Potential linkage between sedimentary oxygen consumption and benthic flux of biogenic elements in a coastal scallop farming area, North Yellow Sea

Highlights

• Benthic release of biogenic elements was explored by static incubation experiments.

• Variations in their interface fluxes with dissolved oxygen were evaluated.

• Potential influence of scallop farming on their benthic release was analyzed.

• Sediment served as a key source of biogenic elements for overlying-water ecosystem.

Abstract

Surface sediments were collected from a mariculture area adjacent to the Yangma Island suffering from hypoxia in summer, and a laboratory static incubation was conducted to study the sedimentary oxygen consumption (SOC) and the benthic fluxes of nutrients and fluorescent dissolved organic matter (FDOM). Compared with some coastal areas, the SOC of the studied area was relatively low in summer with the value of 2.34–6.03 mmol m⁻² d⁻¹. Sediment acted as an important source of nutrients (except for nitrate) and FDOM for the overlying water. Dissolved oxygen (DO) in the overlying water could affect the decomposition mode of sedimentary organic matter (SOM), i.e. aerobic and anaerobic decomposition and subsequently dominated the release of nutrients and FDOM. When DO > 50 μmol l⁻¹, it was beneficial to the release of ammonium, silicate and FDOM. In contrast, low oxygen conditions, i.e. DO < 100 μmol l⁻¹, stimulated sediment phosphate efflux. In addition, scallop farming activities also affected the SOC and benthic flux of nutrients and FDOM mainly through biological deposition.

Introduction

Shellfish aquaculture is one of the fastest growing sectors of the food industry (FAO, 2016). While meeting human needs for food, excessive shellfish farming in recent years has a remarkable eco-environmental impact on coastal waters, e.g. the occurrence of water hypoxia (Lee et al., 2016) and transformation of carbon, nitrogen, phosphorus and silicon metabolism (Daniele et al., 2006; Valérie et al., 2007; Xia et al., 2019).

As one of the important processes, the biogeochemical cycle of sedimentary biogenic elements under the influence of shellfish farming has attracted widespread attention in recent years (Lee et al., 2016; Huang et al., 2018). The increased sedimentary organic matter (SOM) affected by shellfish farming activities has been shown to significantly increase sediment oxygen consumption (SOC) and the turnover of nutrients (Matos et al., 2016). These regenerated nutrients play an important role in phytoplankton production and could meet up to 100% of phytoplankton demand in some coastal waters (Van Broekhoven et al., 2014; Foster and Fulweiler, 2019; Kim et al., 2020). The biogeochemistry of SOM is mainly mediated by bacteria and depends on complex physico-chemical and biological diagenetic processes (e.g. aerobic and anaerobic respiration), which is affected by many factors, e.g. redox characteristics, temperature and hydrodynamic conditions.

It is well known that temperature increase can significantly promote the metabolic rate of sediment microorganisms, thereby accelerating the nutrient turnover in the ecosystem (Zhou et al., 2017). Dissolved oxygen (DO) level can affect the decomposition modes of organic matter (OM), i.e. aerobic and anaerobic decomposition, and subsequent oxidation reactions (Hantush, 2007; Zhou et al., 2017; Foster and Fulweiler, 2019). Generally, OM is mineralized to carbon dioxide (CO₂) through aerobic respiration under oxygen-rich environment, while it can be transformed into methane (CH₄) or low molecular weight OM instead of the CO₂ via fermentation in anaerobic conditions. Generally, the above process will be accompanied by the release of dissolved organic matter (DOM) (Gan et al., 2020); however, the characteristic and flux magnitude of this portion of DOM in different environments are presently unknown.

Furthermore, organic nitrogen (ON) can be converted to ammonium (NH₄⁺) through ammoniating under low oxygen condition (Gao, 2019). Meanwhile, nitrate (NO₃⁻) in anaerobic environments can be removed through the denitrification process (Rysgaard et al., 2004; Matos et al., 2016; Foster and Fulweiler, 2019). It is widely believed that DO is one of the most important parameters controlling the biogeochemical cycle of phosphorus (P) in aquatic ecosystems, which is mainly driven by the iron-bound P (Fe–P) migration in sediments (Liu et al., 2020). Generally, oxygen-rich conditions are favorable for the formation of Fe–P. In contrast, large amounts of P can be released back into water due to reductive dissolution of Fe/Mn oxides under hypoxic conditions. In addition, some sedimentary material can be dispersed by currents, which affects the release of nutrients from the sediment, mainly depending on hydrodynamic conditions at the site (Tang et al., 2020).

The coastal waters around the Yangma Island, an important mariculture zone in the North Yellow Sea, are located next to the north coastline of Shandong Peninsula. The area is significantly affected by scallop farming activities. For example, the SOM content in this area was significantly higher than that in the surrounding area (Yang et al., 2018). Bottom water hypoxia in summer was observed in this area; it generally started to develop in July, reached its maximum in August, and gradually disappeared in autumn (Yang and Gao, 2019). The alternating changes of DO concentration in the bottom water could significantly affect SOC and other diagenetic processes (Yang et al., 2018, 2020a). The trophic level of water in this area was characterized as oligotrophy, and the sediment was one of the important sources of nutrients for the overlying water (Yang et al., 2020a). In addition, previous results showed that aerobic microbial degradation of sinking biological particles could be an important source of fluorescent dissolved organic matter (FDOM) based on the correlation between apparent oxygen utilization (AOU) and FDOM, especially for humic-like components (Yang and Gao, 2019). To sum up, amounts and characteristics of nutrients and DOM are greatly influenced by the SOM biodegradation process in this area.

Although the degradation process of SOM in coastal waters has been extensively studied (Derrien et al., 2019; Liu and Peng, 2019), as far as we know, little is known about the response relationship between the overlying-water DO conditions switching from being oxic to hypoxic and the SOM degradation modes, as well as the accompanying migration and transformation processes of biogenic elements, e.g. nutrients and DOM. Information about these is crucial especially in coastal waters where the overlying-water DO level changes intensely and frequently due to the influence of human activities. Thus, the data set of DO, dissolved nutrients, i.e. dissolved inorganic phosphorus (PO₄³⁻), nitrogen (DIN) and silicate (SiO₃²⁻), and FDOM fluxes across the sediment-water interface determined by a sediment static incubation were provided in this study. The main objectives were: (1) to identify the relationship between the overlying-water DO conditions and the degradation modes of SOM in summer in the coastal waters around the Yangma Island, and (2) to clarify the benthic fluxes of nutrients and FDOM under different SOM degradation modes and their main influencing factors.