Itaparica Reservoir Research Articles

The snakes of the flooded region of Itaparica hydroeletric dam, Northeastern Brazil: “development” based on the drowning of wildlife

The previously underestimated biodiversity of the Caatinga underwent a transformation with the emergence of new studies, intensifying interest in the biota of this complex dry forest. Studies on snakes from the Caatinga have increased recently, resulting in 112 recorded species. Contributing to this, we present a detailed list of 23 species rescued from the Itaparica reservoir implementation in the 1980s. Collected specimens are deposited in the Herpetological and Paleoherpetological Collection at the Universidade Federal Rural de Pernambuco. The recorded snake assemblage is typical of the dry Caatingas of the Sertaneja Depression. We discuss the impacts of flooding and specimen relocation without adequate planning or monitoring on snake diversity. Finally, we conclude by questioning the sustainability aspects of development and interventions in nature.

Transposição do Rio São Francisco: avaliação da influência do Rio Pajeú na qualidade da água da captação do eixo leste

The Sao Francisco River is a main river that flows through seven states. In Pernambuco, the Pajeu River is the largest tributary and it flows into the Itaparica reservoir. The Brazilian semi-arid region suffers from a great shortage of water and the federal government has implemented two water transposition axes of the Sao Francisco Rivers to minimize this problem. The objective of this study was to evaluate the influence of the Pajeu River on the water quality of the Itaparica reservoir, where the catchment point of the East Transposition axis is located. The water quality data refer to the period from January / 2011 to September / 2014, totaling four collections in the dry period and four in the rainy period. A Principal Component Analysis (PCA) was developed between the monitored points, where the first two main components explained 52.6% of the total variability. For the most significant parameters, bloxplot graphs were elaborated and the data analyzed. Although the flow of the Pajeu River is not high, with its characteristics of greater electrical conductivity, BOD, COD and nitrate, the contribution of its waters to the Itaparica reservoir tends to contribute negatively to the water quality of the East axis.

Spatial-temporal evaluation of water quality in Brazilian semiarid reservoirs

Abstract Reservoirs are used in the Brazilian semiarid for human supply, power generation, irrigated agriculture and more recently, intensive fish farming, the Sobradinho reservoir being the largest of this region. In the last decade, the semiarid region has suffered a strong drought, in opposite direction to the increasing demand of multiple water uses. The present work aims to study the spatial-temporal variation of water quality in two reservoirs in the Brazilian semiarid region (Sobradinho and Itaparica reservoirs). For this, samples were collected in the dry and wet periods between 2011 and 2014, and the results were analyzed through boxplot charts and Principal Component Analysis (PCA). For both reservoirs, our results showed that the PCAs did not evidence a specific seasonal change in the water quality characteristics, distinguishing the wet and dry periods through the four years studied. A clear temporal pattern was detected in both reservoirs, with higher values of turbidity, dissolved oxygen and total phosphorus in the wet period and higher values of pH and alkalinity in the dry period. An upstream reduction of nutrients and turbidity was not found in this study, as is reported in literature for consecutive reservoirs, probably due to the irrigation and aquaculture activities, mainly in Itaparica reservoir.

Management of Tropical River Basins and Reservoirs under Water Stress: Experiences from Northeast Brazil

Due to global warming, a reduction in available water will occur in many watersheds and conflicts concerning water use will take place. This situation is already typical in semi-arid areas, where many reservoirs have been constructed for water storage. Increased energy demands and climate change have led to severe and increasing pressure on aquatic systems. Today, the environmental policies of many countries, such as Brazil, give priority to constructing new reservoirs for hydropower use, and an adopted reservoir and river basin management plan must minimize environmental impacts. Moreover, the production of energy plants will promote more requirements for new dam projects. The Itaparica reservoir is 30 years old, located in the São Francisco river in Northeast Brazil, and is the focus of an environmental study. The article focus of investigations is on the environmental and social impacts after the construction of Itaparica reservoir, governance difficulties, and adopted actions to minimize those impacts. Significant environmental impacts are recognizable, such as increased sedimentation in the inflow area, damage to the lakeshore zone by operational water level variation, water losses by evaporation and infiltration, and degradation of inundated vegetation. Furthermore, a trophic upsurge has been registered with severe eutrophication processes, such as the occurrence of cyanobacteria, oxygen deficit in the hypolimnion, and mass development of macrophytes (Egeria densa). With the creation of the dam there was compulsory displacement of the population of the municipalities around Itaparica reservoir with consequent difficulties of adaptation in the new spaces. Furthermore, there was de-structuring of social relation networks, loss of arable land and improvements, and small and insufficient indemnities for land. In this context, concepts for an advanced reservoir management under consideration of water scarcity are presented and discussed. This study aims to contribute to sustainable reservoir management

Grazing reduces plant species diversity of Caatinga dry forests in northeastern Brazil

AbstractQuestionsIn many arid and semi‐arid regions of the world, livestock husbandry is the main land use and grazing is one of the main challenges for biodiversity conservation. This also applies to the Caatinga, a unique seasonally dry tropical forest in northeastern Brazil. It is considered one of the most threatened ecosystems of Brazil. To protect semi‐arid regions from biodiversity loss, degradation, and desertification, sustainable management practices are urgently needed. However, the effect of different grazing intensities on plant diversity is not yet fully understood. In this study, we examined the effect of different grazing intensities on plant species composition, alpha and beta diversity.LocationVicinity of the Itaparica Reservoir, northeastern Brazil.MethodsWe assessed the effects of different grazing intensities (high, intermediate, no/low) on species composition, alpha and beta diversity of the tree and shrub, and the herb layer in a Caatinga area. Plant diversity of the tree and shrub layer was assessed on 45 study plots (each 20 m × 20 m), while the diversity of herbs was studied on a subset of 36 plots (four subplots, each 2 m × 2 m).ResultsIn total, we recorded 140 plant taxa including morphospecies. Grazing at high intensities significantly reduced almost all measures of alpha and beta diversity (tree and shrub layer) compared to areas with no or low grazing. In contrast, species composition was not significantly related to grazing but instead to altitude, which serves as an indicator of reduced anthropogenic impact such as slash‐and‐burn agriculture or tree harvesting.ConclusionOverall, the study results show that the current livestock management has negative effects on plant diversity in Caatinga ecosystems. These results have been incorporated into guidelines provided to regional stakeholders including recommendations for adapted land‐management practices that protect and foster the biodiversity of dry forests while sustaining local livelihoods.

Toxic degree evaluation for fish farming in Pernambuco (Brasil)

The Sao Francisco Sub-Middle excerpt is considered favorable for fish farming region. Despite the economic and social importance of this activity, the use of antibiotics, hormones for reversal of fingerlings and the nitrogen and phosphate addition through the feed, represent potentially generating sources of environmental impacts. Ecotoxicological studies associated with physicochemical and biological assessments provide information about the wastewater of fish farming and its impacts, contributing to the formulation of public policies for monitoring wastewater released in water bodies. Ahead this, the present study was carried out on an excaved tank fish farming with Oreochromis niloticus cultivation in Pernambuco (Brazil), in the semi-arid region. Ecotoxicological parameters were evaluated, physico-chemical and biological agents on six sampling points. The results showed that when comparing the data points in the wastewater before and after the treatment by phytoremediation it is observed that there was a values decrease for the physical and chemical parameters, with high rate of dissolved oxygen. However, for the biological variables there was registered an increase in, calling attention to Itaparica reservoir, with cyanobacteria densities above the limit permitted by CONAMA Resolution 357/05. Since many of these parameters indicate eutrophication and may interfere with the water and in fish quality. Water and wastewater from fish farming in excavated tank, as well as oxytetracycline antibiotic, not accounted for toxicity for microcrustaceous Daphnia similis and Danio rerio fish used in ecotoxicological tests. However, sublethal effects observed in D. rerio call attention to the impacts continued in a long-term exposure. The 17α-methyltestosterone hormone caused acute toxicity to D. similis and D. rerio , showing direct impact on the aquatic ecosystem. The results emphasize the need to monitor the ecotoxicological effects of fish farming activity and its impacts on water resources, as well as the regulation of pharmaceutical products used in crops.

Modeling the impacts of climate extremes and multiple water uses to support water management in the Icó-Mandantes Bay, Northeast Brazil

Abstract The hydropower production, water supply and aquaculture services of the Itaparica Reservoir are of immense importance for the Brazilian Northeast. Uncontrolled water resources consumption (e.g. irrigation, water supply), climate and land use change effects deteriorated the water quantity and quality in the reservoir, leading to socio-economic and environmental problems. In this work, a depth-averaged shallow water model was set up for the Icó-Mandantes Bay, one major branch of the reservoir, using the open TELEMAC-MASCARET system. The aim was to assess the impacts of the newly built water diversion channel, as well as the effects of a flood and tracer transport from an intermittent tributary, both located in the bay. An alternative approach to estimate the water retention times was additionally implemented. The simulations showed that though the diversion channel did not significantly influence the hydrodynamics of the bay, it is necessary to continuously monitor water quality parameters in the withdrawal, especially during rainy periods after droughts, because of the nutrient inputs from the tributary and the overflows of the nearby drainage systems. Management measures adapting to the continuously changing natural conditions and anthropogenic impacts are thus indispensable and the model presented can be a valuable supporting tool for this purpose.

Water management and aquatic ecosystem services of a tropical reservoir (Itaparica, São Francisco, Brazil)

Reservoirs have a wide variety of uses that have led to frequent conflicts over ecological conservation and contamination, especially as land management has intensified. Oligotrophication must be implemented in numerous tropical reservoirs that experience advanced eutrophication to maintain aquatic ecosystem functions. To quantify impacts on ecosystem functions and to develop an adaptive management policy, multiple studies have been conducted on the Itaparica Reservoir, Sao Francisco River, in the semi-arid north-eastern region of Brazil. Here, we add to that existing body of knowledge through investigating how nutrient accumulation is affected by water exchange between the main river flow and Ico-Mandantes Bay. Operational water-level fluctuations in the reservoir create large desiccated littoral areas that release high amounts of nutrients when they are rewetted. In particular, water-level variation promotes proliferation of Egeria densa, a noxious weed, thus elevating trophic levels of the Itaparica Reservoir and Ico-Mandantes Bay. Analysis with a P efficiency model determined 25 μg P L−1 to be the critical concentration and further indicated that the critical load in both bodies of water have been exceeded. Moreover, intensive fish aquaculture using net cages has led to further overtaxing of the reservoir. We conclude that an effective ecological reservoir management policy must involve oligotrophication, harvesting of noxious water weeds for use as soil amendment in agriculture or biogas production, “blue” aquaculture, and limiting hydroelectric power production based on current water availability.

Artisanal fisheries of the Itaparica reservoir, São Francisco River, Brazil: socioeconomic profile, environmental dynamics, and management recommendations

Currently, artisanal fisheries in fresh water reservoirs have been losing ground to commercial fish farming and a great deal of fishery resources are being threatened by human interventions, such as reservoir management and increasing net-cage aquaculture. This study aims to understand the impacts of environmental changes on fishery resources and analyse the migratory fish breeding season as a missing link for fishery management in the semi-arid area of the Sao Francisco River Basin. Among the 870 fishermen operating in the Pernambuco part of the Itaparica Reservoir, 10% were interviewed and affirmed that fish stocks have been compromised due to the closed fishing period’s incoherence, exotic species insertion, and changes to the river flow caused by reservoir construction and operation. A significant correlation was observed between the Reproductive Activity Index (RAI) and precipitation (p = 0.745) as well as between the RAI and the river’s flow (p = −0.909). This shows how important it is to consider both the semi-arid climate setting and reservoir operation in determining the closed period which should safeguard fish reproduction. Monitoring the fish reproduction period and research on the reproductive biology of native fish species is needed in the Sao Francisco River Basin’s different stretches in order to guarantee valuable fish stocks and fisheries maintenance. Integrated action between the fish resource users, civil society, and federal bodies/agencies is essential in order to mitigate impacts and improve fish production systems, not only in Brazil, but in other regions experiencing a similar scenario.

Greenhouse gas emissions from a semi-arid tropical reservoir in northeastern Brazil

We estimated carbon dioxide (CO2) and methane (CH4) emissions by diffusion, ebullition, and degassing in turbines from a semi-arid hydropower reservoir in northeastern Brazil. Sampling sites were allocated within the littoral and deeper waters of one embayment, the main-stream, and at turbines. Annual carbon emissions were estimated at 2.3 × 105 ± 7.45 × 104 t C year−1, or in CO2-equivalents (CO2-eq) at 1.33 × 106 ± 4.5 × 105 t CO2-eq year−1. Diffusion across the water surface was the main pathway accounting for 96% of total carbon emissions. Ebullition was limited to littoral areas. A slight accumulation of CO2, but not of CH4, in bottom waters close to the turbines inlet led to degassing emissions about 8 × 103 t C year−1. Emissions in littoral areas were higher than in main-stream and contribute to 40% of the total carbon. Carbon (C) emissions per electricity generated, at 60% of installed capacity, is 0.05 t C-CO2-eq MWh−1. The ratio increases to 0.09 t C-CO2 MWh−1, equating 80% of the emissions from natural gas and 40% of diesel or coal power plants. Retention time and benthic metabolism were identified as main drivers for carbon emissions in littoral areas, while water column mixing and rapid water flow are important factors preventing CH4 accumulation and loss by degassing. Our results indicate that Itaparica Reservoir, located in the semi-arid region of Northeastern Brazil, acts as a source of GHGs. Management measurements are needed to prevent emissions to raise in the future.

Three-Dimensional Modeling of Wind- and Temperature-Induced Flows in the Icó-Mandantes Bay, Itaparica Reservoir, NE Brazil

The Icó-Mandantes Bay is one of the major branches of the Itaparica Reservoir (Sub-Middle São Francisco River, Northeast Brazil) and is the focus of this study. Besides the harmful algae blooms (HAB) and a severe prolonged drought, the bay has a strategic importance—e.g., the eastern channel of the newly built water diversion will withdraw water from it (drinking water). This article presents the implementation of a three-dimensional (3D) numerical model—pioneering for the region—using TELEMAC-3D. The aim was to investigate the 3D flows induced by moderate or extreme winds as well as by heating of the water surface. The findings showed that a windstorm increased the flow velocities (at least one order of magnitude, i.e., up to 10−1–10−2 m/s) without altering significantly the circulation patterns; this occurred substantially for the heating scenario, which had, in contrast, a lower effect on velocities. In terms of the bay’s management, the main implications are: (1) the withdrawals for drinking water and irrigation agriculture should stop working during windstorms and at least three days afterwards; (2) a heating of the water surface would likely increase the risk of development of HAB in the shallow areas, so that further assessments with a water quality module are needed to support advanced remediation measures; (3) the 3D model proves to be a necessary tool to identify high risk contamination areas e.g., for installation of new aquaculture systems.

ENVIRONMENTAL CONFLICTS IN THE IRRIGATED SCHEME FROM PETROLÂNDIA MUNICIPALITY: SUBSIDY TO UNDERSTAND THE LAND MANAGEMENT AT THE MIDDLE-LOW SÃO FRANCISCO BASIN

The Itaparica region located between Pernambuco and Bahia had got part of his territory flooded due to the construction of the reservoir of the Luiz Gonzaga Hydroeletric, built in 1988. About 10,500 families were transferred to resettlement areas as result of the reservoir construction. More than 21,000 people lived in urban areas and more than 19,000 in rural areas. The resettlement were used construct new cities and areas with irrigated settlements and small villages (agrovilas). The resettlements of Apolônio Sales, Icó-Mandantes and Barreiras Bl2 are the oldest in the Petrolândia Municipality, only the first present different infrastructure. This research aims to discuss the challenges in land management of the irrigated schemes of Chesf in Petrolândia after the Itaparica reservoir construction. Field works, including interviews and workshops using the Constellation Analysis methodology was used as tool in the elaboration of this study. Currently the agrovilas are with an outdated water supply and treatment system most likely causing health issues for the resettled people. The agrovila area presents a disorderly urban increase, normally caused because of immigrants who came to the irrigated scheme to work in the farming or to invest in the agriculture. They also expand new irrigated schemes through inappropriate areas, leaving residential garbage in the native vegetation and using water uncontrolled, causing reduction in the water supply. The debate over the water and land access and the effective action of the public power together with the producers are urgent to solve the existing problems in the resettlements.

Management of sludge from fish ponds at the edge of the Itaparica Reservoir (Brazil): an alternative to improve agricultural production

Sludge generated in intensively managed tilapia fingerling breeding ponds near the Itaparica Dam in the semi-arid Brazilian northeastern region was tested as a soil conditioner to produce lettuce in an effort to mitigate the present environmental impact of the deposition of this sludge, thereby giving it a productive destiny. A greenhouse experiment was performed by mixing the sludge with a Haplic Planosol topsoil, a characteristic soil of the region, so that the sludge corresponded to 0, 25, 50, 75, and 100% of the mixture (substrate). The experiment was set up as a randomized block design with five blocks, each with three pots (5-kg substrate per pot) of the five sludge proportion treatments. One lettuce plant was transplanted into each pot, maintained under greenhouse conditions, and harvested 35 days after transplanting. Sludge, soil, and the substrates were evaluated for nutrient concentration and physical characteristics. Green and dry weights, stem diameter, and nitrogen, phosphorus, and potassium concentrations in the aboveground parts of the lettuce plants were determined. Sludge incorporation into the substrate improved its chemical and physical characteristics. The lettuce plants grew best in the substrate with 75% sludge, increasing its biomass production by 50% and more than doubling its nitrogen uptake.

Nutrient load concept-reservoir vs. bay impacts: a case study from a semi-arid watershed.

Large flow-through reservoirs and lakes possess environmental gradients and monitoring programs are mostly adapted for cost and time effectiveness. Bay areas are often more isolated from the main water body and are likely to have unobserved different environmental processes and impacts. This study was performed at the Itaparica Reservoir, São Francisco River, located in semi-arid Northeast Brazil, with dendritic form. Water residence time in the Icó-Mandantes Bay was estimated by hydrodynamic flow and transport simulations. The P-chlorophyll a relationship was used to develop the P use efficiency coefficient for critical P load estimation of 25 μg P L-1. Phosphorus sources and input rates into a bay and the respective reservoir were calculated and compared regarding their different origins for the period after flooding (1988) and for 2013. After impoundment, the P load highly exceeded the carrying capacity because of leaching and mineralization processes. In 2013, P inputs were still above this threshold, whereas inflow and sub-basin P export during the rainy season were crucial. But eutrophication processes have increased in the bay relative to the main water body. Hence, water in hydraulic isolated parts is prone to eutrophication processes, thus, bays have to be specially considered in water resource management.

Simulations of nutrient emissions from a net cage aquaculture system in a Brazilian bay.

Hydrodynamics and transport simulations were conducted with the modeling software TELEMAC-2D on Icó-Mandantes bay, a branch of the Itaparica reservoir. The bay has a maximal operational water level amplitude of 5 m and is suffering from eutrophication and algae bloom. Therefore, we investigated low and high water level scenarios with two different high resolution meshes, with the purpose to deeper understand their impact on transport of substances and to improve the watershed management. In particular, nutrient emissions from a hypothetical net cage aquaculture system located in the bay were investigated on half-year cycles. We observed a relevant impact on water quality for a tilapia production of 130 t y(-1); i.e. after 6 months' simulation we obtained around 8 μgP L(-1) and 6 μgP L(-1) at the source of emissions, for low and high water scenarios, respectively.

Grazing deteriorates the soil carbon stocks of Caatinga forest ecosystems in Brazil

Grazing by domestic ungulates can have substantial impacts on forests in arid and semi-arid regions, possibly including severe loss of carbon from the soil. Predicting net livestock impacts on soil organic carbon stocks remains challenging, however, due to the dependence on animal loads and on soil and environmental parameters. The objective of this study was to better understand grazing effects on soil organic carbon in seasonal tropical dry forests of north-eastern Brazil (Caatinga) by quantifying carbon stocks of the upper soil profile (0–5cm depth) and greater soil depths (>5cm depth down to bedrock) along a gradient of grazing intensity while accounting for other influencing factors such as soil texture, vegetation, landscape topography, and water availability. We analysed soil organic carbon, soil clay content, altitude above sea level, soil depth to bedrock, distance to the nearest permanent water body, species diversity of perennial plants and aboveground biomass on 45 study plots located in the vicinity of the Itaparica Reservoir, Pernambuco, Brazil. Livestock (mainly goats and cattle) are unevenly distributed in the studied ecosystem, thus grazing intensity was accounted for based on the weight of livestock droppings per square metre and classified as no or light, intermediate, or heavy grazing. The mean soil organic carbon in the area was 16.86±1.28Mgha−1C with approximately one-quarter found in the upper 5cm of the soil profile (4.14±0.43Mgha−1C) and the remainder (12.57±0.97Mgha−1C) in greater soil depths (>5cm). Heavy grazing led to significantly lower soil organic carbon stocks in the upper 5cm, whereas no effect on soil organic carbon of the soil overall or in greater soil depths was detectable. The soil’s clay content and the altitude proved to be the most relevant factors influencing overall soil organic carbon stocks and those in greater soil depths (>5cm). Our findings suggest that grazing causes substantial release of carbon from Brazilian dry forest soils, which should be addressed through improved grazing management via a legally compulsory rotation system. This would ultimately contribute to the conservation of a unique forest system and associated ecosystem services.

Is crop yield related to weed species diversity and biomass in coconut and banana fields of northeastern Brazil?

The ability to accommodate crop production for an ever-growing human population and achieve conservation of rapidly declining biodiversity remains a challenging task worldwide. In agroecosystems, weed diversity and biomass are frequently assumed to be negatively related to crop yield and biomass. However, positive effects of weed species (pollinator and parasitoid attraction) and different resource acquisition strategies may reduce the competitive character of weeds—a potential that can be exploited within land-sharing approaches (i.e., biodiversity conservation and agriculture on the same site). This study aimed at analyzing the relationships of weed diversity and biomass to crop yield and biomass in coconut and banana fields within an irrigation farming scheme established in former Caatinga seasonal dry forest ecosystems around the Itaparica Reservoir, Pernambuco, Brazil. Within each of 21 selected crop fields, we collected weed diversity and biomass data in the fields’ center and edge along with general information on crop yield and the use of fertilizers and other agrochemical inputs. We found no evidence for a negative relationship of crop yield or biomass and weed diversity. On the contrary, crop yield and weed alpha diversity were significantly positively correlated (Shannon and Simpson indices, evenness). In contrast, weed biomass showed a significant negative correlation to crop yield. The use of organic fertilizer had a significant positive effect on crop yield, whereas no impact of herbicides or insecticides was detected. In addition, the field edge provided habitat for more weed species than the field center. Overall, our data show that in perennial tropical crop fields high yield is not opposed to high weed diversity. Moreover, the data suggest that organic farming in the area will likely not lead to yield losses. Nevertheless, the related weed assemblages inhabited only a few typical species of the native dry forest vegetation which makes their contribution to biodiversity conservation at the landscape scale debatable.

Comportamento espacial da clorofila-a no reservatório de Itaparica, rio São Francisco

RESUMO Na análise do estado trófico de um corpo hídrico torna-se fundamental o conhecimento da concentração de clorofila-a. Esse trabalho tem como objetivo determinar e avaliar o comportamento da clorofila-a no reservatório de Itaparica, localizado no sub-médio São Francisco. Para esse fim, utilizou-se de imagem Landsat-TM, na qual foram utilizadas as bandas de 1 a 5 e 7. O modelo aplicado foi escrito em linguagem LEGAL-SPRING 5.2. A partir da imagem da clorofila-a realizou-se o fatiamento do corpo hídrico em seis classes concentração. O valor mínimo foi de <1 mg/L e o maior foi de 249,5 mg/L. As classes que obtiveram maior área foram de 0 a 5 mg/L com 27,4%, seguida da classe 5 a 10 mg/L com 24,6% da área total do reservatório, conforme observado por histograma. Por meio da análise gráfica de pontos situados ao longo do reservatório verifica-se que a concentração da clorofila-a aumenta da região fluvial para região lacustre e a partir dos contatos dos riachos com o reservatório. Em futuros estudos há necessidade de se validar os valores encontrados com dados de campo de forma a verificar a precisão do mapeamento para o reservatório em estudo, observando dia e horário da passagem do sensor.

Phytoplankton composition of the Itaparica and Xingó reservoirs, São Francisco River, Brazil.

The installation of dams causes changes to the integrity of rivers and to the water cycle, performing an instrumental role in the organization of biological communities, including that of phytoplankton. In the present study, we analyzed the taxonomic composition of phytoplankton in two hydroelectric reservoirs on the São Francisco River, Itaparica and Xingó reservoirs. Samples were collected at quarterly intervals between December 2007 and September 2009, at 12 sampling stations in each reservoir, totaling 92 samples. We identified 110 species in the Itaparica reservoir and 136 in the Xingó reservoir, of which diatoms followed by green algae, played a major contribution to both reservoirs. Most of the species is rare and/or occasional. In the Itaparica reservoir, there were no very frequent species, although in the Xingó this category was represented by the diatoms Aulacoseira granulata (Ehrenberg) Simonsen and Fragilaria crotonensis Kitton. These results show that, despite the similarity in the composition of phytoplankton, the reservoirs studied certainly differed regarding their environmental conditions.

Carrying capacity limits of net cage aquaculture in brazilian reservoirs

Finfish aquaculture in net cages is widely used in Brazilian reservoirs, mainlyfor tilapia production. There is a large and increasing potential for productionin the Sao Francisco river basin, and particularly the Itaparica reservoir. Tilapiaproduction amounts to 24,000 t y-1, with a licensed amount of 43,267 t y-1.This intensive fish production in net cages is responsible for a significantbiological oxygen demand, and phosphorus and nitrogen load on the reservoir,which promotes eutrophication. Particulate organic matter released from thenet cages accumulates beneath the net cages, and a minimum water depthbeneath the net cages of 10 m is required to limit the sediment increase toa few millimetres per year. Modeling of Ico-Mandantes bay has identified areduced water exchange within the bay. Modeling of the effect of net cageaquaculture within the Ico-Mandantes bay points out clearly the significantincrease in dissolved phosphorus and the accumulation inside the bay area.The carrying capacity of the reservoir was determined using the P loadmodel, with a critical P concentration based on the phosphorus use efficiency.The critical P concentration amounts 25 μg L-1, and the critical P load of thereservoir amounts 2.84 g m-2 y-1; the actual load is already 3.30 g m-2 y-1,such the reservoir is already overcharged by nutrients. A sustainable “blue”aquaculture must be implemented based on use of advanced systems, speciesselection, fish feed, and linked production systems.