CITERES

The Pantanal of Mato Grosso, Brazil, is a large, seasonal wetland, which exhibits high macrophyte productivity at the beginning of the rainy season, when the floodplain becomes flooded. During inundation, from December through May, there is rapid turnover of decomposing macrophyte litter, which is subsequently colonized and consumed by various organisms. In this paper, the variation in the carbon and nitrogen isotope signatures of decomposing macrophytes and detritus was determined to provide an isotopic baseline for the elucidation of higher trophic levels. Seven abundant macrophyte species, Cyperaceae sp., Pontederia lanceolata, Cabomba furcata, Salvinia auriculata, Eichhornia crassipes, Nymphaea amazonum and Paspalum repens, were exposed in mesocosm decomposition experiments lasting 21 or 100 days. Stable isotope ratios of carbon and nitrogen and the atomic C/N ratios were determined for decomposing plant material, particulate organic matter (POM), the microbial film, and aquatic invertebrate larvae. The δ13C values for the macrophytes did not change during decomposition. However, the variability of δ15N was high (range of ± 6 ‰) due to microbial activity. There was no consistent difference in the isotopic signatures of macrophytes and POM. C/N ratios decreased from 17 to 50 in macrophytes, to 7 to 12 in POM. The isotopic signatures and C/N ratios of the microbial film were the same as those of POM. We concluded that heterotrophic processes did not fractionate stable carbon isotopes but caused an increase in the variability of stable nitrogen ratios and a change in the C/N ratios in our experimental system. Therefore, it was not possible to distinguish fresh and senescent material or even POM when used as a food source. The δ13C values of the aquatic larvae were closely coupled to those of the carbon source provided.

1.Many tropical streams are situated in geologically old, weathered landscapes that are prone to erosion. Permanent or seasonally pulsing inputs of eroded material into the streams may have significant effects on benthos and habitats, even if chemical water quality remains unaltered.2.This study presents data from the Cerrado Region, Brazil, where the semideciduous forests have been largely converted into agricultural areas with considerable erosion problems. In order to quantify impacts of erosion on the benthic community, a simple, inexpensive, standardized artificial substrate was exposed on the stream bed above and below the mouth of an erosion gully in two experiments at the beginning and at the end of the rainy season.3.While the abundance and biomass of most of the biota in the reference sites did not change during the rainy season, it decreased significantly at the impact sites below the mouth of the erosion gully. It was the combination of rainfall-driven flood pulses and increased load of suspended particles from erosion which caused the decline of the benthic colonization in these streams rather than the hydrological disturbance alone.4.Site-to-site comparisons revealed a highly significant reduction in density, biomass and taxon richness of the benthic invertebrates caused by siltation. All benthic insect taxa studied showed the same pattern, indicating a general impact of erosion on habitat quality and food sources. Semi-aquatic insects adapted to shifting habitat conditions, terrestrial food sources and aerial respiration were the most resistant invertebrate group.5.Restoration schemes for the stream catchments are urgently needed to reduce local population extinctions due to impassable stream sections.Copyright © 2006 John Wiley & Sons, Ltd.Many tropical streams are situated in geologically old, weathered landscapes that are prone to erosion. Permanent or seasonally pulsing inputs of eroded material into the streams may have significant effects on benthos and habitats, even if chemical water quality remains unaltered.This study presents data from the Cerrado Region, Brazil, where the semideciduous forests have been largely converted into agricultural areas with considerable erosion problems. In order to quantify impacts of erosion on the benthic community, a simple, inexpensive, standardized artificial substrate was exposed on the stream bed above and below the mouth of an erosion gully in two experiments at the beginning and at the end of the rainy season.While the abundance and biomass of most of the biota in the reference sites did not change during the rainy season, it decreased significantly at the impact sites below the mouth of the erosion gully. It was the combination of rainfall-driven flood pulses and increased load of suspended particles from erosion which caused the decline of the benthic colonization in these streams rather than the hydrological disturbance alone.Site-to-site comparisons revealed a highly significant reduction in density, biomass and taxon richness of the benthic invertebrates caused by siltation. All benthic insect taxa studied showed the same pattern, indicating a general impact of erosion on habitat quality and food sources. Semi-aquatic insects adapted to shifting habitat conditions, terrestrial food sources and aerial respiration were the most resistant invertebrate group.Restoration schemes for the stream catchments are urgently needed to reduce local population extinctions due to impassable stream sections.

Seasonal inundations shape the floodplain characteristics of the Pantanal, a large wetland in Central South America. In the first study combining stable carbon and nitrogen isotope analysis with classical stomach content analysis in this region, we investigated the influence of the annual inundation on diet and isotopic composition of floodplain fish. Apart from potential food items, 33 fish species from the Coqueiro Lake were analyzed, 10 of which were present during both the wet and dry season 1999. A δ13C and δ15N plot of the floodplain ecosystem allowed us to assess a foodchain of 3–4 trophic levels. However, the wide overlap of nitrogen values suggested that the organisms act on trophic continua rather than on distinct levels. The foodweb was based mainly on C3-plant carbon. However, fish species capable of feeding on terrestrial invertebrates (e.g., Brycon microlepis) had δ13C values above –25‰, indicating 13–30% intake of C4-plant based carbon during the flooding period. The novel use of vector coordinates and 2-dimensional ANOVA showed that the seasonal isotopic shifts of δ13C and δ15N were highly significant for some feeding guilds. δ15N values increased from the wet to dry season in most fish species, and these shifts were highly significant for omnivores (Astyanax bimaculatus, Triportheus nematurus, Tetragonopterus argenteus and Moenkhausia dichroura), and significant for invertivores (Gymnogeophagus balzanii and Poptella paraguayensis) and carnivores (Serrasalmus spilopleura). Average carbon isotope ratios decreased at the same time in the herbivores (Methynnis mola: 3.4‰) and detritivores (Psectrogaster curviventris: 5.3‰), but they did not change in the hypostomatic herbivore Sturisoma robustum. We explain these shifts by abundant and variable food sources during the inundation period and increasing carnivory and starvation during the dry season when the lake is confined to its central basin. Isotopic shifts between seasons were more prominent in less specialized species of omnivores, invertivores and some carnivores, whereas more specialized herbivores and detritivores appeared to be more influenced by changes in the carbon isotope ratio of the diet affected by biogeochemical processes such as respiration and methanogenesis. A general model for the interpretation of isotope data of floodplain fish considering different time-scales is given.

1.Principles and practices of conservation from temperate, developed-world regions can generally be applied to tropical, developing regions, but the specific solutions are likely to be determined by regional ecological and socio-economic factors.2.This can be seen in the contributions to the Special Issue ‘Conservation of Tropical Streams’: principles of catchment protection, hydrological connectivity and biodiversity conservation are broadly applicable, but regional planning and management need to be based on specific studies.3.Although specific studies and monitoring are necessary, they should not delay the conservation process; in many cases the specific studies need to parallel the management actions and refine the conservation planning.Copyright © 2006 John Wiley & Sons, Ltd.Principles and practices of conservation from temperate, developed-world regions can generally be applied to tropical, developing regions, but the specific solutions are likely to be determined by regional ecological and socio-economic factors.This can be seen in the contributions to the Special Issue ‘Conservation of Tropical Streams’: principles of catchment protection, hydrological connectivity and biodiversity conservation are broadly applicable, but regional planning and management need to be based on specific studies.Although specific studies and monitoring are necessary, they should not delay the conservation process; in many cases the specific studies need to parallel the management actions and refine the conservation planning.

The decomposition of leaf litter is controlled by several factors. One factor that may play an important role is the content of condensed tannins (proanthocyanidins). Here we designed a combined method to isolate proanthocyanidins from leaf extracts, to convert them to anthocyanidins, and to quantify individual anthocyanidins exactly with a new, simple, but sensitive high-performance liquid chromatography method. We used this method to show composition of proanthocyanidins and to monitor degradation of proanthocyanidins and individual constituents in leaf litter in an aquatic environment over time. Despite the rapid decrease in the initial concentrations, a fraction of the proanthocyanidins remained detectable for several weeks.

Contrary to streams, decomposition processes of terrestrial leaf litter are still poorly understood in lakes. Here, we examined the decomposition of two leaf species, beech (Fagus sylvatica) and poplar (Populus nigra ‘italica’) in the littoral zone of a large pre-alpine lake at a wave exposed site. We focussed on the shredding impact of benthic invertebrates in a field experiment and on the effects of wave-induced disturbances under field and mesocosm conditions. In contrast to our expectations, benthic shredders did not reveal an important role in leaf processing under the conditions of the field experiment (early spring time, wave impact zone). Strong wave turbulence during storm events significantly reduced leaf mass, FPOM and invertebrate densities at field conditions. Several reasons can explain the low importance of shredders in our field study: (a) phenology of the shredder species, (b) feeding preferences and alternative food sources for gammarids, (c) generally low abundance of the native gammarid species due to the recent occurrence of an invasive predator, (d) disturbance of shredder activity due to high wave impact and (e) relatively low food value of the offered leaves. We suggest that leaf litter decomposition in lakes occurs in specific process domains, which largely depend on the hydraulic characteristics and on water-level fluctuations.

Since the 1980s the Ponto-Caspian gammarid Dikerogammarus villosus has spread throughout Europe while displacing native species and is predicted to invade further continents. After it was introduced into Europe in the 1890s the North American crayfish Orconectes limosus spread throughout Europe and served as a vector to displace native crayfish as well. In Lake Constance (Germany) the previously dominant gammarid Gammarus roeselii is subjected to both of these invasive crustaceans. In our experiments both species placed predation pressure on G. roeselii. Kairomone perception tests in a Y-maze revealed the capability of the gammarids G. roeselii and D. villosus to perceive and avoid the scent of the predator crayfish O. limosus. Both species also avoided the kairomone of the other gammarid, but did not avoid the scent of their own species. This taxa specific behavior suggests that taxa specific signals are used. This behavior can help the gammarids avoid shelters previously occupied by predators.

The Pantanal of Mato Grosso, Brazil, is famous for its luxurious plant and animal life. We combine a literature review with recent work and show that species diversity is large but that most major plant and animal groups contain a large number of not wetland-specific species that depend on permanently terrestrial habitats within the Pantanal, or are restricted to dry areas during the low water period. These species occur also in the neighbouring biomes of Cerrado, Amazon Forest or Chaco. Until now, very few endemic species have been described, however, there are large populations of species in the Pantanal that are considered rare or endangered in South America. The number of trees adapted to long term flooding is low in comparison with the Amazon River floodplain. We hypothesize that the reason for the lack of local endemisms and the occurrence of a large number of species with a large ecological amplitude is the climatic instability of the region of the Pantanal, which suffered severe drought during glacial periods. The instability of the actual climate, which is characterized by multi-annual wet and dry periods, has a strong impact on distribution, community structure and population size of many plant and animal species and hinders spatial segregation of populations. The dependence of the system on the flood pulse makes the Pantanal very vulnerable to human induced changes in hydrology and the predicted changes in global climate.

1.The Cerrado, a seasonal savanna vegetation, is one of the largest biomes in the Neotropics. Conversion of Cerrado into agroecosystems has caused severe alterations in the species composition, functioning and habitat structure of streams and rivers.2.As a case study for the Cerrado, the environmental quality of stream-valley systems was investigated in the catchment of the Tenente Amaral River near the city of Jaciara, Mato Grosso, Brazil.3.Interpreting a Landsat-7 image of the area, and using a rapid assessment protocol with airborne and field observations, 481 first-order streams were identified in the catchment area (875.36 km2), equivalent to an average of 0.55 springs per square kilometre. The total stream density was 0.83 km km−2.4.Only the uppermost headwater zones of the streams were unimpaired (8.5% of the stream channels). The largest part of the studied stream sectors were moderately (59.3%) or strongly (32.2%) impaired.5.The greatest impact was caused by erosion. Sediments released by gully erosion alone were calculated as 2.6 × 106 m3 of sediments in the upper zone of one tributary of the Tenente Amaral River. The sediments had deleterious effects on fauna and flora.6.The highly erosion-prone stream valleys in the Cerrado require full protection. In the transition area between agroecosystems and hillslopes, buffer zones consisting of a replanted interfluvial Cerrado tree vegetation and a vegetated buffer strip should be planted. Apart from erosion protection, these strips can provide valuable income for the farmers through selling non-wood products.7.The legal situation providing the baseline for the protection of the Cerrado streams is reviewed, including the efforts of the Brazilian states and collaboration of UNESCO.8.We suggest that it is possible to transfer this management strategy to similar biomes in the seasonal tropics provided that the tree species lists are carefully adapted to the regional flora. Copyright © 2006 John Wiley & Sons, Ltd.The Cerrado, a seasonal savanna vegetation, is one of the largest biomes in the Neotropics. Conversion of Cerrado into agroecosystems has caused severe alterations in the species composition, functioning and habitat structure of streams and rivers.As a case study for the Cerrado, the environmental quality of stream-valley systems was investigated in the catchment of the Tenente Amaral River near the city of Jaciara, Mato Grosso, Brazil.Interpreting a Landsat-7 image of the area, and using a rapid assessment protocol with airborne and field observations, 481 first-order streams were identified in the catchment area (875.36 km2), equivalent to an average of 0.55 springs per square kilometre. The total stream density was 0.83 km km−2.Only the uppermost headwater zones of the streams were unimpaired (8.5% of the stream channels). The largest part of the studied stream sectors were moderately (59.3%) or strongly (32.2%) impaired.The greatest impact was caused by erosion. Sediments released by gully erosion alone were calculated as 2.6 × 106 m3 of sediments in the upper zone of one tributary of the Tenente Amaral River. The sediments had deleterious effects on fauna and flora.The highly erosion-prone stream valleys in the Cerrado require full protection. In the transition area between agroecosystems and hillslopes, buffer zones consisting of a replanted interfluvial Cerrado tree vegetation and a vegetated buffer strip should be planted. Apart from erosion protection, these strips can provide valuable income for the farmers through selling non-wood products.The legal situation providing the baseline for the protection of the Cerrado streams is reviewed, including the efforts of the Brazilian states and collaboration of UNESCO.We suggest that it is possible to transfer this management strategy to similar biomes in the seasonal tropics provided that the tree species lists are carefully adapted to the regional flora.

Contrary to streams, decomposition processes of terrestrial leaf litter are still poorly understood in lakes. Here, we examined the decomposition of two leaf species, beech (Fagus sylvatica) and poplar (Populus nigra ‘italica’) in the littoral zone of a large pre-alpine lake at a wave exposed site. We focussed on the shredding impact of benthic invertebrates in a field experiment and on the effects of wave-induced disturbances under field and mesocosm conditions. In contrast to our expectations, benthic shredders did not reveal an important role in leaf processing under the conditions of the field experiment (early spring time, wave impact zone). Strong wave turbulence during storm events significantly reduced leaf mass, FPOM and invertebrate densities at field conditions. Several reasons can explain the low importance of shredders in our field study: (a) phenology of the shredder species, (b) feeding preferences and alternative food sources for gammarids, (c) generally low abundance of the native gammarid species due to the recent occurrence of an invasive predator, (d) disturbance of shredder activity due to high wave impact and (e) relatively low food value of the offered leaves. We suggest that leaf litter decomposition in lakes occurs in specific process domains, which largely depend on the hydraulic characteristics and on water-level fluctuations.