Abstract
The spatiotemporal variation of phytoplankton and their relationship with environmental variables were analyzed in the Saigon River-a tropical river in Southern Vietnam. Two longitudinal profiles were conducted during dry and rainy season at 18 sampling sites covering more than 60 km long in the river. Besides, a bi-weekly monitoring conducted in the upstream, urban area (Ho Chi Minh City-HCMC), and downstream of Saigon River was organized from December 2016 to November 2017. The major phytoplankton were diatoms (e.g., Cyclotella cf. meneghiniana, Leptocylindrus danicus, Aulacoseira granulata), cyanobacteria (Microcystis spp., Raphidiopsis raciborskii, Pseudanabaena sp.), and euglenoids (Trachelomonas volvocina). Commonly freshwater phytoplankton species and sometimes brackish water species were dominant during the monitoring. Phytoplankton abundances in dry season were much higher than in rainy season (>100 times) which was explained by a shorter riverine water residence time and higher flushing capacity during the dry season. There was a clear separation of phytoplankton abundance between the urban area and the remaining area of Saigon River because of polluted urban emissions of HCMC. Redundancy analysis shows that the environmental variables (TOC, nitrogen, pH, salinity, Mo, Mn) were the driving factors related to the dominance of L. danicus and Cyclotella cf. meneghiniana in the upstream river and urban section of Saigon River. The dominance of cyanobacterium Microcystis spp. in the downstream of Saigon River was related to higher salinity, Mg, Cu concentrations, and lower concentrations of nutrients, Mn, Co, and Mo. The dominance of potentially toxic cyanobacteria in Saigon River possesses health risk to local residents especially upon the increasing temperature context and nutrient loading into the river in the next decades.
Highlights
Anthropogenic activities cause water quality degradation that are often accelerated by global warming and can enhance the Responsible Editor: Thomas HeinMarseille, France eutrophication of water bodies, leading to cyanobacterial mass development and ecological health problems of rivers (Paerl and Huisman 2009; Machado et al 2018)
Particulate organic carbon (POC) was measured on suspended matter retained on a GF/ F filter using combustion in a LECO CS125 analyzer (EPOC laboratory, France) with precision better than 5%
The dominant phytoplankton species, genera, and their densities varied among the sites and were globally characterized by diatoms (e.g., A. granulata, Navicula spp., L. danicus, Cyclotella cf. meneghiniana), cyanobacteria (e.g., Microcystis sp., R. raciborskii, Pseudanabaena sp.), green algae (e.g., S. acuminatus), euglenoids (e.g., T. volvocina), and dinoflagellates (e.g., Peridinium sp.) (Table 1, Fig. 4)
Summary
Anthropogenic activities cause water quality degradation that are often accelerated by global warming and can enhance the Responsible Editor: Thomas Hein. The dynamics of phytoplankton in rivers from Southeast Asia are poorly reported and understood (Hoang et al 2018; Duong et al 2019), and some environmental parameters like trace metals are rarely considered during the investigations (Wang et al 2020). The provinces in the Saigon River basin (e.g., Ho Chi Minh City, Tay Ninh, and Binh Duong Provinces) are fastly developing in terms of economy, industry, and urbanization, but those activities contribute to the deterioration of the water quality, mainly by the anthropogenic activities and their release to the river (Strady et al 2017; Lahens et al 2018; Nguyen et al 2019a). We investigated the spatial and temporal variation of phytoplankton and their relationship with environmental variables in the Saigon River in the context of fast urbanization of Ho Chi Minh City (HCMC). The main objectives are (i) to assess the structure of phytoplankton and their diversity in the Saigon River as a tropical estuary, (ii) to characterize the main environmental drivers explaining the temporal and spatial structure of phytoplankton by statistical approach, and (iii) to discuss potential risk of the urban contamination and management issues on the phytoplankton dynamics
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