Figure 2 demonstrated a wide geographical representation in the study, with contributions from 22 countries and regions. Nevertheless, research on this topic exhibits uneven distribution worldwide. Among coastal regions, China, the USA, India, and Russia exhibited the highest number of publications in this review, accounting for 12, 8, 8, and 3 articles, respectively. These countries contributed approximately 52.5% of the total studies. The underlying reason for this phenomenon could be attributed to the dominant role of power generation in these countries on a global scale, thereby amplifying the environmental challenges associated with power plant emissions. According to data released by the International Energy Agency (IEA) as of 2024, these four countries were ranked 1, 2, 3, and 4 in terms of power generation (IEA, 2024). Among the countries examined, the United States stood out as the first to focus on the impact of thermal discharge from power plants on marine benthic organisms (Kolehmainen et al., 1974; Burton et al., 1976). In relative terms, China’s research in this field commenced more than two decades later. However, it is noteworthy that China has increasingly devoted attention to this research direction in recent years (Cai et al., 2023; Qiao et al., 2023). Concurrently, research in India predominantly concentrated on assessing the effects of the Madras Atomic Power Station on marine life, including benthic organisms (Murugesan et al., 2011).

This study identified a scarcity of research addressing the impact of power plant discharge on marine benthic organisms within tropical regions, with current literature primarily limited to India (8), Brazil (3), Mexico (2), and Malaysia (1). Interestingly, all 14 articles identified within these regions exclusively examined the effects of thermal discharge. Conversely, there were limited studies investigating the impacts of cold discharge from power plants on marine benthic organisms in tropical areas. Furthermore, on a global scale, only two studies were located concerning the effects of cold discharge from power plants on marine life, with a primary emphasis on fish populations (Oshimi et al., 2021) and phytoplankton dynamics (Giraud et al., 2019). This notable absence of research on the effects of cold discharge from power plants, particularly Ocean Thermal Energy Conversion (OTEC) plants, underscored a significant gap in understanding the potential ecological impacts within tropical marine ecosystems.

Figure 3 presented a time series keyword map delineating the study of power plant impacts on coastal benthic organisms spanning the years 2014 to 2023. This visualization encompassed 197 high-frequency words interconnected by 622 lines. The density of the network map is 0.0322, indicating that 3.22% of the potential relationships in the network have been realized. This result underscores the limited interaction among those keywords. Notably, the modularity degree Q attained a value of 0.8055, surpassing the threshold of 0.3, thereby indicating a reasonable level of structural organization within the dataset. Furthermore, the silhouette score, registering at 0.957, exceeded the threshold of 0.4, thereby supporting the credibility and coherence of Figure 3 .

Figure 3 highlights high-frequency keywords (frequency > 7) over the past decade, with the size of each circle representing the co-occurrence frequency of the corresponding keyword. Notably, during this period, nine keywords — “assemblages”, “benthic community”, “benthic macrofauna”, “benthic foraminifera”, “benthic invertebrates”, “coral reefs”, “east coast”, “bay”, and “climate change”—were observed with high frequency. These keywords respectively highlight the most researched organism groups, biological indices, study sites, and environmental issues contributing to the increase in water temperatures.

The high frequency with which the term “assemblages”, “benthic community”, and “benthic invertebrates” appeared in the 2014 and 2015 keyword clouds indicates a substantial volume of research focused on the impact of power plant thermal emissions on the benthic community and assemblages of benthic invertebrates (Bryan et al., 2014; Cardoso-Mohedano et al., 2015).

The frequent use of the term “climate change” reflects its role in exacerbating the impact of thermal emissions from power plants on benthic organisms. This is due to the fact that the burning of fossil fuels in power plants is a significant contributor to climate change (Osman et al., 2023). Furthermore, climate change has accelerated ocean surface warming (Garcia-Soto et al., 2021; Oh et al., 2024; Dalpadado et al., 2024), thereby intensifying the effects of thermal emissions on benthic organisms (Farshchi et al., 2020; Wasti et al., 2022).

Between 2022 and 2023, the most prevalent keywords are “ benthic macrofauna “ and “coral reefs”. This trend underscored the growing importance of studying the influence exerted by coastal power plant wastewater on the composition and dynamics of macrobenthic and coral reef communities in the specified region (West et al., 2021; Qiao et al., 2023).

Of the 58 articles, 56 specifically highlighted temperature as a crucial factor in the observed alterations of ecological indicators among benthic organisms. These articles further conducted qualitative investigations into these phenomena. The most cited biodiversity parameters among those that suffered alterations were related to changes in the structure and composition of populations, communities, and ecosystems, namely: abundance (20.5%), community structure (13.94%), distribution (13.12%), species richness (9.84%), density (8.2%), and species evenness (6.56%), which together accounted for >70% of the total parameters affected ( Figure 4 ).

There is clear evidence that the abundance, community structure, and distribution of benthic organisms are influenced by thermal discharges. A meta-analysis of 75 studies conducted by Guimarães et al. (2023) demonstrated a significant increase in water temperature near nuclear power plants due to thermal emissions compared to reference areas, with a mean increase of 4.38°C (95% CI = 3.72-5.03). This increase in temperature can harm benthic communities, potentially leading to habitat loss, decreased biodiversity indices, or even species extinction. These findings are consistent with those of Farshchi et al. (2020), who studied macrobenthos near the outlet of the Neka power plant in the southern Caspian Sea. Farshchi et al. (2020) determined that thermal emissions from the power plant significantly affected macroinvertebrate abundance, species richness, species composition, and assemblage structure when comparing impacted and control stations. Although there is limited research on the impact of cold emissions on benthic community structure, studies examining seasonal variations have shown that lower ambient temperatures can lead to significant shifts in community structure (Li et al., 2020) and diversity (Bacouillard et al., 2020), and a marked decrease in biomass (Ying et al., 2020).

Temperature affects not only the ecological indices of benthic communities but also significantly influences the physiological activities of individual organisms (Deldicq et al., 2021). Vaquer-Sunyer and Duarte (2011) demonstrated that higher temperatures reduce the mean survival time of marine benthic organisms by more than 50% under anoxic conditions. Similarly, Kim et al. (2017) reported that proximity to thermal wastewater outlets results in decreased abundance and smaller sizes of Crassostrea gigas, alongside significantly elevated levels of heat shock proteins hsp70 and hsp90 mRNA (Kim et al., 2017). Conversely, excessively low temperatures have been shown to hinder the growth and development of benthic invertebrates’ eggs and larvae (Zeng et al., 2020; Everall et al., 2015), and even cause high mortality rates of benthic invertebrates (Colella et al., 2012; Thieltges et al., 2004). Furthermore, the metamorphosis of marine benthos is temperature-dependent, with colder water significantly prolonging the time to metamorphosis (Gangur and Marshall, 2020; Gall et al., 2021).

The effects of elevated temperatures linked to power plant discharge were frequently discerned through alterations in the composition of biological communities, facilitating the assessment and observation of ecological parameters about community composition and structure. These parameters included abundance, spatial distribution, dominance, density, and species richness (Farshchi et al., 2020).

Utilizing data extracted from 56 published articles investigating alterations in benthic organisms near power plant outfalls, it was noted that Mollusca garnered the highest citation frequency (35.71%), followed by Arthropoda (28.57%), and Annelida (20.24%). Collectively, these taxa accounted for over 84.52% of the total benthic community impacted by power plant effluents ( Figure 5 ).

The significant alteration of benthic community structure by power plant discharge could be attributed to the limited ability of sessile and slow-moving benthic organisms to migrate to less stressful environments, rendering them particularly vulnerable to acute temperature stress (Smith et al., 2023). Elevated temperatures were associated with mass mortality events in Gastropoda and Foraminifera (Arieli et al., 2011; Schiel et al., 2004; Titelboim et al., 2016). Additionally, these temperature increases could result in reduced egg production in scallops and crabs, posing challenges to recovering losses incurred through fishing activities (Caputi et al., 2019). Nevertheless, thermal discharge could create a conducive environment for thermal-tolerant species of Mytilopsis leucophaeata (Florin et al., 2013), Periwinkles, and Cthamalid barnacles (Suresh et al., 1993). Conversely, lower water temperatures influence benthic community dynamics. Heip and Craeymeersch (1995) investigated benthic organisms in the same marine region and observed that macrofaunal body weight, density, and diversity increased linearly as water temperatures decreased. In contrast, the distribution patterns and trends within the meiofauna differed significantly, which had a notable impact on the overall structure of the benthic community.

Considerable evidence attests to the significant elevation of temperature within thermal discharge zones in contrast to reference regions (Farshchi et al., 2020; Cai et al., 2023; Bozorgchenani et al., 2018). Guimarães et al. (2023), through a meticulous meta-analysis, demonstrated a substantial increase of 4.38°C in water temperature adjacent to nuclear power plant outlet. Furthermore, their findings revealed a correlation between temperature fluctuations and the geographical latitude of power plant installations (Guimarães et al., 2023). Nevertheless, consensus remained elusive regarding the precise impact of variables such as temperature differentials, ambient environmental conditions, and the volume of power plant effluent on benthic organisms. Our research utilized the Spearman correlation coefficient to examine the impact of several variables, including generation capacity, cooling water discharge, distance between discharge points and sampling locations, ambient water temperature, and temperature differential between influence and reference points, on variations in benthic abundance. Findings indicated that plant age, temperature difference (P < 0.05) and ambient temperature (P < 0.01) exhibited statistically significant effects, displaying a negative correlation ( Figure 6 ). Notably, the analysis revealed no statistically significant associations between changes in benthic abundance and factors such as generation capacity, cooling water discharge, or the distance between discharge points and sampling points.

This study revealed a direct correlation between the magnitude of the temperature variance between the impacted and control sites and the extent of benthic abundance decline. Indeed, as far back as 1984, GESAMP emphasized that to uphold the ecological integrity of marine environments, temperature differentials near power plants should not have surpassed 7°C in subtropical waters and 5°C in tropical waters (GESAMP, 1984). Arai et al. (2015) demonstrated that the composition of benthic communities was substantially influenced by temperature disparities.

Benthos had a limited tolerance for high temperatures, with a maximum water temperature range of approximately 35-45.8°C (Saraswat et al., 2011; Hamilton and Gosselin, 2020). Different species had different temperature preferences and tolerances, and changes in water temperature affected their growth, reproduction, and survival (Jones et al., 2010; Cardoso-Mohedano et al., 2015). In tropical regions where surface sea temperatures frequently exceed 30°C, even a temperature increase of less than 5°C also adversely affect certain organisms, pushing them beyond their thermal tolerance limits (Kimmerer and Weaver, 2013; Farshchi et al., 2020). Yang et al. (2020) investigated the effects of various environmental factors on benthic organisms and found that temperature variation is the most critical factor influencing changes in benthic abundance and tolerance characteristics. Specifically, benthic abundance was lower in summer compared to spring, when decreased temperatures led to an increase in abundance.

From Figure 6 , there is a significant negative correlation between benthic abundance and power plant operational time (P < 0.05). This indicates that as the operational time of the power plants increases, benthic abundance decreases. In the initial years of operation, thermal discharges from power plants can significantly alter the community structure and abundance of benthic organisms; however, these changes typically stabilize over time. The Daya Bay Nuclear Power Station has been operational since 1994, and Wang et al. (2008) conducted an extensive assessment of ecological indicators in Daya Bay over a 22-year period (1982-2004). Their findings revealed a marked decline in both the average biomass and species richness of benthic animals near the nuclear power plant, with biomass decreasing from 317.9 g/m² in 1991 to 45.24 g/m² in 2004, and species richness declining from 250 species in 1991 to 177 species in 2004. However, field investigations conducted in 2021 and 2023 indicated that thermal discharges from the power plant had minimal impact on larger benthic organisms in the surrounding marine environment (Rao et al., 2021; Cai et al., 2023). This phenomenon may be attributed to the substantial influence of domestic and industrial sewage discharge, as well as mariculture activities, on the ecological integrity of Daya Bay, alongside the gradual stabilization of community structure and abundance trends over time. Consequently, no significant differences were observed in the composition and functional diversity of the macrobenthic community between the thermal discharge area and the control area (Rao et al., 2021).

Similarly, the Madras Atomic Power Station, which began full operations in 1984, has been the subject of seven studies examining the effects of thermal emissions on benthic communities from 1992 to 2010. Research conducted in 1992 and 1993 documented a 25% reduction in species richness at the outlet, with significant seasonal variations. During this period, ambient temperatures ranged from 37.0 to 37.6°C, resulting in near-total mortality of macrobenthos, with the exceptions of periwinkles and Chthamalid barnacles (Ahamed et al., 1992; Sasikumar et al., 1993; Suresh et al., 1993). Subsequently, the community structure evolved steadily, leading to a focus on the thermal tolerance and physiological characteristics of individual species. From 1996 to 2003, studies primarily investigated the thermal tolerance and physiological responses of copepods (Suresh et al., 1996), bivalves (Masilamoni et al., 2002), and oysters (Rajagopal et al., 2003) in proximity to the emission outlet. Additionally, Hussain et al. (2010) reported that the population of Donax cuneatus was notably rare within 100 meters of the mixing zone, while the control group exhibited a density of 64.0 ± 3.6 to 88.3 ± 9.6 ind/m².

In the realm of Ocean Thermal Energy Conversion (OTEC), where the process involved the uptake of deep, nutrient-rich seawater, a pertinent phenomenon emerges wherein the disparity in density between the discharged plume and its ambient surroundings triggered a tendency for the plume to either ascend or descend to an equilibrium depth. This intricate interplay gave rise to an artificially induced zone enriched with nutrients, as observed in the study by Comfort and Vega (2011). When this plume equilibrium occurs within the zone of light penetration, there is the potential to stimulate phytoplankton and algal blooms (Giraud et al., 2019).

Simultaneously, the discharge of thermal effluent from power plants introduced a marked temperature elevation in proximity to the outfall. This not only reduces water viscosity and increases water vapor pressure, directly influencing the dissolved oxygen-carrying capacity of seawater, as suggested by Ali et al. (2020), but also indirectly influenced the dissolved oxygen levels by affecting microbial activity due to the temperature elevation. Breitburg et al. (2018) observed that rising oceanic temperature, coupled with heightened nutrient discharge into coastal waters, synergistically accelerated microbial respiration-driven oxygen consumption. This process reduced oxygen solubility in water, and prolonged the replenishment duration of atmospheric oxygen to the aquatic environment.

Nevertheless, oxygen was vital for benthic organisms to conduct metabolic processes and sustain their physiological functions. Persistent exposure to environments with low dissolved oxygen content posed significant constraints on the life activities and behaviors of benthic organisms (Chan et al., 2008). Many benthic species could not thrive in oxygen-depleted waters, and those that managed to survive have experienced diminished growth rates, reproductive failures, and heightened vulnerability to diseases (Hyvärinen et al., 2022; Briggs et al., 2021). Furthermore, the decline in species diversity resulting from diminishing oxygen levels could have triggered cascading impacts on marine ecosystems, leading to the loss of food sources and disruptions in nutrient cycling (Sun et al., 2022). Singh et al. (2021) documented a positive correlation between the diversity index of Hanzawaia concentrica, Globocassidulina subglobosa, and Cancris sagra and dissolved oxygen levels in aquatic environments, underscoring the crucial role of dissolved oxygen in sustaining the survival and diversity of benthic communities (Singh et al., 2021).

According to Henry’s law, the solubility of gases in water decreases as temperature rises, so higher temperatures reduce the solubility of carbon dioxide. In regions with warm ambient water, thermal discharge from power plants can increase pH levels (Jebure and Meshjel, 2019). However, in other areas, thermal discharge may lower seawater pH ( Table 2 ). The impact of power plant discharges on water bodies is influenced by both the discharge volume and the temperature difference from ambient water. Larger discharge volumes and higher temperature differences can significantly elevate local water temperatures, affecting wider areas of the water body (Raptis et al., 2016; Issakhov and Zhandaulet, 2021). In semi-enclosed bodies, like estuaries and bays with limited water exchange, thermal discharges can impact the entire system, as poor circulation exacerbates temperature retention and affects local ecosystems more extensively (Raptis et al., 2016). Similarly, cold discharges with large volumes and high temperature differences have greater effects, determining the extent of cooling and the area impacted (Issakhov and Zhandaulet, 2021).

Changes in pH can alter the chemical properties of the water and directly affect the physiology, behavior, and distribution of marine organisms ( Table 3 ). Kroeker et al. (2013) comprehensively analyzed the results of 228 studies examining the biological effects of ocean acidification. Their results suggest that acidification leads to decreased survival, calcification, growth, reproduction and development in a wide range of marine organisms, as well as significant character-mediated variation between taxa and enhanced sensitivity to early life history stages. In addition, their results indicate that mollusks are significantly sensitive to acidification, especially during the larval stage, and that vulnerability to acidification increases with simultaneous exposure of multiple organisms to elevated sea temperature.

pH levels can impact the availability of dissolved minerals and nutrients that are essential for the growth and reproduction of benthic organisms. Additionally, high or low pH levels can directly affect the acidity or alkalinity of the water, which can be harmful to benthic organisms if it exceeds their tolerance range (Fabricius et al., 2014).

The pH tolerance range of benthic organisms has been extensively studied, with research defining specific thresholds that delineate conditions for optimal health and survival across various taxa (Feugere et al., 2021; Dong et al., 2020). Benthic macroinvertebrates, particularly sensitive to pH fluctuations, generally experience adverse effects when pH levels fall below 5 or exceed 9 (Yuan, 2004). The pH of a fluid is positively correlated with the concentration of carbonate, bicarbonate, and other related salts in the aquatic environment. Calcifying invertebrates, such as corals, gastropods, and bivalves, rely on calcium carbonate for structural formation. Ocean acidification, by reducing the availability of carbonate ions necessary for calcification, weakens these structures, often increasing mortality rates (Hoegh-Guldberg et al., 2017; Beesley et al., 2008; Vargas et al., 2015). When the pH value of seawater decreases below 7, it can have severe impacts on marine organisms such as shrimp, snails, and bivalves with calcium carbonate shells, which experience difficulties in surviving. Additionally, if the pH value drops below 6.5, the benthic population will degrade, and their reproductive capacity will be significantly reduced (Busch and McElhany, 2016). Dong et al. (2020) investigated the growth, development, and community structure changes of various types of foraminifera (hyaline, porcelaneous, and agglutinated) in different pH environments. The study revealed that the species richness and individual growth of hyaline and porcelaneous foraminifera were positively correlated with pH, while the agglutinated foraminifera exhibited a negative correlation (Dong et al., 2020).

In a key experimental study, Kroeker et al. (2011) specifically investigated pH tolerance among invertebrates, conducting a controlled trial on Castello Aragonese d’Ischia, a small island, in May and September. By releasing carbon dioxide at a depth of 0.5–3 meters around the site, the researchers adjusted pH levels and assessed species abundances near the vents, sampling over 15,000 individuals across 82 taxonomic families. This study quantified suitable pH ranges for different invertebrate taxa, finding that a pH of 8.1 ± 0.1 was optimal for 4% of gastropods, 22% of decapods, 10% of amphipods, 29% of tanaids, 38% of isopods, 47% of polychaetes, and 75% of sipunculids. For 26% of gastropods, 30% of bivalves, 39% of decapods, 7% of amphipods, and 38% of isopods, a broader pH range of 7.8 ± 0.3 to 8.1 ± 0.1 was suitable, while amphipods, 29% of tanaids, 13% of isopods, and 7% of polychaetes tolerated a range of 6.6 ± 0.5 to 8.1 ± 0.1.

From the above, the impact of pH on the benthic community is contingent upon several factors, including the extent of pH fluctuations, the type of pre-existing benthic community, and the ecological backdrop of the ecosystem.

The process of piping large quantities of cooling water from power plants into the ocean contributes to the transport of coastal sediment (Venugopalan et al., 2011; Wither et al., 2012). Bozorgchenani et al. (2018) conducted a parallel investigation, which revealed a prevailing dominance of clay and silt in the sediment composition at both discharge outlets of power plants. This observation was derived from year-round monitoring of sediment particle sizes. In contrast, the control group exhibited a prevailing dominance of sand. Furthermore, Bozorgchenani et al. (2018) noted a marked elevation in the total organic matter (TOM) content at the discharge point in comparison to the control point. In tandem, studies have substantiated that the elevated temperatures at the thermal discharge points played a pivotal role in fostering an increase in organic matter content. Interestingly, in certain regions, there was a notable increase in organic matter content during summer compared to winter. This phenomenon is attributed to the augmented primary production resulting from higher temperatures (Cheng et al., 2004; Sarkar et al., 2005). The impact of power plant discharge extended significantly to sediment composition.