Abstract
To identify key factors that control primary production (P.P.) and trigger cyanobacterial harmful algal blooms (cHABs), we investigated spatio-temporal variations in P.P. in a continuous weir system in the Nakdong River once or twice a month from April to October 2018. P.P. was measured through an in-situ incubation experiment using a 13C tracer. Relative proportion of pigment-based phytoplankton composition was calculated by the CHEMTAX program based on pigment analysis using a high-performance liquid chromatography (HPLC). P.P. was higher in spring (1130 ± 1140 mg C m−2 d−1) and summer (1060 ± 814 mg C m−2 d−1) than autumn (180 ± 220 mg C m−2 d−1), and tended to increase downstream. P.P. was negatively related to PO43− (r = −0.41, p < 0.01) due to utilization by phytoplankton during the spring and summer when it was high. The relative proportion of pigment-based cyanobacteria (mainly Microcystis sp.) was positively correlated with water temperature (r = 0.79, p < 0.01) and hydraulic retention time (HRT, r = 0.67, p < 0.01), suggesting that these two factors should affect cHABs in summer. Therefore, to control HRT could be one of the solutions for reducing cHABs in a continuous weir system.
Highlights
Primary productivity is controlled by various physical, chemical, and biological factors [1]
Higher primary production over 1000 mg C m−2 d−1 were detected four times at the middle study sites (ND-3 and ND-4), five times at ND-5, and three times at the lower study site (ND-6) (Figure 5). These results indicate that primary production increased from the upper to lower sites of the continuous weir system in the Nakdong River, with the exception of ND-6
Since ND-6 is located at the furthest site (44 km) from ND-5, environment conditions could be different at ND-6 which could be influenced by much larger inflow of the water from local tributaries compared to other study sites (Table 1)
Summary
Primary productivity is controlled by various physical, chemical, and biological factors [1]. Phytoplankton could contribute extremely to primary productivity in aquatic environments where the distributions of attached algae and aquatic plants are limited due to water-level fluctuations caused by weirs or dams [1]. The significant change of the amplitude of water-level fluctuations, causing strong disturbances in aquatic environments [12,13], such as phytoplankton community composition and densities by affecting the amount of suspended sediments, light availability, and diluting biomass [6,11]. Algal blooms deteriorate water quality by increasing organic matter and microbial activity. The increase in cyanobacterial harmful algal blooms (cHABs) caused by climate change and nutrient inputs in recent decades is of global concern [4]. Proliferation of cyanobacteria changes the color of the water, creates scums and odors, and produces harmful toxins (e.g., microcystins, MCs) that are hazardous to drink and can potentially affect aquatic organisms [15,16]
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