Elsevier

Chemosphere

Volume 247, May 2020, 125960
Chemosphere

Nutrient addition bioassay and phytoplankton community structure monitored during autumn in Xiangxi Bay of Three Gorges Reservoir, China

https://doi.org/10.1016/j.chemosphere.2020.125960Get rights and content

Highlights

  • Episodic HABs events are driving by increasing nutrient budget and constantly changing climate.

  • Increasing nutrient paradigm greatly affects biological diversity and exacerbates competition and succession.

  • Trace metal addition causes a community transition from Cyanobacteria to Chlorophyte dominated system.

  • Combined N, P and Si addition stimulates growth rate and optimizes biomass production (as chla).

Abstract

The increasing freshwater ecosystem nutrient budget is a critical anthropogenic factor promoting freshwater eutrophication and episodic bloom of harmful algae which threaten water quality and public health. To understand how the eutrophic freshwater ecosystem responds in term of phytoplankton community structure dynamics to a sudden rise in nutrient concentrations, a microcosm study by nutrient addition bioassay was implemented in Xiangxi Bay (XXB) of Three Gorges Reservoir, China. Our results showed that dissolved trace elements supply adequately altered the phytoplankton community structure creating a regime shift from cyanobacteria-dominated to essentially Chlorophytes-dominated system, relative abundance (>70%). Combined N, P, and Si led to maximum growth stimulation accompanied by the highest chlorophyll yield (82.7 ± 14.01 μgL−1) and growth rate (1.098 ± 0.12 μgL−1d−1). N separate additions resulted in growth responses which did not differ while P -addition differed significantly (p∠0.05) with the control justifying a P limited system. Si enrichment stimulated diatom growth, relative abundance (20.62%) and maximum utility rate (USi = 83.37 ± 0.33%). This study also reveals that increasing nutrient loading from anthropogenic sources adequately decrease the ecological diversity (H < 1) and community overlap (CC ≤ 0.5) intensifying competition and succession which then select the fast-growing taxa to dominate and expand. Result points to the need for multiple nutrient control of N, P and Si loading into XXB through a prudent nutrient management protocol for lasting bloom mitigation in the tributary bay.

Introduction

Harmful algal blooms (HABs) are a pervasive environmental problem with an increasing trend in both severity and geographic range regularly induced by several factors. Anthropogenic nutrient input into freshwater-marine continuum has dramatically increased the current state of episodic HABs (Nwankwegu et al., 2019). The constantly changing climate, extreme hydrologic events and weather conditions such as rainfall, droughts, cyclones, and heavy storms, as well as atmospheric depositions, largely impact HAB and nutrient dynamics (Huo et al., 2019; Li et al., 2019; Pesce et al., 2018; Val et al., 2016). In the past, the phosphorus (P) was recognized as the only nutrient ultimately controlling freshwater eutrophication and bloom accretions (Paerl, 2008). A critical consideration was consequently given to P-alone input reduction in an attempt to mitigating the episodic outbreak of HABs in most aquatic systems. This management approach stemmed from the initial conclusion that the process of denitrification was sufficient to keep in check the excess N- input into freshwaters.

Subsequently, several studies essentially demonstrated that varying phytoplankton taxa respond differently to a wide range of nutrients varieties including N, P and most micronutrients in concert (Binding et al., 2018; Coronado-franco et al., 2018; Kumar et al., 2018; Wu et al., 2018) and a single nutrient management protocol would no longer gurantee a lasting solution to the HAB outbreaks. Frequently, cases of species-specific nutrient metabolism occur, significantly providing an advantage for ecosystem dominance and while some algal species are sensitive to P limitation others remain unaffected except N becomes limiting (Franklin et al., 2018, 2018; Silkin et al., 2019; Weisse et al., 2016). The sensitivity of phytoplankton community structure to the increasing nutrient concentrations in aquatic systems has received much attention (Klose et al., 2015). This outcome raised global concern on the need to resolve the relationship between phytoplankton structures and the aggregate nutrient disturbances rather than just P in the eutrophic freshwaters. The nutrient budget in a eutrophic system essentially cut across the macronutrients (N &P) including their dissolved, oxidized and reduced forms, and the micronutrients notably; iron (Fe), silicon (Si), manganese (Mn), zinc (Zn), and copper (Cu). Thus a comprehensive study on nutrient pool driving phytoplankton dynamics and bloom accretions should take into cognizance the extended nutrient pool.

In the Xiangxi Bay (XXB), a tributary in the Yangtze River, China (the world’s third-largest river), severe eutrophication and harmful algal bloom has remained unabated since the impoundment of the world’s largest hydroelectric project, the Three Gorges Dam (TGD). Several studies while attempting to provide a sustainable bloom management strategy have evaluated the hydrodynamic influences on the bloom condition including current characteristics (Yang et al., 2018), vertical mixing (Johnson et al., 2012), isotopic nutrient distributions (Yang et al., 2015), as well as sediment P release (Zhijing et al., 2012). However, the information on how sudden repletion in the critical nutrients controlling eutrophication induced by anthropogenic actions from mining activities in the surrounding towns affect the phytoplankton community structure has not been considered. The relationship between macro-microelements and phytoplankton structure dynamics as they affect the aquatic ecosystem functioning are often very difficult to recognize and no study of such at present has been carried out in the eutrophic XXB. Also, since aquatic nutrient budget shows characteristic dynamic fluctuations, the study on the phytoplankton responses to the sharp increase in the nutrient pool would adequately complement the strategies for bloom control. This could help in enacting a prudent mitigation step that can guarantee a lasting solution to the expanding phytoplankton bloom in the tributary. Hence, the aim of the present study was to examine the dynamics in the phytoplankton community structure under abrupt nutrient repletion while elaborating macro-micronutrient synergy on growth stimulations through the deployment of field microcosm study involving nutrient addition bioassay. It also evaluated the dominance and pattern of taxonomic shift controlled by nutrient dynamics. The sensitivity of a bioassay informed its relevance in the present study since it is used to track the immediate assessment of nutrient-algae interactions through rapid biomass growth responses under controlled conditions usually 3–5 days incubation. Further, we hypothesized that nutrient additions would significantly affect the phytoplankton community structure and ecological diversity in the eutrophic Xingxi Bay.

Section snippets

Study area

Xiangxi Bay (XXB) is the largest tributary in the lower reach of Three Gorges Reservoir (TGR) Yangtze River, China, located within the latitude 31°04/- 31°34 N/and longitude 110°25/- 111°06E/. It has a length of 94 km, the average annual flow of 47.4 m3/s and the watershed area of 3099 km2 (Gao et al., 2018). It is 32 km from Three Gorges Dam (TGD), and 38 km upstream of Yangtze River. It originates from Shennongjia National Park, Northwestern Hubei province, China. The XXB (Fig. 1) flows

The ambient XXB physicochemical and biological conditions

The initial properties of XXB surface water prior to bioassay are presented in Table 1. The tributary bay at the time of the study showed ambient temperature characteristics (WT = 28 ± 2 °C). The water was considerably clear at some depth, with average depth 39 0 ± 1.0 m and DO (11.54 ± 0.23 mgL−1). The pH was slightly alkaline which did not significantly change (p ≥ 0.05) throughout the study. The system showed TN/TP ratio of 11.50 (TN = 1.49 ± 0.31 mgL−1, TP = 0.13 ± 0.01 mgL−1). This

Conclusions

The present study compares the responses of different phytoplankton communities in Xiangxi Bay to the increasing concentrations of N, P Si, and some trace metals. It also quantified the biomass production (as chla) and estimated the nutrient disappearance (uptake) rates with time. This provides comprehensive insights into underscoring the effect of increasing anthropogenic nutrient inputs on phytoplankton community structure and biological diversity exacerbated by indiscriminate sewage

CRediT authorship contribution statement

Amechi S. Nwankwegu: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Writing - original draft. Yiping Li: Conceptualization, Funding acquisition, Supervision. Yanan Huang: Formal analysis, Visualization. Jin Wei: Formal analysis, Visualization. Eyram Norgbey: Formal analysis, Visualization. Qiuying Lai: Formal analysis, Visualization. Linda Sarpong: Formal analysis, Visualization. Kai Wang: Formal analysis, Visualization. Daobin Ji: Investigation, Formal analysis.

Acknowledgements

Authors are especially grateful to the Research Group of Ecological Water Conservancy of the Three Gorges University for providing experimental platform used during the study. Equally appreciated are Zhao Xingxing and Huang Jiawei for their assistance and commitments during the field experiment. This study was financially supported by National Key Research and Development Program of China (2017YFC0405203), the fundamental research funds for central universities (2018B48214, 2017B20514) and

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