Impacts of Environmental Change and Human Activities on Aquatic Ecosystems

1. Introduction

With the ongoing changes in the global climate system and the continuous intensification of human activities, aquatic ecosystems face unprecedented stresses from multiple sources [1,2]. As a vital foundation for maintaining water security, regulating regional climate, supporting biodiversity, and providing ecosystem services, the health of aquatic ecosystems is directly linked to overall environmental quality and the sustainable development of human society [2,3]. Recently, natural ecological changes such as global warming, altered precipitation patterns, and the increasing frequency of extreme climate events (e.g., floods, droughts, and heatwaves) have induced profound changes in the hydrological processes and ecological patterns of water bodies. For example, rising water temperatures may alter species’ life cycles and metabolic rates. At the same time, extreme rainfall or drought can lead to wetland shrinkage, river flow interruption, or sudden increases in runoff, triggering additional changes in nutrient loads and degradation of ecological functions [4,5]. In addition, prolonged environmental stress has gradually reduced ecosystems’ capacity for self-recovery, as evidenced by a decline in biodiversity and increased instability in ecological processes [6].

Human activities are degrading aquatic ecosystems at an increasing rate. In particular, urbanization and industrialization have led to the discharge of both point-source and non-point-source pollutants. Extensive use of chemical fertilizers and pesticides in agriculture can result in eutrophication, excessive algal blooms, and the deterioration of benthic habitats [7,8]. Furthermore, the construction of hydraulic engineering projects and the mismanagement of water resources have disrupted water bodies’ natural connectivity and ecological flow regimes. These alterations affect the migration routes and habitat structures of aquatic organisms, ultimately leading to ecosystem degradation and the loss of essential ecological services [9,10]. In the face of the complex challenges posed by the combined effects of natural environmental changes and human activities, it is imperative to adopt a systems perspective to facilitate a deep understanding of the impact mechanisms of various stressors on the structure and function of aquatic ecosystems. There is an urgent need to establish integrated models for ecological risk assessment and pollution control, and to develop adaptive, practical strategies for ecological restoration and sustainable management to enhance the resilience and stability of aquatic ecosystems, ensure water quality safety, and maintain ecological integrity [11,12].

This Special Issue focuses on the environmental challenges facing aquatic ecosystems, aiming to promote interdisciplinary scientific research and provide theoretical foundations and practical guidance for understanding and addressing aquatic ecosystem crises in the context of global change. It also seeks to offer feasible scientific recommendations for policymakers and environmental managers to jointly advancing the sustainable development of aquatic ecosystems and deepening the progress of regional ecological civilization.

2. Overview of Advanced Developments

An et al. (Contribution 1) provided a comprehensive review of how external environmental factors influence the distribution and migration of microplastics in aquatic systems, focusing on meteorological conditions, ecosystem characteristics, and the physical properties of water bodies. The study revealed that rainfall introduces land-based microplastics into water bodies and facilitates vertical and horizontal transport; sunlight accelerates microplastic aging and fragmentation, altering their density, surface activity, and adsorption capacity; and wind promotes transboundary movement by disturbing the water surface or carrying particles. Additionally, aquatic plants, animal ingestion and metabolism, and microbial production of extracellular polymeric substances (EPSs) affect microplastics’ morphology and transport pathways. Geomorphological features and suspended particulate matter in lakes and rivers also govern sedimentation, aggregation, and resuspension behaviors. This work systematically analyzes the drivers of microplastic transport from multiple dimensions, with particular focus on the often-overlooked role of natural environmental conditions such as climate and ecological context, providing strong support for water resource management and microplastic pollution control.

Bo et al. (Contribution 2) emphasized the unique role of rice paddies in protecting aquatic biodiversity. They examined the impacts of climate change, such as rising water temperatures and altered hydrological cycles, on aquatic ecosystems. The results revealed that climate change profoundly alters the character and dynamics of natural systems globally. Increased water temperatures and disruptions in the hydrological cycle have profound implications for biodiversity in freshwater environments, and may also be evident in artificial aquatic agro-ecosystems. Strategies such as reducing irrigation requirements, promoting the cultivation of drought-tolerant crops, and utilizing precision agriculture techniques are proposed to promote a sustainable balance between agriculture and ecological conservation.

Chen et al. (Contribution 3) classified the ecosystem services of the Xin’an River Basin into provisioning, regulating, and cultural services. They estimated the basin’s total service value using ecosystem service valuation (ESV). The total ESV was estimated at CNY 70.271 billion, with provisioning services accounting for 22.7%, regulating services 24.6%, and cultural services 52.7%. Based on different compensation scopes, ecological compensation’s upper and lower limits were calculated at CNY 4.085 billion and CNY 1.438 billion, respectively. By integrating theoretical modeling with empirical analysis, this study offers a practical framework and financial assessment for ecological compensation, supporting refined management and precise compensation in the basin.

Dong et al. (Contribution 4) systematically monitored the spatial and temporal distribution of nine antibiotics—including roxithromycin, oxytetracycline, sulfamethoxazole, and ofloxacin—in the influent and effluent of wastewater treatment plants (WWTPs) in urban and suburban areas of Tangshan City. They used risk quotient methods to assess the potential ecological risks these antibiotics pose to aquatic ecosystems and explored how antibiotic distribution correlates with factors such as season, temperature, pH, hydrolysis properties, usage patterns, and treatment processes. The results showed significant seasonal and regional variations in antibiotic concentrations. In urban WWTPs, roxithromycin was the main pollutant in spring, sulfamethoxazole dominated in summer and autumn, and ofloxacin peaked in winter. In suburban WWTPs, sulfamethoxazole and norfloxacin were the primary contaminants. Sulfamethoxazole had the highest per capita pollution load for most of the year, with ofloxacin peaking in winter. Ecological risk assessment indicated winter was the season with the highest antibiotic risk, while the other three seasons posed low risks. Roxithromycin, oxytetracycline, tetracycline, and chlortetracycline in urban WWTPs, and roxithromycin and oxytetracycline in suburban WWTPs, presented moderate risks. Key factors influencing antibiotic distribution included hydrolysis rate, temperature, pH, and sludge adsorption capacity. This study revealed multidimensional variations in antibiotic distribution, providing valuable data and theoretical foundations for assessing antibiotic pollution loads, implementing tiered ecological risk management, and improving wastewater treatment processes in both urban and suburban areas.

Kasem et al. (Contribution 5) focused on nitrate (NO₃⁻) contamination in shallow groundwater in the arid southeastern Nile Delta, Egypt. They systematically applied stable isotope techniques (δ¹⁵N/δ¹⁸O–NO₃⁻ and δ²H/δ¹⁸O–H₂O) combined with hydrochemical analysis to identify nitrate sources and transformation mechanisms. The results showed that groundwater recharge is primarily controlled by Nile River water, with irrigation water leakage in the west and mixing with deep groundwater in the east. Some samples exceeded safe limits for TDS, SO₄²⁻, NO₃⁻, and Mn²⁺, with NO₃⁻ concentrations reaching up to 652 mg/L, far above WHO drinking water standards. The most severe pollution occurred in the central unconfined aquifer, spreading along groundwater flow paths to deeper and eastern zones. In the west and east, nitrate mainly originated from soil organic nitrogen (SON) and fertilizer nitrification products (CFs). At the same time, the central region was heavily impacted by domestic sewage inputs accompanied by denitrification closely linked to manganese biogeochemical cycling. This study is the first in the area to integrate multiple stable isotope methods and hydrochemical indicators to systematically identify nitrate sources and transformation pathways. It proposed a coupled mechanism of denitrification and manganese cycling, expanding our understanding of groundwater pollution–biogeochemical interactions and offering a replicable approach to pollution identification and control strategies in the Nile Delta and other arid groundwater systems.

In July 2023, Krupa et al. (Contribution 6) conducted systematic monitoring of the physicochemical parameters (transparency, temperature, pH, salinity (TDS), dissolved oxygen), pollutant concentrations, and biological indicators (zooplankton species richness, abundance, biomass, diversity index, etc.) in the upstream (Black Irtysh River) and downstream (Pavlodar region) sections of the Irtysh River in Kazakhstan. The results showed that nitrate nitrogen and phosphate were generally clean or lightly polluted, and ammonia nitrogen was below moderate pollution in most areas. In contrast, nitrite nitrogen (NO₂⁻) exhibited moderate pollution across most regions and reached high pollution levels in Pavlodar and Aksu. Among the nine heavy metals, iron (Fe), copper (Cu), and manganese (Mn) were commonly exceeded, with localized exceedances of zinc (Zn) and chromium (Cr). Overall, the water quality ranged from clean to lightly polluted, with localized heavy pollution downstream. The community structure indicated high organic pollution but relatively low heavy metal toxicity. Elevated iron levels in the Black Irtysh River likely resulted from geological leaching, while water quality deterioration downstream was mainly attributed to industrial and urban wastewater discharge. High flow velocity, sufficient dissolved oxygen, adsorption of heavy metals by suspended particles and clay along riverbanks, and uptake of nutrients and heavy metals by aquatic plants contributed to the river’s self-purification capacity. This study provides a comprehensive ecological assessment of the Irtysh River in Kazakhstan and offers scientific support for watershed management and transboundary water cooperation.

Li et al. (Contribution 7) investigated the impacts of four small cascade hydropower stations on microbial plankton community structure and ecological functions in tributaries of the Pearl River. Using flow cytometry, microbial populations were classified, and environmental indicators such as photosynthetic autotrophic capacity (PAC), bacterial activity index (BAI), viral regulatory capacity (VRC), and fungal metabolic capacity (FMC) were calculated. The results showed that along the cascade, dissolved oxygen (DO) and electrical conductivity (EC) increased, while the oxidation reduction potential (ORP) and total organic carbon (TOC) decreased. Abundances of viruses, low-nucleic-acid (LNA) bacteria, and fungi declined by 30.9%, 30.5%, and 34.9%, respectively. EC, TOC, and nitrate nitrogen (NO₃⁻-N) were key drivers influencing microbial abundance changes. Carbon and nitrogen nutrient levels significantly affected ecological indicators. The cascade hydropower stations significantly affected the PAC, BAI, and VRC, but had a relatively minor impact on FMC. The downward trend in VRC suggests that the stations had a weakening viral regulatory effect. This study applied high-throughput flow cytometry for microbial community classification, providing a novel technical approach, and comprehensively assessed the ecological impact of cascade hydropower stations by integrating environmental factors with ecological evaluation metrics.

Sim et al. (Contribution 8) analyzed water quality and phytoplankton community changes from 2013 to 2016 across a series of interconnected dams and reservoirs on the North Han River (Uiam, Cheongpyeong, Sambong-ri, and Paldang Lakes). The results showed that during drought periods, prolonged water residence time promoted nutrient accumulation and recycling within reservoirs, exacerbating eutrophication and water quality deterioration. Cyanobacteria became dominant, triggering harmful algal blooms. Notably, changes in upstream dam discharges directly impacted downstream reservoir water quality and ecosystem health. This study deeply explored the effects of climate change-induced extreme drought on water quality and phytoplankton dynamics, providing scientific insight into the impacts of climate-driven drought and informing integrated water management strategies.

Yang et al. (Contribution 9) investigated the concentration, distribution, and ecological risk of rare earth elements (REEs) in surface sediments of the Eastern Tiaoxi River (ETX) in eastern China. The study found that total REE concentrations ranged from 133.62 to 222.92 mg/kg, characterized by enrichment of middle rare earth elements (MREEs) and depletion of heavy rare earth elements (HREEs). REE concentrations and distributions were closely associated with elements such as Ca, Fe, Mg, and Mn, likely reflecting the influence of clay minerals, Fe-Mn oxides, and specific heavy minerals. The research also revealed significantly elevated REE levels near urbanized areas, while natural processes like soil transport and chemical weathering primarily drove REE variations elsewhere. Ecological risk assessment highlighted notable REE enrichment and moderate ecological risk in sediments near urban zones, with relatively minor impacts from agricultural areas. This study elucidates the combined effects of urbanization and natural processes on REE distribution and ecological risk in the ETX watershed, providing a scientific basis for environmental management and pollution control.

Zhao et al. (Contribution 10) focused on water quality pollution on the Qinghai–Tibet Plateau, investigating the spatial–temporal distribution patterns of water quality parameters, key influencing factors, and associated water quality risks. The study found significant differences in average water quality concentrations between the flood and dry seasons, with cadmium (Cd) levels meeting Class II water standards during the flood season, indicating that the water quality was relatively better at that time. Heavy metal risks exhibited distinct spatial patterns among different rivers—for example, the Sìgōu River showed higher heavy metal risks, highlighting uneven pollution levels across rivers. Pollution differences were mainly attributed to discharges from livestock farms and industrial enterprises, especially heavy metals. Meteorological factors had a noticeable impact on water quality, with seasonal variations between flood and dry periods further confirming climate regulation effects. By comprehensively analyzing spatial–temporal water quality variations across seasons and rivers, and applying Boruta and IFN-SPA algorithms to identify major pollutants, this study thoroughly revealed water quality dynamics and emphasized the significant role of meteorological factors, providing crucial insights for water quality management under climate change scenarios.

3. Conclusions

The papers in the Special Issue “Impacts of Environmental Change and Human Activities on Aquatic Ecosystems” in Water provide a contemporary snapshot of research trends in this field. The ten research articles in this collection cover studies on pollutant source identification and migration mechanisms (Contributions 1, 3, 4, and 10), water quality assessment and ecological risk evaluation (Contributions 2, 4, and 6), the impacts of climate change and extreme weather on aquatic ecosystems (Contributions 3, 6, and 9), ecosystem functions and ecological compensation (Contribution 5), as well as ecological responses and changes in aquatic biological communities (Contributions 2, 7, and 8). These works focus on surface waters, including reservoirs/rivers/lakes, groundwater, urban wastewater, integrated watersheds (ecosystem services), and agricultural ecosystems, covering transboundary rivers, plateau water systems, and urban water bodies. The studied pollutants are diverse, including nitrates (NO₃⁻)/nitrogen and phosphorus nutrients, heavy metals, microplastics, antibiotics, and rare earth elements. The research methods and technical approaches employed are mature and include isotope tracing, risk assessment (RQ/ecological risk index), hydrochemical and statistical analyses, bioindicator methods (plankton/microorganisms), high-throughput technologies (flow cytometry), and algorithm-assisted analyses (Boruta/IFN-SPA). The study regions are typical and internationally representative, including Chinese river basins (Pearl River, Diaoxi River, Xin’anjiang River), international water bodies (Italian paddy fields, the Nile River in Egypt, the Irtysh River, and the Korean Peninsula border), as well as typical environments in plateau and arid regions.

Current research has made significant progress in identifying multi-source pollution. Combining isotope techniques and ecological indicators has effectively enhanced the quantitative analysis of pollution sources, becoming a hot topic in water environment studies. At the same time, the impact of climate factors on water quality and ecosystems has received increasing attention, reflecting new trends in water environment risks in the context of global change. At the management and decision-making level, introducing ecosystem assessment and compensation mechanisms provides a quantifiable basis for formulating scientifically sound policies. With advances in technology, biological response indicators such as microorganisms and plankton are gradually becoming important tools for evaluating ecological health. Overall, related studies commonly adopt multi-scale, multi-factor integrated analytical frameworks, showing a development trend of “cross-interdisciplinary integration”.

Therefore, future research is recommended to further strengthen the synergistic analysis and integrated management of multiple pollutants, deeply explore the behavioral mechanisms of emerging pollutants (such as microplastics, antibiotics, and rare earth elements) in the environment, and promote the development of coupled ecological–water quality modeling technologies to support intelligent watershed governance. At the same time, water ecological protection and ecological compensation strategies tailored to regional ecological differences should be formulated to improve the systematization and precision of water environment management.