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 (S_250–450, S_275–295, S_350–450, S_R 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 (S_250–450 and S_R) 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.

We investigated how allochthonous and autochthonous sources of dissolved organic matter (DOM) affected the optical conditions and chemical characteristics of two contrasting tropical freshwater systems (Dom Helvécio-DH and Pampulha reservoir) in a dry and rainy period in 2013. We analyzed PAR (photosynthetically active radiation) and UV (ultraviolet) attenuation coefficients, nutrients, chlorophyll-a (Chl-a), dissolved organic matter (DOC) and spectral characteristics of CDOM (colored dissolved organic matter). Significant differences in CDOM sources and quantity were observed, with a dominantly terrestrial input in DH during the rainy period with approximately 50% higher DOC and Chl-a levels, and a doubling in total nitrogen (TN) and total phosphorus (TP) compared to the dry winter period. The eutrophic Pampulha had several fold higher levels of DOC, Chl-a, TN and TP, with organic matter of mostly originating from phytoplankton in both seasons. Differences in source and quantity had strong implications on water transparency, DOC concentrations, CDOM quality and its susceptibility to photo- and biodegradation. DH was several fold clearer in both the UV and PAR spectrum. In DH transparency to both UV and PAR radiation was highest during the summer, suggesting elevated photo- and biodegradation during stratification. Pampulha was most transparent in the dry period even during period of algal bloom. In both systems we observed seasonal variations in concentrations of nutrients and Chl-a, and in DH differences were also found in DOC concentrations as well as the specific UV absorbance (SUVA254) and molecular size (M). Our results documents that different sources of DOM and seasonal inputs reflect in the seasonality of apparent and inherent optical properties and nutrients availability with implications for water quality and aquatic community.

We studied the effects of particulate and dissolved optically active components on the attenuation of photosynthetic active radiation (PAR) in a tropical lake. The temporal and spatial distribution of tripton, Chl-a and a_CDOM(440) and their relative contribution to the diffuse PAR attenuation coefficient (Kd) was investigated at 21 sites (dry and wet seasons and two intermediate periods) and at monthly interval at 1 pelagic site. Higher values of ​​ Kd were observed during the mixing period, characterized by a higher concentration of tripton and Chl-a compared to the stratified rainy season. In the spatial sampling PAR attenuation was dominated by tripton absorption/scattering (average relative contribution of 79%), followed by Chl-a (average 11.6%). In the monthly sampling tripton and Chl-a accounted for most of the Kd with relative contributions of 47.8% and 35.6%, respectively. Multiple linear regression analysis showed that Chl-a and tripton in combination explained 97% of the monthly variation in Kd (p<0.001), but Chl-a had more influence (higher regression coefficient). Thus, although most of light attenuation was due to tripton, seasonal variations in phytoplankton abundance were responsible for most of the temporal fluctuations in Kd.