Investigations on the ecology and production-biology of the "floating meadows" (Paspalo-Echinochloetum) on the Middle Amazon. Part II. The aquatic fauna in the root zone of floating vegetation

Besides their importance for primary production, (JUNK 1970) the floating vegetation in the varzea region of the Amazon must be taken into consideration as a very important factor for the secondary production in the waters of this region as well. The floating stands represent a biotope which, with few exceptions, offers very good living conditions for aquatic animals. Accordingly they are populated with a large number of individuals and species. The maximum values found using a net of mesh size 223 microns were 780 000 individuals per m² which is equal to 11.6 g dry weight /m² (62 g fresh weight). The plants are used by the animals as a substratum and partly for nutrition. Experiments with excelsior as a substratum have shown that phytoplankton and detritus, trapped in the roots, are very important for nutrition for the population. High quantities of inorganic suspended material in combination with current (flowing white water) strongly reduce the number of individuals especially the filtering forms. Under such conditions, Cyclestheria hislopi BAIRD (Fam. Conchostraca) was not found. In dense floating islands, which are several years old, the oxygen content can be the limiting factor for the population of aquatic animals. In general we can say that the development of the aquatic fauna is influenced more by local than regional factors. Because of the complex composition of the fauna it can be shown that the short term variations of the ecological factors are shown better by the number of individuals than by the biomass. Animals with a fast succession of generations react to changes in the ecological conditions, and have a greater influence on the number rather than the biomass of individuals, due to their small size. From the point of view of the colonization by aquatic invertebrate fauna the floating vegetation could be divided into three different types, which are briefly characterized in Table 12. The factor which lead to this division and their effects on the animals have been investigated and discussed extensively. For further clarification of the relationship between fauna and substratum, experiments have been conducted with an artificial substratum, excelsior

Sedimentological studies of the "Ilha de Marchantaria" in the Solimões/Amazon River near Manaus

Sedimentological studies on the Ilha de Marchantaria an island in the Amazon river near Manaus reveal the existence of four different structural main units. A: Sandy channel bars consisting of giant ripples constitute deposits for the formation of islands or newly formed areas of the Varzea. B: Gradual accumulation on the channel bars leads to the formation of point bar ridges which consist chiefly of small-scale ripples. C: Between point bar ridges of different ages there exist swales. During rising water level the river water flows from the downward side into the swales where finegrained sediments are deposited. D: Permanent lakes are formed mainly in the centre of the islands. During low water periods the lakes may be cut of from the river. During high water periods when the whole island is flooded by the river, additional sedimentation takes place especially in the upstream area of the island. There is however, extensive erosion of the banks on the upstream side of the island. By erosion of the upstream end and sedimentation processes on the downstream end the island moves slowly downstream

Nutrient dynamics of decomposing leaves from Amazonian floodplain forest species in water

Decomposition experiments were performed in freshwater tanks using fresh leaves of four Amazonian tree species from blackwater and whitewater floodplain forests. Weight loss, loss of the major elements, Na, K, Mg, Ca, N and P from the leaves, and release of these elements into the water were studied during a four month period. Based on the nutrient contents of fresh multispecies leaf litter and data on the shedding of this litter, nutrient inputs from leaves into bodies of blackwater and whitewater in the forests during flood period were calculated. The input of dissolved inorganic N, P and K may be as great or even greater than input from river water, indicating the importance of the floodplain forest as nutrient pump from sediments into the water

The trophic status of the fish fauna in Lago Camaleão, a macrophyte dominated floodplain lake in the middle Amazon

Food offer and consumption by the fish fauna were studied at medium and high waterlevels in a macrophyte dominated and strongly hypoxic floodplain lake of the middle Amazon. Detritus was shown to be the main food item, followed by terrestrial invertebrates and periphyton. Low aquatic macrophyte consumption is related to its low nutrient value in comparison with other abundant foods and its seasonal availability

Herbaceous plants of the Amazon floodplain near Manaus: Species diversity and adaptations to the flood pulse

In the floodplain of the Amazon river near Manaus, 388 herbaceous plant species, excepting epiphytes, were collected belonging to 64 families and 182 genera. 330 species are considered terrestrial, 34 aquatic, the others have an intermediate status. Most of the species occur in relatively small numbers. Only 17 species formed large monospecific stands. The largest number of species (273) was found during the dry phase in disturbed areas on the levees, as for instance in abandoned fields, because of a reduced impact of the flood and high light intensity. Low numbers were recorded during low water period from the floor of floodplain forest (25) because of insufficient light conditions and from low lying lake beds (26), which were dominated by a few highly adapted species. The following attributes were found to favour the occurrence of herbaceous plants in the Amazon floodplain: resistance of seeds and spores to flooding and dessication; short reproductive cycles; high reproduction rates; high primary production; tolerance of adult plants to flooding and drought; adaptations to waterlevel fluctuations (for example a floating way of life). Short life cycles and high reproduction rates allow the quick colonization of disturbed habitats and the substitution of population losses (r-strategy). This strategy is supported by the elevated nutrient status of the Amazon river floodplain in comparison with the floodplain of the Negro River, where herbaceous plants are scarce. The number of ruderal species and weeds including a rising number of neophytes is large. The great species diversity is related to great habitat diversity, fertility of sediments and water, predictability of the floodpulse and the reduction of interspecifìc competition due to the annual set back of the populations by the flooding and drought. The observations are in concordance with the predictions of the floodpulse concept

Seasonal variation in the major cation (Na, K, Mg, and Ca) content of the water of Lago Camaleão, an Amazonian floodplain- lake near Manaus, Brazil

Changes in the major cation content (Na, K, Mg and Ca) of the Lago Camaleão, one of the várzea lakes on an island in the Rio Solimões/Amazonas near Manaus were followed during one year. Comparative changes were also followed in the Rio Solimões. Changes in the electrolyte content of the lake water, presented as total maior cation content, are extremely great during the period of investigation. Nov. 1980 to Nov. 1981, and show a pronounced annual periodicity. During the low water period (Oct. 1981) the ion concentration reached its maximum and was more than 15 times higher than in the river at the same time. With rising water level, in flow of comparatively electrolyte poor water from the river and simultaneously increasing rain fall dilute the water in the lake. In the following months there is a relatively sudden reduction of the electrolyte content of the lake water. By the time of high water the levels have fallen almost to those of the river. With falling water level concentrations of major cations at first increase very slowly followed by an abrupt change within one month, from approx. 20 mg major cations/l to approx. 230 mg/l. Investigation of different lake sections showed that the electrolyte content is not uniform throughout the lake. Concentrations rise from the lake entrance to the inner part of the lake, so that the lake sections distant from the river are up to fifteen times as rich in electrolytes as the river itself. The changes in total ion content are accompanied by marked changes in the chemical composition of the lake water during the year, i. e. in the relative proportions of each of the four elements (Na, K, Mg and Ca). Greatest changes are shown by the element K. The deeper water of the lake during the high water period is richer in K probably due to the release of elements from the decomposition of K-rich terrestrial and aquatic macrophytes. The uptake of large quantities of nutrients from the system for the growth of macrophytic vegetation, the accumulation of nutrients in macrophytes and sediment, and the release of these nutrients during decomposition and resuspension are among the most important factors contributing to the electrolyte budget in the Lago Camaleão