Responses of Functional and Taxonomic Phytoplankton Diversity to Environmental Gradients in Subtropical and Tropical Reservoirs

Understanding the influence of environmental conditions on biodiversity is a major task in ecology. We investigated how phytoplankton taxonomic (TD) and functional (FD) diversities vary with environmental factors in eight subtropical and tropical reservoirs. We hypothesized that i) environmental variables affect phytoplankton TD and FD; ii) FD provides better relationships to environmental changes than TD, and; iii) indices based on biomass are better related to the environment than those based on identities. The relationships between phytoplankton diversities and environmental drivers were assessed through generalized linear models. Our hypotheses were partly confirmed. TD and FD were, in fact, dependent on the environment, with higher values occurring in warmer, clearer, and more enriched systems, under lower zooplankton grazing pressure; but FD based on identities was not predicted better from environmental conditions than TD based on identities. As expected, indices based on biomass are better related to the environment than their counterpart based on identities

IMPACTOS DAINTRODUÇÃODA TILÁPIA DO NILO, OREOCHROMIS NILOTICUS, SOBRE A ESTRUTURA TRÓFICA DOS ECOSSISTEMAS AQUÁTICOS DO BIOMA CAATINGA

The Nile tilapia, Oreochromis niloticus (Linnaeus 1757), is an omnivorous fish which has been introduced into the aquatic ecosystems of the Caatinga Biome since the 70´s to improve fisheries and aquaculture. However, the environmental impacts of this species introduction are still poorly known. The aim of this work was to experimentally investigate possible mechanisms by which this species might negatively affect other fish species. Two field experiments were conducted where the abundance of tilapia was manipulated and the phyto- and zooplankton dynamics were monitored together with the variation in water transparency. We tested the hypothesis that the Nile tilapia would decrease the abundance of zooplankton, increase the abundance of phytoplankton and consequently decrease water transparency. Results from both experiments confirm the hypothesis but suggest that only larger zooplankton is negatively affected while only smaller algae are enhanced by tilapias. Even though the Nile tilapia is an omnivorous species feeding on both zooplankton and phytoplankton, its effects on plankton communities were very similar to the effects of strictly zooplanktivorous fish. Results also show a strong negative effect of tilapias on water transparency due to its bioturbation or ressuspension of sediments. Therefore, through its negative effects on zooplankton abundance and water transparency, the Nile tilapia may inhibit the recruitment of other fish species that feed mainly on zooplankton and are visually oriented to locate and capture their prey, at least in earlier stages of their lives.   A tilápia do Nilo, Oreochromis niloticus (Linnaeus 1757), é um peixe onívoro que vem sendo introduzido nos ecossistemas aquáticos do Bioma Caatinga desde a década de 70 para fins de pesca e piscicultura. No entanto, os impactos ambientais oriundos da introdução desta espécie ainda permanecem pouco conhecidos. O objetivo deste trabalho foi investigar experimentalmente possíveis mecanismos pelos quais a espécie pode afetar negativamente outras espécies de peixes. Dois experimentos de campo foram realizados onde as densidades de tilápias foram manipuladas e as dinâmicas do fito e zooplâncton foram monitoradas assim como a variação na transparência da água. A hipótese testada neste trabalho é a de que a tilápia do Nilo reduz a abundância do zooplâncton, aumenta a abundância do fitoplâncton e conseqüentemente diminui a transparência da água. Os resultados dos dois experimentos confirmam a hipótese, mas sugerem que apenas os organismos zooplanctônicos de maior tamanho são negativamente afetados enquanto que apenas as algas planctônicas de menor tamanho são favorecidas pelas tilápias. Apesar da tilápia do Nilo ser uma espécie onívora e se alimentar tanto de zooplâncton quanto de fitoplâncton, seus efeitos sobre as comunidades planctônicas foram muito semelhantes aos efeitos de peixes estritamente zooplanctívoros. Os resultados ainda demonstram um forte efeito negativo das tilápias sobre a transparência da água devido a sua ação de bioturbação ou ressuspensão de sedimentos. Portanto, através de seus efeitos negativos sobre a abundância do zooplâncton e a transparência da água, a tilápia do Nilo pode inibir o recrutamento de outras espécies de peixes que se alimentam principalmente de zooplâncton e se orientam visualmente para localizar e capturar suas presas, ao menos nos estágios iniciais de suas vidas

<b>Local ecological knowledge is not a useful source of information concerning impacts caused by non-native Nile tilapia on fishery stocks</b> - doi 10.4025/actascibiolsci.v35i3.18418

Exotic freshwater fish can have deleterious effects on local biodiversity, although these impacts often only become apparent many years after the introduction. Local Ecological Knowledge (LEK) may be a useful source of information in situations where formal technical studies are insufficient, but few works have examined the reliability of information generated through this approach. We examined the reliability of LEK by investigating the impacts of Nile tilapia on fishery stocks in an artificial reservoir in northeastern Brazil. We gathered LEK from 29 experienced fishermen and then confronted this information with official fishery statistics from the same site. Twenty-two fishermen stated that total catch in the Gargalheiras Reservoir had declined over the years, 68% (N=15) of them began fishing before 1976 (the year Nile tilapia was introduced into reservoir). Of those 15 fishermen, 87% (N=13) stated that tilapia has not negatively affected other species, which ran counter to analyses of fishery statistics. Our study suggests that the LEK of fishermen is not a useful source of information concerning the impacts caused by exotic tilapia. However, the LEK added an overfishing hypothesis of the decline in fishery stocks in the Gargalheiras reservoir.  

Effect of suspended clay on growth rates of the cyanobacterium Cylindrospermopsis raciborskii

Recent studies have shown that sediment resuspension may lead to the collapse of C. raciborskii dominance, which suggests that clay might have a negative effect on the growth of C. raciborskii. To test the hypothesis that suspended clay creates an unfavorable environment for growth of C. raciborskii, we exposed four different strains of this species to various concentrations of the clays kaolinite and bentonite, and monitored the biomass of each strain over the course of 1-week microcosm experiments. Contrary to our hypothesis, C. raciborskii was able to grow in suspensions of both clays. While kaolinite clay caused higher turbidity than bentonite, the growth rates of all four C. raciborskii strains were higher in kaolinite than in bentonite suspensions. C. raciborskii could still grow in clay concentrations that cause turbidity far above the levels found in natural lakes. Our study suggests that the reported collapse of C. raciborskii blooms with high concentrations of suspended sediments in tropical shallow lakes is probably not caused by the effects of suspended clay on light attenuation, but rather is a consequence of cell sinking or, possibly a response to disturbance events responsible for sediment suspension

Aquatic systems in semi-arid Brazil: limnology and management Ecossistemas aquáticos do semi-árido brasileiro: aspectos limnológicos e manejo

Aquatic systems in semi-arid Brazil include natural shallow lakes, artificial reservoirs and intermittent streams and rivers. These systems are distinctive features in the semi-arid landscape and comprise a range of associated systems functioning as an ever-changing mosaic of dry/wet patches. Lakes and reservoirs in semi-arid Brazil are subject to important periods of water shortages, whereas rivers and streams are characterized as highly variable and driven by the extremes of water flow and its absence. Within this view a catchment-scale approach must be used to create a holistic model to conceptualize and comprehend these aquatic systems, since the aquatic environment types in the semi-arid region of Brazil incorporate broader aspects within the catchment scale such as geomorphology, vegetation, climate and land use. This paper summarizes some of the information on the aquatic systems of the Brazilian semi-arid region and shows the importance of limnological studies in this region. It also attempts to establish perspectives for future research considering the catchment as a scale for surveying biological processes and limnological characteristics of the various aquatic systems. It is presented information on their overall structure and functioning, as well as characteristics of some biological communities, such as phytoplankton, periphyton, aquatic macrophytes, benthic invertebrates and fish. The importance of the understanding of eutrophication in reservoirs and the role of the dry phase in streams is emphasized, and information on possible actions of planning and management to improve water quality of reservoirs are presented.<br>Os ecossistemas no semiárido brasileiro englobam lagos rasos naturais, reservatórios artificiais e rios e riachos intermitentes. Estes sistemas são particularidades na paisagem do semiárida e compreendem uma grande variedade de sistemas associados, funcionando como um mosaico em constante mudança entre épocas de seca e chuva. Lagos e reservatórios no semiárido brasileiro estão sujeitos a importantes períodos de escassez de água, enquanto que os rios e riachos são caracterizados como altamente variáveis e impulsionados pelos extremos de fluxo de água e sua ausência. Dentro desta perspectiva uma abordagem em escala de bacia hidrográfica deve ser usada para se criar um modelo holístico para conceituar esses sistemas aquáticos, uma vez que os tipos de ambientes aquáticos na região semiárida do Brasil incorporam aspectos mais amplos dentro da escala de bacia hidrográfica como geomorfologia, clima, vegetação e uso da terra. Este artigo sintetiza algumas das informações sobre os sistemas aquáticos da região semiárida brasileira e mostra a importância de estudos limnológicos nesta região. Ele também tenta estabelecer perspectivas para pesquisas futuras considerando a bacia hidrográfica como uma escala para levantamento de processos biológicos e as características limnológicas dos vários sistemas aquáticos. É apresentada informações sobre sua estrutura e funcionamento geral, bem como características de algumas comunidades biológicas, tais como o fitoplâncton, perifíton, macrofitas aquáticas, invertebrados bentônicos e peixes. A importância do entendimento da eutrofização em reservatórios e o papel da fase de seca em riachos são enfatizados, e informações sobre possíveis ações de planejamento e gestão para melhorar a qualidade da água dos reservatórios são apresentados

Aquatic systems in semi-arid Brazil: limnology and management

Aquatic systems in semi-arid Brazil include natural shallow lakes, artificial reservoirs and intermittent streams and rivers. These systems are distinctive features in the semi-arid landscape and comprise a range of associated systems functioning as an ever-changing mosaic of dry/wet patches. Lakes and reservoirs in semi-arid Brazil are subject to important periods of water shortages, whereas rivers and streams are characterized as highly variable and driven by the extremes of water flow and its absence. Within this view a catchment-scale approach must be used to create a holistic model to conceptualize and comprehend these aquatic systems, since the aquatic environment types in the semi-arid region of Brazil incorporate broader aspects within the catchment scale such as geomorphology, vegetation, climate and land use. This paper summarizes some of the information on the aquatic systems of the Brazilian semi-arid region and shows the importance of limnological studies in this region. It also attempts to establish perspectives for future research considering the catchment as a scale for surveying biological processes and limnological characteristics of the various aquatic systems. It is presented information on their overall structure and functioning, as well as characteristics of some biological communities, such as phytoplankton, periphyton, aquatic macrophytes, benthic invertebrates and fish. The importance of the understanding of eutrophication in reservoirs and the role of the dry phase in streams is emphasized, and information on possible actions of planning and management to improve water quality of reservoirs are presented