Dynamics of phosphorus–iron–sulfur at the sediment–water interface influenced by algae blooms decomposition

Highlights

• In-situ, high resolution DGT techniques were employed to characterize the dynamics of P, Fe, and S at the sediment-water interface during algal decomposition.

• A simultaneous release of P and S occurred from degraded algal, resulting in bidirectional diffusion fluxes to sediment and overlying water.

• The sediment remained to be a major source of labile Fe to the overlying water.

• The sources of Fe and S in formation of the black waters in eutrophic lakes were the sediment and the degraded algal, respectively.

Abstract

This study addresses the previously unknown effects of algae blooms on the dynamics of phosphorus (P), iron (Fe) and sulfur (S) across a lacustrine sediment-water interface (SWI). A mesocosm experiment was conducted in-situ to investigate these effects based on two recently-developed diffusive gradients in thin-films techniques (DGT). Soluble P, Fe(II), and S(-II) exhibited similar changing trends in a water column subject to the algae addition. Peak concentrations appeared on day 7 of the 16-day experiment. The lowest Eh occurred at the experiment’s midway point indicating a strong algae degradation. A maximum increase in DGT-labile S appeared on day 8 near the SWI, while the DGT-labile P and Fe exhibited persistent increases almost to the end of experiment. Significantly positive correlations of labile P were observed switching from between labile Fe and labile S in sediments, suggesting a significant change in original Fe-coupled dynamics of P under algae decomposition. Apparent fluxes were calculated based on DGT profiles where a simultaneous release of P and S occurred from degraded algae, resulting in bidirectional diffusion fluxes from sediment to overlying water. In contrast, sediment acted as a major source of labile Fe due to added depth and apparently positive fluxes.

Introduction

Over the last few decades, ecological deterioration induced by algae blooms in the aquatic ecosystem has been a global concern, and nutrient cycling is also a primary concern due to its direct effect on productivity [1]. The internal loads of nutrients and pollutants from the sediment can continuously contribute to water eutrophication after a reduction of external anthropogenic inputs [2], [3]. Sediment plays an essential environmental role due to its capacity to store or release various compounds from or into the water column [4], [5]. This is especially true of the sediment–water interface, which has frequent substance turnover and nutrient exchange due to its susceptibility to various physical, chemical, and biological interactions [6], [7].

Phosphorus (P), iron (Fe), and sulfur (S) are essential nutrients for living organisms and have major environmental impacts on the aquatic ecosystem. P has attracted great attention for its significant contribution to eutrophication and algae blooms in aquatic ecosystems [8], [9]. Fe is the most prevalent redox-sensitive metal, which has a paramount importance in the oxidation of organic matter [10] and the formation of phytoplankton blooms [11]. Soluble sulfide is highly toxic, and may have relevant impacts at the ecosystem level [12], [13]. Furthermore, the interactions among sedimentary cycles of P, S, and Fe can affect their availability and mobility as they relate to each other. It is widely accepted that P lability in sediments is primarily controlled by the Fe redox cycling, and reduction and dissolution of Fe(III) oxyhydroxides is a major mechanism responsible for the release of P [14]. Sulphate reduction to S(-II) can limit Fe(II) diffusion rates in anoxic sediments through the formation of insoluble iron–sulphide precipitates (FeS/FeS₂) and thereby indirectly control P availability [1]. Understanding the dynamic interaction among Fe, P, and S is the key to understanding their biogeochemical fates and endogenous contributions to water eutrophication in lakes.

Increased input of nutrients in waters can lead to excessive algae growth, followed by their decomposition, deficiency of dissolved oxygen, and finally eutrophication and ecological deterioration. Following periods of algae decomposition, algae can precipitate and settle on the lake floor where abundant particulate/soluble/colloidal nutrients can be released into the surrounding regions [15], [16]. Recent studies revealed that the course of algae bloom decomposition directly contributes to a higher availability of P, Fe, and S [17], [18]. Besides, the aggregation and decomposition of abundant algae has the potential to significantly change the physical and biological features of the benthic environment such as DO, pH, and Eh, turbidity and particulate matter, which can strongly modify the nutrients cycling [19], [20]. Accordingly, the algae decomposition may have a profound effect on the sediment and water quality, which would consequently influence the ecology and biogeochemical cycling. For example, the most well-known ecological problem known as “black bloom” always appears after the severe algae bloom, and its formation is closely related to the post-bloom appearance of amorphous ferrous sulphide [20], [21].

Up to now, our understanding of the cycling of P, Fe, and S at the SWI has been limited due to the lack of in situ high-resolution monitoring technologies. Most previous studies were based on invasive ex situ sampling methods such as collection of sediment samples from the water bottom followed by slicing and centrifugation, which can easily change the original properties of samples thereby causing considerable analytical errors [22]. DGT technique is a dynamical sampling technique established based on the Fick’s First Law [22], [23], [24]. It can minimize the problems associated with conventional sampling methods such as analyte contamination and analyte speciation change during the sampling process [25], [26]. This technique has unique advantages in allowing in situ measurements of a number of solutes at a high spatial resolution, which enables it to be a powerful tool in interpretation of biogeochemical processes related to key elements [16], [24], [27], [28]. Recently, Motellca Hein et al. [29] and Robertson et al. [30], [31] successfully applied a combined DGT to simultaneous measurements of Fe and S at a millimeter resolution [29], [30], [31]. Furthermore, two types of combined DGT (i.e., ZrO-Chelex and ZrO-AgI DGT) were developed for simultaneous measurements of P and Fe, and P and S, respectively [32], [33]. These combined DGTs can provide a much more detailed insight into P, Fe, and S dynamics and related environmental processes at the SWI, than, for example, traditional destructive centimeter-resolution approaches.

This work seeks to advance the understanding of the effects of algae decomposition on dynamics of P, Fe, and S at the SWI based on laboratory incubation experiments. Two in situ, high-resolution technologies including microelectrodes and two combined DGTs (ZrO-Chelex and ZrO-AgI DGTs) were used for the first time. It will provide detailed data information of P, Fe, and S and relevant environmental factors at fine-scale, which would be beneficial to reassess the cycling of these elements during algae decomposition. The results are helpful toward clarifying the contributions of algae bloom decomposition to nutrient cycling in eutrophic lakes.