Suzanne J. Sippel

Passive microwave observations of inundation area and the area/stage relation in the Amazon River floodplain

Inundation patterns in Amazon River floodplains are revealed by analysis of the 37 GHz polarization difference observed by the Scanning Multichannel Microwave Radiometer on the Nimbus-7 satellite. Flooded area is estimated at monthly intervals for January 1979 through August 1987 using mixing models that account for the major landscape units with distinctive microwave emission characteristics. Results are presented separately for 12 longitudinal reaches along the Amazon River main stem in Brazil as well as for three major tributaries (the Jurua, Purus and Madeira rivers). The total area along the Amazon River main stem that was flooded (including both floodplain and open water) varied between 19000 and 91 000km2. The correlation between flooded area and river stage is used to develop a predictive relationship and reconstruct regional inundation patterns in the floodplain of the Amazon River main stem over the past 94 years of stage records (1903-1996). The mean flooded area along the Amazon River during t...

Oxygen depletion and carbon dioxide and methane production in waters of the Pantanal wetland of Brazil

This study examines dissolved O2, CO2 and CH4 in waters of the Pantanal, a vast savanna floodplain in Brazil. Measurements 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. Dissolved O2 is often strongly depleted, particularly in waters filled with emergent vascular plants, which are the most extensive aquatic environment of the region. Median O2 concentrations were 35 μM for vegetated waters, 116 μM for the Paraguay River, 95 μM for tributary rivers, and 165 μM for open lakes (atmospheric equilibrium, 230–290 μM). Airwater diffusive fluxes were calculated from dissolved gas concentrations for representative vegetated floodplain waters, based on data collected over the course of an annual cycle. These fluxes reveal about twice as much CO2 evasion as can be accounted for by invasion of O2 (overall means in nmol cm-2 s-1: O2 0.18, CO2 0.34, and CH4 0.017). Methanogenesis is estimated to account for ca. 20% of the total heterotrophic metabolism in the water column and sediments, with the remainder likely due mostly to aerobic respiration. Anaerobic respiration is limited by the low concentrations of alternate electron acceptors. We hypothesize that O2 transported through the stems of emergent plants is consumed in aerobic respiration by plant tissues or microorganisms, producing CO2 that preferentially dissolves into the water, and thus explaining most of the excess CO2 evasion. This hypothesis is supported by measurements of gases in submersed stems of emergent plants.

Determination of inundation area in the Amazon River floodplain using the SMMR 37 GHz polarization difference

Abstract The 37 GHz polarization difference observed by the Scanning Multichannel Microwave Radiometer (SMMR), which was operated on board the Nimbus-7 satellite, provides a sensitive indicator of surface water. These data can provide information on seasonal inundation patterns in large tropical wetlands such as the Amazon River floodplain. Although the SMMR data are of low resolution, we were able to estimate the area inundated within a group of pixels by using linear mixing models which incorporate the major end-members of the observed microwave signatures. The models were then used to estimate seasonal changes in inundation area over a 7-year period for a 34,550 km2 area along the Amazon River near Manaus. The seasonal changes in inundation area determined using mixing models correlate well with changes in river stage.

Identifying the spatial scale of land use that most strongly influences overall river ecosystem health score.

Catchment and riparian degradation has resulted in declining ecosystem health of streams worldwide. With restoration a priority in many regions, there is an increasing interest in the scale at which land use influences stream ecosystem health. Our goal was to use a substantial data set collected as part of a monitoring program (the Southeast Queensland, Australia, Ecological Health Monitoring Program data set, collected at 116 sites over six years) to identify the spatial scale of land use, or the combination of spatial scales, that most strongly influences overall ecosystem health. In addition, we aimed to determine whether the most influential scale differed for different aspects of ecosystem health. We used linear-mixed models and a Bayesian model-averaging approach to generate models for the overall aggregated ecosystem health score and for each of the five component indicators (fish, macroinvertebrates, water quality, nutrients, and ecosystem processes) that make up the score. Dense forest close to the survey site, mid-dense forest in the hydrologically active near-stream areas of the catchment, urbanization in the riparian buffer, and tree cover at the reach scale were all significant in explaining ecosystem health, suggesting an overriding influence of forest cover, particularly close to the stream. Season and antecedent rainfall were also important explanatory variables, with some land-use variables showing significant seasonal interactions. There were also differential influences of land use for each of the component indicators. Our approach is useful given that restoring general ecosystem health is the focus of many stream restoration projects; it allowed us to predict the scale and catchment position of restoration that would result in the greatest improvement of ecosystem health in the regions streams and rivers. The models we generated suggested that good ecosystem health can be maintained in catchments where 80% of hydrologically active areas in close proximity to the stream have mid-dense forest cover and moderate health can be obtained with 60% cover.

Remote sensing of lakes and floodplains in the Amazon basin

Abstract Remote sensing of the lakes and floodplains in the Amazon basin has provided quantitative information on open water, floating macrophytes and flooded forests, and regional inundation. Side‐looking airborne radar (X‐band) imagery permitted measurement of lake shapes and areas. Satellite altimeters permitted estimation of stage for large rivers. Application of spectral mixture analysis to Landsat multispectral scanner and thematic mapper imagery permitted calculation of concentrations of suspended sediments in surface waters. Videography from low‐flying aircraft allowed repetitive estimation of areal extent of floating macrophytes and sediment‐laden flood waters, while clouds and smoke reduced the availability of such estimates from Landsat imagery to very few scenes per year. Application of linear mixing models to thermal emissions measured by satellite‐borne passive microwave sensors permitted determination of inundation area on a regional scale in the Amazon basin. Synthetic aperture radar senso...

Plant-mediated transport and isotopic composition of methane from shallow tropical wetlands

Abstract The methane (CH4) content of gas bubbles in sediments and CH4 emission from plants in the Pantanal wetland of Brazil was examined across a depth gradient to ascertain the relative importance of plant-mediated CH4 emission versus ebullition. Sediment bubbles contained >50% CH4 at sites with water depths >1 m but often contained <50% CH4 in shallower waters. Previous work has shown that bubble % CH4 is related to ebullition rates, and therefore these patterns are suggestive of greater CH4 emission by ebullition from deeper waters compared to waters <1 m in depth. Stable isotope ratios of carbon and hydrogen in the CH4 showed evidence for significant CH4 oxidation but no apparent relation with water depth. Shallower waters typically contain rooted emergent plants with erect, vertical stems or petioles, whereas in waters deeper than ~1 m, emergent aquatic plants are limited to rooted forms with long stems that float at the water surface. Rooted emergent plants typical of deeper waters had low rates of CH4 emission (0.0–0.2 mg d−1 CH4 for individual plants) compared to species typical of inundation depths <1 m (1.6–7.5 mg d−1 CH4). These observations suggest that tropical savanna floodplains like the Pantanal with inundation depths >1 m have low rates of CH4 emission via rooted emergent and floating-leaf plants, in contrast to results from shallower rice fields and temperate wetlands in which plant-mediated transport dominates CH4 emission from vegetated waters.