Aquatic biogeochemistry of the Orinoco River floodplain (Venezuela) and the Pantanal wetland (Brazil)

This dissertation includes three distinct studies of the aquatic biogeochemistry of tropical floodplain environments. The first study examines the relation between basin morphology and chemical and ecological characteristics of lakes on the Orinoco River floodplain of Venezuela. Data are presented on major solutes, nutrients, particulates, and chlorophyll. The diverse characteristics of floodplain lakes are shown to be partly explained by variation in the morphology, which affects their rates of water exchange, the relative importance of different water sources, and the degree to which processes operating within the lakes affect the chemistry of the water.; The second study uses stable isotopes of carbon and nitrogen as tracers of organic matter sources and energy flow through consumer food webs in the Orinoco River floodplain. A new method for separation and purification of fine particulate organic matter for isotopic analysis was developed. Algae were particularly variable in their isotopic ratios but are the only major source of organic matter relatively depleted in carbon-13. Many of the aquatic animals show relatively depleted carbon-13 ratios, indicating reliance on carbon from algal sources. This finding contradicts the conventional view that vascular plants sustain consumer food webs in wetlands.; The third study examines dissolved gases in waters of the Pantanal wetland of Brazil. Measurements of dissolved gases and major solutes are presented for 540 samples from throughout the region, ranging from areas of sheet flooding to sluggish marsh streams to the major rivers of the region. The data reveal that dissolved oxygen is strongly depleted in most waters of the Pantanal. Comparison of carbon dioxide and oxygen concentrations and air-water diffusive fluxes shows considerably more carbon dioxide accumulation than can be accounted for by consumption of dissolved oxygen. Evidence is presented to support the hypothesis that oxygen transported through the stems of emergent plants is consumed in aerobic respiration by plant tissues or microorganisms, producing carbon dioxide that preferentially dissolves into the water. Methane is found at high concentrations in surface waters.