Atmospheric Deposition of Multi-Class Substances into the Ocean: Synthesis of Fluxes, Seasonal Spatial Patterns and Ecological Risks

Abstract

Atmospheric deposition is increasingly recognized as a significant pathway transporting diverse substances from land to the ocean. However, significant uncertainties persist regarding the magnitude, spatial variability, and ecological implications of these inputs into the ocean. This study compiles and standardizes observational datasets from published sources to provide a cross-substance synthesis of atmospheric concentrations, deposition fluxes, seasonal patterns, and ecological risks. The analysis covers major substance categories, including nutrients, trace metals, microplastics, POPs and other emerging pollutants. The novelty of this work lies in its cross-pollutant approach and the integration of seasonal dynamics, particularly for winter deposition. Global results show widespread deposition across the world’s oceans, with consistently elevated concentrations in densely populated coastal regions and detectable levels even in remote areas, underscoring the role of long-range transport. Our analysis reveals pronounced winter peaks in regions like the Bohai Sea and the Baltic Sea, highlighting a critical but often overlooked seasonal window. Strong nearshore-to-offshore gradients across most substances indicate dominant influences from coastal anthropogenic emissions. Ecological risk assessment using the Risk Quotient method suggests that risks are generally low but spatially heterogeneous, with hotspots in regions of intensive human activity. Overall, this synthesis highlights the importance of atmospheric pathways in shaping marine substance distributions and emphasizes the need for improved monitoring and modeling to better quantify episodic deposition processes under future environmental change.

1. Introduction

Marine pollution has emerged as a pervasive environmental challenge, with diverse anthropogenic contaminants now detected from heavily impacted coastal margins to the most remote ocean basins [1,2]. Growing evidence has demonstrated that atmospheric inputs can substantially modify seawater chemistry, alter biogeochemical cycles, and influence ecosystem structure and function [3,4]. Although the number of observations has increased, significant uncertainties remain regarding the amount of material the atmosphere delivers to the ocean, the relative contribution of different deposition pathways, and their impacts on marine ecosystems [5]. Previous reviews on atmospheric deposition have primarily focused on individual substance types [6]. While some studies, such as those by [7], have expanded the scope to include multiple substances, few have integrated seasonal analyses or assessed the ecological risks associated with atmospheric deposition. As a result, significant gaps remain in our understanding of the magnitude, variability, and ecological consequences of atmospheric deposition to the ocean, particularly at regional and global scales.

Atmospheric deposition represents a major vector linking terrestrial emissions with ocean biogeochemistry [8]. Substances are transferred to the ocean surface via dry deposition, wet deposition, and aerosol settling, which differ in their physicochemical mechanisms yet jointly regulate the air–sea exchange [9]. Dry deposition drives continuous gravitational settling and gas absorption, whereas wet deposition delivers substances episodically through precipitation scavenging, a process that efficiently removes airborne particles and gases from the atmosphere during rain and snow events. Together, these pathways introduce both micronutrients, such as N, P, Si and Fe that can stimulate primary production [10], and harmful contaminants including microplastics, Hg and POPs [11]. For example, atmospheric N deposition can contribute more than 50% of dissolved inorganic nitrogen (DIN) in marginal seas such as the Bohai Sea [12], while large-scale dust transport supplies bioavailable iron to regions with high levels of nutrients and low quantities of chlorophyll [13]. Conversely, mercury deposition has been shown to induce marked bioaccumulation in estuarine fish species [14], underscoring the contrasting ecological roles of atmospheric inputs, which range from essential nutrient supply to toxic contamination. These atmospheric pathways determine the spatial distribution and seasonal variability of substances in marine environments.

However, current knowledge of atmospheric deposition remains fragmented. Most studies have focused on specific substances or regions, and as a result, the global perspective is fragmented. This has led to uncertainties when it comes to quantifying how much material the atmosphere delivers to the ocean and how this varies by region, season, and substance type. Additionally, many studies have not adequately addressed the seasonal dynamics of deposition. The impact of winter deposition, in particular, has often been overlooked, despite evidence that it can contribute substantially to pollutant loading in ice-covered seas [15]. Furthermore, the ecological consequences of these atmospheric inputs are still poorly understood; there is only limited research focused on integrating the impacts of various pollutants and analyzing their effects on marine ecosystems. Seasonal variations and ecological implications after deposition remain underexplored, making it difficult to accurately assess the risks atmospheric substances pose to marine life.

To address these issues, this study developed a unified framework to quantify the contribution of atmospheric deposition to surface–ocean contamination. The objectives of this study were: (1) to establish a harmonized database that consolidates fragmented observations across major substance categories; (2) to evaluate the effectiveness of consistent normalization procedures in quantifying fluxes and concentrations; (3) to characterize seasonal and spatial variability of the concentrations and deposition fluxes of various substances; and (4) to evaluate ecological risks associated with short-term atmospheric inputs. By integrating diverse substance categories, this work would provide the first cross-pollutant synthesis of atmospheric deposition to the ocean. This synthesis will provide a quantitative basis for constraining regional atmospheric inputs and for evaluating their implications for ecosystem functioning in data limited marine environments.

2. Materials and Methods

2.1. Literature Compilation

A bibliometric analysis of atmospheric deposition studies was conducted by searching the Web of Science (WOS) core database, with “atmospheric deposition” as the key word. The workflow of the literature screening process is described below. Initially, a total of 4749 relevant documents were searched, and another 77 relevant studies were collected from the reference lists of these documents. Of the 4826 documents, 3035 were excluded, as a review of their abstracts revealed that they were irrelevant to the research topic. As for the remaining 1791 documents, another 1532 were excluded as they did not contain data that could be used for the analysis in this study. From the 259 documents retained for detailed examination, the data from 144 documents could not be unified in units, or the magnitudes were inconsistent, so they were also excluded. For example, one study reported nitrogen deposition in g·kg⁻¹ (as a concentration in collected dust) but failed to report the total mass of dust collected over the sampling period or the surface area of the sampler. Because the essential metadata for converting concentration into a standard flux (g·d⁻¹·km⁻²) was missing, this document could not be utilized for quantitative comparison. Finally, usable data for analysis were collected from the remaining 115 documents [10,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128]. Comprehensive details of the 115 included studies are provided in the Supplementary Materials S1 to ensure data transparency.

Atmospheric deposition includes two primary pathways: dry deposition, involving the continuous settling of particles and gas absorption, and wet deposition, characterized by the episodic removal of substances through precipitation. Unless otherwise specified, the results presented in the following sections represent total atmospheric deposition, encompassing both dry and wet components without further differentiation.

2.2. Data Standardization and Flux-to-Concentration Conversion Methods

Atmospheric deposition flux and concentration are the basic parameters used to describe atmospheric deposition characteristics. Atmospheric deposition flux refers to the mass of particulate matter falling on a unit area per unit time. The units of measurement used are tons per square kilometer per day (t·km⁻²·d⁻¹), grams per square meter per day (g·m⁻²·d⁻¹), kilograms per square meter per year (kg·m⁻²·year⁻¹), etc. Atmospheric concentration refers to the mass of particles per unit volume. The unit of measurements used are grams per liter (g·L⁻¹), kilograms per cubic meter (kg·m⁻³), etc. In order to better present the distribution of various substances, the unit of each substance was standardized during data collection and chart creation. Based on the relative molecular mass of the substances and the collected literature data, the atmospheric concentration of nutrients was unified to the unit of nmol·m⁻³, which was used in most of the studies we referenced. The concentration of mercury was standardized to ng·m⁻³. The unit for Fe flux was standardized to nmol·m⁻²·d⁻¹. The abundance of microplastics was standardized to item·m⁻³. The concentration of persistent organic pollutants was standardized to ng·m⁻³. While collecting the data, the latitude and longitude of the data collection sites were also recorded to accurately locate them on the global substance distribution map.

Because the collection of flux data was based on atmospheric values, the substance deposition fluxes were converted to the resultant increase in water-column concentration over a short time period—that is, the Predicted Environmental Concentration (PEC)—to substitute for the actual concentration (MEC). Accordingly, the following equation was used for the conversion:

$$PEC={\displaystyle \frac{F\times t}{h\times \rho }}$$

(1)

F is the deposition flux (g·m⁻²·d⁻¹); t is the deposition duration (d); h is the assumed water depth into which the deposited pollutant is mixed (m); and ρ is the seawater density.

2.3. Seasonal Analysis

The atmospheric deposition of several nutrients and metals in different seasons is estimated using the following equations [41,129]:

$$D_d=(F_d+F_w)\times A\times D$$

(2)

$$F_d=V_d\times C$$

(3)

$$F_w=R_w\times C\times P$$

(4)

where D_d is the total deposition (kg), A is the deposition area (m²), and D is the number of deposition days (d). F_d is the dry deposition flux (g·m⁻²·d⁻¹), V_d is the deposition rate of atmospheric particles simulated using the Williams model (m·s⁻¹), and C is the atmospheric concentration (g·m⁻³). F_w is the wet deposition flux (g·m⁻²·d⁻¹), R_w is the scavenging ratio, and P is the precipitation (m·d⁻¹). For regional application, the Bohai Sea and the Baltic Sea were selected as representative marine systems, with deposition fluxes for nutrients and selected metals extracted from studies. The Bohai Sea covers about 77,000 square kilometers and the Baltic Sea covers an area of 377,000 square kilometers. These two areas were utilized as case studies for the calculation of seasonal fluxes, assuming a duration of 90 days per season. The detailed datasets of these areas are provided in Tables S1 and S2 of Supplementary Materials S2.

2.4. Risk Assessment

The Risk Quotient (RQ) method [130] is used to evaluate the potential ecological risk associated with substance exposure. This method assesses ecological hazards by comparing the measured environmental concentration (MEC) of a substance with its Predicted No-Effect Concentration (PNEC). Representative organisms from three trophic levels—algae, invertebrates (e.g., Daphnia), and vertebrates (fish)—are commonly used to derive toxicity thresholds. The highest risk level among the three nutritional levels was selected for the risk assessment. The underlying principle is that the ratio between MEC and PNEC reflects the likelihood of adverse ecological effects.

(1) Single-Factor Risk Quotient (RQ)

$$RQ={\displaystyle \frac{MEC}{PNEC}}$$

(5)

where MEC is the actual detected concentration of substances in the environment (g·m⁻³), typically using the maximum value or a conservative estimate (half the detection limit) when undetected. In this study, the concentration values are those standardized to uniform units. PNEC is the Predicted No-Effect Concentration, which represents the concentration that will not cause harmful effects to organisms under long-term exposure (g·m⁻³). It is typically derived by dividing toxicity data (such as EC50, LC50, NOEC, etc.) by an assessment factor (AF). The formula is as follows:

$$PNEC={\displaystyle \frac{{EC}_{50} or {LC}_{50 }or NOEC}{AF}}$$

(6)

The assessment factor (AF) follows standard practice, with AF = 100 for chronic toxicity data and AF = 1000 for acute data.

(2) Mixed Risk Quotient (MRQ)

The Mixed Risk Quotient (MRQ) is used to evaluate the combined ecological risk posed by multiple co-occurring substances [131]. The calculation equation is as follows:

$$MRQ={\sum }_{i=1}^n{\displaystyle \frac{{MEC}_i}{{PNEC}_i}}$$

(7)

where MEC_i and PNEC_i correspond to the environmental concentration and no-effect concentration of substance i, respectively. Risk levels are interpreted based on RQ values as follows: RQ < 0.01, no risk; RQ < 0.1, low risk; 0.1 ≤ RQ < 1, moderate risk; RQ ≥ 1, high risk.

In this study, PNEC values are obtained from the literature and used to calculate the RQ and MRQ. PNEC_Hg = 0.39 μg·L⁻¹, PNEC_NO3 = 1 mg·L⁻¹, PNEC_PO4 = 0.1 mg·L⁻¹, PNEC_NH4 = 0.5 mg·L⁻¹, PNEC_MP = 12,000 particls·m⁻³, PNEC_POPs = 1,060,000 ng·m⁻³ [132,133,134].

The figures in the article were all created using Origin 2024.

3. Results

3.1. Overview of Atmospheric Substance Categories and Monitoring Status

Atmospheric deposition could transport a wide spectrum of substances to marine environments. In this synthesis, these substances were categorized into nutrients (e.g., nitrogen, phosphorus, silicon), organic pollutants (e.g., polycyclic aromatic hydrocarbons, organophosphate esters, organochlorine pesticides), heavy metals (e.g., mercury, iron), and microplastics (in Figure 1). Across both coastal and open-ocean regions, long-term observational datasets and direct measurement techniques were far more common than short-term campaigns or model-based estimates. Existing studies have covered a wide range of spatial and temporal scales, ranging from nearshore estuaries to open-ocean basins, and from single-event measurements to multi-year monitoring.

3.2. Deposition Flux of Substances

A global distribution map of nitrate, ammonium, and phosphate concentrations was created using atmospheric data from the literature (Figure 2A,B). The nitrate concentration range was 3.85–244 nmol·m⁻³, the ammonium was 0.38–339 nmol·m⁻³, and the phosphate was 0.027–2.52 nmol·m⁻³. These nutrients were reported in various marine regions, including China’s coastal waters (Bohai Sea, Yellow Sea, East China Sea, South China Sea), the U.S. coasts, the Northwest Pacific, the Indian Ocean, the Atlantic Ocean, the Arabian Sea, and the Red Sea. China’s coastal areas generally have higher concentrations than other regions, with elevated ammonium and nitrate levels in North America’s densely populated areas.

Figure 2C,D show the atmospheric deposition fluxes of iron (Fe) and concentration of mercury (Hg), respectively. The iron deposition flux ranged from 2 to 690 nmol·m⁻²·d⁻¹, with high-flux regions primarily observed in the North Atlantic, the Yellow Sea of China and the Southern Indian Ocean. The atmospheric mercury concentration ranged from 0.07 to 29.3 ng·m⁻³. Atmospheric Hg concentrations were observed across coastal China and the Indian, Atlantic, and Pacific Oceans, with notably high levels concentrated in the Bohai Sea of China and the Baltic Sea.

The abundance of microplastics ranged from 0.39 to 933 items·100 m⁻³. Concentrations have been reported in coastal China, the western Pacific, the U.S. coasts, European seas, and near Antarctica, indicating a global distribution (Figure 2E). Atmospheric concentrations of POPs, including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs), have been observed in various ship-based and coastal atmospheric sampling stations (Figure 2F). The POPs concentrations ranged from 0.005 to 117 ng·m⁻³, with high levels observed along the coasts of western Africa, the Europe and the United States. Variations in study designs and target analytes resulted in inter-site variability, with some studies focusing on pesticides, while others included PAHs and PCBs. POPs have been detected even in remote regions like the Arctic.

3.3. Seasonal and Spatial Variation in Atmospheric Deposition Substances

3.3.1. Seasonal Variation

To investigate the seasonal variability of atmospheric deposition, available data were compiled to generate a seasonal distribution map of substances. As shown in Figure 3A, B, atmospheric concentrations and deposition fluxes of ammonium, Cu, Pb, Zn, Cd, and As were elevated in winter compared to other seasons. Quantitative estimates for the Bohai Sea and Baltic Sea further illustrated this winter pattern (Figure 4). In the Bohai Sea, winter deposition of ammonium reached 1.6 × 10⁷ kg approximately, while the deposition of heavy metals included around 3.3 × 10⁵ kg of Cu, 4.6 × 10⁵ kg of Pb, 1.4 × 10⁶ kg of Zn, 7.1 × 10³ kg of Cd and 1.5 × 10⁵ kg of As. In the Baltic Sea, winter nitrogen deposition was estimated at approximately 1 × 10⁷ kg, with heavy metals deposition amounts of about 1.5 × 10⁵ kg for Cu, 1.5 × 10⁴ kg for Ni, 2.5 × 10⁵ kg for Mn, and 3 × 10⁴ kg for V.

3.3.2. Spatial Patterns of Substance Concentrations Along the Shore-to-Offshore Gradient

Spatial variations in atmospheric deposition-derived substances showed a clear nearshore–offshore gradient (Figure 5). Concentrations of PO₄³⁻ and NH₄⁺ decreased significantly with distance from the coast (both p < 0.0001), indicating stronger coastal influences from anthropogenic sources. While PO₄³⁻ concentrations at offshore stations often approach zero, the fitted function provides a quantitative description of the land-to-ocean transition and the spatial extent of coastal nutrient influence. Trace metals, such as Fe, also declined steeply offshore (p < 0.0001), supporting land-based contributions like industrial emissions and dust. Microplastics and POPs followed a similar pattern, with concentrations decreasing offshore (p < 0.0001 for microplastics, p = 0.0006 for POPs), reflecting terrestrial origins and coastal emissions. The spatial distribution of Hg was also examined; however, as the fitting was not statistically significant (p > 0.05), the plot is provided in Figure S1 (Supplementary Materials S2) for reference. Overall, except for Hg, all substances exhibited significant decreases with distance, highlighting the dominant role of coastal human activities in atmospheric substance distribution.

3.4. Ecological Implications

RQ values were calculated for each substance to characterize the global spatial pattern of ecological risk (Figure 6). Here, the quotient reflects the incremental risk contributed by one day of deposition (t = 1 day), and a mixed-layer depth of 10 m was assumed (h = 10 m). The RQ value of POPs was calculated using the MRQ method. The RQ values for microplastics ranged from 2.74 × 10⁻⁶ to 6.60 × 10⁻³. The RQ values for NO₃ ranged from 9.56 × 10⁻⁵ to 6.06 × 10⁻³. The RQ values for NH₄ ranged from 1.96 × 10⁻⁶ to 1.75 × 10⁻³. The RQ values for PO₄ ranged from 9.37 × 10⁻⁶ to 8.74 × 10⁻⁴. The RQ values for Hg ranged from 7.51 × 10⁻⁶ to 3.17 × 10⁻³. The RQ values for POPs ranged from 3.98 × 10⁻⁸ to 9.30 × 10⁻⁴. The calculated RQ values for all studied components remained below 0.01, indicating a negligible ecological risk based on current atmospheric deposition levels. Since these RQ values were relatively small, a uniform scaling factor of 600 was used during visualization to allow all substances to be displayed within a single plot. The global distribution of atmospheric RQ values exhibited strong spatial heterogeneity and revealed several hotspot regions, particularly along the coasts of East and South Asia, Europe, and the Northwest Atlantic. These patterns reflected the combined influence of population density, urbanization, industrial emissions, and atmospheric transport.

4. Discussion

4.1. Atmospheric Deposition Relative to Other Substance Input Pathways

Compared with river discharge, coastal runoff, and wastewater effluents, atmospheric deposition constitutes a diffuse, non-point source of pollution that is far more difficult to quantify or control [135]. However, as a key pathway linking the atmosphere and the ocean, atmospheric deposition delivers substantial quantities of substances to marine ecosystems. Studies have shown that, in nearshore regions, the total substance load delivered by atmospheric deposition can be considerable, sometimes even comparable to direct terrestrial inputs. In China, for example, the annual land–sea transport of nitrogen is estimated at 8 Tg, with atmospheric deposition accounting for over one-third (~3 Tg) of this total [136]. The Bohai Sea receives the highest atmospheric nitrogen input (~70 kg·ha⁻¹), while the Yellow Sea, East China Sea, and South China Sea receive 8–16 kg·ha⁻¹ [137]. In Europe, the deposition fluxes of N in the Western Mediterranean region are 0.92 Tg·year⁻¹, while in the Eastern Mediterranean region they are 1.10 Tg·year⁻¹, and in the Black Sea region they are 0.36 Tg(N)·year⁻¹ [59]. A study estimated the global annual emissions of MP atmosphere to be approximately 16.1 Tg by combining observational data from the western United States with modeling methods [138]. In the Russian Arctic Ocean, the monthly input of PAHs into the atmosphere was estimated at 1040 tons. The monthly input of mercury into the atmosphere was approximately 530 tons [139].

In the open ocean, where riverine and land-based anthropogenic point sources are essentially absent, apart from the pollution caused by ships, atmospheric deposition becomes a major external pathway supplying both nutrients and contaminants [10,140]. For trace and micro-pollutants such as Hg, POPs, and airborne microplastics, atmospheric inputs frequently represent the primary or even prevailing source to marine ecosystems. In this study, the short-term concentration of Hg deposited into the open ocean via atmospheric deposition was estimated at 0.29 ng·L⁻¹, closely matching the baseline concentration of 0.3 ng·L⁻¹ reported for uncontaminated seawater by the U.S. Toxicological Profile for Mercury. POPs have also been detected in the Arctic, despite the absence of direct terrestrial sources. The occurrence of microplastics in Antarctica further supports atmospheric transport as an important pathway for these particles [141]. These diverse atmospheric inputs operate across spatial scales determined by particle size, chemical reactivity, and atmospheric residence time. As a result, even open-ocean regions far from emission sources are no longer isolated from human influence.

4.2. Transport Distances of Atmospheric Deposition Across Substance Categories

Spatially, all substances except Hg exhibited decreasing concentrations with increasing distance from the shore. This nearshore-to-offshore gradient is consistent with earlier studies [142,143] and reflects the combined influence of terrestrial emissions, air mass movement, and coastal meteorological processes such as sea fog, which collectively enhanced deposition near the coastline. Despite this shared spatial pattern, the magnitude of the gradient and the underlying atmospheric processes differed substantially among nutrients, trace metals, microplastics and POPs, primarily due to differences in physicochemical properties and atmospheric residence times.

Reactive nitrogen in the atmosphere is transported primarily as nitrate (NO₃⁻) and ammonium (NH₄⁺). Their relatively short atmospheric lifetimes (from hours to several days) restrict their transport to regional scales, producing strong land–sea concentration gradients [144]. Wet deposition dominates in monsoon-affected regions, efficiently removing N species near their source regions. Phosphorus is often associated with coarse particles, and its atmospheric residence time is also short (from days to two weeks), reinforcing its steep nearshore decline [145]. Microplastics’ low density and small particle size facilitate uplift and atmospheric transport across regional and even intercontinental scales [146]. Microplastics are removed from the atmosphere through both dry deposition and precipitation scavenging, explaining their widespread detection across coastal seas, open-ocean environments, and even polar regions [38]. Once deposited into the ocean, they are subject to vertical transport and can accumulate from the surface waters to the benthic sediments [147], contributing to microplastic pollution that is both widespread and persistent in the marine environment.

Mercury differs fundamentally from other metals. Mercury enters the atmosphere from both natural and anthropogenic sources, predominantly as gaseous elemental mercury (GEM), which constitutes over 95% of atmospheric Hg [97]. GEM’s low solubility and long atmospheric lifetime enable its long-range transport. Through oxidation, GEM converts to reactive gaseous mercury (GOM), which subsequently deposits via wet and dry processes. These atmospheric transformation pathways may explain Hg’s relatively uniform spatial distribution compared with other substances.

4.3. Ecological Consequences in the Ocean

Overall, the ecological risks associated with atmospheric deposition were negligible, with most Risk Quotient (RQ) values remaining below 0.01—substantially lower than those reported for aquatic water columns or sediments. Similar low-risk patterns have been documented for Hg, OCPs, and PAHs in surface waters across different regions [132,148,149]. It should be noted that these risk estimates are sensitive to the assumed mixing depth (10 m). While a 10 m depth is representative of the well-mixed surface layer in the ocean, a shallower mixing depth would linearly increase the predicted concentrations. Furthermore, the 10 m model is a simplified first-order screening tool that does not explicitly account for complex oceanic processes such as advection, stratification, and biological uptake. However, by ignoring these removal and dilution pathways, our approach provides a highly conservative estimate of ecological risk. Even with a significantly reduced mixing volume or under these worst-case scenario assumptions, the RQ values remain well below the threshold of concern (0.1), further confirming the robustness of the negligible risk conclusion. Nevertheless, elevated RQs were consistently observed in densely populated coastal regions, including southeastern China, Japan, Southeast Asia, eastern North America, and Europe. These spatial hotspots coincided with areas of intense anthropogenic emissions and enhanced atmospheric inputs, indicating that while risks remain low at a global scale, localized ecological pressure may still occur in human-impacted coastal systems.

Seasonal processes, particularly winter deposition, may generate short-term ecological stress that is not fully captured by annual-average risk metrics. In seasonally ice-covered seas such as the Bohai Sea, atmospheric substances accumulate on the surface of the ice during winter and are rapidly released into the water column during melt periods, producing abrupt concentration increases over short timescales [150]. Reported post-melt concentrations of dissolved inorganic nitrogen and phosphorus, as well as trace metals, reached levels comparable to or exceeding background seawater values [151,152]. These episodic inputs may temporarily elevate nutrient availability or metal exposure, potentially intensifying ecological pressure during sensitive seasonal windows, even when long-term risks remain low.

Beyond short-term pulses, sustained atmospheric deposition may exert meaningful ecological impacts through long-term accumulation and chronic exposure. Continuous inputs of nitrogen and phosphorus can gradually alter nutrient stoichiometry, reinforce eutrophication tendencies, and promote shifts toward bloom-forming or opportunistic phytoplankton taxa in coastal waters [153,154]. For trace metals, atmospheric Hg poses risk mainly through post-depositional transformation and bioaccumulation rather than instantaneous concentrations, leading to long-term ecological and human health concerns [155,156]. Similarly, microplastics and POPs may induce cumulative biological stress through persistent exposure, trophic transfer, and biomagnification, affecting organism health, the food-web structure, and ecosystem functioning [157,158,159,160,161]. These long-term processes highlight that low RQ values based on short-term exposure do not preclude significant ecological consequences over extended timescales.

Beyond traditional pollutants, emerging airborne contaminants, including pharmaceuticals, personal care products, and PFAS, have been detected in atmospheric depositions with increasing frequency. These compounds exhibit persistence, bioaccumulation potential, and diverse toxicological effects, even at low concentrations [162]. Their ecological implications may include altered microbial processes, impaired organismal health, and risks to humans via food-web transfer. Current monitoring remains limited, highlighting substantial knowledge gaps regarding their atmospheric transport and marine impacts.

4.4. Advantages, Limitations and Knowledge Gaps

This study synthesizes atmospheric deposition data across marginal seas and ocean basins, integrating major substance groups, nutrients, trace metals, microplastics, and POPs, into a single framework. By merging multi-source observations with published flux estimates, spatial and seasonal patterns were resolved with greater consistency. Our cross-substance analysis underscores the dominant role of atmospheric deposition in driving trace and micro-pollutant inputs to offshore environments, effectively linking terrestrial emissions to marine biogeochemistry.

However, several uncertainties limit the quantitative precision of these findings. Most notably, deposition flux estimates are heavily constrained by the scarcity of dry-deposition measurements, particularly in the open ocean. While our synthesis is based on a wide range of observational data, it is must be acknowledged that the spatial density of measurements is higher in coastal margins than in open-ocean regions, which may influence the precision of global spatial generalizations. This study primarily focuses on the spatial comparison of contemporary datasets; future research should address the sparse data coverage in remote marine environments to further refine our understanding of global atmospheric input patterns. Furthermore, the use of heterogeneous datasets—varying in sampling strategy and analytical methods—introduces potential methodological inconsistencies. Specifically, although various substances have been normalized into a single comparative framework, it should be noted that microplastics and nutrients exhibit fundamentally different physicochemical behaviors. Our analysis provides a cross-pollutant synthesis of total atmospheric loading to offer a holistic perspective on the cumulative pressure of atmospheric inputs on marine systems, but the specific environmental fate of each substance category must be interpreted with caution. Finally, due to the lack of detailed speciation data in global atmospheric monitoring, our risk assessment is based on total concentrations; while this provides a robust first-order, conservative screening of ecological pressure, future research must prioritize pollutant speciation to further refine the precision of toxicity and bioavailability interpretations. It should be noted that the current analysis does not account for bidirectional air–sea exchange of semi-volatile species (such as Hg) or post-depositional processes, which adds uncertainty to the net flux estimates.

Moving forward, bridging the gap between observation and process understanding is critical. There is an urgent need for high-resolution, long-term deposition measurements to replace the current reliance on model extrapolation in open-ocean regions. Future research must also better characterize pollutant speciation and chemical aging to determine bioavailability. Ultimately, the field requires integrated ocean–atmosphere models that can capture multi-pollutant interactions and the ecological impacts of deposition, from altering microbial communities to facilitating trophic transfer.

5. Conclusions

This study achieves a comprehensive synthesis of atmospheric deposition for major substance groups across global marine environments. A key strength of this work is the quantitative confirmation of land-to-sea gradients for almost all studied substances, which underscores the overwhelming dominance of terrestrial anthropogenic sources, except in the case of Hg. Even remote Arctic ecosystems are now clearly coupled to mid-latitude emissions via long-range transport. Seasonally, our findings point to a critical gap in understanding winter deposition, where sea ice acts as a temporary reservoir, potentially leading to shock loads of pollutants during melt seasons. Although current ecological risks are assessed as negligible (RQ < 0.01), the cumulative impact in coastal hotspots remains a concern. Looking forward, it is imperative to address the scarcity of data on ice-mediated transport through targeted field campaigns, especially in rapidly changing polar regions. Furthermore, future efforts should leverage integrated biogeochemical modeling to bridge the gap between static deposition fluxes and their dynamic biological impacts on marine ecosystems.