Sidinei Magela Thomaz

Floods increase similarity among aquatic habitats in river-floodplain systems

During low water levels, habitats in river-floodplain systems are isolated from each other and from the main river. Oppositely, floods tend to connect water bodies with distinct hydrological characteristics and, as a result, ecological processes and biological communities tend to be more similar among the distinct habitats that comprise a river-floodplain system. Based on a literature review and also using unpublished data obtained in tropical floodplains, the aim of this paper is to highlight the effects of floods as a process that reduce spatial variability. The usual negative relationship between the coefficient of variation of any ecological indicator (e.g., chlorophyll-a or total phosphorus) and water level is the main result demonstrating a reduction in spatial variability due to floods. Considering physical, chemical or biotic data gathered in distinct habitats within the floodplain, this pattern was found in temperate and tropical regions, subjected to distinct levels of anthropogenic impacts, and at different spatial extents. The main mechanism that accounts for this pattern may be stated as follow. During low water level, the biological communities of each habitat in the floodplain (e.g. lagoons, backwater, streams) follow distinct temporal trajectories due to the effects of local driving forces (e.g. an efficient predator trapped in a lagoon but not in another). Management plans and biodiversity conservation in river floodplain systems will benefit by considering the effects of flood homogenization and increased connectivity peculiar to these unique ecosystems.

Global diversity of aquatic macrophytes in freshwater

Aquatic macrophytes are aquatic photosynthetic organisms, large enough to see with the naked eye, that actively grow permanently or periodically submerged below, floating on, or growing up through the water surface. Aquatic macrophytes are represented in seven plant divisions: Cyanobacteria, Chlorophyta, Rhodophyta, Xanthophyta, Bryophyta, Pteridophyta and Spermatophyta. Species composition and distribution of aquatic macrophytes in the more primitive divisions are less well known than for the vascular macrophytes (Pteridophyta and Spermatophyta), which are represented by 33 orders and 88 families with about 2,614 species in c. 412 genera. These c. 2,614 aquatic species of Pteridophyta and Spermatophyta evolved from land plants and represent only a small fraction (∼1%) of the total number of vascular plants. Our analysis of the numbers and distribution of vascular macrophytes showed that whilst many species have broad ranges, species diversity is highest in the Neotropics, intermediate in the Oriental, Nearctic and Afrotropics, lower in the Palearctic and Australasia, lower again in the Pacific Oceanic Islands, and lowest in the Antarctic region. About 39% of the c. 412 genera containing aquatic vascular macrophytes are endemic to a single biogeographic region, with 61–64% of all aquatic vascular plant species found in the Afrotropics and Neotropics being endemic to those regions. Aquatic macrophytes play an important role in the structure and function of aquatic ecosystems and certain macrophyte species (e.g., rice) are cultivated for human consumption, yet several of the worst invasive weeds in the world are aquatic plants. Many of the threats to fresh waters (e.g., climate change, eutrophication) will result in reduced macrophyte diversity and will, in turn, threaten the faunal diversity of aquatic ecosystems and favour the establishment of exotic species, at the expense of native species.

Habitat complexity: approaches and future directions

Habitat complexity is one of the most important factors structuring biotic assemblages, yet we still lack basic understanding of the underlying mechanisms. Although it is one of the primary targets in conservation management, no methods are available for comparing complexity across ecosystems, and system-specific qualitative assessment predominates. Despite its overwhelming importance for faunal diversity and abundance, there has been surprisingly little interest in examining its effects on other community and ecosystem attributes. We discuss possibilities of such effects, outlining potentially fruitful areas for future research, and argue that complexity may be implicated in community persistence and ecosystem stability by acting as a decoupling mechanism in predator–prey interactions. We provide a brief overview of methods used to quantify complexity in different ecosystems, highlighting contributions of the current issue of Hydrobiologia, and discuss potential application of these approaches for cross-ecosystem comparisons. Better understanding of the role of habitat complexity resulting from such comparisons is critically important for preservation of biodiversity and ecosystem function in an era of unprecedented habitat loss.

Aquatic macrophyte distribution in relation to water and sediment conditions in the Itaipu Reservoir, Brazil

Aquatic macrophyte community distribution along the eastern shoreline of the Itaipu Reservoir (one of the South Americas largest impoundments) is described in relation to limnological and sedimentological factors. The central body of the reservoir is mesotrophic, while the arms (flooded influent river valleys) along the eastern shore may be oligo-mesotrophic to eutrophic, depending on time of year and sub-catchment characteristics. Macrophyte community composition and species cover were surveyed at 30 sites in four arms, in relation to sediment total P and organic matter; underwater light regime; and water total P and Kjeldahl N concentration, alkalinity, conductivity, depth and pH. Seventeen euhydrophyte and six emergent macrophyte species were recorded. Large stands of Egeria najas dominated the euhydrophyte vegetation, together with free-floating weed species (Pistia stratiotes Linn., Salvinia auriculata Aublet and Eichhornia crassipes (Mart.) Solms.). Canonical Correspondence Analysis of the data showed that two sets of variables were important predictors of aquatic macrophyte community structure. Floating macrophyte assemblage was closely related to concentration of nutrients in both water and sediment, while light penetration was the strongest predictor of submerged species occurrence. Although a large number of potential nuisance species were present, dense growths were restricted to shallower areas of the Itaipu Reservoir, causing localised problems. The possibility of increasing interference by these plants with fisheries, recreational use, transport and hydroelectricity generation suggests a need for continued monitoring of weed distribution and abundance, and investigation of appropriate management measures.

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.

Temporal and spatial patterns of aquatic macrophyte diversity in the Upper Paraná River floodplain

Although the importance of long-term data has been emphasized by ecologists in recent years, little is known about how communities may change over time. In this study, we describe the general patterns of aquatic macrophyte diversity in the Paraná River floodplain observed during six years of study. Temporal changes in community composition were also evaluated. Data on the presence or absence of aquatic macrophytes were collected between March 2002 and March 2008, in six lakes associated with three rivers. Different analytical strategies were used to evaluate the dynamics of aquatic macrophyte communities between the different systems in the floodplain. The composition of aquatic macrophytes differed among the rivers, mainly with respect to the different vegetation life forms (floating, submersed, emergent and rooted with floating stems). The temporal similarity of species composition during the six years and the beta-diversity index indicated that the month-to-month species turnover was, in general, lower in the connected lakes, which are directly influenced by the river. Probably the water level fluctuation is a selective force that contributes to maintaining diversity or richness. Our findings indicated the importance of abiotic characteristics and connectivity of the lakes in determining macrophyte composition and community stability over a long time frame.

Aquatic plant communities and predictors of diversity in a sub-tropical river floodplain: the upper Rio Paraná, Brazil

Abstract Analysis of field survey data collected during 1999, from waterbodies (river channel, secondary channels and backwaters, temporary lagoons, permanent lagoons) within the riverine floodplain wetlands of the Upper Rio Parana in southern Brazil showed: (i) strong dominance by five aquatic macrophyte species (most important being Eichhornia azurea), out of 28 species recorded at the survey stations; (ii) based on TWINSPAN clustering, the existence of three major aquatic vegetation types. These comprised an E. azurea–Limnobium laevigatum sub-community (at 20% of stations sampled); an E. azurea–Salvinia sub-community (commonest, at 67% of stations); plus a Nymphaea amazonum–Myriophyllum aquaticum community (rarest, at 13%) which occurred in habitats showing significant between-type differences in environmental conditions for water (depth, underwater light regime, conductivity, pH, alkalinity, phosphate) and sediments (redox potential, calcium, total Kjeldahl nitrogen, phosphate, iron); (iii) predictable patterns of variation in plant size and shape, among populations of the dominant plant species present: E. azurea, Cyperus diffusus, Polygonum ferrugineum, Salvinia spp., Eichhornia crassipes, N. amazonum, along gradients of water and sediment physico-chemistry; (iv) significant variation in both macrophyte community abundance and species diversity between the three vegetation types, with the highest species richness occurring in the E. azurea–L. laevigatum sub-community, but highest community biomass in the E. azurea–Salvinia sub-community. Minimal linear models are presented which use a combination of environmental and vegetation attribute data as predictor variables to predict species richness and biomass of aquatic vegetation for the floodplain waterbodies. Such models can be used to assess vegetation response to likely scenarios of future change in river conditions in sub-tropical areas, for example modification of flow regime by dam construction.

Aquatic invasive species: challenges for the future

Humans have effectively transported thousands of species around the globe and, with accelerated trade; the rate of introductions has increased over time. Aquatic ecosystems seem at particular risk from invasive species because of threats to biodiversity and human needs for water resources. Here, we review some known aspects of aquatic invasive species (AIS) and explore several new questions. We describe impacts of AIS, factors limiting their dispersal, and the role that humans play in transporting AIS. We also review the characteristics of species that should be the greatest threat for future invasions, including those that pave the way for invasions by other species (“invasional meltdown”). Susceptible aquatic communities, such as reservoirs, may serve as stepping stones for invasions of new landscapes. Some microbes disperse long distance, infect new hosts and grow in the external aquatic medium, a process that has consequences for human health. We also discuss the interaction between species invasions and other human impacts (climate change, landscape conversion), as well as the possible connection of invasions with regime shifts in lakes. Since many invaders become permanent features of the environment, we discuss how humans live with invasive species, and conclude with questions for future research.

Aquatic macrophytes of Itaipu Reservoir, Brazil: survey of species and ecological considerations

In a survey of the aquatic macrophytes of the Itaipu Reservoir, we identified 62 taxa in 25 families and 42 genera. The highest number taxa was observed for the emergent macrophytes (40 taxa). Reduced fluctuation in water level, increased light penetration, and sediment enrichment by nutrients and organic matter following the formation of the reservoir favored the appearance of a species-rich community of submerged macrophytes (23% of the taxa identified). The aquatic macrophytes were found mainly near the mouths of the main tributaries of the reservoir, in shallow area of depth less than 2 meters. In addition to the shallow depth, the greater nutrient input from the tributaries and relative protection from wind explained this distribution. Among the species found, Egeria najas merits mention for its occurrence in all localities sampled, with biomass values varying between 98 and 186 gDW/m2. Some potential nuisance species such as Eichhornia crassipes, Salvinia auriculata, and Pistia stratiotes also deserve attention, since they were also observed to be covering large areas of Itaipu Reservoir.

Decomposition of Eichhornia azurea from limnologically different environments of the Upper Paraná River floodplain

Morphometrically distinct populations of Eichhornia azurea (Swartz) Kunth from environments with different limnological characteristics (Cortado Canal and Guaraná Lake, both in the Upper Paraná River floodplain), were incubated in the laboratory for 90 days. During this period, they were allowed to decompose in water of their original environment, or in water from reciprocally exchanged environments. The objectives were (a) to determine decomposition rates and changes in concentration of carbon, nitrogen and phosphorus in plant tissue and (b) to verify whether these rates and changes in tissue were affected by plant origin or by the characteristics of the water in which they were decomposing. Three distinct phases of decomposition were observed. In the first phase, ash content was rapidly lost. In the second, nitrogen increased and phosphorus decreased. In the third, both nitrogen and phosphorus increased. The results suggest that (a) changes in ash content and phosphorus concentration were similar in both environments; (b) the concentration of nitrogen remaining in the plant tissue was affected differently (ANOVA-Repeated, p<0.05) by the water from Cortado Canal and Guaraná Lake; and (c) plants from different environments lost weight at different rates.