Distinctive effects of allochthonous and autochthonous organic matter on CDOM spectra in a tropical lake

Luciana Pena Mello Brandão,Philippe Massicotte,Francisco Antônio Rodrigues Barbosa,Ludmila Silva Brighenti,Eero Asmala,Diego Pujoni,Denise Tonetta,José Fernandes Bezerra-Neto,Peter Anton Staehr

doi:10.5194/bg-15-2931-2018

Abstract

Abstract. Despite the increasing understanding about differences in carbon cycling between temperate and tropical freshwater systems, our knowledge on the importance of organic matter (OM) pools on light absorption properties in tropical lakes is very scarce. We performed a factorial mesocosm experiment in a tropical lake (Minas Gerais, Brazil) to evaluate the effects of increased concentrations of allochthonous and autochthonous OM, and differences in light availability on the light absorption characteristics of chromophoric dissolved organic matter (CDOM). Autochthonous OM deriving from phytoplankton ( ∼ Chl a) was stimulated by addition of nutrients, while OM from degradation of terrestrial leaves increased allochthonous OM, and neutral shading was used to manipulate light availability. Effects of the additions and shading on DOC, Chl a, nutrients, total suspended solid concentrations (TSM) and spectral CDOM absorption were monitored every 3 days. CDOM quality was characterized by spectral indices (S250–450, S275–295, S350–450, SR and SUVA254). Effects of carbon sources and shading on the spectral CDOM absorption was investigated through principal component (PCA) and redundancy (RDA) analyses. The two different OM sources affected CDOM quality very differently and shading had minor effects on OM levels, but significant effects on OM quality, especially in combination with nutrient additions. Spectral indices (S250–450 and SR) were mostly affected by allochthonous OM addition. The PCA showed that enrichment by allochthonous carbon had a strong effect on the CDOM spectra in the range between 300 and 400 nm, while the increase in autochthonous carbon increased absorption at wavelengths below 350 nm. Our study shows that small inputs of allochthonous OM can have large effects on the spectral light absorption compared to large production of autochthonous OM, with important implications for carbon cycling in tropical lakes.

Highlights

Organic matter (OM) consists of particulate organic matter (POM; organic compounds represented by aquatic communities and detritus) and dissolved organic matter (DOM – in most DOM studies it is the compounds smaller than 0.2 or 0.7 μm), which is by far the largest pool of organic carbon in aquatic systems (Hedges, 1992)
Our study shows that small inputs of allochthonous OM can have large effects on the spectral light absorption compared to large production of autochthonous OM, with important implications for carbon cycling in tropical lakes
The results showed a significant effect for the interaction between nutrients and shade for aCDOM254 and aCDOM440

Summary

Introduction

Organic matter (OM) consists of particulate organic matter (POM; organic compounds represented by aquatic communities and detritus) and dissolved organic matter (DOM – in most DOM studies it is the compounds smaller than 0.2 or 0.7 μm), which is by far the largest pool of organic carbon in aquatic systems (Hedges, 1992). Brandão et al.: Distinctive effects of allochthonous and autochthonous organic matter

Methods

Results

Discussion

Conclusion