Human Activities Caused Hypoxia Expansion in a Large Eutrophic Estuary: Non-negligible Role of Riverine Suspended Sediments

Abstract. Increase in riverine nutrient loads was generally recognized as the primary cause of coastal deoxygenation, whereas the role of other riverine factors, especially suspended sediments, has received less attention. This study aims to discern the impacts of anthropogenic alterations in various riverine inputs on the subsurface deoxygenation over the past three decades in a large river-dominated estuary, the Pearl River Estuary (PRE). By utilizing the physical-biogeochemical model, we reproduced the observed dissolved oxygen (DO) conditions off the PRE in the historical period (the 1990s with high-suspended sediments-DO and low-nutrient inputs) and the present period (the 2010s with low-suspended sediments-DO and high-nutrient inputs). Due to the decadal changes in riverine inputs, the PRE has witnessed more extensive and persistent low-oxygen events during summer in the 2020s, with larger spatial extents of ~2926 km² for low oxygen (DO < 4 mg/L, increased by ~148 % relative to the 1990s) and 617 km² for hypoxia (DO < 3 mg/L, by 192 %) and longer duration (by ~15–35 days), evolving into three distinct hypoxic centers controlled by different factors. Model experiments suggested that the decreased riverine DO content (46 %) has led to a low-oxygen expansion in the upper regions, accounting for 44 % to the total increment. Meanwhile, the increased nutrient levels (100 % in nitrogen and 225 % in phosphorus) and the declined suspended sediment concentration (60 %) have jointly promoted the primary production and bottom oxygen consumptions (dominated by sediment oxygen uptake), thus resulting in a substantial enlargement of low-oxygen area (104 %) and hypoxic area (192 %) in the lower reaches. Our results revealed a more critical role of the riverine suspended sediment decline in the exacerbation of eutrophication and deoxygenation off the PRE via improving light conditions to support higher local productivity, which could further amplify the effect combined with the growth in nutrients and confound the effectiveness of hypoxia mitigation under nutrient controls. Overall, in the context of global changes in riverine suspended sediments, it is imperative to reassess the contribution of riverine inputs to the coastal deoxygenation worldwide over the past decades, given that the impact of suspended sediments has been constantly overlooked in relevant investigations.