Quality transformation of dissolved organic carbon during water transit through lakes: contrasting controls by photochemical and biological processes

Abstract

Abstract. Dissolved organic carbon (DOC) may be removed, transformed, or added during water transit through lakes, resulting in changes in DOC composition and pigmentation (color). However, the process-based understanding of these changes is incomplete, especially for headwater lakes. We hypothesized that because heterotrophic bacteria preferentially consume noncolored DOC, while photochemical processing removes colored fractions, the overall changes in DOC color upon water passage through a lake depend on the relative importance of these two processes, accordingly. To test this hypothesis we combined laboratory experiments with field studies in nine boreal lakes, assessing both the relative importance of different DOC decay processes (biological or photochemical) and the loss of color during water transit time (WTT) through the lakes. We found that influence from photo-decay dominated changes in DOC quality in the epilimnia of relatively clear headwater lakes, resulting in systematic and selective net losses of colored DOC. However, in highly pigmented brown-water lakes (absorbance at 420 nm > 7 m −1 ) biological processes dominated, and there was no systematic relationship between color loss and WTT. Moreover, in situ data and dark experiments supported our hypothesis on the selective microbial removal of nonpigmented DOC, mainly of low molecular weight, leading to persistent water color in these highly colored lakes. Our study shows that brown headwater lakes may not conform to the commonly reported pattern of the selective removal of colored constituents in freshwaters, as DOC can show a sustained degree of pigmentation upon transit through these lakes.

Systematic microbial production of optically active dissolved organic matter in subarctic lake water

“…Such observations of increasing CDOM in dark bacterial incubations have only rarely been observed before (Berggren et al ). However, it has been long known that bacterioplankton can excrete or by other means produce CDOM (Tranvik ; Shimotori et al ), at the same time as they tend to prefer consuming noncolored DOM fractions (Asmala et al ; Hansen et al ; Berggren et al ). Theoretically, this makes it possible for bacteria to make a net contribution to CDOM, especially in lakes where noncolored algal DOC may be a main carbon source.…”

“…The fractions of DOM that are optically active, that is, the light‐absorbing “colored” (CDOM) and the “fluorescent” (FDOM components) fractions are relatively persistent in the environment (Kellerman et al ; Berggren et al ), compared to the smaller pool of fast cycled noncolored DOM in freshwaters (Berggren et al ). However, due to their commonly large concentrations, CDOM and FDOM are associated with significant quantities of reactive compounds (Wetzel ; Lapierre and del Giorgio ), including microbially degradable DOM fractions of importance for the landscape carbon cycling (Lapierre et al ).…”

“…Aquatic microbial production of CDOM is evident from laboratory (Tranvik ; Shimotori et al ) and marine (Rowe et al ) studies, but corresponding observations in natural freshwaters are scarce. In brown‐water lakes, CDOM tends to be consumed by bacteria faster that it is produced (Berggren et al ). Regarding FDOM, a study of lakes and streams in southern Québec (Guillemette and del Giorgio ) found that most fluorescence components were net produced in clear‐water systems where significant fractions of the DOM were of autochthonous origin, whereas in systems dominated by allochthonous DOM the FDOM tended to be net consumed.…”

“…However, little is known about the interaction between lake browning and the balance between internal microbial production and degradation of optically active constituents. Such interactions, which are nonlinear by nature (Berggren et al ), can potentially exacerbate or dampen the terrestrial influence on the optical environment in lakes with even small changes, thus influence a wealth of ecological and biogeochemical processes.…”

How Do Spatial Patterns Impact Regulation of Water-Related Ecosystem Services? Insights from a New Town Development in the Yangtze River Delta, China

“…Indicator 3.4 of Bp acknowledges the role of biological processes in mitigating eutrophic processes through a healthy food web in aquatic ecosystems [18,52]. A water body that can largely purify itself through an input of micro-organism treatment and self-perpetuating biological communities was graded 5 [18,47].…”