Indirect effect of different fish communities on nutrient chlorophyll relationship in shallow hypertrophic water quality reservoirs

Effects of different fish communities on the proportion of different nitrogen and phosphorous forms and the amount of phytoplankton (chlorophyll a) were examined in two consecutive years (1992–1993) in three Hungarian shallow water reservoirs (Cassette and outer reservoir of the Kis–Balaton Water Protection System, and Marcali reservoir). Possible interactions between nutrient concentrations and the amount of phytoplankton in these reservoirs were also examined. Considerable differences in the proportions of different nutrient forms were observed between the three test sites, which could be explained by the presence of different fish stocks in these reservoirs. In the Cassette, the fish biomass necessary for a water quality improvement was around 50 kg ha⁻¹. Phytoplankton biomass was controlled by the zooplankton, consequently chlorophyll a concentrations decreased considerably, while those of dissolved nutrients significantly increased. In the outer reservoir, phytoplankton was controlled bottom-up, since the 250 kg ha⁻¹ fish biomass was larger than the critical value due to the high proportion of planktivorous species. Chlorophyll a concentrations were high, and nutrients were mainly in particulate form (in algal cells). In the Marcali reservoir, the recently introduced silver carp population could not control fully the phytoplankton. The biomass of phytoplankton decreased only slightly, while its composition changed considerably. Although biomanipulation with silver carp is suitable for ceasing cyanobacterial blooms, reduction of the amount of planktivorous fish seems to be a more adequate method for increasing water transparency, rather than introduction of phytoplankton feeding fish.

     

Strong effects of occasional drying on subsequent water clarity and cyanobacterial blooms in cool tropical reservoirs

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Reducing harmful algae in raw water by light-shading

The occurrence of harmful algal bloom in water source poses a serious water safety problem to local water supply systems. In order to ensure the raw water quality, the feasibility of an in situ light-shading measure was investigated through enclosure experiment and pilot-scale experiment. The results showed that harmful algal bloom could be controlled by light-shading lasting for 6–9 days, with water quality being partially improved. When aeration was added, the reduction of algal biomass could be enhanced, and water quality was further improved compared to that without aeration. These experimental results offered an attractive in situ algal control measure for lakes or reservoirs suffered from harmful algal bloom.     

Fish community response to the longitudinal environmental gradient in Czech deep-valley reservoirs: Implications for ecological monitoring and management

Ecological quality assessment of non-natural water bodies is, in contrast to natural systems, less developed and requires determining biological indicators that reliably reflect environmental conditions and anthropogenic pressures. This study was motivated to propose fish indicators appropriate for assessment of reservoir ecosystems in central Europe. We analysed changes in water quality, total biomass and the taxonomic, trophic and size composition of fish communities along the longitudinal axes of four elongated, deep-valley reservoirs. Due to high nutrient inputs from their catchments, the reservoirs exhibited pronounced within-system gradients in primary productivity and water transparency. Although fish communities were similar among the reservoirs and dominated by few native species, the community structure and biomass systematically changed along the longitudinal axes of the reservoirs. The biomass and proportion of planktivores/benthivores in the fish community were highest at eutrophic sites near the river inflow and declined substantially towards deep, more oligotrophic sites close to the dam. The biomass and proportion of piscivores significantly increased downstream within the reservoirs alongside improving water quality. At species level, perch Perca fluviatilis and bream Abramis brama responded most sensitively, although in opposite directions, to the longitudinal environmental gradient. The major longitudinal changes in fish community characteristics were found to be consistent between pelagic and benthic habitats. The results of this study suggest that fish communities are appropriate indicators of eutrophication and can be used for ecological quality assessment of non-natural lentic water bodies, such as reservoirs. Moreover, our results underline the necessity to consider within-system gradients in water quality and the fish community when planning sampling programmes for deep-valley reservoirs.     

典型亚热带热分层水库秋季细菌群落垂直分布

水库在我国东南沿海地区是重要的饮用水水源地,对地区经济发展和社会稳定起到重要作用。选择亚热带地区典型的热分层水库——福建莆田东圳水库,于2011年秋季稳定分层期,以水体温度的垂直变化特征为依据进行分层采样。应用PCRDGGE和克隆测序的方法研究浮游细菌群落的垂直分布特征,利用多元统计分析揭示细菌群落与热分层水体理化指标之间的关系。结果显示:溶解氧、电导率、叶绿素a、总氮、氨氮及硝氮在上下层水体中的分布有显著差异,下层缺氧区细菌的ShannonWiener指数和DGGE条带数明显高于上层好氧区,表明东圳水库热分层水体中存在明显的物理、化学及生物分层现象。测序结果表明β-变形菌可能是东圳水库中占优势的细菌类群,统计结果提示溶解氧是显著影响细菌群落组成的环境因子。热分层水体的物理化学分层与水体细菌群落结构密切相关,提示水库生态学研究应对水体热分层给予重视。     

Hydrobiological consequences of the addition of phosphate precipitants to inlet water of lakes

Summary In three model reservoirs (LUND, 1975) a method reducing bluegreen algal blooms in lakes was studied. Iron or aluminium were added to inlet waters for chemically binding the inflowing phosphorus.The research program, started in 1975, includes intensive monitoring of many chemical and hydrobiological variables, the determination of water and mass balances and since 1977 measurements or primary production rates with¹⁴C. In this paper only the results found in 1977 are discussed. An attempt is made to describe quantitatively how growth rates and changes in biomass are interconnected and how phosphorus precipitation changes these variables.In all reservoirs a large discrepancy was observed between the actual rate of increase in the algal population and the relative production rate. The latter appeared to be higher by one order of magnitude. The relative death rate due to grazing can account for the large difference between these growth rates only when selective grazing of zooplankton on phytoplankton is assumed.It can be concluded that treatment of inlet water with AVR, an aluminium salt, is unsuccessful in reducing algal development. Treatment with ironsulphate may be successful, but a reduction of the relative growth rates was not observed. The effects of grazing of zooplankton andDreissena polymorpha need further investigation.     

Relationship between epipelon,epiphyton and phytoplankton in two limnological phases in a shallow tropical reservoir with high Nymphaea coverage

Macrophytes and phytoplankton are recognized as having roles in determining alternative stable states in shallow lakes and reservoirs, while the role of periphyton has been poorly investigated. Temporal and spatial variation of phytoplankton, epipelon and epiphyton was examined in a shallow reservoir with high abundance of aquatic macrophytes. The relationships between algae communities and abiotic factors, macrophyte coverage and zooplankton density were also analyzed. Monthly sampling was performed in three zones of the depth gradient of the reservoir. Two phases of algal dominance were found: a phytoplankton phase and epipelon phase. The phase of phytoplankton dominance was characterized by high macrophyte coverage. Rotifera was the dominant zooplankton group in all the zones. Flagellate algae were dominant in phytoplankton, epipelon and epiphyton. Macrophyte coverage was found to be a predictor for algal biomass. Changes in biomass and species composition were associated with macrophyte cover variation, mainly the Nymphaea. In addition to the abiotic factors, the macrophyte coverage was a determining factor for changes to the algal community, contributing to the alternation between dominance phases of phytoplankton and epipelon. The macrophyte–phytoplankton–periphyton relationship needs to be further known in shallow reservoirs, especially the role of epipelon as an alternate stable state.

     

Effects of invasive zebra mussels on phytoplankton,turbidity, and dissolved nutrients in reservoirs

Few experiments have quantified the effects of invasive zebra mussels (Dreissena polymorpha) on man-made reservoirs relative to other aquatic habitats. Reservoirs, however, are the dominate water body type in many of the states that are at the current front of the zebra mussel invasion into the western United States. The objective of this research, therefore, was to determine how zebra mussels affected phytoplankton, turbidity, and dissolved nutrients in water that was collected from three Kansas reservoirs that varied in trophic state (mesotrophic to hypereutrophic), but all experienced frequent cyanobacterial blooms. Laboratory mesocosm experiments were conducted to document the effects of zebra mussels on cyanobacteria and general water quality characteristics in the reservoir water. Zebra mussels significantly reduced algal biomass, and the total biovolume of cyanobacteria (communities were dominated by Anabaena) in each reservoir experiment. The effects of zebra mussels on other major algal groups (diatoms, flagellates, and green algae) and algal diversity were less consistent and varied between the three reservoir experiments. Similarly, the effects of zebra mussels on nutrient concentrations varied between experiments. Zebra mussels increased dissolved phosphorus concentrations in two of the reservoir experiments, but there was no effect of zebra mussels on dissolved phosphorus in the mesotrophic reservoir experiment. Combined, our results strongly suggest that zebra mussels have the potential to significantly impact reservoirs as they continue to expand throughout the western United States. Moreover, the magnitude of these effects may be context dependent and vary depending on the trophic state and/or resident phytoplankton communities of individual reservoirs as has similarly been reported for natural lakes.     

Interactions of fish,algae, and abiotic factors in a shallow,tropical pond

Protecting and restoring shallow tropical lakes and wetlands requires a knowledge of what shapes and controls algal dynamics and primary productivity in these systems. Algal community structure and composition can be regulated either through biotic or abiotic controls. Large-scale changes in fish populations can affect algal communities by altering food web dynamics and the physical and chemical properties of the aquatic environment. A reduction in fish biomass can lead to a reduction in algal biomass because of increased grazing by zooplankton and reduced availablity of nutrients. However, the omnivorous fish common in tropical systems often consume algae, and their reduction can increase algal biomass. There is a need for more information on the effect of fish removals/reductions in tropical systems. In a five-year study of a shallow, tropical pond in Hawaii, I investigated the water quality effects of tilapia removal following the occurrence of two natural fish die-offs. I describe the concurrent impacts of water-level fluctuations and the fish die-offs on the physical and chemical conditions of the pond and the subsequent changes in the algal community. Overall, nutrients, suspended sediment, organic matter, and algal biomass were significantly reduced and light availability significantly increased in the absence of tilapia.

     

不同营养类型水库大型底栖动物的群落结构特征及其水质评价

2006～2007年对湖北省4座不同营养类型水库的大型底栖动物群落结构和多样性进行周年研究,并进行了水质评价.共采集到底栖动物39种,其中寡毛类14种、水生昆虫19种、软体动物6种.以金沙河的底栖动物种类最多,达24种,其次为徐家河19种,道观河和桃园河各10种.4座水库优势种类各异,金沙河为多毛管水蚓(Aulodrilus pluriseta)、隐摇蚊(Cryptochironomus sp.)、多足摇蚊(Polypedilum sp.);徐家河为瑞士水丝蚓(Limnodrilus helveticus)、前突摇蚊(Procladius sp.);桃园河为多毛管水蚓(Aulodrilus pluriseta)、长跗摇蚊(Tanytarsus sp.);水质污染比较严重的道观河水库优势种类为霍甫水丝蚓(Limnodrilus hoffmeisteri)、大红德永摇蚊(Tokunagayusurika akamusi).各库底栖动物年平均密度和生物量分别为:金沙河316 8ind · m-2,1294.3mg · m-2 ;道观河318.2ind · m-2,430.7mg · m-2 ;徐家河330.3ind · m-2,517.4mg · m-2;桃园河209ind · m-2,325 3mg · m-2 .TN、TP、COD平均含量变幅分别为0.392～1.018、0.011～0.042、3.505～9.166mg · L-1;采用理化分析、Goodnight-Whitley指数、Shannon-Wiener指数、Margalef指数、Simpson指数、Pielou均匀度指数对水库的水质进行了综合评价,结果表明:金沙河水库属中营养型;徐家河和桃园河属中-富营养型;道观河水库属富营养型.对水库主要的物理化学因子与寡毛类、水生昆虫、软体动物密度,以及各类底栖动物之间进行矩阵分析,发现水生昆虫与水深呈负相关关系,随着水深的增加,其密度和种类下降,水深超过10m,下降的幅度更明显;寡毛类有从属于TN和TP含量的趋势, TP变化对底栖动物的影响更大;寡毛类、水生昆虫、软体动物三者之间无相关性.     

ALGAL‐RELATED TASTES AND ODORS IN PHOENIX WATER SUPPLY: PRELIMINARY REPORT

Frequent episodes of algal‐related tastes and odors (T & O) in drinking waters in metropolitan Phoenix, Arizona prompted initiation of a three‐year project in July 1999 to investigate the occurrence of T & O metabolites and to develop a comprehensive management strategy to reduce the problems in drinking water supplies in arid environments. Two metabolites, 2‐methylisoborneol (MIB) and geosmin, have been identified as compounds responsible for the earthy‐musty tastes and odors in water supplies. Both were detected in the water supply system, including source rivers, reservoirs, canal delivery system and water treatment plants. Higher concentrations of MIB and geosmin occurred in distribution canals than in the upstream reservoirs indicating that significant production of the T & O compounds occurs within the canal system. A baseline‐monitoring program has been established for the complex water supply system, with special emphasis on the canal system. Efforts are underway to investigate possible correlations between physical/chemical parameters, algal composition and biomass, with the occurrence of MIB and geosmin. In addition, several physical and chemical treatments are planned for the canal system to reduce algal growth and related MIB and geosmin concentrations.     

El Niño Southern Oscillation and climate trends impact reservoir water quality

Low dissolved oxygen concentration in bottom layers of lakes and reservoirs usually indicates low water quality. In lakes, empirical models predicting anoxia are almost entirely based on the decay of plankton biomass, while in reservoirs recent findings suggest a prominent role of streamflow and load of organic carbon. This suggests a potential link between water quality in reservoirs and climate processes affecting streamflow. Here we support this hypothesis presenting evidence that both interannual climate variability and recent climate change, mainly consisting in a significant increase in potential evapotranspiration in the upstream basin, affected the oxygen content in a Mediterranean reservoir (Sau Reservoir, Spain). Using a 44‐year monthly record, we found strong and consistent signatures of El Niño Southern Oscillation in the inflow and reservoir oxygen content. Spectral and wavelet techniques showed that the El Niño, streamflow, and reservoir oxygen content series oscillated in common periods, which coincided with the main El Niño variability modes. An empirical model explaining the annual oxygen content in the reservoir suggested that a decreasing streamflow trend reduced the oxygen content of the reservoir by about 20%, counteracting remediation measures implemented at the basin upstream the reservoir. Our results provide the first quantitative evidence of climate change effects on reservoir water quality using long‐term instrumental data, and indicate that streamflow should be considered as a key variable in assessing climate change impact on reservoir water quality. These results are especially relevant in regions of the world where reservoirs are abundant and most climate models predict a decrease in runoff during the next decades. Both the expected trends and the sensitivity of reservoir water quality to global interannual climate variability should be considered for a correct management of water resources in the present and to design adaptation policies in the future.     

River–reservoir transition zones are nitrogen fixation hot spots regardless of ecosystem trophic state

Reservoir hydrodynamics may create heterogeneity in nitrogen (N) fixation along the riverine–transition–lacustrine gradient. In particular, N fixation may be highest in reservoir transition zones where phytoplankton biomass is also expected to be relatively high. We investigated spatial patterns of N fixation in three Texas (USA) reservoirs of varying trophic state. We sampled 6–9 stations along the longitudinal axes of the major inflows and measured N fixation using the acetylene reduction method. Total N, total phosphorus (P), and algal biomass (as chlorophyll-a) were also measured at each sample location. Measurable N fixation was observed in all reservoirs and was light-dependent. Nitrogen fixation was consistently low in the riverine zone, highest in the transition zone, and low in lacustrine zone of all reservoirs. The absolute magnitude of N fixation was similar in two relatively unproductive reservoirs and an order of magnitude higher in the eutrophic reservoir. A similar pattern was observed in mean nutrient and chlorophyll-a concentrations among reservoirs. However, chlorophyll-a concentrations were highest in the riverine zone of each reservoir and exhibited a monotonic decrease in the downstream direction. Maximum chlorophyll-a concentrations did not coincide with maximum N fixation rates. Results of our study indicate that reservoir transition zones can be biogeochemical hot spots for planktonic N fixation, regardless of trophic state. Therefore, transition zones may be the most at risk locations for water quality degradation associated with increased reservoir productivity. Water quality managers and aquatic scientists should consider the spatial heterogeneity imposed by unique hydrodynamic controls in reservoir ecosystems. Handling editor: Luigi Naselli-Flores     

A SYSTEMATIC STUDY OF GIGAR-TINACEAE FROM PACIFIC NORTH AMERICA BASED ON MOLECULAR AND MORPHOLOGICAL EVIDENCE

Frequent episodes of algal-related tastes and odors (T & O) in drinking waters in metropolitan Phoenix, Arizona prompted initiation of a three-year project in July 1999 to investigate the occurrence of T & O metabolites and to develop a comprehensive management strategy to reduce the problems in drinking water supplies in arid environments. Two metabolites, 2-methylisoborneol (MIB) and geosmin, have been identified as compounds responsible for the earthy-musty tastes and odors in water supplies. Both were detected in the water supply system, including source rivers, reservoirs, canal delivery system and water treatment plants. Higher concentrations of MIB and geosmin occurred in distribution canals than in the upstream reservoirs indicating that significant production of the T & O compounds occurs within the canal system. A baseline-monitoring program has been established for the complex water supply system, with special emphasis on the canal system. Efforts are underway to investigate possible correlations between physical/chemical parameters, algal composition and biomass, with the occurrence of MIB and geosmin. In addition, several physical and chemical treatments are planned for the canal system to reduce algal growth and related MIB and geosmin concentrations.     

Some aspects of photosynthetic characteristics in a set of perennial irrigation reservoirs located in five river basins in Sri Lanka

Phytoplankton primary productivity of eleven irrigation reservoirs located in five river basins in Sri Lanka was determined on a single occasion together with light climate and nutrient concentrations. Although area-based gross primary productivity (1.43–11.65 g O₂ m^–2 d^–1) falls within the range already established for tropical water bodies, net daily rate was negative in three water bodies. Light-saturated optimum rates were found in water bodies, with relatively high algal biomass, but photosynthetic efficiency or specific rates were higher in water bodies with low algal biomass, indicating nutrient limitation or physiological adaptation of phytoplankton. Concentrations of micronutrients and algal biomass in the reservoirs are largely altered by high flushing rate resulting from irrigation release. Underwater light climate and nutrient availability control the rate of photosynthesis and subsequent area-based primary production to a great extent. However, morpho-edephic index or euphotic algal biomass in the most productive stratum of the water column is not a good predictor of photosynthetic capacity or daily rate of primary production of these shallow tropical irrigation reservoirs.     

Phytoplankton and bacterioplankton production in a reed-covered water body

Between 1996 and 1998 phytoplanktonic primary production and bacterioplankton production were measured monthly at five sampling stations in the lower Kis-Balaton reservoir. The open water area of the reservoir was rich in phytoplankton and had hypertrophic characteristics, but inside the reed stand (80% of the surface area) phytoplankton biomass and production were substantially (30–50 times) lower. The algal removal efficiency of the lower Kis-Balaton reservoir was 96%. The reservoir had a considerably smaller effect on bacterioplankton removal than on the phytoplankton. The decrease of biomass and production of bacterioplankton in the through-flowing water was approximately 60%. Inside the reed stand the biomass and the production of planktonic bacteria exceeded that of the phytoplankton by several times, suggesting that the release of biodegradable dissolved organic (humic) substances from macrophytes stimulated the metabolism of bacterioplankton. The significant reduction of phytoplankton inside the dense reed stand was primarily the result of the shading effect of the reeds. In the open water area a shading experiment demonstrated that a 1-week residence period for planktonic algae in the reed-covered area was sufficient for their complete elimination. The decomposition of planktonic algae, reed material and the lack of primary production inside the reed stand created oxygen-deficient and phosphorus-rich conditions during the vegetative period. These results suggest that reed-covered water bodies can effectively retain suspended solids and planktonic algae, but because of decomposition processes they cannot retain biologically-available phosphorus.     

Influence of drought on algal biofilms and meiofaunal assemblages of temperate reservoirs and rivers

The role of hydrological droughts in shaping meiofauna abundance through alterations in biofilm biomass and composition was investigated. In January 2005, continental Portugal was under a moderate to severe drought resulting from a 40% to 60% decrease in rainfall during the previous 12 months relative to the long-term average (1961–1990). Reservoir capacity was reduced by 30–50% relative to average values and the width of streams was reduced by 20–80% in the Zêzere River Basin (central Portugal). Algal biomass and algal class composition of biofilms was assessed through quantification of algal pigments in three reservoir and six river locations. During drought, habitat alterations are expected to be sharp in rivers while, in the absence of water quality deterioration, the habitat characteristics of reservoirs are expected to remain fairly unaffected. Chlorophylls and carotenoid pigments were extracted from biofilm samples and analysed using high performance liquid chromatography (HPLC). In the winter of 2003, during the period of average rainfall, biofilm biomass did not exceed 5 μg chlorophyll a cm⁻² at any location. River biofilm biomass was roughly half of that measured in the reservoirs. In the winter of 2005 (drought), biofilm biomass increased by more than 5-fold in river locations and remained low or decreased in the reservoirs. Algal biofilms were either dominated by Bacillariophyceae or by Chlorophyceae regardless of the existence of drought. The relative contribution of Bacillariophyceae to total biofilm biomass was higher during the drought than under average hydrological conditions. The abundance of harpacticoids, cladocerans and ostracods was favoured by the drought only in the reservoirs where an increase in diatom proportion in biofilms was observed. The increase in the abundance of cyclopoid copepods, turbellarians, nematodes and chironomids in rivers during the drought could be explained by algal class composition and biomass of biofilms and environmental variables (organic matter sediment content, phosphorus availability content and sediment granulometry). The hydrological drought appears to regulate meiofauna abundance only in river locations, possibly through the promotion of the growth of biofilms and the availability of organic matter deposited in rivers during the drought. Handling editor: D. Ryder     

Responses of the planktonic diatom Asterionella formosa Hassall to abiotic environmental factors in a reservoir complex (south-eastern France)

The River Durance and its main tributary, the Verdon, are both highly regulated rivers flowing in south-eastern France. The course of both rivers is interrupted by a series of reservoirs with quite different geographical, morphometric, climatic, hydrodynamic and chemical characteristics. The planktonic diatom Asterionella formosa Hassall, which has undesirable cyclic effects from the water management point of view, was studied in this complex of reservoirs located in the Mediterranean region. The results indicate that only the monomictic calcareous reservoirs show a bimodal pattern of Asterionella formosa abundance. The population dynamics of this algal species was found to depend on both the morphometric features and the hydraulic mode of management (retention time) used at each reservoir, as well as on the physical (temperature, suspended matter) and chemical (nitrogen, silica, calcium) characteristics of the water.     

Chemical and physical characteristics of the Salton Sea, California

A 1-year sampling program was conducted to assess current chemical and physical conditions in the Salton Sea. Analyses included general physical conditions and a suite of water quality parameters, including nutrients, trophic state variables, major cations and anions, trace metals and organic compounds. Samples were collected from three locations in the main body of the lake and from the three major tributaries. Nutrient concentrations in the Salton Sea are high and lead to frequent algal blooms, which in turn contribute to low dissolved oxygen concentrations. The tributaries consist primarily of agricultural return flows with high nutrient levels. Concentrations of trace metals and organic compounds do not appear to be of major concern. Two geochemical models, PHRQPITZ and PHREEQC, were used to evaluate potential chemical reactions limiting the solubility of selected water quality variables. Modeling indicated that the Salton Sea is supersaturated with respect to calcite, gypsum, and other minerals. Precipitation of these minerals may serve as a sink for phosphorus and limit the rate of salt accumulation in the Salton Sea.     

Longitudinal changes in protistan bacterivory and bacterial production in two canyon-shaped reservoirs of different trophic status

The drastic interactions of weather as El Niño events with catchment and hydrological processes can cause unexpected changes in physical, chemical and biological properties of freshwater aquatic ecosystems. The severe drought during 1998–1999 in the northeastern region of Brazil induced ecological changes in numerous reservoirs as in Tapacurá reservoir, one of the biggest drinking-water suppliers in Pernambuco state. Investigations were based on monthly sampling over 2 years (May 1998–May 2000) conducted at 3 representative stations with 3 sampled depths through the water column (0.5 m, middle and 0.5 m above the bottom). Temporal changes in ecological processes, especially stratification, were driven by two major precipitation patterns, with an initial marked dry period (period 1) followed by a rainy season (period 2). Dissolved oxygen and pH variations, higher conductivity and alkalinity values, higher concentrations of particulate organic material (carbon, nitrogen and phosphorus) and higher levels of algal biomass (chlorophyll a) characterized the dry period (May 1998–May 1999). During this phase of low water level when the reservoir storage capacity reached a minimum of 3.9%, the concentrations of chlorophyll a gradually increased with a cyanobacterial bloom (Cylindrospermopsis raciborskii) noted in April 1999. The decline in chlorophyll a and particulate organic matter were observed as a result of the first rains in May–June 1999, with the drastic changes of quality of matter (higher particulate C/N ratio). After a phase characterized by the entire water column turning anoxic, a second phase in the stratification process could be identified from June 1999 with the pronounced rainfalls accompanied by an overturn event. Annual rainfall deficit and lack of reservoir water renewal in 1998–1999 linked to the 1997 El Niño consequences were important determinants of high eutrophication levels and drastic ecological modifications in Tapacurá reservoir.