Natália Fernanda Santana

Limnology in the Upper Paraná River floodplain: large-scale spatial and temporal patterns, and the influence of reservoirs

Knowledge of abiotic limnological factors is important to monitor changes caused by humans, and to explain the structure and dynamics of populations and communities in a variety of inland water ecosystems. In this study, we used a long term data-set (eight years) collected in 10 habitats with different features (river channels, and connected and isolated lakes) to describe the spatial and temporal patterns of some of the principal limnological factors. In general, the degree of connectivity of the lakes, together with the rivers to which the lakes are connected, were important determinants of their limnological characteristics. These differences are expected, because rivers entering the floodplain come from different geological regions and are subject to different human impacts. At large spatial scales, these differences contribute to the increased habitat diversity of the floodplain and thus to its high biodiversity. With regard to temporal variation, Secchi-disk transparency increased, and total phosphorus decreased in the Paraná River main channel during the last 20 years. Although these changes are directly attributed to the several reservoir cascades located upstream, the closing of the Porto Primavera dam in 1998 enhanced this effect. The increase in water transparency explains biotic changes within the floodplain. The lower-phosphorus Paraná River water probably dilutes concentrations of this element in the floodplain waterbodies during major floods, with future consequences for their productivity.

Impact of fish cage-culture on the community structure of zooplankton in a tropical reservoir

O objetivo deste estudo foi investigar o impacto da piscicultura em tanques-rede sobre a estrutura da comunidade zooplanctonica em um reservatorio tropical. Acredita-se que a abundância do zooplâncton e maior perto dos tanques-rede e aumenta ao longo do tempo devido ao aumento da disponibilidade de alimento. As coletas foram realizadas proximo aos tanques-rede, bem como a montante e a jusante dos mesmos, antes e apos a instalacao dos tanques. A abundância do zooplâncton aumentou 15 dias depois da instalacao dos tanques, seguida por uma reducao e posterior aumento no fim do experimento. A abundância dos rotiferos apresentou diferencas significativas entre os pontos (p<0,05) e periodos de amostragem (p<0,001). Diferencas significativas tambem foram observadas para a abundância do zooplâncton total e de cladoceros (p<0,001). A variacao espacial e temporal das variaveis fisicas e quimicas apresentou uma relacao indireta com a estrutura e dinâmica da comunidade zooplanctonica, pois essa variacao indicou a producao primaria no ambiente. A hipotese foi rejeitada, pois o zooplâncton foi mais abundante no ponto de controle. Apenas os rotiferos apresentaram maior abundância perto dos tanques, devido a influencia da disponibilidade de alimento. No entanto, a dinâmica da comunidade durante o experimento tambem foi relacionada com a disponibilidade de alimento. Os resultados sugerem o impacto da piscicultura em tanques-rede na comunidade zooplanctonica.

Bacterioplankton features and its relations with doc characteristics and other limnological variables in Paraná river floodplain environments (PR/MS-Brazil).

Since the introduction of the Microbial Loop concept, many studies aimed to explain the role of bacterioplankton and dissolved organic carbon (DOC) in aquatic ecosystems. Paraná River floodplain system is a very complex environment where these subjects were little explored. The aim of this work was to characterize bacterial community in terms of density, biomass and biovolume in some water bodies of this floodplain and to verify its temporal variation and its relation with some limnological variables, including some indicators of DOC quality, obtained through Ultraviolet-visible (UV-VIS) and fluorescence spectroscopic analysis. Bacterial density, biomass and biovolume are similar to those from other freshwater environments and both density and biomass were higher in the period with less rain. The limnological and spectroscopic features that showed any relation with bacterioplankton were the concentrations of N-NH4 and P-PO4, water transparency, and some indicators of DOC quality and origin. The analysis of these relations showed a possible competition between bacterioplankton and phytoplankton for inorganic nutrients and that the DOC used by bacterioplankton is labile and probably from aquatic macrophytes.

Main predictors of periphyton species richness depend on adherence strategy and cell size

Periphytic algae are important components of aquatic ecosystems. However, the factors driving periphyton species richness variation remain largely unexplored. Here, we used data from a subtropical floodplain (Upper Paraná River floodplain, Brazil) to quantify the influence of environmental variables (total suspended matter, temperature, conductivity, nutrient concentrations, hydrology, phytoplankton biomass, phytoplankton species richness, aquatic macrophyte species richness and zooplankton density) on overall periphytic algal species richness and on the richness of different algal groups defined by morphological traits (cell size and adherence strategy). We expected that the coefficients of determination of the models estimated for different trait-based groups would be higher than the model coefficient of determination of the entire algal community. We also expected that the relative importance of explanatory variables in predicting species richness would differ among algal groups. The coefficient of determination for the model used to predict overall periphytic algal species richness was higher than the ones obtained for models used to predict the species richness of the different groups. Thus, our first prediction was not supported. Species richness of aquatic macrophytes was the main predictor of periphyton species richness of the entire community and a significant predictor of the species richness of small mobile, large mobile and small-loosely attached algae. Abiotic variables, phytoplankton species richness, chlorophyll-a concentration, and hydrology were also significant predictors, depending on the group. These results suggest that habitat heterogeneity (as proxied by aquatic macrophytes richness) is important for maintaining periphyton species richness in floodplain environments. However, other factors played a role, suggesting that the analysis of species richness of different trait-based groups unveils relationships that were not detectable when the entire community was analysed together.

Eichhornia azurea decomposition and the bacterial dynamic: an experimental research.

Organic decomposition is a complex interaction between chemical, physical and biological processes, where the variety of aquatic vascular plants is essential for the trophic dynamics of freshwater ecosystems. The goal of this study was to determine the aquatic macrophyte Eichhornia azurea (Sw.) Kunth decomposition rate, the time relation with the limnological parameters, and whether this relationship is a result of decomposition processes. To that end, we collected water and leaves of E. azurea in Surf Leopoldo, PR. The experiment consisted of two treatments: 25 containers with 450 mL of water and 0.8 g of biomass dry weight were used with or without the addition of macrophytes. Samples were collected in triplicate at times 0, 3 h, 6 h, 12 h, 24 h, 72 h, 120 h, 168 h and 240 h. When the container was removed, the plant material was dried in an oven. After 48 h, the material was measured to obtain the final dry weight. Analyses of pH, conductivity, dissolved oxygen, total phosphorus N-ammonia (NH4), soluble reactive phosphorus (PO4) and dissolved organic carbon were performed, and the decomposition rate was calculated. The results showed significant temporal variation of limnological parameters in the study. Additionally, dissolved oxygen, conductivity, dissolved organic carbon and total phosphorus were correlated with the dry weight of the biomass, suggesting that E. azurea decomposition significantly interferes with the dynamics of these variables.

Herbivory effects on the periphytic algal functional diversity in lake ecosystems: an experimental approach

Herbivory can determine the rates of ecosystem functions and is key factor to periphytic algae development. Here, we aimed to evaluate the effects of herbivory of zooplankton and omnivorous fish in periphytic algae of lake ecosystems. Based on prediction of grazer:prey size ratio, we tested that periphytic algal biomass and density will be lower in the fish treatments. Based on prediction of the size-selective predation, size-efficiency hypothesis, and general allometric equations, and that both herbivores will feed with distinct mode, we tested the hypothesis that periphytic algal functional diversity will be higher in the mixed treatment. There were four treatments (Control, T1-zooplankton, T2-fish, T3-zooplankton+fish). We evaluated algae biomass, density, functional traits, functional diversity, and the herbivory effect. Our results show that the presence of both herbivores were related to higher functional diversity, lower density values, and prostrate periphytic algae, and in T2 occurs higher periphytic algae biomass via nutrient facilitation. Fish was responsible for the higher consumer effect size, and was more important modifier than zooplankton of the periphytic algae community. The greater variation in producer species traits was directly related to mixed foraging patterns of herbivores together, which allow rare species to persist, decreasing the dominance of stronger competitive species.