Denitrification in Hangzhou Bay: The role of nutrient dynamics in a tidal and human-impacted estuarine system
1.Introduction
Present in most living organisms, nitrogen is essential for the biosynthesis of nucleic acids and proteins, thereby playing a funda- mental role in cellular processes (Mikhail and Sverjensky, 2014). It is a vital nutrient for ecosystem sustainability and supporting agricultural productivity. Nitrogen undergoes a series of transformations into
various forms—such as ammonia (NH), nitrate (NO ), nitrite (NO ),
and atmospheric nitrogen (N2)—via processes including nitrification, denitrification, and ammonification (Kieber et al., 2005). Although ni- trogen is an essential nutrient, its environmental impact varies signifi- cantly depending on its speciation and concentration, the ecosystem’s nutrient demand, and the availability of other key elements (Hinkle and Tesoriero, 2014). In marine environments, nitrogen stimulates primary
production and carbon fixation; however, excessive nitrogen inputs can lead to eutrophication, which is a critical ecological issue.
Anthropogenic activities, particularly the combustion of fossil fuels and the extensive use of nitrogen-containing fertilizers in agriculture have significantly increased nitrogen inputs into aquatic systems over recent decades (Han et al., 2014). The anthropogenic nitrogen entered the marine system mainly via riverine discharge. The reactive nitrogen flux was projected to increase from 118 Tg N yr − 1 in the early 1990s to 150 Tg N yr− 1 by 2050, with anthropogenic nitrogen inputs significantly exceeding those from natural processes (Galloway et al., 2004; Galloway et al., 2008). Coastal estuaries, which are critical endpoints for reactive nitrogen from both groundwater and surface runoff, have seen anthro- pogenic nitrogen inputs rise by over 150 % since the 20th century (Malone and Newton, 2020). For this reason, estuarine eutrophication,
- Corresponding authors at: Second Institute of Oceanography, Hangzhou, China.
E-mail addresses: liyangjie@sio.org.cn (Y. Li), lihongliang@sio.org.cn (H. Li).
https://doi.org/10.1016/j.marpolbul.2025.118783
Received 31 August 2025; Received in revised form 25 September 2025; Accepted 26 September 2025
Available online 6 October 2025
0025-326X/© 2025 Elsevier Ltd. All rights are reserved, including those for text and data mining, AI training, and similar technologies.
which refers to excessive richness of nitrogen in estuaries, causes a dense growth of plant life that consumes huge amount of the oxygen, has become a growing global issue, as it disrupts several key ecological functions of estuaries (Wang et al., 2021). For example, this phenome- non contributes to various environmental challenges, including oxygen depletion, harmful algal blooms, loss of biodiversity, ocean acidifica- tion, and even the establishment of invasive species (Cai et al., 2016). Notably, severe eutrophication and hypoxia have been observed in the Gulf of Mexico (Laurent et al., 2018), Changjiang Bay (Li et al., 2024), and the Black Sea (Strokal et al., 2014) over the past two decades.
Situated in the southern part of Changjiang Bay, Hangzhou Bay is a large estuarine area characterized by strong tidal currents, high turbidity, and elevated nutrient concentrations. This bay receives sub- stantial nitrogen inputs from the Qiantang River (Yang et al., 2024). Notably, the Qiantang Estuary in Hangzhou Bay is one of the few estu- aries in the world that experiences a tidal bore—a phenomenon where seawater advances like a vertical wall due to the faster movement of surface water relative to the bottom layer (Xie, 2018). Two main geographic features contribute to this phenomenon: the bell-mouth shape of the estuary, which funnels seawater into a narrowing space, and a shallow sandbank along the Qiantang River that intensifies tidal bores (Deng et al., 2020). Despite its unique characteristics, limited research has focused on Hangzhou Bay, and the impacts ofthe tidal bore on nutrient distribution and trends remain poorly understood.
In recent years, there has been growing concern from both national and local government sides about the nitrogen cycle in coastal ecosys- tems due to its effects on water quality and marine life (Yang et al., 2024). Denitrification is particularly important in this region, as it helps remove excess nitrogen and reduces the risks of eutrophication and hypoxia (Liu et al., 2018). However, studies indicate that the denitrifi- cation rate in Hangzhou Bay is relatively low due to distinctive envi- ronmental factors (Li et al., 2024). For instance, the tidal bore accelerates oxygen exchange between the atmosphere and surface water, which may inhibit conditions favorable for denitrification. Additionally, high turbidity reduces sunlight penetration, limiting algae and phytoplankton growth, and thereby limiting the nitrogen fixation (Wang et al., 2019). Tidal activities also lead to algae accumulation in intertidal zones, potentially altering nutrient dynamics (Zhang et al., 2015). By focusing on the denitrification rate, we can gain valuable insights into the nitrogen cycle in Hangzhou Bay, facilitating the development of effective environmental management strategies.
This study examines nitrogen dynamics in Hangzhou Bay with three objectives: (1) mapping spatial and seasonal nutrient patterns from the Qiantang River to the coast, (2) quantifying denitrification rates during two contrasting seasons, and (3) identifying the key environmental drivers of nitrogen removal using statistical modeling. The sampling strategy was designed to capture the major spatial variability of nutrient and environmental conditions across the bay, while the two survey
periods represent distinct hydroclimatic seasons that encompass much of the annual range in conditions. Together, this framework advances understanding of nitrogen cycling in this dynamic coastal ecosystem and provides insights to support sustainable management strategies for maintaining the ecological health of Hangzhou Bay.
- Materials & methods：
2.1. Study area
Hangzhou Bay (30.4◦N, 121.5◦ E) spans approximately 5800 km2. Situated west of the East China Sea and south of the Yangtze River Es- tuary, the bay has an average depth of 8–10 m (Fig. 1). This dynamic region is characterized by complex hydrodynamics (Fang et al., 2024), elevated concentrations of suspended particulate matter (SPM), and nutrient enrichment (Wu et al., 2019). The bay receives substantial freshwater inflow from the Qiantang River (QTR) and the Cao’e River, together contributing an annual runoff of 49 km3 and a sediment load of 2.07 million tons (Liu et al., 2017). Furthermore, the Changjiang River strongly influences surface water and sediment dynamics, with an annual runoff of 770 km3 and sediment transport of 66.5 million tons (Zheng et al., 2012). The region experiences a subtropical monsoon climate, with a mean annual temperature of 17 ◦ C and around1600 mm of rainfall, most of which occurs between April and September. Hy- drodynamics are further shaped by a semidiurnal tide, with maximum current velocities reaching 3.0 m s− 1, driving horizontal water and SPM transport (Fan et al., 2023). Residual flow and SPM dynamics show distinct spatial patterns, being flood-dominant in the northern part of the bay and ebb-dominant in the southern part (Liang et al., 2022).
2.2. Sampling & in-situ measurements
Sampling was conducted aboard the research vessel Zhe Dai Fishery 03720. Based on distinct geographical and hydrological characteristics, the sampling sites were categorized into three regional units: the Qiantang River (QR), with 10 sites sampled in March and 12 in June; the Water Mouth (WM), with 18 sites in March and 13 in June; and the Seashore Area (SA), with 17 sites in March and 15 in June (Fig. 1). Surface sediments (0–10 cm) were collected in triplicate from 45 sta- tions in March and 40 stations in June using stainless steel cutting rings as sub cores from box cores. The sediment samples were immediately sealed in sterile plastic bags and stored at 4 ◦ C in the dark. In the lab- oratory, one part of the fresh sediment sample was incubated immedi- ately via slurry experiments to measure the rates of the denitrification processes. Remaining sediment samples were used to analyze their physicochemical properties.
Seawater temperature and salinity were measured using an RBR Maestro multiparameter water quality monitor (RBRmaestro3, RBR,Canada). At each station, seawater samples were collected using Niskin bottles. The pH was determined using an Orion pH 3 Star analyzer equipped with an electrode calibrated against three NIST standard buffers (pH 4.01, 7.00, and 10.01). Dissolved oxygen (DO) measure- ments were conducted onboard by fixing and titrating samples from the Niskin bottles using the classic Winkler procedure. For fluorescence analysis, a 50 mL aliquot of surface seawater was filtered through a 25 mm GF/F filter and analyzed using a fluorometer (Turner-Designs Tril- ogy™). Additionally, a 100 mL portion of seawater was filtered on-site through a pre-weighed 47 mm GF/F filter to determine suspended particulate matter concentrations. The filtered seawater was then transferred to HDPE bottles and stored for subsequent nutrient analysis.
