Fatty acid and stable isotope (δ13C, δ15N) signatures of particulate organic matter in the lower Amazon River: Seasonal contrasts and connectivity between floodplain lakes and the mainstem

Fatty acid (FA) composition and stable isotope (delta C-13, delta N-15) signatures of four aquatic plants, plankton, sediment, soil and suspended particulate organic matter (SPOM) collected from open floodplain lakes (Varzea) and rivers of the central Brazilian Amazon basin were gathered during high and low water stages in 2009. SPOM from Varzea had a major contribution of autochthonous material from phytoplankton and C-3 aquatic plants. As shown from stable isotope composition of SPOM (delta C-13 -31.3 +/- 3.2 parts per thousand; delta N-15 3.6 +/- 1.5 parts per thousand), the C-4 aquatic phanerogam (delta C-13 -13.1 +/- 0.5 parts per thousand; delta N-15 4.1 +/- 1.7 parts per thousand) contribution appeared to be weak, although these plants were the most abundant macrophyte in the Varzea. During low water season, increasing concentration of 18:3 omega 3 was recorded in the SPOM of lakes. This FA, abundant mainly in the Varzea plants (up to 49% of total FAs), was due to the accumulation of their detritus in the ecosystem. This dry season, when connectivity with the river mainstem was restricted, was also characterized by a high concentration in the SPOM of the cyanobacteria marker 16:1 omega 7 (up to 21% of total FAs). The FA compositions of SPOM from the Amazon River also exhibited significant seasonal differences, in particular a higher concentration of 16:1 omega 7 and 18:3 omega 3 during the dry season. This suggests a seasonal contribution of autochthonous material produced in Varzea to the Amazon River SPOM. (C) 2011 Elsevier Ltd. All rights reserved.