Use of optical absorption indices to assess seasonal variability of dissolved organic matter in Amazon floodplain lakes

Maria Paula Da Silva,Evlyn Novo,Daniel S F Jorge,Lino A Sander De Carvalho,Claudio C F Barbosa

doi:10.5194/bg-17-5355-2020

Abstract

Abstract. Given the importance of dissolved organic matter (DOM) in the carbon cycling of aquatic ecosystems, information on its seasonal variability is crucial. In this study we assess the use of optical absorption indices available in the literature based on in situ data to both characterize the seasonal variability of DOM in a highly complex environment and for application in large-scale studies using remote sensing data. The study area comprises four lakes located in the Mamirauá Sustainable Development Reserve (MSDR). Samples for the determination of colored dissolved organic matter (CDOM) and measurements of remote sensing reflectance (Rrs) were acquired in situ. The Rrs was used to simulate the response of the visible bands of the Sentinel-2 MultiSpectral Instrument (MSI), which was used in the proposed models. Differences between lakes were tested using the CDOM indices. The results highlight the role of the flood pulse in the DOM dynamics at the floodplain lakes. The validation results show that the use of the absorption coefficient of CDOM (aCDOM) as a proxy of the spectral slope between 275 and 295 nm (S275–295) during rising water is worthwhile, demonstrating its potential application to Sentinel-2 MSI imagery data for studying DOM dynamics on the large scale.

Highlights

Floodplain is a type of wetland that is characterized by a mosaic of landscapes, and it oscillates periodically between aquatic and terrestrial systems
After the removal of the Buabuá and Mamirauá samples acquired in March and April, the Kruskal–Wallis results showed no significant differences in absorption coefficient of CDOM (aCDOM)(440) values between both lakes and hydrograph stages (p = 0.51)
The lakes surrounded by flood forest and close to the Solimões River have more dissolved organic matter (DOM), which is indicated by a higher dissolved organic carbon (DOC) concentration and a higher aCDOM(440), compared with those located close to Japurá River

Summary

Introduction

Floodplain is a type of wetland that is characterized by a mosaic of landscapes, and it oscillates periodically between aquatic and terrestrial systems. This oscillation represents a key aspect in the biogeochemistry, ecology and hydrology of floodplain lakes (Junk et al, 1989; Moreira-Turq et al, 2003). DOM represents the largest pool of organic carbon in the aquatic environment, and it has an important role in the ecosystem carbon budgets (Seekell et al, 2018; Tranvik et al, 2009; Richey et al, 2002). The role of DOM in the environment changes according to its quality (Inamdar et al, 2012)

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