Particulate Organic Matter Distribution along the Lower Amazon River: Addressing Aquatic Ecology Concepts Using Fatty Acids

Abstract

One of the greatest challenges in understanding the Amazon basin functioning is to ascertain the role played by floodplains in the organic matter (OM) cycle, crucial for a large spectrum of ecological mechanisms. Fatty acids (FAs) were combined with environmental descriptors and analyzed through multivariate and spatial tools (asymmetric eigenvector maps, AEM and principal coordinates of neighbor matrices, PCNM). This challenge allowed investigating the distribution of suspended particulate organic matter (SPOM), in order to trace its seasonal origin and quality, along a 800 km section of the Amazon river-floodplain system. Statistical analysis confirmed that large amounts of saturated FAs (15:0, 18:0, 24:0, 25:0 and 26:0), an indication of refractory OM, were concomitantly recorded with high pCO2 in rivers, during the high water season (HW). Contrastingly, FAs marker which may be attributed in this ecosystem to aquatic plants (18:2ω6 and 18:3ω3) and cyanobacteria (16:1ω7), were correlated with higher O2, chlorophyll a and pheopigments in floodplains, due to a high primary production during low waters (LW). Decreasing concentrations of unsaturated FAs, that characterize labile OM, were recorded during HW, from upstream to downstream. Furthermore, using PCNM and AEM spatial methods, FAs compositions of SPOM displayed an upstream-downstream gradient during HW, which was attributed to OM retention and the extent of flooded forest in floodplains. Discrimination of OM quality between the Amazon River and floodplains corroborate higher autotrophic production in the latter and transfer of OM to rivers at LW season. Together, these gradients demonstrate the validity of FAs as predictors of spatial and temporal changes in OM quality. These spatial and temporal trends are explained by 1) downstream change in landscape morphology as predicted by the River Continuum Concept; 2) enhanced primary production during LW when the water level decreased and its residence time increased as predicted by the Flood Pulse Concept.

The compositional evolution of dissolved and particulate organic matter along the lower Amazon River—Óbidos to the ocean

“…Recent efforts to make chemical measurements downstream of Óbidos have increased (e.g. Moreira-Turcq et al, 2003;Mortillaro et al, 2011;Mortillaro et al, 2012), but relatively few have ventured all the way to the river mouth (Ward et al, 2013;Medeiros et al, 2015). These relatively understudied reaches near the mouths of large rivers represent essential conduits in the delivery of organic and inorganic components from rivers to the ocean (Wehrli, 2013).…”

The role of hydrodynamic and biogeochemistry on CO<sub>2</sub> flux and <i>p</i>CO<sub>2</sub> at the Amazon River mouth

“…The higher CO2 outgassing observed at falling water (Fig. 3) could be attributed to the influence of organic matter retention in floodplains, were intense autotrophic production occurs and when the water level start to decrease the transfer of organic matter to rivers increased (Mortillaro et al, 2012).…”

Process catalyzers in Amazonian rivers: large woody debris modifies ecosystem processes across freshwater habitats

“…In contrast to the river, floodplain lakes typically have high water column primary productivity (Mortillaro et al. ), which may limit benthic productivity but also be the ultimate source of benthic AFDM accumulation (Cotner et al. , Mortillaro et al.…”

“…Rather than being controlled by animals or sedimentation, benthic primary productivity was primarily limited by turbidity, as expected in a large white water river with high sediment load. In contrast to the river, floodplain lakes typically have high water column primary productivity (Mortillaro et al 2012), which may limit benthic productivity but also be the ultimate source of benthic AFDM accumulation (Cotner et al 2006, Mortillaro et al 2011. Isotopic studies from the Orinoco and other Amazon floodplain lakes support the notion that phytoplankton is an important contributor to benthic consumer energy budgets (Forsberg et al 1993, Roach et al 2009).…”