Abstract
Background and aims – Biomonitoring is an important tool for assessing river water quality, but is not routinely applied in tropical rivers. Marked hydrological changes can occur between wet and dry season conditions in the tropics. Thus, a prerequisite for ecological assessment is that the influence of ‘natural’ hydrological change on biota can be distinguished from variability driven by water quality parameters of interest. Here we aimed to (a) assess seasonal changes in water quality, diatoms and algal assemblages from river phytoplankton and artificial substrates through the dry-wet season transition (February–July 2018) in the Red River close to Hanoi and (b) evaluate the potential for microscopic counts and high-performance liquid chromatography (HPLC) analysis of chlorophyll and carotenoid pigments for biomonitoring in large tropical rivers.
 Methods – River water (phytoplankton) and biofilms grown on artificial glass substrates were sampled monthly through the dry (February–April) to wet (May–August) season transition and analysed via microscopic and HPLC techniques.
 Key results – All phototrophic communities shifted markedly between the dry and wet seasons. Phytoplankton concentrations were low (c. thousands of cells/mL) and declined as the wet season progressed. The dominant phytoplankton taxa were centric diatoms (Aulacoseira granulata and Aulacoseira distans) and chlorophytes (Scenedesmus and Pediastrum spp.), with chlorophytes becoming more dominant in the wet season. Biofilm diatoms were dominated by Melosira varians, and areal densities declined in the wet season when fast-growing pioneer diatom taxa (e.g. Achnanthidium minutissimum, Planothidium lanceolatum) and non-degraded Chlorophyll a concentrations increased, suggesting active phytobenthos growth in response to scour damage. Otherwise, a-phorbins were very abundant in river seston and biofilms indicating in situ Chlorophyll a degradation which may be typical of tropical river environments. The very large range of total suspended solids (reaching > 120 mg/L) and turbidity appears to be a key driver of photoautotrophs through control of light availability.
 Conclusions – Hydrological change and associated turbidity conditions exceed nutrient influences on photoautotrophs at inter-seasonal scales in this part of the Red River. Inter-seasonal differences might be a useful measure for biomonitoring to help track how changes in suspended solids, a major water quality issue in tropical rivers, interact with other variables of interest.
Highlights
Increasing pressure is being placed upon river systems due to the higher demand for water resources, following population growth and greater urbanization (Vörösmarty et al 2010)
Whilst biological monitoring has been widely employed as a tool for water quality assessment in some nations, Chlorophyll a (Chl a) and coliforms are the only biotic variables currently included in the Vietnam surface water monitoring guidelines
Concentrations of dissolved inorganic nitrogen (DIN) and silicate consistently exceeded 1.4 mg/L and 4.5 mg/L respectively, whereas phosphate concentrations ranged between 0.003 and 0.019 mg/L with the majority of P being present in particulate and/or soluble unreactive forms (0.075– 0.130 mg/L total phosphorus; TP)
Summary
Increasing pressure is being placed upon river systems due to the higher demand for water resources, following population growth and greater urbanization (Vörösmarty et al 2010). Typical of many developing regions, the Red River Delta in northern Vietnam is undergoing rapid changes in industrialization, urbanization, agricultural expansion, irrigation and dam construction This has led to environmental degradation of the Red River, including pollution by nutrients and toxins, and major changes to sediment loads (Le et al 2007, 2015, 2018, Dang et al 2010, Vinh et al 2014). In such a context, the need for monitoring and assessment of water quality in Vietnam has been recognised, leading to recent developments in legislation (Nguyen 2013). The potential for the application of diatom-based biomonitoring has been demonstrated in the Red River Delta (Duong et al 2012, 2014, Hoang et al 2018), but empirical data linking diatom assemblages with environmental conditions are limited, and other algal monitoring techniques such as chlorophyll and carotenoid pigment analysis have not been explored
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