Photochemical reactivities of dissolved organic matter (DOM) in a sub-alpine lake revealed by EEM-PARAFAC: An insight into the fate of allochthonous DOM in alpine lakes affected by climate change

Abstract

Due to climate change, tree line advance is occurring in many alpine regions. Within the next 50 to 100years, alpine lake catchments are expected to develop increased vegetation cover similar to that of sub-alpine lake catchments which currently exist below the tree line. Such changes in vegetation could trigger increased allochthonous DOM inputs to alpine lakes. To understand the fate of allochthonous DOM in alpine lakes impacted by climate change, the photochemical reactivity of DOM in sub-alpine Lake Tiancai (located 200m below the tree line) was investigated by excitation emission matrix fluorescence combined with parallel factor analysis (EEM-PARAFAC) and UV-Vis spectra analysis. With photo-exposure, a decrease in apparent DOM molecular weight was observed and 32% DOM was photomineralized to CO2. Interestingly, the aromaticity of DOM increased after photodegradation, as evidenced by increases in both the specific UV absorbance at 254nm (SUVA254) and the humification index (HIX). Five EEM-PARAFAC components were identified, including four terrestrially-derived substances (C1, C2, C3 and C4; allochthonous) and one tryptophan-like substance (C5; autochthonous). Generally, allochthonous DOM represented by C2 and C3 exhibited greater photoreactivity than autochthonous DOM represented by C5. C4 was identified as a possible photoproduct with relatively high aromaticity and photorefractive tendencies and contributed to the observed increase in SUVA254 and HIX. UV light facilitated the photodegradation of DOM and had the greatest effect on the removal of C3. This study provides information on the transformation of EEM-PARAFAC components in a sub-alpine lake, which is important in understanding the fate of increased allochthonous DOM inputs to alpine lakes impacted by climate change.

Selective Adsorption of Terrestrial Dissolved Organic Matter to Inorganic Surfaces Along a Boreal Inland Water Continuum

“…These results show the same pattern as a study where soil and leaf extracts had been adsorbed onto goethite and gibbsite minerals, and the humic‐like PARAFAC components decreased with 67%–91%, whereas the protein‐like components decreased with only 26%–52% (Banaitis et al, ). Long emission wavelength fluorescing DOM compounds are known to be lost due to a variety of processes (e.g., naturally occurring processes on a catchment scale [Kothawala et al, ]; photochemical degradation [Du et al, ]; FeCl 3 coagulation [Lavonen et al, ]). The tendency for long emission wavelength FDOM to be lost also manifests itself in the observed shifts in peaks A and C (Figures b and c).…”

Climate‐induced hydrological variation controls the transformation of dissolved organic matter in a subalpine lake

“…In deep, clear-water lakes, UV radiation is a fundamental environmental factor (Sommaruga 2001) that determines both primary productivity (Sommaruga and Augustin 2006;H€ ader et al 2014) and DOM photo-bleaching (Du et al 2016). At the same time, phytoplankton and chromophoric DOM are two main factors controlling light attenuation (and thereby also UV radiation) (Morris et al 1995;Sommaruga and Psenner 1997).…”

“…Its sources and reactivity are directly controlled by environmental drivers such as solar radiation and hydrology and it can therefore be regarded as an important mediator of the lake carbon cycling response to climate variability (Mladenov et al ). Solar radiation can have multiple interactive effects on both DOM quantity and quality: (1) ultraviolet (UV) radiation can break aromatic molecules down to lower‐molecular‐weight compounds (Du et al ); (2) photosynthetically active radiation (PAR) favors primary production as an autochthonous source of DOM (Panzenböck ), typically consisting of short‐chain aliphatic compounds (Flynn et al ) of increased biodegradability (Pérez and Sommaruga ); and (3) increased water temperature resulting from longwave radiation can accelerate the metabolic rates of heterotrophic microbial communities degrading DOM. The balance between these different pathways ultimately determines the magnitude and direction of in‐lake DOM transformation.…”

Characteristics of Dissolved Organic Matter and Its Role in Lake Eutrophication at the Early Stage of Algal Blooms—A Case Study of Lake Taihu, China

“…The HIX values of DOM in the overlying water of Lake Taihu ranged from 0.26 to 1.94, with a mean value of 0.66, which is far less than 4. Additionally, the HIX value was also significantly lower than that of Lake Tiancai (4.25) [59], Lake Baihua (2.88-5.63) [60], Changshou Lake (3.10-6.47) [61], and Lake Michigan (2.33) [62], indicating a weak humification degree for DOM. The difference of HIX between Lake Taihu and the other lakes was mainly caused by the source and component of DOM.…”