Jukka Salo

Fluvial Perturbance in the Western Amazon Basin: Regulation by Long-Term Sub-Andean Tectonics

Haffers refuge theory proposes that during the arid climatic phases of the late Pleistocene, tropical lowland forests of Amazonia were reduced to isolated patches contributing to the high species richness of the present-day forest. The theory was developed because no obvious historic or modern geomorphic isolation barriers were recorded in Amazonia. Analyses of radar images combined with stratigraphical data show that in the basinal forelands of the tectonically active Andes the geological setting causes long-term fluvial perturbance. This leads to a temporally structured highly complex mosaic of fossil and present floodplains. These dynamics have been present with varying activity and geographic range during the Tertiary and Quaternary, providing site-turnover that has not been recognized by the biogeographic tradition of the Amazon basin.

New site formation and colonizing vegetation in primary succession on the western Amazon floodplains.

The major proportion of western Amazon forests grow on fluvial deposits and thus originated in floodplain environments. The fluviodynamic character of the sites initially colonized by plants was studied along different river types, and this information was combined with botanical observations from the same areas. Special emphasis was given to colonizing plant distribution and survival in relation to the abiotic environment. Four frequently occurring landform types, each rich in microforms, were recognized in relation to the colonization process : fluvial bars, swales, abandoned channels and riverbanks (...)

Recent and ancient fluvial deposition systems in the Amazonian foreland basin, Peru

Still active Sub-Andean foreland deformation is suggested to have syndepositionally modified the fluvial depositional environments in the Peruvian Amazonian foreland basin throughout Neogene-Quaternary time. Modern fluvial aggradation continues to proceed on a large scale ( c . 120 000 km 2 ) in two differing depositional systems. Firstly, various multistoried floodbasin deposits are derived from the meandering and anastomosing rivers within the subsiding intraforeland basins. Secondly, in the northern part of the Pastaza-Maranon basin the largest known Holocene alluvial fan-like formation ( c . 60 000 km 2 ) composed of reworked, volcaniclastic debris derived from active Ecuadorian volcanoes, has been identified. The widespread, poorly known, dissected surface alluvium ( terra firme ) which covers the main part of the Peruvian Amazonian foreland basin shows further evidence of long-term foreland deformation, and terraces indicate both the effects of tectonism and Pleistocene climatic oscillations. In northern Peru, the surface alluvium was deposited by a Tertiary fluvial system with palaeocurrents to the west and northwest into the Andean foreland basin. In southern Peru, the respective surficial alluvium was part of a post-Miocene fluvial system flowing northeast into the main Amazon basin. Both systems were gradually abandoned when the eastward migrating Andean foreland deformation led to the more distinctive partitioning of the intraforeland basins, and the modern drainage system was created.

River types, site evolution and successional vegetation patterns in Peruvian Amazonia

Lateral migration of Amazonian lowland rivers causes a vegetation succession on recently deposited fluvial sediments. The forest ground is formed gradually in se- quence, giving rise to age-zonation of the successional stages. Using Landsat MSS maps and field observations, we studied variations in fluvial landform evolution and river character. The widely used characterizations of the rivers ac- cording to their suspension-load character and channel pattern were evaluated in relation to the vegetation succes- sion. Sequential successional forests appear extensively along meandering white-water rivers, which are rich in sus- pended sediments and are characterized by mobile channels. Conversely, vegetation zonation is less pronounced at the margins of slowly eroding suspension-poor rivers. Line transects were established to document meander develop- ment along eight different rivers. The chemical composition of the recently deposited alluvium differs markedly both among rivers and along the transects. Concentrations of organic carbon and nitrogen increase toward the meander neck along with the acid reaction of the soil. The rate of riv- erine forest regeneration in Peruvian lowland Amazonia was extrapolated on the basis of erosion data from four different rivers, and suggests that approximately 130 km2 of forest is annually eroded and replaced by successional vegetation, the equivalent of 0.2% of the present floodplain area of this region.

Selective predation and habitat shift in a copepod species — support for the predation hypothesis

SummaryIn studying the relevance of the predation hypothesis as an explanation for the vertical migration of zooplankton two predictions were tested:1)The egg sacs of Eurytemora are sufficient to cause different predation pressure in food selection experiments with visually hunting planktivores as predators and ovigerous vs. non-ovigerous Eurytemora females as prey.2)If copepods avoid predation by vertical migration, there should occur differences in the vertical distribution pattern of ovigerous and non-ovigerous females according to selective predation. The results showed that, when ovigerous and non-ovigerous females were presented to predators, a significant preference for ovigerous females was found in high densities of prey. No preference was found in experiments with males vs. non-ovigerous females and at low copepod densities. In a field study we found that non-ovigerous females distributed throughout the water column preferred the deeper parts. Ovigerous females were also abundant in deeper waters but almost totally avoided the surface layer above 20 m. Thus our results support the predation hypothesis when the adaptive value of vertical migration is considered.On the basis of our results we deduced testable predictions on the evolutionary effects of predation:1)In heavily foraged communities there is strong coevolution among the prey to resemble each other in terms of which are critical in selective predation.2)If the carrying of an egg sac is considered as parental care, it is likely that parental care occurs more seldom in environments with high predation.

High local species richness of parasitic wasps (Hymenoptera: Ichneumonidae; Pimplinae and Rhyssinae) from the lowland rainforests of Peruvian Amazonia

Abstract.  1. The parasitic wasp family Ichneumonidae (Hymenoptera) is of great interest because it has been claimed that its species richness does not increase with decreasing latitude.

Riparian flooded forests of the Orinoco and Amazon basins: a comparative review

This paper compares the non-deltaic, riparian-flooded forests of the Orinoco and Amazon River basins. Ecological relationships between these forests and their environments that can be useful in establishing schemes for biodiversity conservation are identified. Adaptations of species to flow seasonality, flooding intensity, sedimentation pattern and nutrient depletion are described. The variability and diversity of riparian-flooded forests is related to (i) landscape evolution (regional-scale, long-term), (ii) water quality (basin scale, long-term) and (iii) hydrology and geomorphology (sector-scale, medium-term). The floristic analysis has produced a preliminary list of 242 tree species common to the riparian-flooded forests of both basins. This relatively high number of species is related to connectivity between the riparian corridors of both basins and the effective operation of dispersal mechanisms. Highly oligotrophic environments add uniqueness at the regional scale through the evolution of endemic species presenting adaptations not only to flooding but also to nutrient depletion. The process of genetic diversification and the evolution of genotypes adapted to flooding are suggested to explain longitudinal gradients at tributary junctions and floodplain-upland ecotones where current fluvial dynamics are unpredictable over ecological time scales. The paper presents information that may be used to devise appropriate measures to evaluate sites for riparian biodiversity conservation and management.

Holocene floodplain lake sediments in the Amazon: 14C dating and palaeoecological use

Abstract The Holocene biostratigraphic record of Amazon lowland rain forests is fragmentary and is based on restricted number of cores, the majority representing sedimentation in a floodplain environment. These biostratigraphic data have often been interpreted as demonstrating climatic oscillation between wetter and drier conditions, and the subsequent vegetation changes. Here, we survey and 14C date a further six western Amazon floodplain lakes and examine their fluvially controlled sedimentation processes. The lakes, which are of the cutoff type, are located along the rivers Manu, Tambopata and Amazon (Solimoes) in the Peruvian Amazon lowlands. All the lakes cored are under the influence of current overbank sedimentation of the parent suspension-rich whitewater rivers and receive an annual fill of allochthonous inorganic suspension during the flood. Most of the cores collected were rich in minerogenic sediments (gyttja clay, loss on ignition We think that the observed bias towards older 14C ages documented in the gyttja clays is caused by the high proportion of old (fine particulate) organic matter being recirculated and resedimented within the suspension load. The strong fluvial control of lake sedimentation and the unreliability of the 14C dates from the Amazon suspension-rich floodplains call for caution in palaeoecological interpretation. Furthermore, owing to the low number of within-lake replicate cores in the Amazon Holocene stratigraphy analyses, we consider the postulated Holocene climatic oscillation schemes to be tentative only.

Recent history of sedimentation and biotic communities in Lake Pyhäjärvi, SW Finland

The sedimentary record covering the last 150 years was studied in the productive clear water lake Pyhäjärvi in SW Finland. The lake has faced significant human-induced changes: (1) the water level was lowered by almost 2 metres in the early 1850s; (2) planktivorous coregonid fishes were successively introduced, commencing in 1908; and (3) nutrient input from intensified agriculture has increased during this century.Sediments were sampled from the 25 m deep depression of the otherwise shallow lake by freeze-corer and were date by 210Pb-chronology and pollen stratigraphy. According to litho-, chemo-, 210Pb- and pollen stratigraphies, the sedimentary sequence consists of five different sedimentary facies, each representing a different depositional environment resulting both from the lowering of water level and different stages of final deposition. The sediments in the depression are believed to have been deposited orderly, but, as a result of resuspension, they have a substantial littoral sediment input.After the lowering of the lake level, oxygen content in the depression is believed to have decreased on the basis of black coloration (sulfides) of the sediment from 1870–1880 onwards. The oxygen deficit worsened after the 1940s when e.g. iron, zinc, calcium and phosphorus were increasingly liberated into the water body. In contrast, diatoms, chironomids and cladoceran communities were notably stable, with the most important biotic changes being: 1) the decreased body size of the cladoceran Bosmina coregoni, apparently due to intensive selective predation by the introduced whitefish; and 2) increased abundances of the diatom Fragilaria crotonensis, and the profundal chironomid Chironomus plumosus f. semirectus after the 1950s, suggesting an increase in the trophic status of the lake. Eutrophication was probably in response to increased nutrient supply due to intensified cultivation and use of industrial fertilizers in the lakes drainage.

LAGO AMAZONAS: FACT OR FANCY?

It has been suggested that a huge lake, Lago Amazonas, covered a large part of the Amazon basin until as recently as two thousand years ago. According to this hypothesis, the topmost sediments in western Amazonia are almost universally young deposite of lacustrine and deltaic origin. The hypothesis has gained some attention among biologists because of its implications for biological phenomena in Amazonia, especially biogeography and biodiversity. According to the available geological data, however, Amazonia is geologically far more complex than assumed by the lake hypothesis. In the following discussion we will point out the weaknesses of the Lago Amazonas hypothesis, and indicate alternative explanations of the surface geology that are based on tectonically controlled fluvial deposition.