaena · Zooplankton Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 57: [137][138][139][140][141][142][143][144][145][146][147][148][149] 2009 reports of surface scum have become much more numerous since the mid-20th century. Harmful algal blooms caused by cyanobacteria (CyanoHABs) are one of the most severe problems in freshwater ecosystems nowadays. These often-toxic blooms and the dense surface scums are indicative of water quality deterioration and food-web changes, and can be responsible for the mortality of fish, domestic animals and even humans (Paerl & Huisman 2009).Cyanobacteria blooms occur in waters during calm stratified conditions with high temperatures and adequate nutrient supplies (Huszar et al. 2000). Cyanobacterial dominance is mostly related to favourable bottom-up factors (Briand et al. 2002, Marinho & Huszar 2002, since they are widely known for their ability to minimize grazing pressure, through different effects on zooplankton community: mechanical interference (difficulty in manipulating and ingesting large colonies or filaments), assimilation (low amounts of essential nutrients) and toxicity (De Bernardi & Giussani 1990). However, some zooplankton species have developed physiological resistance to cyanotoxins (Fulton & Paerl 1988...

Funil Reservoir receives inflow from a highly industrialized region and acts as a natural sink to pollutants. Among the consequences of the uncontrolled nutrient loading is the constant presence and periodic heavy blooms of Microcystis aeruginosa. This study verified limnological features and zooplankton assemblage of the Funil Reservoir focusing on the environmental‐indicator properties of rotifers and cladocerans. The summer bloom of M. aeruginosa caused reductions in water transparency, nitrate and orthophosphate concentrations and raises in chlorophyll a, pH, dissolved oxygen and chemical oxygen demand values. All zooplankton species presented spatial and temporal variations with the exception of the copepods, which were present in all samples. According to canonical correspondence analysis, ammonium was the variable most related to zooplankton variability and different Rotifera and Cladocera assemblages indicated distinct environment conditions. Rotifer taxa associated with increases of water temperature and chlorophyll a concentration were found with high densities during M. aeruginosa blooms. Species of rotifer and cladocerans are suggested as indicators that can be used to identify different physical and chemical gradients or eutrophic increases in Funil Reservoir.

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a b s t r a c tLongitudinal heterogeneity in reservoirs is especially related to increase in sedimentation and water transparency along the river/dam axis. Consequently, primary production tends to reach higher values in intermediate regions where there is a balance between the availability of the main resources (light and nutrients) suitable for phytoplankton growth. Many factors such as reservoir morphometry, retention time, thermal stratification and geographical location can affect the boundaries between these regions. The tropical Funil Reservoir (Brazil), despite a low retention time, has experienced severe eutrophication in recent decades, with persistent cyanobacteria blooms. During the course of 1 year, samples were collected at four stations along the reservoir (fluvial, intermediate and lentic compartments) to evaluate if spatial heterogeneity could affect the occurrence and distribution of these blooms along the reservoir. Although the reservoir has a short annual retention time (mean 41.5 days), the typical zonation pattern was observed for the main abiotic variables and phytoplankton abundance. However, higher biomass occurred in the lentic compartment rather than in the intermediate zone. Despite the peculiar heterogeneity in total biomass, the phytoplankton composition and seasonal variability were very similar along the entire reservoir, with a few marked differences only in the fluvial zone. Phytoplankton total biomass in Funil Reservoir was high, even in periods of lower seasonal retention time (around 15 days), and was especially related to high input of nutrients. Moreover, retention time directly affects the spatial heterogeneity of phytoplankton biomass, since strong variability was only observed during the cold-dry season, corresponding to periods of longer retention time (around 80 days). While high availability of nutrients promoted high cyanobacterial biomass in the entire system, the few periods of heterogeneous spatiality seemed to be related to changes in retention time.

Hydroelectric reservoirs can stratify, producing favorable conditions for mercury methylation in the hypolimnion. The methylmercury (MeHg) can be exported downstream, increasing its bioavailability below the dam. Our objective was to assess the mercury levels in plankton, suspended particulate matter (SPM) and fish collected upstream (UP) and downstream (DW) from the Reservatório de Samuel dam, an Amazonian reservoir that stratifies during half of the year. Mercury concentrations in both SPM and plankton were similar between the two sites, which could indicate there are no conditions favoring methylation at the moment of sampling (absence of stratification). Almost all mercury found in the muscle of fishes was in organic form, and differences of mercury levels between sites were dependent on the fishes trophic level. Herbivores showed similar mean organic mercury levels (UP = 117 lg g -1 ; DW = 120 lg g -1 ; n = 12), whereas omnivores (UP = 142 lg g -1 ; DW = 534 lg g -1 ; n = 27) and carnivores (UP = 545 lg g -1 ; DW = 1,366 lg g -1 ; n = 69) showed significantly higher values below the dam. The absence of a reservoir effect in herbivores is expected, since they feed on grassy vegetation, near the riverbanks, which is not much influenced by mercury in aquatic systems. On the other hand, the higher mercury levels below the dam observed for omnivores and carnivores suggest a possible influence of the reservoir since they feed on items that could be contaminated by MeHg exported from upstream. The results highlight the necessity of assessing areas downstream of reservoirs.

Concentrations of organic (OrgHg) and inorganic mercury (InorgHg) were assessed in different fish tissues (liver, muscle, kidney, gut and gonads) and trophic levels collected in an impacted tropical reservoir in southeastern Brazil. Organic mercury concentrations in muscle were remarkably higher in the carnivorous species Hoplias malabaricus and Oligosarcus hepsetus. The ratios of OrgHg in relation to total mercury (%OrgHg) in muscle also varied according to the species trophic level: 93% for carnivores, 84% for omnivores, 73% for algivores/planktivores and 58% for detritivores. The %OrgHg in the gut tissue of carnivores (78%) was much higher than that found in omnivores (30%), possibly reflecting a process of trophic biomagnification in the reservoir. On the other hand, the InorgHg concentrations in muscle decreased with the trophic level increase, suggesting that this form of mercury did not biomagnify through the food web. Gonads contained the least total mercury, and approximately all of this mercury was represented by the organic form (83 to 98%). The kidney and the liver of all fish species contained less than 50% OrgHg. We suggest that the low %OrgHg in the liver is related to different capacities or strategies of OrgHg detoxification by the fish.Concentrações de mercúrio orgânico (OrgHg) e inorgânico (InorgHg) foram avaliadas em diferentes tecidos e níveis tróficos de peixes (fígado, músculo, rim, trato digestivo e gônadas) coletados em um reservatório tropical impactado, no sudeste do Brasil. Concentrações de OrgHg no músculo foram notavelmente maiores em carnívoros (Hoplias malabaricus e Oligosarcus hepsetus).As porcentagens de OrgHg em relação ao mercúrio total (%OrgHg) no músculo também variaram de acordo com o nível trófico das espécies: 93% para os carnívoros, 84% para os onívoros, 73% para os algívoros/planctívoros e 58% para os peixes detritívoros. Além disso, a %OrgHg encontrada no trato digestivo dos peixes carnívoros (78%) foi substancialmente superior a encontrada nos onívoros (30%), possivelmente refletindo um processo de biomagnificação trófica no reservatório. Por outro lado, as concentrações de InorgHg no músculo diminuíram com o aumento do nível trófico, sugerindo que esta forma do mercúrio não biomagnificou ao longo da cadeia alimentar. As gônadas apresentaram as menores concentrações de mercúrio total e grande parte deste estava na forma orgânica (83 a 98%). Por outro lado, rins e fígado de todas as espécies de peixes apresentaram menos que 50% de OrgHg. Sugere-se que a baixa %OrgHg no fígado possa estar relacionada às diferentes capacidades ou estratégias de destoxificação do OrgHg nesses peixes.