Maritime and Port Contributions to Coastal Nutrient Loading in the Baltic Sea: Apportionment and Regulatory Implications

Abstract

Eutrophication caused by excessive nitrogen and phosphorus input remains the most severe environmental threat to the Baltic Sea. While nutrient sources in general are widely studied and regulated, the relative importance of maritime nutrient inputs and their regulatory treatment remain insufficiently integrated into land-based nutrient assessments. This study applies a load-based source apportionment approach and quantifies the maritime- and port-related nutrient inputs to a Baltic Sea coastal system, in relation to other nutrient contributors (riverine, municipal, and industrial sources). Additionally, the stringency of the regulatory frameworks governing each source is assessed using a qualitative regulatory classification scale and compared to the proportion of each nutrient source. The results show that riverine inputs dominate total nutrient loading, accounting for over 90% of both nitrogen and phosphorus. Maritime sources contribute only a small share overall. However, fertilizer cargo handling constitutes the largest nitrogen point source, while ship wastewater inputs are negligible. In contrast, ship wastewater is subject to the strictest regulatory controls, whereas fertilizer handling operates under permits lacking explicit nutrient discharge limits. The findings reveal a governance mismatch between nutrient pressures and regulatory focus and highlight the need to better align nutrient management priorities with actual environmental pressures in semi-enclosed seas.

1. Introduction

1.1. State of the Baltic Sea

The Baltic Sea is a shallow, semi-enclosed sea in northern Europe with limited exchange with the World Ocean and minimal tides. Situated between maritime and continental climate zones and influenced by the North Atlantic and Arctic, it exhibits strong climatic variability. Large river inflows from its extensive catchment create a pronounced salinity gradient, from about 20 in the Danish Straits to less than 2 in the northern and eastern areas, resulting in brackish conditions with marine species dominating in the southwest and freshwater species in the northeast [1].

The catchment area is roughly four times larger than the sea itself, covering nearly 20% of Europe and spanning from temperate, densely populated regions to subarctic areas. It encompasses parts of 14 countries and is home to approximately 85 million people [1]. It is one of the most intensively trafficked maritime regions worldwide with approximately 3500–5500 ships sailing its waters each month [2].

The Baltic Sea Region has experienced prolonged periods of excessive fertilizer application, particularly from the 1950s to the 1990s, alongside substantial urban nutrient inputs [3]. As a densely populated and highly utilized marine area, the sea is strongly influenced by human activities, including agriculture, aquaculture, fisheries, river regulation, chemical pollution, tourism, and coastal management [4].

High external nutrient loads have resulted in widespread eutrophication, including recurrent phytoplankton and cyanobacterial blooms, reduced water clarity, and oxygen depletion (hypoxia) in bottom waters [5]. These impacts are amplified by the long residence time: because the Baltic Sea is connected to the Atlantic Ocean only through the narrow Danish Straits, complete water exchange takes around 30 years [6], slowing recovery once nutrients accumulate. Eutrophication is currently considered the most serious environmental pressure affecting the Baltic Sea [7], posing a persistent challenge to the environmental sustainability of coastal ecosystems and maritime activities in the region.

1.2. Nutrient Sources

The majority of nutrient inputs to the Baltic Sea originate from the surrounding catchment. Long-term assessments indicate that approximately 75% of nitrogen and at least 95% of phosphorus loads reach the sea via riverine inflows or direct waterborne discharges. Agriculture constitutes the dominant source, while additional contributions arise from upstream point sources, municipalities, wastewater treatment plants, industry, and transport [7].

Model-based estimates indicate that shipping accounts for about 1–3% of total nitrogen and less than 1% of total phosphorus inputs to the Baltic Sea [8,9]. These estimates primarily account for ship-generated wastewater discharges and atmospheric deposition originating from ship emissions. However, several potentially relevant maritime and port-related nutrient pathways are poorly represented in regional nutrient inventories. These include fertilizer and other nutrient-containing cargo handling in ports, contaminated stormwater runoff from terminal areas, and operational discharges regulated under maritime conventions [9,10].

Fertilizer cargo handling deserves particular attention. Annually, more than 45 million tons of agricultural fertilizers are handled in Baltic Sea ports [11], often in close proximity to sensitive coastal waters. Empirical data on nutrient losses during loading, unloading, and storage operations remain scarce, and such discharges are typically absent from river-based monitoring programmes and national nutrient load compilations. As a result, port-related nutrient inputs may be underestimated despite their potentially high local relevance [10].

In contrast, ship-generated sewage discharges have been studied more extensively. Recent estimates indicate that ship-borne black and grey water discharges to the Baltic Sea have decreased markedly following regulatory restrictions, particularly for passenger vessels [9,12].

In addition to maritime point-source reductions, land-based point-source discharges have been substantially reduced since the 1990s in the Baltic Sea but also in other sea areas [13]. Diffuse agricultural sources remain the dominant contributor, particularly for nitrogen, and continue to challenge eutrophication management across the Baltic Sea region and globally [14,15,16,17,18].

1.3. Regulatory Context

Nutrient inputs to the marine environment are governed through multiple regulatory layers. Marine-source discharges are primarily regulated through international conventions, which are implemented to and complemented by national legislation. Land-based nutrient discharges are regulated through European Union directives and national legislation, and major point sources are controlled through environmental permits that specify monitoring and reporting obligations. Ports are affected by both maritime and shore regulations. In addition, nutrient sources are subject to Baltic Sea wide regional requirements.

Ship-generated sewage (black water) is regulated internationally under MARPOL Annex IV [19] and, in certain areas, by additional national requirements. Since the prohibition of sewage discharge from passenger vessels to the Baltic Sea by MARPOL Annex IV, both black water and grey water discharge has decreased by approximately 50% [9]. In the studied Finnish coastal waters, the discharge of sewage—including treated effluent—is prohibited from all ships under the Maritime Environmental Protection Act since 1 July 2025 [20]. Greywater is not regulated internationally under MARPOL Annex IV, but national rules may apply; in Finnish territorial waters, discharge of greywater will be prohibited from 1 January 2030 [20]. Operational discharges and wastes beyond wastewater are regulated mainly under MARPOL Annex V (garbage), including provisions for food waste and cargo residues [19]. Cargo residues that are not classified as harmful to the Marine Environment (HME) can be discharged to the Baltic Sea under certain conditions. Fertilizers are not classified as HME despite their well-known potential to contribute to eutrophication. Consequently, fertilizer residues can be discharged at sea when regulatory distance and operational requirements are met [19].

Land-based nutrient inputs are addressed through European Union and national water and agriculture policy. The EU Nitrates Directive constrains nitrogen and phosphorus use and management on agricultural land [21], and the EU Water Framework Directive [22] establishes objectives and programmes of measures for achieving good ecological status in surface waters through coordinated monitoring, planning, and programmes of measures implemented at the river basin level.

Implementation of these directives occurs through national legislation and policy in-struments. In Finland, agricultural nutrient management measures include restrictions on fertilizer use, phosphorus balance requirements for cultivated fields, and other agri-environmental measures designed to limit nutrient losses from agricultural land to rivers and ultimately to coastal waters such as the Baltic Sea. In 2023, the Finnish national legislation governing the use of phosphorus-containing fertilizers in agriculture was tightened [23]. The regulation imposes stricter limits on the application of mineral phosphorus fertilizers to reduce the risk of phosphorus runoff into watercourses. The National Phosphorus Decree tightened restrictions on mineral phosphorus fertilizers but allows higher phosphorus application to cultivated fields through manure under certain conditions, raising concerns about its effectiveness in preventing long-term phosphorus accumulation in soils and subsequent runoff [24].

Municipal wastewater treatment plants and industrial facilities that discharge to water operate under environmental permits. These permits typically specify numeric limits for nitrogen and phosphorus, require routine monitoring, and oblige operators to report results to the competent authority [25].

In addition to EU and national measures, the Baltic Sea coastal states cooperate through the Baltic Marine Environment Protection Commission (Helsinki Commission, HELCOM) to achieve a good environmental status of the Baltic Sea. HELCOM regulates nutrient input mainly by giving recommendations, setting regional reduction targets, coordinating policies among Baltic countries, monitoring pollution levels, and promoting best practices [7].

HELCOM’s nutrient reduction approach defines Maximum Allowable Inputs (MAI) and allocates reduction responsibilities among countries. For the Gulf of Finland, the MAI is 101,800 t of nitrogen and 3600 t of phosphorus annually [26]. In 2022, total nitrogen input to the Gulf of Finland was 126% of the MAI, and total phosphorus was 137% of the MAI [27]. According to HELCOM, the required annual reductions in nutrient inputs to the Gulf of Finland are 6000 tons of nitrogen and 200 tons of phosphorus The Country-Allocated Reduction Target (CART) for the Gulf of Finland is 1199 t of nitrogen and 146 t of phosphorus per year [28].

Beyond regulatory programmes, non-governmental initiatives support practical nutrient reduction measures. For example, Baltic Sea Action Group (BSAG) [29], the John Nurminen Foundation [30], and Race for the Baltic [31] coordinate projects and voluntary commitments that target hotspots such as wastewater treatment upgrades, manure and nutrient recycling solutions, and catchment restoration measures, complementing public policy and accelerating implementation.

1.4. Research Gap and Rationale

A comprehensive, source-to-sea assessment of nutrient inputs to coastal waters is currently lacking. In particular, port- and shipping-related discharges are seldom evaluated alongside traditional land-based sources in a unified framework. Fertilizer and other nutrient-contributing cargo handling discharges are typically absent from published nutrient load assessments and official discharge databases, despite evidence that they can be locally important inputs.

Different nutrient sources are regulated by separate regulatory frameworks and layers and monitored by separate authorities. A systematic mapping of regulatory overlaps and gaps is therefore needed to understand how differing regimes influence nutrient inputs.

1.5. What We Study

The case study focuses on the Hamina–Kotka–Pyhtää coastal waters on the Finnish coast of the eastern Gulf of Finland. This region was selected for four main reasons.

The first reason is that the eastern Gulf of Finland is among the most severely impacted areas in the Baltic Sea in terms of nitrogen and phosphorus loads [27,28], making the area particularly relevant for detailed investigation.

Second, the Hamina–Kotka–Pyhtää area reflects the broader Baltic Sea system well, as it encompasses all major, commonly identified sources of nutrient loading within a single, well-defined area. The area receives nutrient inputs from the Kymi River (via several branches) and from smaller local rivers [32,33], as well as direct coastal discharges from municipal and industrial point sources [32,34,35]. In addition, the area hosts intensive maritime activities associated with one of the largest ports in Finland, which represents a potential source of nutrient inputs related to shipping operations and cargo handling [10,36].

Third, uniquely detailed data on cargo handling—typically unavailable elsewhere—is accessible for this region. Fertilizer cargo-related nutrient discharges were quantified in our previous study [10], allowing us to now integrate these with other relevant datasets to produce a comprehensive assessment of total nutrient inputs.

Finally, the regulatory framework governing the area spans international, EU-level, regional, and national layers, providing a comprehensive context for regulatory analysis.

Figure 1 shows the location of the study area in the Gulf of Finland and studied nutrient sources: red dot marks a riverine diffuse source, purple square marks a land-based point source, orange square marks maritime or port-related point source. Rivers (red dots) from west to east are Taasia, Kymi/Ahvenkoski, Kymi/Pyhtää, Kymi/Koivukoski, Kymi/Korkeakoski, Summa, Vehka. The point sources (purple and orange squares) from north to south are Stora Enso: Sunila Mill, Kotkamills Oy, Mussalo Wastewater treatment plant, fertilizer berth at the Port of HaminaKotka Oy and maritime traffic.

The Port of HaminaKotka is the biggest general port in Finland, with annual cargo throughput of 12–18 million tons [37]. Its berths are distributed in Hamina–Kotka region and it hosts about 2500 ship calls annually. Around 2–3 million tons of fertilizers are loaded to ships in Mussalo annually [37,38], most of them being nitrogen fertilizers such as urea.

Mussalo Wastewater Treatment Plant collects and treats wastewaters from the cities and counties of Kotka, Kouvola, Pyhtää and Hamina with 156,000 habitants [39] and in several additional industrial facilities [32]. It is the fourth biggest wastewater treatment plant in Finland, treating over 10 million cubic meters of water annually [34]. Other industrial nutrient point-source contributors are Stora Enso, Sunila Mill, which handles the wastewaters of the former pulp mill, and the paper and cartonboard factory Kotkamills Oy [35].

This study (i) quantifies total nitrogen and phosphorus inputs to the Hamina–Kotka–Pyhtää coastal waters in 2021 and apportions them among riverine inputs, land-based point sources, port cargo handling, and ship operations; (ii) maps the regulatory instruments and permits governing each source category; and (iii) identifies potential governance gaps where loads are substantial but controls or monitoring are weak or where loads are negligible but controls are strict. The main focus is on the maritime-related nutrient sources, including port operations. Port activities can be viewed as both land-based point sources and maritime-related sources, as they occupy an intermediate position in terms of both the origin of the discharge and regulatory framework.

Our research questions are as follows:

RQ1. What are the shares of maritime and port activities in relation to total nutrient input?

RQ2. Which regulations and permits control each studied nutrient source?

RQ3. Based on the results, are there regulatory gaps related to the discharges?

To find the answers to these questions, we evaluated nutrient discharges from identified maritime and shore sources using previous studies and measurements, surveyed the related regulations and compared the nutrient load sources to related regulations to identify overlaps and potential gaps.

This paper is structured as follows: Section 1 provides an introduction, giving an overview of this study and outlining the research questions. It also presents the background of this topic based on a literature review. Section 2 describes the methodology used to assess the total nutrient input and the contribution of shipping and port activities. The results are presented in Section 3. Section 4 discusses the results and compares them with the results of other studies. Finally, Section 5 presents the conclusions and recommendations for future actions and research.

2. Materials and Methods

2.1. Study Area and Scope of the Assessment

This case study focuses on nutrient inputs in the Hamina–Kotka–Pyhtää coastal waters during the year 2021 and compares annual nitrogen and phosphorus loads from three main source categories:

(i)

Riverine inputs (representing combined diffuse and upstream point sources);

(ii)

Local land-based point sources (municipal wastewater treatment and industrial facilities);

(iii)

Maritime-related sources, separating diffuse ship waste waters from fertilizer cargo handling point source at port terminals.

The analysis was limited to the most significant and quantifiable nutrient sources. Minor or poorly quantifiable sources, such as food waste from ships and solid cargo residues, were excluded. It is nevertheless acknowledged that fertilizer residues deposited on ship decks during loading and unloading operations may constitute a locally relevant nutrient source; however, reliable quantitative estimates for this pathway are currently unavailable.

Nutrient discharges from industries, municipalities, and agricultural activities located in the lower reaches of the river catchments were included in the riverine input category, as their discharges reach the coastal waters indirectly via river transport. In contrast, industries and municipalities discharging treated effluents directly into the coastal waters were assessed separately as land-based point sources. Wastewaters generated on board cargo ships were included in the analysis even though they are not necessarily discharged in the immediate vicinity of the study area. These loads were considered to illustrate the potential theoretical maximum nutrient input associated with all cargo vessels visiting the ports within the study area.

In addition to quantifying nutrient loads, the study examines the regulatory context governing each identified nutrient source. This includes: (iv) international, regional, and national regulatory frameworks applicable to the different source categories; and (v) environmental permits.

To ensure transparency and reproducibility, a detailed traceability table is provided as Supplementary Materials. For each quantified nutrient source, the table specifies the dataset used, temporal coverage, spatial location, sampling frequency or calculation basis, analytical method, and key assumptions underlying the annual nitrogen and phosphorus load estimates.

2.2. Data Sources

The assessment integrates multiple complementary data sources to quantify nutrient inputs and to support the regulatory analysis. The data sources comprise: (i) regional nutrient load data produced by environmental authorities [33]; (ii) environmental monitoring reports [32,40,41]; (iii) peer-reviewed scientific literature on maritime nutrient discharges and mitigation measures, including the authors’ previous studies [10,34]; (iv) European Union, international, and national legislation governing nutrient discharges from both land-based and maritime activities [19,20,21,22,23]; and (v) environmental permits of operators located in the study area [42,43,44,45].

As this study is based on observational data, “n” refers to the number of observations, reports, or time steps underlying each annual estimate. These include monitored time series such as monthly river measurements, operational or compliance reports such as quarterly wastewater treatment plant reports, modeled daily estimates for fertilizer terminal discharges, and annual estimates derived from the literature for ship-generated wastewater. The distinction between raw observations, aggregated annual values, and secondary estimates is documented in the traceability table in the Supplementary Materials of this article.

2.3. Analytical Approach to Nutrient Source Apportionment

The analysis applied a load-based nutrient source apportionment approach consistent with the HELCOM Pollution Load Compilation (PLC) methodology. Annual nitrogen and phosphorus loads from riverine, land-based point, and maritime-related sources were compiled from monitoring data, operator reports, and previous studies, harmonized to comparable units, and assessed using a comparative load assessment. This approach enables a transparent comparison of the relative magnitude of nutrient inputs from different source categories without modelling nutrient transport or ecological responses.

The compiled nutrient loads were further benchmarked against HELCOM Country-Allocated Reductions Targets (CART) to contextualize their relative significance with respect to regional eutrophication reduction targets in the Gulf of Finland set to Finland. This comparison provides a policy-relevant reference for assessing the contribution of local nutrient sources in relation to basin-scale environmental objectives and the sustainability of nutrient management strategies in semi-enclosed seas.

2.4. Unit Harmonization, Uncertainty and Limitations

All nutrient inputs were harmonized to annual nitrogen and phosphorus loads (tons year⁻¹) to enable comparison across heterogeneous data sources. Where monitoring data were reported as concentrations and flows, annual loads were calculated by aggregation over the study year. Reported annual discharges from permits and operational reports were used directly. Modeled or estimated sources were calculated using published coefficients and assumptions applied consistently for the year 2021.

Uncertainty varies by source category. Riverine and land-based point-source loads are based on regulatory monitoring and are primarily subject to analytical and flow-measurement uncertainty. Fertilizer terminal discharges include an estimated analytical uncertainty of approximately 15% due to dilution requirements during sample analysis. Ship-generated wastewater loads represent theoretical maximum estimates based on published per capita generation rates, vessel characteristics, and operational assumptions. Given the compilative nature of the study and the heterogeneity of data sources, uncertainty is reported in the supplemented traceability table descriptively by category instead of through formal error propagation or inferential statistical analysis.

Double counting between source categories was avoided by assigning nutrient loads to mutually exclusive pathways. Riverine loads include all upstream diffuse and point-source inputs transported to the coast via rivers and exclude discharges entering coastal waters directly. Municipal and industrial point sources were included only when effluents were discharged directly into the coastal area. Fertilizer terminal discharges represent on-site stormwater and cargo handling losses and do not overlap with river monitoring stations or municipal drainage systems. Ship-generated wastewater loads were treated separately and represent potential offshore discharges distributed across the Baltic Sea rather than localized coastal inputs.

2.5. Regulatory Analysis

In parallel, the regulatory analysis was informed by the DPSIR (Drivers–Pressures–State–Impact–Response) framework, focusing on the linkage between nutrient pressures and regulatory responses. To compare the relative stringency of regulatory control applied to different nutrient sources, a qualitative regulatory classification scale ranging from 0 to 4 was developed for this study using predefined criteria based on the presence and strength of legally binding regulatory instruments. The scale reflects the type and strength of regulatory instruments governing nutrient discharges, ranging from the absence of direct controls to explicit discharge prohibitions (

Table 1. Qualitative regulatory classification scale.

Regulatory Level	Criteria
0	No restrictions
1	No numerical discharge limits Monitoring and mitigation requirements
2	Numerical discharge or input limits by regulations in general Monitoring and mitigation requirements
3	Specific numerical discharge limits by environmental permits Monitoring and mitigation requirements
4	Discharge prohibited

). Regulatory sources consulted included international conventions, European Union directives, national legislation, and site-specific environmental permits. Each source category was assigned a regulatory level corresponding to the most stringent applicable instrument. Ambiguous cases were handled conservatively by assigning the lower regulatory level unless explicit numeric discharge limits or prohibitions were present.

Level 0 depicts that there are no regulatory instruments directly addressing nutrient releases from the source category. Level 1 implies that environmental requirements focus on operational practices or preventive measures (e.g., cargo handling procedures, stormwater management), without numeric nutrient discharge limits. Level 2 indicates that nutrient discharges are addressed through broader policy instruments or management programmes (e.g., catchment-level water protection programmes or agricultural nutrient management rules). Numeric limits exist based on the rules, but a specific environmental permit is not required. Level 3 signals that environmental permits establish source-specific numeric discharge limits for nitrogen and/or phosphorus, combined with monitoring, reporting, and regulatory oversight. Level 4 shows that regulatory frameworks include explicit discharge prohibitions or highly restrictive rules governing discharges.

3. Results

3.1. Nutrient Load Quantification

The total input of nitrogen was 7035 tons, and the input of phosphorus was 225 tons in 2021. Across all assessed sources, riverine inputs dominated total nutrient loading to the Hamina–Kotka–Pyhtää coastal waters in 2021 (93.7% of nitrogen and 91.7% of phosphorus). Local land-based point sources contributed 2.4% of nitrogen and 8.1% of phosphorus. Maritime-related sources accounted for the remaining small fraction: port activities contributed 3.9% of nitrogen and 0.1% of phosphorus, whereas ship wastewaters contributed <0.1% of both nutrients (

Table 2. Nitrogen and phosphorus discharges in Hamina–Kotka–Pyhtää coastal area in 2021.

Nitrogen Source	Nitrogen (Tons)	Point Source	Data Sources	Phosphorus Source	Phosphorus (Tons)	Point Source	Data Sources
Kymi River Ahvenkoski	3171.03	no	[32,33]	Kymi River Ahvenkoski	89.49	no	[32,33]
Kymi River Koivukoski	1352.25	no	[32,33]	Kymi River Korkeakoski	38.45	no	[32,33]
Kymi River Korkeakoski	1325.21	no	[32,33]	Kymi River Koivukoski	32.83	no	[32,33]
Taasia River	283.97	no	[32,33]	Taasia River	25.86	no	[32,33]
Fertilizer loading at port	272.90	yes	[10]	Summa River	12.37	no	[32,33]
Summa River	256.93	no	[32,33]	Industry: Kotkamills	8.03	yes	[32]
Kymi River Pyhtää	114.94	no	[33]	Industry: Sunila Mill	7.43	yes	[41]
Vehka River	87.90	no	[32,33]	Vehka River	4.50	no	[32,33]
Municipalities: WWTP	77.20	yes	[40]	Kymi River Pyhtää	3.37	no	[33]
Industry: Kotkamills	60.59	yes	[32]	Municipalities: WWTP	2.81	yes	[40]
Industry: Sunila Mill	31.54	yes	[41]	Fertilizer loading at port	0.20	yes	[10]
Ships’ Black waters	0.61	yes	[36]	Ships’ Grey Waters	0.07	yes	[36]
Ships’ Grey Waters	0.17	yes	[36]	Ships’ Black waters	0.06	yes	[36]
Total Nitrogen load	7035.24			Total Phosphorus load	225.45

; Figure 2 and Figure 3).

Figure 2 illustrates that the largest branches of the Kymi River are by far the dominant sources of nitrogen discharge. However, one point source—fertilizer cargo loading—is among the riverine sources in terms of magnitude. Ship wastewater contributes only a very small share compared to the other sources. Figure 3 shows that, in addition to the largest branches of the Kymi River, the smaller Taasia River is also a major contributor to phosphorus loading. Similar to the nitrogen loads, ship wastewater represents only a very minor share of the total phosphorus load.

3.2. Regulatory Mapping

The regulatory mapping revealed substantial differences in how nutrient discharges from the assessed source categories are governed. To compare the relative stringency of regulatory control applied to different nutrient sources, a qualitative regulatory classification scale (0–4) was developed for this study (see Section 2). The scale reflects the type and strength of regulatory instruments governing nutrient discharges, ranging from the absence of specific regulation (0) to strict regulatory control with explicit discharge prohibitions (4).

Riverine nutrient loads entering the Hamina–Kotka–Pyhtää coastal area originate from both point sources and diffuse agricultural sources within the catchment area. Upstream municipal wastewater treatment plants and industrial facilities discharging into rivers operate under environmental permits that establish facility-specific limits for nitrogen and phosphorus discharges. In contrast, diffuse agricultural discharges within the catchment are regulated primarily through nutrient management requirements, such as restrictions on fertilizer application. These measures regulate agricultural practices rather than establishing quantified discharge limits for nutrients. Consequently, riverine nutrient loads entering coastal waters are controlled indirectly through catchment-level management measures rather than through direct limits on total riverine nutrient inputs. Because riverine nutrient loads originate from a combination of sources subject to both numerical permit limits and more general practice-based regulations, this source category was assigned a score of 2 on the regulatory scale.

Three major point sources (Mussalo wastewater treatment plant, Sunila Milland Kotkamills) discharging directly into coastal waters are regulated through environmental permits issued by Finnish environmental authorities. These permits include explicit numeric limits for nitrogen and phosphorus discharges and may require advanced nutrient removal technologies. Permit conditions also include monitoring and reporting requirements that enable regulatory authorities to verify compliance. Compared with other nutrient sources in the study area, these sources are therefore subject to direct and quantifiable regulatory control. Accordingly, these point sources were assigned a score of 3 on the regulatory scale.

Fertilizer cargo handling at port is subject to environmental permits that require preventive measures and monitoring but do not impose numeric nitrogen or phosphorus discharge limits; therefore, this source category was assigned level 1.

Ship-generated wastewater is regulated through a multi-level framework consisting of international maritime conventions and national implementation measures. MARPOL Annex IV restricts the discharge of sewage but does not regulate grey water. From the perspective of international regulation alone, grey water discharges would therefore correspond to level 0. However, national regulations in Finland impose stricter requirements: the discharge of sewage from ships is prohibited in Finnish coastal waters, and the discharge of grey water is scheduled to be prohibited from 2030 onwards. Considering this stricter national regulatory framework, ship-generated wastewater was assigned the highest regulatory score of 4.

Table 3. Regulatory mapping of major nutrient source categories in the study area.

Source Group	Source Category	Examples in This Study	Main Regulatory Instruments	Regulatory Level (0–4)
Land-based sources	Riverine and diffuse catchment inputs including point sources via rivers	Kymi River branches; Taasia, Summa and Vehka Rivers	EU Water Framework Directive programmes of measures; Nitrates Directive; national agri-environmental regulations, environmental permits for industrial facilities along the river with numeric nitrogen and phosphorus discharge limits	2
	Municipal point sources	Mussalo wastewater treatment plant	Environmental permit with numeric nitrogen and phosphorus discharge limits; monitoring and reporting obligations	3
	Industrial point sources	Kotkamills; Sunila Mill	Environmental permit with numeric nitrogen and phosphorus discharge limits; monitoring and reporting obligations	3
Maritime and port-related sources	Port cargo handling	Fertilizer loading terminals; contaminated stormwater	Environmental permits for port and terminal operations; operational requirements (generally without numeric nutrient limits)	1
	Ship sewage (blackwater)	Cargo vessel sewage	MARPOL Annex IV; national prohibition of sewage discharge in Finnish territorial waters	4
	Ship greywater	Cargo vessel greywater	(greywater not regulated under MARPOL Annex IV); national prohibition in Finnish territorial waters from 2030	4

summarizes the main regulatory frameworks governing nutrient discharges from the different source categories analyzed in this study.

Figure 4 illustrates that rivers and fertilizer handling represent the largest sources of nitrogen input in the studied coastal area. In contrast, ship-generated wastewater is subject to the most stringent regulatory control, particularly when accounting for the forthcoming prohibition of grey water discharges in Finnish territorial waters. Figure 5 demonstrates a similar pattern for phosphorus inputs, where sources contributing relatively small nutrient loads are regulated more strictly than larger contributors.

Figure 6 highlights a mismatch between nutrient load apportionment and regulatory stringency regarding the point sources. Fertilizer handling accounts for a larger share of nitrogen input than the other assessed point sources combined, yet it is subject to a lower level of regulatory control compared with others. In contrast, Figure 7 indicates that for phosphorus inputs the distribution of loads is more closely aligned with the regulatory level applied to each point source.

4. Discussion

4.1. Contextualizing Maritime Contribution

The main findings of this study regarding nutrient apportionment (RQ1) were the following:

• Riverine inputs dominate total nutrient delivery by 94% of N (nitrogen) and 92% of P (phosphorus) in 2021;
• Ships’ wastewaters are minor contributors in magnitude (<0.1% of N and <0.1% of P) and distributed outside coastal waters;
• Fertilizer cargo loading at port contributes 3.9% of N and 0.1% of P, being the biggest identified point source of nitrogen exceeding other point sources combined.

The dominance of riverine nutrient inputs observed in the Hamina–Kotka–Pyhtää coastal waters is consistent with long-term assessments in the Baltic Sea and other semi-enclosed coastal systems. Numerous studies have demonstrated that land-based sources, particularly agriculture and upstream diffuse inputs, account for the majority of nitrogen and phosphorus loads delivered to coastal waters, in which agriculture commonly contributes roughly 40 to 60% of total nutrient loads and municipal wastewater remains much more variable but still far larger than shipping in most systems [14,16,36,46,47,48,49,50,51]. Despite significant reductions in municipal and industrial discharges since the 1990s, nutrient legacies in soils and continued diffuse losses from catchments remain the primary drivers of eutrophication pressure in the Baltic Sea.

What distinguishes this study from earlier assessments is the integration of maritime and port-related nutrient pathways into the source apportionment. While shipping-related nutrient inputs have been examined at the basin scale using modeling approaches [8,9], such analyses typically focus on ship-generated wastewaters and atmospheric deposition and do not resolve port cargo handling or terminal-level discharges.

Empirical studies of ship sewage and grey water confirm that these inputs are quantitatively small, contributing approximately <0.1–3% for both nitrogen and phosphorus [36,47,52,53]. The present results corroborate these findings for the eastern Gulf of Finland. In addition, it should be noted that while the loads from fertilizer application, municipal wastewater treatment plants, and industry are localized point sources, the nutrient load from ships’ wastewaters is dispersed throughout the open waters of the Baltic Sea during their voyages.

In contrast, fertilizer cargo handling at ports emerges as a locally significant nutrient source that is largely absent from regional nutrient inventories. Previous work has demonstrated that nutrient losses from fertilizer terminals can be substantial under certain operational and meteorological conditions and may constitute a non-negligible fraction of local nitrogen inputs [10]. The present study confirms that, while small in relation to total catchment loads, fertilizer handling represents the largest maritime-related nitrogen source and the largest identified point source in the study area. This finding aligns with broader observations that spatially concentrated sources, even when modest in absolute terms, can play a disproportionate role in shaping coastal water quality, particularly in semi-enclosed systems with limited water exchange [13,54].

4.2. Regulatory Implications

The main findings of this study regarding the regulatory framework and gaps regarding nutrient discharges (RQ2 and RQ3) were the following:

• Nutrient inputs to the study area are governed by a multi-layered regulatory framework spanning international conventions, EU legislation, national laws, and local environmental permits.
• Regulatory stringency is not aligned with the magnitude of nutrient pressures: sources contributing the largest nutrient loads are often regulated less strictly than sources with minor quantitative importance.

  ○

  Diffuse agricultural sources, which dominate total nitrogen and phosphorus inputs via rivers, remain ineffectively regulated despite extensive policy frameworks.

  ○

  Ship-generated wastewater contributes a negligible share of nutrient inputs but is subject to the strictest regulatory controls, including discharge prohibitions to certain ship types (passenger vessels by international MARPOL) and to certain locations (all ships in Finnish territorial waters by national legislation).

  ○

  Fertilizer cargo handling at ports constitutes the largest maritime-related nitrogen source and the largest identified nitrogen point source, yet environmental permits lack explicit nutrient discharge limits.

4.2.1. Ineffective Regulation of Dominant Diffuse Sources

The regulatory patterns observed in this case study reflect challenges documented in Baltic Sea environmental governance literature: policy frameworks are extensive and targets are clear, but the alignment between pressures and regulatory instruments is uneven, particularly where diffuse sources dominate and governance spans multiple levels [55,56]. In the studied area, the dominant nutrient inputs originate from the river system, reflecting diffuse agricultural losses and upstream point sources. Despite decades of regulation under the EU Nitrates Directive, Water Framework Directive, and the HELCOM Baltic Sea Action Plan (BSAP), agriculture remains the most challenging sector for nutrient abatement. Numerous studies demonstrate that reductions from diffuse sources are slower, more uncertain, and more costly than reductions from point sources [57,58,59].

Similarly, European nutrient discharge analyses show that policies aimed at point sources were more effectively implemented than those targeting diffuse sources [60]. Projected EU policy implementation to 2050 is expected to reduce domestic/industrial nitrogen discharges by 14% and phosphorus by 25%, whereas agricultural reductions are more modest at 5% for nitrogen and 2% for phosphorus [61].

4.2.2. Fertilizer Cargo Handling: An Overlooked but Manageable Source

Fertilizer cargo handling at ports represents the largest maritime-related nitrogen source and the largest identified nitrogen point source in the study area, yet it is subject to comparatively weak regulatory control. This imbalance reflects a broader governance mismatch in which port-based and land–sea interface activities lack clear nutrient-specific requirements and consistent performance standards.

Despite their localized environmental relevance, nutrient discharges associated with fertilizer loading are typically regulated through environmental permits that emphasize operational practices and general pollution prevention rather than explicit nitrogen or phosphorus discharge limits. As a result, fertilizer handling exceeds the nitrogen loads of several other point sources that are subject to stricter, limit-based regulation. This gap persists even though effective technical and operational mitigation measures—such as covered storage, enclosed conveyors, containment structures, and advanced stormwater management systems—are well established and technically feasible.

The findings illustrate a recurring governance challenge at the land–sea interface, where port activities fall between terrestrial and maritime regulatory regimes. Such boundary zones have been identified in Baltic Sea governance research as weak points in environmental management, characterized by fragmented responsibilities and diffuse accountability [62,63]. In practice, this institutional ambiguity limits the translation of nutrient reduction objectives into enforceable permit conditions for port operations.

From a policy-efficiency perspective, this regulatory gap is notable. Unlike diffuse agricultural runoff, port-related discharges are spatially confined, operationally well defined, and technically controllable. Economic and governance analyses indicate that nutrient abatement is most cost-effective when applied to sources that are observable, enforceable, and associated with clear institutional responsibility [64,65]. Strengthening port environmental permits through explicit nutrient discharge limits, systematic monitoring, and mandatory implementation of best available techniques would therefore complement catchment-scale measures and address a currently under-regulated but high-leverage source.

Finland’s environmental permitting system closely resembles those of other Baltic countries, including Sweden, Denmark, Norway, Iceland, and Estonia [66], suggesting that similar regulatory gaps related to fertilizer handling potentially exist elsewhere in the Baltic Sea region.

4.2.3. Inefficiency of Additional Ship Wastewater Restrictions

In contrast to fertilizer handling, ship-generated wastewater is subject to the strictest regulatory controls despite its negligible contribution to total nutrient loads. Earlier studies show that nutrient inputs from ship wastewater were already very small prior to the most recent regulatory restrictions [8,36] and have decreased further following their implementation [9,12].

The regulatory framework governing ship wastewater is fragmented across international and national levels. Under MARPOL Annex IV, the discharge of sewage from passenger ships is prohibited in the Baltic Sea, while greywater is not regulated at the international level. Finnish national legislation, in contrast, prohibits the discharge of sewage—including treated effluent—from all ships in territorial waters and will extend the prohibition to greywater from 2030 onwards. As a result, nationally imposed requirements go beyond internationally agreed standards and apply unevenly across wastewater streams. This outcome reflects a well-documented tendency in Baltic Sea governance to prioritize sources that are administratively clear and internationally harmonized, even when their environmental impact is limited [55,65]. From a sustainability perspective, further tightening of ship wastewater regulations is unlikely to deliver meaningful additional reductions in coastal eutrophication. Moreover, increasing divergence between national maritime regulations may create compliance challenges for international shipping without proportional environmental benefit. This finding supports calls in the literature to better align regulatory effort with environmental effectiveness rather than regulatory convenience [56].

4.3. Alignment with HELCOM Targets

According the HELCOM country allocated reduction target (CART), Finland should reduce the input of nitrogen by 1199 tons and the input of phosphorus by 146 tons per year in the Gulf of Finland in order to reach good environmental status of the sea. In the studied area, achieving these targets relies mainly on reductions from agriculture and other catchment sources, complemented by improved targeted controls for locally important point sources. The importance of local contributions is illustrated by the Taasia River, a small, local catchment that alone contributed to nearly 18% of the total phosphorus CART for the whole Gulf of Finland. For nitrogen, fertilizer cargo loading at a port accounts for approximately 23% of the nitrogen CART for the Gulf of Finland. It is notable that a reduction of only 17% in total nitrogen inputs within the Hamina–Kotka–Pyhtää area would be sufficient to meet the entire nitrogen CART for the Gulf of Finland. In contrast, achieving the phosphorus reduction target would require a substantially larger reduction of approximately 65% in phosphorus inputs from the studied area.

These findings reinforce conclusions from Baltic Sea governance research: while point-source controls have delivered substantial reductions historically, achieving remaining targets—particularly for phosphorus—will depend primarily on diffuse agricultural abatement, complemented by targeted regulation of locally significant point sources [26,58,67].

5. Conclusions

This study quantified the relative contribution of maritime- and port-related nutrient sources to total nitrogen and phosphorus inputs in the Hamina–Kotka–Pyhtää coastal waters and mapped the regulatory regime governing each source category. Riverine inputs dominated total loading in 2021, while maritime sources were minor overall; however, fertilizer cargo handling and associated stormwater runoff formed the primary maritime-related pathway and the largest identified nitrogen point source. In regulatory terms, shipborne wastewaters are subject to stringent controls, and municipal and industrial point sources operate under permits with numeric limits and monitoring. In contrast, excess amount of manure can be inserted into cultivated fields due to the manure exception in the phosphorus degree, and fertilizer terminal discharges can lack explicit nutrient limits, indicating a governance gap. Catchment-scale measures remain essential for achieving HELCOM CART objectives, and targeted permit updates for fertilizer terminals offer a practical local opportunity for additional reductions.

From a broader perspective, these results highlight a recurring challenge in coastal eutrophication management: the dominant nutrient sources are often diffuse and difficult to regulate, whereas smaller but more manageable sources at the land–sea interface tend to receive disproportionate analytical and regulatory attention. Studies from estuaries and coastal seas worldwide emphasize that effective nutrient management requires not only addressing the largest sources but also identifying leverage points where targeted interventions can yield measurable local benefits. By integrating port-related activities into the nutrient source framework, this study contributes to closing a persistent gap between basin-scale nutrient assessments and local coastal management.

The Hamina–Kotka–Pyhtää case exemplifies dynamics that extend well beyond the Baltic Sea. Many coastal regions combine large agricultural catchments, intensive port operations, and dense maritime traffic. The approach applied here, which quantifies heterogeneous nutrient sources alongside a structured regulatory comparison, provides a transferable template for identifying where governance efforts are aligned with environmental pressures and where such mismatches may weaken eutrophication mitigation in semi-enclosed coastal systems worldwide.