Jiayi Pan ^1* Yeqiang Shu ² Zhe-Wen Zheng ³ Adam T. Devlin ⁴ Majid Nazeer ⁵ Andrew M. Fischer ⁶

• ¹ Jiangxi Normal University, Nanchang, China
• ² South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China
• ³ National Taiwan Normal University, Taipei City, Taipei County, Taiwan
• ⁴ University of Hawaii, Honolulu, Hawaii, United States
• ⁵ Hong Kong Polytechnic University, Kowloon, Hong Kong, SAR China
• ⁶ University of Tasmania, Hobart, Tasmania, Australia

Over the past few decades, coastal and estuarine ecological environments have undergone significant and rapid transformations, driven by a range of factors, with the variability in hydrodynamic conditions being one of the most prominent (Elliott et al., 2019;Statham, 2012;Da et al., 2024). These dynamic forces influence water movement, sediment transport, and nutrient cycling, all of which are critical components affecting the health and stability of these ecosystems (van Vliet et al., 2023;Li et al., 2023). Consequently, understanding and interpreting hydrodynamic processes in coastal and estuarine waters are fundamental to unraveling the mechanisms behind ecological changes. This task requires a multidisciplinary approach, incorporating advanced methodologies such as remote sensing, in-situ observations, and numerical modeling. Each of these methods offers unique insights, from large-scale monitoring to fine-scale process modeling, providing a comprehensive understanding of these complex systems. While the immediate effects of local hydrodynamic conditions are significant, they do not act in isolation. Broader regional or even global events, such as climate change, sea-level rise, and extreme weather patterns, can exacerbate or modify these local processes. Therefore, analyzing hydrodynamic variability across different temporal and spatial scales is crucial to fully grasp how ecological environments evolve in response to various external forces, both immediate and long-term. Gaining a deeper understanding of both the ecological changes and the dynamic mechanisms driving them is essential for developing effective conservation and management strategies. Clarifying these relationships will enhance our ability to protect coastal and estuarine ecosystems from further degradation and to anticipate future changes. Furthermore, such knowledge is invaluable for environmental managers seeking to adopt appropriate mitigation and adaptation measures, ensuring the sustainability of these vital ecosystems. In this special issue, we present a collection of six papers that highlight cutting-edge research on ecological environment changes in response to hydrodynamic variability in coastal and estuarine waters. The titles of these papers are listed in Table 1.Table 1 The papers published in this issue. We will provide brief descriptions of these studies in the following section, offering insights into their key findings and contributions to the understanding of ecological environment changes in coastal and estuarine waters. Each study presents a unique perspective on the interplay between hydrodynamic variability and ecosystem dynamics, highlighting innovative methodologies and novel research approaches. The seasonal transition of the Western Boundary Current (WBC) in the southern South China Sea (SCS) from summer to winter is a complex process that typically begins in early October and lasts for about two weeks. Tian et al. (2023, this issue) analyzed this process using data from the Hybrid Coordinate Ocean Model (HYCOM) and AVISO altimeter measurements. They found that the transition occurs uniformly above the thermocline (~100 m), but lags in deeper layers. This process is mainly governed by geostrophic balance. Above the thermocline, the barotropic pressure gradient drives the transition, while below, both barotropic and baroclinic gradients play a role. Initially, these gradients offset each other in deeper waters, causing a delay in reaching geostrophic balance. This delay increases with depth as it takes time for the barotropic pressure to dominate. Changes in the barotropic gradient are influenced by wind stress curl, while shifts in the baroclinic gradient below the thermocline are linked to deep water warming, downwelling near the slope, and reduced upwelling off Vietnam.Dissolved oxygen (DO) is a critical water quality parameter necessary for supporting aquatic life and is particularly important for the growth and health of salmon. The study by Jeong et al. (2024, this issue) examined hourly DO concentration data from 21 aquaculture sites in British Columbia, Canada, collected between 2015 and 2017. Using a two-stage time-series analysis, the study revealed that 42.3% of the DO data was classified as 'optimal,' 56.5% as 'suboptimal,' and 1.2% as 'stressed.'The occurrence of hypoxic conditions varied significantly depending on the season, site, and even between different cages at the same location. Environmental factors, including temperature, wind direction, and phytoplankton absorption showed strong seasonal associations with DO concentrations. In particular, temperature had a significant impact during summer and winter, while wind and phytoplankton absorption played key roles in winter. The findings provide important baseline information on deviations from recommended DO levels and offer insights for improving water quality monitoring and resource management in aquaculture environments. The key findings showed that anoxia is prevalent at depths of 5-7 meters, with distinct seasonal patterns. Ventilation in the upper water column is notably stronger during the winter months when surface stratification weakens, allowing for better oxygen exchange. Additionally, anoxic water occasionally rises to the surface, though only for brief periods. These results provide critical insights into the oxygen dynamics of the lagoon, offering a baseline for improved environmental management and future mitigation strategies.The organic carbon (OC) to sulfur (S) ratio in sediments is a common paleosalinity indicator used to differentiate between marine and freshwater environments. With this ratio, Gao et al. (2024, this issue) explored the relationship between carbon, nitrogen, and sulfur in Hangzhou Bay (HZB) and adjacent sea areas, where sediment dispersal plays a significant role. An 80-meter sediment core from HZB provides key insights into environmental changes in coastal eastern China since the Last Glacial Maximum. Analysis of 82 subsamples showed that from 33.6 to 12.3 ka BP, low TOC (0.21%), TN (0.02%), and TS (0.06%), with a high TOC/TS ratio (9.1), suggested freshwater organic matter from terrestrial sources during a cold climate. In the Holocene, increases in TOC, TN, TS, and δ13C, along with a lower TOC/TS ratio (2.7), indicated greater seawater influence as the climate warmed post-12.3 ka BP. The TOC/TS ratio effectively tracked seawater intrusion, distinguishing freshwater, transitional, and marine environments, and proving useful for interpreting past environmental and climate changes.Nitrogen is a key factor contributing to water eutrophication, making accurate estimation of nitrogen levels essential for effective environmental monitoring. Zheng et al. (2024, this issue) present a twostep feature extraction method using deep learning to overcome the limitations of satellite spectral bands for nitrogen inversion in Shandong's offshore waters. The first step involves fusing spectral bands for manual feature extraction to enhance the dataset. Next, a one-dimensional convolutional residual network (ResNet-1D) is used to automatically extract deep features, with residual learning simplifying the training process. This approach reduces the Mean Relative Error (MRE) by 10% in both test and validation datasets. The study also examines the spatiotemporal distribution of total nitrogen concentration (TNC) in Shandong's coastal waters, revealing higher concentrations near the coast and seasonal variations, with the highest levels occurring in October. The annual average TNC (0.3 mg/L) has shown a gradual decline since 2014.In recent years, there has been growing attention on the darkening of coastal waters, a phenomenon associated with water browning and changes in light conditions. While it is widely accepted that this darkening is influenced by climate shifts, the precise relationship between light attenuation and its driving factors remains debated, as causal links are often drawn from indirect evidence. Aigars et al.(2024, this issue) used high-resolution remote sensing and modeled data, along with in-situ observations, to investigate this issue. The study compared two periods (1998-2007 and 2008-2018) to assess the impact of climate-related factors such as freshwater runoff, dissolved organic carbon mobilization, and shifts in the freshwater-saline water balance on the darkening of coastal waters in the Gulf of Riga, Baltic Sea. The results indicate that milder winter conditions are the most likely cause of the observed darkening. However, the study also emphasized the need for more detailed and time-sensitive data, as the current findings were not entirely conclusive. The studies in this issue offer key insights into coastal and marine environmental processes. Together, these studies underscore the importance of integrating advanced technologies such as remote sensing, machine learning, and high-resolution models to better understand and manage the complex processes affecting coastal environments. As climate change continues to reshape marine and coastal systems, the findings presented here highlight the need for more precise, long-term data collection and adaptive management strategies to protect these vulnerable ecosystems.