Satellite images can now be used to assess river sediment discharge, and systematic studies over rivers and lakes are required to support such applications and document the variability of inland water optical properties at the watershed scale. The optical properties of the Amazon Basin waters were analyzed from in situ measurements of the remote sensing reflectance (R rs ) at 279 stations and downwelling diffuse attenuation coefficients (K d ) at 133 stations. Measurements of the apparent optical properties, suspended particulate matter (SPM) contents, and characteristics and colored dissolved organic matter (CDOM) absorption spectra were performed during 16 cruises along the main Amazonian Rivers draining the Andes and for some tributaries. Surface-suspended sediment granulometry and mineralogy showed a stable distribution at the catchment scale, even over large distances and between tributaries. The particle number-size distribution was best described using a segmented distribution with a slope of 2.2 for the fine range (1-15 μm), and the CDOM absorption coefficient at 440 nm varied from 1.8 to 7.9 m À1 . Overall, both R rs and K d were strongly correlated with SPM, although strong CDOM absorption limited the use of the blue spectrum. Reflectance saturation from blue to red was observed at approximately 100 g m À3 , whereas the near-infrared (NIR) wavelength enabled the monitoring of the full SPM range (5-620 g m À3 ). In contrast, K d showed no saturation for SPM from green to NIR, and a linear model was calculated. The use of the reflectance ratio was investigated and shown to improve the suspended sediment concentration retrieval performance.

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International audienc

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Holocene environments have been reconstructed by sedimentological, mineralogical and organic geochemical analysis of a 270-cm core from Santa Ninha Lake, a floodplain lake in lower Amazonia. Dated by fourteen AMS-radiocarbon dates, the sediment core has a basal age of 5,600 cal years BP and different sedimentary units were identified. These units document various hydrologic phases in the evolution of this lake. Reduced Amazon River influence, with reduced high-water levels of the river, characterized the period between 5,600 and 5,100 cal years BP. Comparison with other Amazonian and Andean paleoclimate studies point to a dryer climate during this phase. After 5,100 cal years BP coarse sediments and quartz increase which suggest a higher inflow of the Amazon River. Between 5,000 and 2,300 cal years BP the coarse sediments and quartz remain high but the organic carbon showed the lowest values. The riverine inflow caused dilution of the organic material produced in the lake and consequently low rates of carbon flux in these phases were recorded. These results show that the hydrodynamics of the Amazon River strongly influence the behavior, productivity and consequently the sedimentation process in the floodplain lakes.

Aim Ongoing and future anthropogenic climate change poses one of the greatest threats to biodiversity, affecting species distributions and ecological interactions. In the Amazon, climatic changes are expected to induce warming, disrupt precipitation patterns and of particular concern, to increase the intensity and frequency of droughts. Yet the response of ecosystems to intense warm, dry events is not well understood. In the Andes the mid‐Holocene dry event (MHDE), c. 9,000 to 4,000 years ago, was the warmest and driest period of the last 100,000 years which coincided with changes in evaporation and precipitation that caused lake levels to drop over most of tropical South America. This event probably approximates our near‐climatic future, and a critical question is: How much did vegetation change in response to this forcing? Location Lake Pata, Brazilian Western Amazonia. Taxon Terrestrial and aquatic plants. Methods We used pollen, charcoal, total organic carbon (TOC), total nitrogen (TN), δ13C and δ15N data from a new high‐resolution core that spans the last c. 7,600 years history of Lake Pata. Results We found that in the wettest section of Amazonia changes associated with the MHDE were detected in the geochemistry analysis but that vegetation changed very little in response to drought during the Holocene. This is the first high‐resolution core without apparent hiatuses that spans most of the Holocene (last 7,600 cal yr bp) from Lake Pata, Brazil. Changes in the organic geochemistry of sediments indicated that between c. 6,500 and 3,600 cal yr bp lake levels dropped. Vegetation, however, showed little change as near‐modern forests were seen throughout the record, evidencing the substantial resilience of this system. Only a few species replacements and minor fluctuations in abundance were observed in the pollen record. Main conclusions The mid‐Holocene warming and reduced precipitation had a limited impact on western Amazonian forests. We attribute much of the resilience to a lack of fire in this system, and that if human‐set fires were to be introduced, the forest destruction from that cause would override that induced by climate alone.

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RESUMOA biota terrestre é considerada como um sumidouro de carbono atmosférico. As fl orestas tropicais apresentam um importante papel nesta assimilação, porém pouco ainda se sabe sobre o papel dos sistemas aquá-ticos continentais neste processo de acumulação bem como sua relação com as mudanças paleoclimáticas. Portanto, o objetivo deste artigo consiste em reunir os principais trabalhos realizados em lagos amazônicos, a fi m de evidenciar a participação dos sistemas aquáticos amazônicos no acúmulo de carbono em função de mudanças paleoclimáticas e antrópicas na Amazônia. Para este estudo foram escolhidos três sistemas lacustres distintos: o primeiro consiste nos lagos de várzea Santa Ninha e Acarabixi, caracterizados por sofrerem infl uência direta do ciclo hidrológico do Rio Amazonas e Rio Negro respectivamente. O segundo engloba os lagos isolados da dinâmica fl uvial amazônica, que apesar de serem pouco extensos em área tem um papel importante no que tange ao entendimento dos processos paleoclimáticos na Amazônia. Foram abordados então os seguintes registros: Lagoa da Pata (AM), Lago Caracaranã (RR), Lago CSN N4 Carajás (PA), Lago de campos inundados em Humaitá (AM). Como exemplo de sistema que recebe infl uência da atividade antrópica foi estudado barragem na região de intensa mudança do uso da terra em Alta Floresta (MT). Através de diferentes indicadores paleoambientais e ambientais (datações dos diferentes ecossistemas fl uviais amazônicos por 210 Pb e 14 C, concentração de carbono orgânico, relação C/N, análise da deposição de partículas de carvão, δ 13 C, δ 15 N, derivados de clorofi la, determinação mineralógica, determinação de mercúrio e cálculo de taxas de acumulação de carbono) foram reconstruídas as condições ambientais pretéritas e suas implicações no funcionamento Os lagos de várzea Santa Ninha e Acarabixi apresentaram elevadas taxas de acumulação de carbono, chegando a atingir valores superiores a 400g/m 2 /ano. Já os lagos isolados apresentaram fl uxos mais reduzidos, onde raramente encontraram-se registros que ultrapassaram os 20g/m 2 /ano. O maior valor de acumulação de carbono foi observado em de barragem em área de intensa mudança do uso da terra em Alta Floresta (MT). A acumulação de carbono alcança o maior fl uxo de 433g/m 2 /ano. Estes dados revelam uma importante participação de diferentes ambientes lacustres amazônicos na acumulação de carbono, que apresentou-se susceptível à alteração climática que ocorreu nos últimos milênios. Palavras-chave: Acúmulo de carbono. lagos de várzea, lagos isolados, Amazônia, mudanças paleoclimáticas. ABSTRACT CARBON ACUMULATION IN AMAZON LAKES AS INDICATORS OF ANTROPIC AND PALEOCLIMATIC EVENTS.Land biota is regarded as a natural sink for atmospheric carbon. Tropical forests have an important role in this assimilation, but little is known about the role of continental aquatic systems in the process of carbon sequestration and its relationship with paleoclimatic changes. The objective of the present article is to bring together the main studies with Amaz...