Influence of predation by fish and water turbidity on a Daphnia gessneri population in an Amazonian floodplain lake,Brazil

The population behavior of Daphnia gessneri Herbst, 1967 in a floodplain lake (Lago Grande) of the lower Rio Solimões was investigated between April 1979 and March 1980 with regard to 1) predation by the fish called tambaqui (Colossoma macropomum, Characidae), 2) water level fluctuation and 3) water transparency. Zooplankton density samples were collected at two sites near mid-lake, where water depth and Secchi disc transparency were measured. In addition, qualitative samples of zooplankton and fish collections were taken at several sites in the adjacent floodplain areas. The author concludes that fluctuations in Daphnia gessneri populations correlate most with intense predation by fish and water turbidity.     

The contribution of long‐term isolated water bodies to floodplain fish diversity

1. Floodplains and their water bodies are typical ecotopes of large lowland rivers. The lowland Oder River, Germany, provided a rare opportunity to study fish assemblages of comparable floodplain water bodies differing by >50 years of isolation history. We hypothesised that true floodplain specialists peak in rarely connected water bodies, while frequently flooded waters support tolerant generalists. 2. Three macrohabitats, main channel (MC), temporarily connected and isolated floodplain water bodies, were sampled by electro‐fishing and their fish assemblage characters recorded. 3. Long‐term isolation of floodplain water bodies had a significant effect on the fish assemblage by promoting species preferring still water. Limnophilic and floodplain specialist species significantly increased with isolation. 4. Fish densities, species richness and diversity clearly differed between MC sites and floodplain water bodies. Shannon’s species diversity index peaked in both the MC and isolated waters and was lowest in the temporarily flooded waters where eurytopic fish dominated. 5. The significant gain in abundance and numbers of limnophilic species in the isolated compared to the temporarily flooded water bodies underpinned the significant contribution of long‐term isolated waters to the gamma‐diversity of large floodplain rivers, which should be considered in floodplain rehabilitation.     

Impact of hydrology on free-living and particle-associated microorganisms in a river floodplain system (Danube, Austria)

1. A floodplain segment of the Danube River downstream of Vienna was studied during the hydrologically most dynamic phase (spring–summer) to evaluate the significance of connection between the main channel and the floodplain segment for particle abundance and quality as well as for bacterial and viral parameters, both free‐living and attached to particles. 2. Hydrological connectivity between the main channel and its floodplain segment (expressed as water age) influenced particle abundance and quality. Polysaccharide‐containing particles [Alcian positive‐stained particles (ASP)] and protein‐containing particles [Coomassie positive‐stained particles (CSP)] each contributed a substantial fraction to total suspended solids and were both positively related to water age. ASP were about twice as abundant as CSP. 3. Water age influenced bacterial and viral abundance and the bacterioplankton productivity in the surrounding water. Free‐living bacterial abundance and their bacterial secondary production (BSP) increased continuously with water age, best described by a linear regression. Water age also significantly impacted BSP and per cell BSP of bacteria attached to particles. The abundance of attached bacteria and viruses was not influenced by water age. 4. Bacteria and viruses on particles were influenced by particle quality. Their abundance on particles was closely related to particle size. Particle‐attached bacteria accounted on average for 30.34% (± 3.09) of the total bacterial abundance. A variable and occasionally significant proportion of viruses, between 0.43% and 35.06%, were associated with particulate material. 5. Bacteria attached to particles were significantly more productive than their free‐living counterparts. Their per‐cell activity was on average 8.6 times higher than that of the free‐living fraction. 6. Hydrological connectivity between the Danube River and its floodplain is crucial not only for the exchange of water, sediment and nutrients, but also for microbiota, thus influencing microbial life, distribution and activity.     

The significance of side-arm connectivity for carbon dynamics of the River Danube, Austria

1. Side‐arms connected to the main stem of the river are key areas for biogeochemical cycling in fluvial landscapes, exhibiting high rates of carbon processing. 2. This work focused on quantifying autochthonous and allochthonous carbon pools and, thereby, on comparing transport and transformation processes in a restored side‐arm system of the River Danube (Regelsbrunn). We established a carbon budget and quantified carbon processing from March to September 2003. In addition, data from previous studies during 1997 to 1999 were assessed. 3. Gross primary production (GPP) and community respiration were estimated by diel oxygen time curves and an oxygen mass balance. Plankton primary production was determined to estimate its contribution to GPP under different hydrological conditions. 4. Based on the degree of connectivity, three hydrological phases were differentiated. Most of the organic matter, dominated by allochthonous carbon, was transported in the main channel and through the side‐arm during floods, while at intermediate and low flows (and thus connectivity), transformation processes became more important and autochthonous carbon dominated the carbon pool. The side‐arm system functioned as a sink for particulate matter [total suspended solids and particulate organic carbon (POC)] and a source of dissolved organic carbon (DOC) and chlorophyll‐a. 5. Autochthonous primary production of 4.2 t C day^?1 in the side‐arm was equivalent to about 20% of the allochthonous inputs of 20 t C day^?1 (POC and DOC) entering the area at mean flow (1% of the discharge of the main channel). Pelagic photosynthesis was generally high at mean flow (1.3–3.8 g C m^?2 day^?1), and contributed up to 90% of system productivity. During long stagnant periods at low discharge, the side‐arm was controlled by biological processes and a shift from planktonic to benthic activity occurred (benthic primary production of 0.4–14 g C m^?2 day^?1). 6. The transformation of the organic matter that passes through the side‐arm under different hydrological conditions, points to the importance of these subsystems in contributing autochthonous carbon to the food web of the main channel.     

Transport of larval fish in the Amazon

In the Amazon near Manaus, larvae of Characiformes, Clupeiformes, Tetraodontiformes, Pleuronectiformes, Gymnotiformes, Belonidae and Sciaenidae were sampled in the river during most of the year, except in June and July, when the water level was at its maximum. Characiformes, Tetraodontiformes and Siluriformes were found in the ichthyoplankton mostly during the rising waters, but Clupeiformes and Sciaenidae drifted in the river almost all year around. Egg abundance was extremely low, suggesting that they do not drift or have a very short residence time. Two types of larval drift seemed to occur: a rising water drift and a lowering water drift. Characiformes, Tetraodontiformes and some Clupeiformes drifted mostly during the rising waters and were more abundant near the banks. Their strategy seemed to be a passive dispersion towards the floodplain with the flood pulse. The high densities near the banks optimized their chances of reaching a floodplain inlet. The groups that drifted during the lowering waters showed an alternative strategy. They were flushed from the floodplain lakes and may have stayed in the main river channel for a few months before returning to the floodplain. Predation in the lakes during the period when water level decline was probably the force behind this drift.     

Temperature and neuromuscular development in the tambaqui

The development of muscle innervation pattern was investigated in larvae of the Amazonian fish, the tambaqui Colossoma macropomum. The time to hatching decreased from 28–29 h at 23.5° C to 11–12 h at 31° C. The larvae hatched after the completion of somitogenesis (38-somite stage) at 23.5° C but only at the 33-somite stage at 28–31° C. Embryos were stained for acetylcholinesterase activity and with an acetylated tubulin antibody in order to visualize neural processes. All muscle fibre types were initially innervated at their myoseptal ends. The development of motor innervation to the trunk muscle was delayed with respect to hatching at higher temperatures. At hatching, muscle fibres were innervated only to somites 16–17 at 28–31° C and somite 23–26 at 23.5–25° C (counting from the head), although the larvae swam vigorously to avoid sinking. In contrast, in newly hatched larvae myofibrils were present right along the trunk at all temperatures in both the superficial and inner muscle fibres. At hatching numerous multi-layered membrane contacts with the ultrastructural characteristics of gap junctions, were found between muscle fibres and at the inter-somite junctions, suggesting the somites were initially electrically coupled. These structures disappeared concomitant with the development of muscle endplates right down the trunk. The larvae started feeding 5 days post-hatch at 28° C. First feeding was associated with a dramatic decrease in the volume density of mitochondria and an increase in the volume density of myofibrils in the inner muscle fibres. The polyneuronal and multi-terminal pattern of innervation characteristic of adult slow-muscle fibres also developed around the time of first feeding.     

Flood regime,dam regulation and fish in the Upper Paraná River: effects on assemblage attributes,reproduction and recruitment

The flood regime is the most important force determining seasonality in neotropical rivers. In the Upper Paran River floodplain, it is the primary factor influencing biological processes. The aim of this paper is to summarize information on the influence of dam-controlled floods on some fish assemblage attributes, reproduction and recruitment in the Upper Paran River floodplain, providing preliminary guidelines for dam operation upstream. Fish were collected in different habitats of the Upper Paran River floodplain (river, channels and lagoons) in the period from 1986 to 2001. The high water period in the Paran River usually occurs from November/December to April/May. Annual variation in the hydrograph affects species with distinct life history strategies differently, and influences the composition and structure of fish assemblages. Large floods were associated with higher species richness. Frequencies of individuals with ripe and partially spent gonads, which indicate spawning, were higher during the period of increasing water level. Dependence on floods seems to be lowest in sedentary species that develop parental care, and highest in large migratory species that spawn in the upper stretches of the basin and use flooded areas as nurseries. Migratory fishes were favored by annual floods that lasted more than 75 days, with longer floods yielding larger populations. The occurrence of high water levels at the beginning of summer is fundamental to the spawning success of migratory species. However, the flood may be less important for recruitment of juveniles if it is of short duration. Dam operation upstream (releasing more water during the raining season) has potential to promote greater floods with appropriate duration improving recruitment, particularly for migratory species.     

浅水通江闸控湖泊越冬水鸟分类和功能多样性对水位变化的响应

长江中下游洪泛平原浅水湖泊是重要的水鸟越冬地,为东亚-澳大利西亚迁徙路线上的越冬水鸟提供了丰富的食物资源和越冬保障。近年来,为满足湖泊周边居民生产和生活的需要,长江中下游多数浅水通江湖泊通过建立闸坝调控湖泊水位,闸控水位是目前通江湖泊最典型的水文扰动方式。水鸟作为湿地生态系统的指示物种,对水位变化敏感,其多样性变化可以反应水位变化对湿地生态系统的影响。基于分类和功能多样性,探究闸控湖泊水位变化对越冬水鸟群落的影响。2021-2022年和2022-2023年水鸟越冬期对武昌湖进行了实地观测,结合遥感资料,获取了水鸟种类、组成和分布以及生境因子数据。基于不同年际水位条件下水鸟群落组成变化,分析了生境变化对越冬水鸟多样性的影响,进一步探讨了越冬水鸟群落对水位变化的响应。结果表明,在武昌湖的两个越冬期,雁形目鸟类均占优势,高水位年份的Shannon-Wiener多样性指数显著较高(P < 0.05),而功能分异度指数显著较低(P < 0.05),这表明两个年度间水鸟群落的种类组成、数量和性状特征存在显著差异。一般线性模型结果表明,生境面积受到水位的影响,水位与浅水面积、泥滩面积和深水面积具有显著的相关性(P < 0.05),而与挺水植物区面积的相关性不显著(P > 0.05)。广义线性混合模型表明,水位引起的生境变化是水鸟群落多样性变化的主要原因,而水位变化也会直接影响越冬水鸟群落多样性,Shannon-Wiener多样性指数和功能丰富度指数与水位之间具有显著的相关性(P < 0.05)。水位、水鸟及其生境之间的相互关系分析,有助于阐明水位变化对越冬水鸟群落的影响,这些研究成果对于加强湖泊生态系统的保护和管理具有重要的指导意义。     

Feeding ecology of three Astyanax species (Characidae, Tetragonopterinae) from a floodplain lake of Mogi-Guaçú River, Paraná River Basin, Brazil

Synopsis The feeding ecology of three characids (A. fasciatus, A. bimaculatus and A. schubarti) was studied monthly during 1988 in Lake Inferñao, a major floodplain lake of the Mogi-Guaçú River in the State of São Paulo. River flooding directly influenced the diet of the omnivorous A. bimaculatus and A. fasciatus which responded to maximum inundation (March) by consuming predominantly allochthonous insects. In contrast, A. schubarti was less influenced by the river flood cycle on qualitative changes in diet and relied basically on aquatic macrophytes and periphytic algae. The importance of zooplankton in the diet of the three species was low, and may be attributed to its low density in the water column (< 1 ind l^–1). Ontogenetic diet changes were evident for the three species. For A. fasciatus and A. bimaculatus the importance of zooplankton was high at the early stages, decreasing with size. A. schubarti at younger stages consumed diversified items which gradually decreased quantitatively up to the size class of 65 mm; from this size onwards, the diet became restricted to the consumption of periphytic algae and macrophytes.     

Lake age and water level affect the turbidity of floodplain lakes along the lower Rhine

SUMMARY 1. We sampled a set of 93 lakes situated in the floodplains of the lower River Rhine in search for morphometric and other factors that explain their variation in clarity.
2. Lakes with a drop in summer water level were less turbid at the time of sampling, mainly because of a lower concentration of inorganic suspended solids (ISS).
3. We also found that older lakes were more turbid than younger lakes and that this was largely because of an increase in phytoplankton.
4. Water clarity was positively related to lake depth and the presence of vegetation.
5. Model calculations indicated that the underwater light climate was strongly affected by chlorophyll and ISS, the latter being the dominant factor affecting Secchi depth. Dissolved organic carbon (DOC) was less important.
6. The high concentration of ISS suggests that intensive resuspension occurs in most of the lakes. Using a simple wave model, and assuming that vegetation protects sediments against resuspension, we could eliminate wind resuspension as an important process in 90% of the lakes, leaving resuspension by benthivorous fish as probably the most important factor determining transparency.
7. Chlorophyll a concentration showed a strong positive correlation to ISS concentration, suggesting that resuspension may also have a positive effect on phytoplankton biomass in these lakes.
8. In conclusion, in-lake processes, rather than river dynamics, seem to be driving the turbidity of floodplain lakes along the lower River Rhine.     

Standing crop and distribution of fish in drifting and attached floating meadow within an Upper Amazonian varzea lake

The standing crop and distribution of fish within the floating meadow habitat of Lago Mamirauá, an upper Amazonian floodplain lake, were studied during December 1993 when the water was rising and had just begun to enter the forest. The ichthyofauna was dominated by six groups, the Anostomidae, Gymnotiformes, Erythrinidae, Serrasalmidae, Synbranchidae and Cichlidae. For anchored meadow, total wet weight was estimated as 312 kg ha ^-1 and total number as 90 400 individuals ha^-1. This biomass estimate was probably below the annual maximum because some fish had already dispersed into the forest, but, the number of individuals was close to the annual maximum because of the recent arrival of juvenile Serrasalmidae and Cichlidae. For drifting islands of meadow fish biomass and density were estimated as 192kg ha^-1 and 12 700 individuals ha^-1 respectively. The Synbranchidae, Gymnotiformes and Erythrinidae avoided drifting meadow. In comparison, the Cichlidae and Anostomidae, while favouring marginal, anchored meadow, were frequently dispersed by drifting islands. The Siluriformes were unique in favouring drifting over anchored meadow. For this group the observed down-stream drift may be a seasonal migration to the main river channels. Juvenile fish avoided drifting islands, presumably to avoid being flushed from the lakes. Their abundance was lowest at the mouth of the lake. The generally abundant Serrasalmus spp. juveniles were absent from meadow at the mouth of the lake.     

Fish mortality and physicochemistry in a managed floodplain wetland

Patterns of fish mortality and associated physicochemical factors werestudied during late spring in a managed wetland canal along the lowerMissouri River, Missouri. Mean dawn dissolved oxygen was lower and meanun-ionized ammonia and turbidity were higher during the fish kill thanbefore or after the kill, or than was observed in a nearby wetland canalwhere no fish kill occurred. Dissolved oxygen at dawn and un-ionizedammonia concentrations were at critically low and high levels respectively,so that both likely contributed to the fish mortality. Timing and magnitudeof observed carcasses suggested that Ameiurus melas Rafinesques wasthe most tolerant species for the sizes observed compared to Ictiobuscyprinellus Valenciennes, Lepomis macrochirus Rafinesque, Cyprinus carpio Linneaus, and Lepomis cyanellus Rafinesque.Decreasing mean lengths of fish carcasses during the fish kill for C.carpio, L. cyanellus, and A. melas, indicate that smaller fishes mayhave been more tolerant of harsh environmental conditions than largerindividuals of the same species. Differential mortalities among species andsizes during drawdowns in actively managed wetland pools may haveintentional and unintentional ramifications on wetland and riverine fishcommunity structure, fish-avian interactions, and implementing anecosystem management perspective to restoring more naturalized riverfloodplain wetland functions. Late summer and early autumn draining ofmanaged wetlands might be used to benefit a wider diversity of wildlife andfishes.