Mechanism of eutrophication process during algal decomposition at the water/sediment interface

Highlights

• The high-resolution system is used to detect the distribution of DO-pH-P-S-Fe.

• The DO and pH values corresponded to the Chla content and photosynthetic activity.

• pH Changes in all profiles and deeper sediments compared to the DO.

• The temporal heterogeneity of P-S-Fe varies with algal decay.

• The spatial heterogeneity of P-Fe-S is affected by the interaction processes.

Abstract

Water eutrophication has become a global environmental issue recently. Investigating the chemical gradient, spatial heterogeneity of dissolved oxygen, pH, phosphorus, sulfur, and iron (DO–pH–P–S–Fe) at the water/sediment interface during different algal decomposition stages is critical for exploring the underlying mechanism of the eutrophication process. We conducted mesocosm experiments to observe the two-dimensional distribution imaging in micro-interface using a high-resolution system (e.g., diffusive gradients in thin films and planar optode probes). The process of algal decomposition includes three stages: algal accumulation, algal growth, and algal decay. The changes in the dissolved oxygen (DO) observed in overlying water and upper sediments (−15 mm) and the changes in pH occurred throughout the profile, with their peak values (245.14 μmol L⁻¹ and 10.7) corresponding to the highest chlorophyll-a content (600 mg m⁻³) and maximum photosynthetic activity, respectively. The distribution of P-S-Fe in overlying water and upper sediments (−20 mm) is mainly affected by the changes in DO and pH caused by algal decomposition, whereas it is mainly affected by microbial activities and other chemical processes in deep sediments. P-S-Fe converged from algae cell and sediments to overlying water during algal accumulation, migrated to sediments in algal growth, and stored in algal decay sediments, while formed “black bloom (FeS)” in overlying water. The findings of this study suggest that light and pH should be controlled during algal accumulation for controlling eutrophication. Moreover, P-S-Fe in overlying water and upper sediments is better to remove in this optimal period, which will contribute a new guideline for the environmental sustainability of water eutrophication caused by cyanobacteria bloom.

Introduction

Water eutrophication often leads to cyanobacterial blooms, which cause toxin release, a sharp decline in aquatic biodiversity, oxygen deficiency, destroy aquatic habitat, and a potential threat to human health (Rozan et al., 2002; Gubelit and Berezina, 2010; Wang et al., 2016; Huisman et al., 2018). During cyanobacterial degradation, the sediment-water interface (SWI) acts as the primary medium for substance turnover and nutrient exchange (Han et al., 2015; Smith et al., 2011; Giles et al., 2016). Here, O₂, pH, P, Fe, and S are critical elements in many physicochemical reactions at the SWI and significantly impact the aquatic ecosystems (Zhu et al., 2006; Oguri et al., 2006; Han et al., 2015).

Oxygen is a key element of aerobic processes in most ecosystems; it acts as a proxy of element migration. The distribution and concentration of dissolved oxygen (DO) across the SWI is significant for nutrient cycling, as well as to the nutrient concentration gradient resulting from algal decay (Zhu et al., 2013; Han et al., 2015; Rickelt et al., 2013; Glud, 2008; Oguri et al., 2006). Algal decomposition also affects the pH, which controls the exchange reactions at the SWI via acid-base balance regulation, precipitation-dissolution, and redox processes (Wang et al., 2015, 2016; Xie et al., 2003). Algal accumulation and decomposition affect the water environment by depleting the DO and increasing pH, creating anaerobic conditions, which subsequently affect the P cycle at the SWI (Wang et al., 2015, 2016; Boers, 1991; Xie et al., 2003; Gao et al., 2013; Chen et al., 2018; Zhu et al., 2013; Chuai et al., 2011). Phosphorus is crucial for the regulation of tropical status in lakes; FeOOH-phosphate complex reduction, ligand-exchange reactions, Fe–P coupling, and organic P mineralization mediate its release from sediments (Smith et al., 2011; Giles et al., 2016; Wang et al., 2016; Zhang et al., 2013; Ding et al., 2016; Chen et al., 2018; Gao et al., 2013; Smolders et al., 2017). Redox-sensitive iron, which forms complexes with O, P, and S, is essential for life and affects the distribution of several oxyanions (Melton et al., 2014; Smolders et al., 2017). The presence of sulfur at the SWI results from the reduction of sulfate (SO₄²⁻) to sulfide (H₂S, HS⁻, and S²⁻) in anoxic conditions, as well as from sulfate-reducing bacteria (SRB) that forms insoluble FeS compounds from Fe²⁺ (Azzoni et al., 2001; Rozan et al., 2002; Kankanamge et al., 2017, 2020). Sulfur is mainly responsible for the black substances during cyanobacterial blooms (Stahl, 1979; Roden and Edmonds, 1997; Feng et al., 2014).

Algal decomposition significantly affects redox conditions, influencing P, Fe, and S exchange (Smith et al., 2011; Chuai et al., 2011; Feng et al., 2014; Wang et al., 2015, 2016; Han et al., 2015). Previous researches reported the interface and pore water concentration gradients of these parameters (Azzoni et al., 2001; Rozan et al., 2002). The one-dimensional (1-D) analysis of the DO, pH, soluble Fe(Ⅱ), as well as the soluble reactive phosphorus (SRP), at the SWI during algal decomposition was measured using microelectrodes, diffusive gradients in thin-film (DGT) probes, and high-resolution dialysis (Han et al., 2015; Chen et al., 2018). Nevertheless, the in situ two-dimensional (2-D) high-resolution information of these parameters must be considered to obtain their vertical profile distribution results. The 2-D distribution of solutes (P and S), DO, pH, and Fe(Ⅱ) at the SWI at the millimeter-scale were successfully obtained by using a high-resolution passive sampling technique and fluorescent imaging sensor system, such as the DGT, diffusive equilibration in thin film (DET), and planar optode (PO) technique (Davison and Zhang, 1994; Glud et al., 1996; Stockdale et al., 2009; Ding et al., 2010; Zhu and Aller, 2012; Santner et al., 2015; Kankanamge et al., 2017). Several researchers have successfully attempted to use a combined DGT or DET-DGT to simultaneously measure the 2-D Fe, and S distribution in sediments (Motelica-heino et al., 2003; Robertson et al., 2008, 2009; Pagès et al., 2011, 2012; Kankanamge et al., 2017, 2020). However, a lack of direct and conclusive evidence regarding the benthic chemical gradients and spatial heterogeneity analysis of critical elements (DO–pH–P–S–Fe) and feedbacks from the sediment-water interface has hindered the understanding of the eutrophication processes during algal decomposition.

Based on the above facts, in this study, a combined DGT–PO system was used in laboratory incubation experiments to measure the 2-D DO–pH–P–S–Fe high-resolution vertical distribution at the sub-millimeter scale in the SWI during different phases of algal growth. Here, AgI and Zr-oxide DGTs were used to investigate labile S and P distributions, respectively. Three PO techniques using the DO, pH, and Fe sensor films were combined and used to analyze the distributions of DO, pH, and Fe(Ⅱ). The finding of the current study will provide elaborate and significant insights on eutrophication mechanisms. Furthermore, it also provides guidelines for the management of algal bloom-related pollution.