Abstract
Organic matter in the water environment can enhance either flocculation or stabilization and, thus, controls the fate and transportation of cohesive sediments and causes seasonal variation in the turbidity of river water, determining floc morphology and settling velocity. The aim of this study was to elucidate the way that biological factors change the organic matter composition and enhances either flocculation or stabilization in different seasons. Jar test experiments were performed using a mixture of standard kaolinite and the filtered river water samples collected (bi-)weekly or monthly from April to December 2015 upstream a constructed weir in Nakdong River, to estimate the flocculation potential of the seasonal river water samples. Chlorophyll-a concentration, algae number concentration, and the fluorescence characteristics of organic matter were used to represent the biological factors. Our results revealed that flocculation potential depended not only on the algal population dynamics, but also the origins (or chemical composition) of organic matter in the river water. Extracellular polymeric substances (EPS), as algal organic matter, enhanced flocculation, while humic substances (HS), as terrestrial organic matter, enhanced stabilization, rather than flocculation. Since flocculation potential reached its maximum around the peaks of algal population, algae-produced EPS likely enhanced flocculation by binding sediment particles in the flocs. This observation supports previous findings of seasonal variation in EPS production and EPS-mediated flocculation. However, when HS was transported from the surrounding basin by a heavy rainfall event, cohesive sediments tended to be rather stabilized. Supplementary flocculation potential tests, which were performed with artificial water containing refined EPS and HS, also showed the opposing effects of EPS and HS.
Highlights
Micrometer-sized cohesive sediments are usually present in suspended forms in rivers, which make the river water turbid
We hypothesized that the organic matter composition of river water affects flocculation potential, and we focused on elucidating how biological domains change the organic matter composition and enhance either flocculation or stabilization of cohesive sediments via one year of monitoring data of low-flow rivers
The algal population peaks were followed by sudden decreases, which were probably caused by nutrient depletion, light attenuation and/or algae washout [38]
Summary
Micrometer-sized cohesive sediments are usually present in suspended forms in rivers, which make the river water turbid. The turbidity of river water may vary seasonally and spatially. A heavy storm can result in more turbid river water due to the production of cohesive sediments from land disturbance and soil runoff [1,2]. Biota-rich rivers tend to be less turbid because biota have the ability to trap cohesive sediments through flocculation, sedimentation, and filtration [3,4,5]. The seasonality of river turbidity is obvious, the associated processes and mechanisms behind this phenomenon remain poorly understood. Flocculation is one of the most important processes for controlling the turbidity of river water [6]
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