1. Introduction
Eutrophication is one of the main environmental threats to the Baltic Sea. Eutrophication, a consequence of excessive nutrient input, particularly nitrogen and phosphorus, disrupts marine ecosystems by promoting excessive algal growth and triggering hypoxic conditions. This phenomenon results in the decline of species diversity, deterioration of water quality, and adverse impacts on fisheries, ultimately undermining the stability and functionality of marine ecosystems [
1]. The main contributor to the nutrient load is agriculture and it has the greatest potential for reduction. Other sources include point sources in the upper reaches of rivers, municipal sources and wastewater treatment plants, and industry and transport [
2].
Shipping contributes roughly 0.3% of the total phosphorus and 1.25–3.3% of the total nitrogen inputs to the Baltic Sea [
3], and potential sources include fertilizer transfer, food waste disposal, gray and black water discharges, bilge and scrubber water discharges, and the discharge of treated ballast water [
4,
5,
6,
7,
8,
9].
Annually, over 45 million tons of fertilizers are transported via cargo ships at Baltic Sea ports [
10]. With an estimated cargo loss of 0.05% during handling operations [
11], approximately 22,000 tonnes of fertilizers may inadvertently enter the sea each year. The primary causes of fertilizer loss include ship loading and unloading processes (particularly when employing antiquated techniques), temporary open storage, and inadequate stormwater management at port facilities [
10]. Fertilizers are managed in more than 70 ports in the Baltic Sea region [
10], with the largest quantities in 2019 being handled at Klaipeda (13 million tonnes), Saint Petersburg (9), Vyborg (4), Gdynia (2), Gdansk (2), HaminaKotka (2), Szczecin (1.7), Tallinn (1.6), Rostock (1.6), Police (1.3), Liepaja (1.1), and Uusikaupunki (0.9) [
12].
Fertilizer discharges from shipping to the Baltic Sea are not currently regulated. While MARPOL Annex V controls the cargo residues from dry bulk carriers classified as harmful to the marine environment (HME), fertilizers do not fall under this category and can be discharged at sea if over 12 nautical miles from land [
11]. According to Finnish National Environmental Protection Act Law [
13], an environmental permit is needed for activities with pollution risks. The case port has an environmental permit [
14] for fertilizer loading activities, which requires certain operational measures. However, no limits are set for cargo emissions or discharges. The updated Baltic Sea Action Plan [
2] establishes nitrogen and phosphorus reduction targets, with actions related to shipping and ports to decrease the nutrient loads from dry bulk fertilizer handling. Thus, although fertilizer terminals are recognized as significant sources of nutrient load, no systematic and reliable monitoring of discharges have been conducted. Neither have the total nutrient loads to the sea from the discharges from fertilizer loading in port areas been systematically assessed.
To fill this gap, this study aimed to quantify the spatio-temporal variability of nutrient discharges due to fertilizer loading. The main research questions of this study are as follows: (1) How can nitrogen and phosphorus discharges from fertilizer loading be estimated? (2) How much phosphorus and nitrogen in kg was generated by the loading/unloading of fertilizers? (3) How can nitrogen and phosphorus discharges be reduced at the port? To answer the questions, the quantities of nitrogen and phosphorus discharges from the identified sources (rainwater) were measured, the environmental variables affecting these emissions were identified and measured (fertilizer application area, rainfall intensity), and a model relating environmental variables and nutrient emissions was built to assess the daily nitrogen and phosphorus emissions at the fertilizer application site over the course of a year. The results were compared with previous studies of nitrogen and phosphorus sources.
This paper is organized as follows:
Section 1 is an introduction that gives the reader an overview of the study, and presents the research questions and background of this study based on a literature review.
Section 2 consists of the methodology for carrying out the monitoring of nitrogen and phosphorus emissions in the fertilizer load area and the construction of a model for predicting daily nutrient discharges in the area. The results are presented in
Section 3, and in
Section 4, the results are discussed and compared to other studies. Conclusions and recommendations for action and further research are described in
Section 5.
4. Discussion
Our study focused on three key research questions: firstly, developing a method to estimate nitrogen and phosphorus emissions from fertilizer application; secondly, quantifying the total amount of these nutrients generated through both fertilizer application and removal; and thirdly, investigating strategies for mitigating these emissions.
Our approach combined in situ monitoring and modeling to provide robust estimates of emissions. We measured nitrogen and phosphorus emissions directly from identified sources, such as rainwater, and identified and measured key environmental variables that influence these emissions, including the area of fertilizer application and rainfall intensity. We then developed a model relating these environmental variables to nutrient emissions. Using this combined approach, our model estimated that the total nitrogen and phosphorus emissions from fertilizer application to the marine environment were 272,906 kg and 196 kg per year, respectively. This methodology not only provided accurate daily estimates, but also provided a comprehensive annual overview of nutrient emissions, demonstrating the effectiveness of integrating direct monitoring with predictive modeling in environmental studies.
The present study demonstrates that machine learning modeling methods are effective tools for predicting the complex realism of various processes, and are capable of reproducing non-linear relationships and identifying tipping points indicative of abrupt discontinuous shifts. The BRT model revealed that as the loading area increased, the total nitrogen discharge increased, while the phosphorus discharge decreased. This latter finding may be attributed to differences in the characteristics of the loading areas utilized by terminals.
Our model effectively integrates spatial and temporal dimensions by using training data from three different loading areas with different catchment characteristics and by collecting nutrient concentration samples at different times. This design captures the essential spatial and temporal variability of nutrient inputs to the sea, which is influenced by factors such as the catchment area, fertilizer application rate, loading timing, and rainfall intensity. By incorporating these spatial and temporal factors as independent variables, our model is able to predict nutrient loads for areas and periods beyond the scope of existing monitoring. This approach significantly improves the robustness and accuracy of our predictions. However, it is important to note that our results, based on data from one specific port, should not be extrapolated to other ports without additional data from those sites. Thus, the current model can be employed in the same port for hindcasting nutrient emissions in previous years or conducting scenario analyses for future port management. Additionally, the model can be applied in other ports, provided it is re-parametrized and validated using port-specific monitoring data. This is necessary due to the unique catchment areas, fertilizer loading management, and rainfall characteristics that distinguish individual ports. Despite this limitation, our study serves as a valuable demonstration of how pollutant loads can be effectively predicted in scenarios where monitoring data do not cover all spatial and temporal extents.
This study relies on actual monitoring measurements of water nutrient concentrations in stormwater, which were used to train the BRT model and estimate the total annual load over one year. Inaccuracies may arise due to the limited number of stormwater samples employed for model training relative to the observed spatio-temporal variability of nutrient concentrations in stormwater. Another source of error stems from nutrient analyses. Thus, it is expected that increasing the sample size can enhance confidence in the results. Despite the considerable variability in the training data and the relatively small sample size, the BRT model showed a high level of accuracy, accounting for over 80% of the variability in the nutrient discharges to the sea. This is further demonstrated by the very small margins of error, as shown in
Figure 4 and
Figure 5. These results highlight the effectiveness of machine learning algorithms in capturing complex patterns and relationships that may not be fully captured by traditional statistical methods. Therefore, while sample size is a consideration, the robust performance of our model mitigates this potential limitation.
Prior to this study, the sampling efforts in the case port were limited to annual samples as per environmental permits. The present study conducted intensive sampling to provide a comprehensive analysis of stormwater quality. The obtained results were then compared with the annual stormwater sampling data collected by the Water and Environment Association of the River Kymijoki [
15]. The phosphorus concentrations observed in the stormwater wells in this study were found to be consistent with the previously reported results. However, notable differences were observed in the nitrogen concentrations, with one stormwater well exhibiting significantly higher levels in this study.
For many nutrient sources entering the Baltic Sea, the nutrient concentrations are relatively low. However, passenger ship wastewater and fertilizer stormwater represent point sources with higher concentrations. Consequently, it is crucial to collect sewage waters from passenger ships using port reception facilities, as mandated by the International Maritime Organization IMO in 2011 [
20]. On the other hand, the total annual amount of nitrogen in the sewage waters of all 2355 cargo ships calling at the case port in 2020 was only 0.7 tons [
21], whereas the BRT modeling estimates an annual nitrogen load of 273 tons from the fertilizer loading area. When comparing these loads to other sectors, the annual nitrogen load at the fertilizer site is equivalent to the emissions produced by 9000 tons of caged finfish in the sea [
22]. Presently, Estonia’s total aquaculture fish production, including freshwater systems, amounts to less than 1000 tons annually [
23], and aquaculture expansion is hindered by the Baltic Sea’s heavy eutrophication. Therefore, it is essential to develop and implement mitigation measures to eliminate nutrient sources related to fertilizer loading.
The updated Baltic Sea Action Plan [
2] establishes reduction targets for nitrogen and phosphorus for the Baltic Sea countries, necessitating numerous actions to meet these goals. For shipping and ports, this includes minimizing nutrient loads from dry bulk fertilizer handling operations. According to the results of this study, the amount of nitrogen discharged to the sea via stormwaters during fertilizer loading contributes to 15% of the total reduction target set by HELCOM at that coastal area.
General techniques have been identified to reduce the emissions during loading, as described by Coalition Clean Baltic in 2019 [
24]. Enclosing conveyors, chutes, and telescoping arm loaders is a simple yet effective measure to reduce dust emissions and further discharges to the sea. Another effective approach is to reduce the distance between the equipment and ship holds, which helps to minimize the freefall of material. In addition, it is recommended to suspend unloading and handling operations during unfavorable weather conditions, such as rain and wind, which can increase the run-off or blowing dust. Dust suppression techniques such as bag house filters, screw conveyors, and vacuum collecting equipment can also be introduced wherever practical. Finally, the regular sweeping of the bulk storage and access/egress areas, and handling swept material, is an important measure to prevent nutrients’ introduction into the Baltic Sea. The combination of these techniques can help to mitigate the impact of loading and unloading activities on air and water quality in port areas.
In recent years, substantial progress has been made in developing environmentally friendly methods for dry bulk handling in ports. Two Baltic ports have investigated potential solutions to reduce the environmental impact of fertilizers in the WISA (Water Innovation System Amplifier) project [
25]. Between 2017 and 2020, the project initiated preventive measures and successfully implemented new processes. In the Port of Åhus, several actions were undertaken, including terminal operator education on the environmental impact of products, spill prevention measures, enhanced cleaning processes and equipment, and spillage reduction initiatives during crane operations. Additionally, the Port of Åhus introduced environmental monitoring, conducting individual meetings with companies operating within the harbor area. The prevention measures described provided good improvements, as the discharge of nitrogen and phosphorus via stormwater was reduced by 60–70%.
In the case port of this study, the implementation of mitigating measures has been discussed with the loading terminals. It has been emphasized to the terminals that compliance with the environmental permit is essential, requiring the covering of stormwater wells during the loading process, ensuring the proper closure of loading grabs, and conducting a thorough cleaning of the berth after loading operations. Furthermore, to validate the effectiveness of the implemented measures, it is recommended to conduct a reassessment of the stormwater sampling period to determine if the discharge levels have indeed been reduced.
In the near future, it is crucial to implement measures to prevent the continued release of emissions into the sea, emphasizing the need for comprehensive actions by all stakeholders. However, the challenge lies in the absence of regulations specifically targeting the reduction of nutrient input to the sea, rendering all actions voluntary. Fertilizers, classified as non-Hazardous Materials (non-HME), are not regulated by the MARPOL convention, and existing environmental permits lack maximum limits for fertilizer discharges into the sea. As a step forward, HELCOM is currently developing recommendations for fertilizer ports to minimize their environmental impact. To enhance effectiveness, it is crucial to investigate alternative regulatory approaches, including potential modifications to environmental permitting processes. The consideration of making mitigation methods mandatory, while also considering their financial feasibility, is paramount. For the operating terminals, implementing mitigation measures, such as containment bags, filters, and weather-related operational breaks, may currently be perceived as economically challenging. In the long run, such measures could prove beneficial for both reducing environmental impacts and aligning with the increasingly stringent demands from stakeholders.