F. Basarı et al. 
cauna, including marine mammals, fish and invertebrates (e.g. 
Aichardson et al., 1995; Wisniewska et al., 2018; Thomsen et al., 2020; 
Juarte et al., 2021; Frankish et al., 2023). It is therefore essential to 
assess the impact of URN on the marine environment to evaluate future 
measures that can be taken to reduce the emission of URN. 
In 2008, the Marine Strategy Framework Directive (MSFD) of the 
European Union (EU) recognized underwater noise as one of eleven 
descriptors that determine the environmental status of marine regions. 
initially aiming to achieve or maintain a Good Environmental Status 
(GES) of the European Seas by 2020, all Member States were required to 
develop strategies or measures to achieve this status (European Com- 
nission, 2008). Those marine strategies also include monitoring and 
assessment of each descriptor that should be established to determine 
the state of all marine waters on a regular basis (Dekeling et al., 2014). 
The MSFD Technical Group on Underwater Noise (TG Noise) along with 
the OSPAR convention require the combined use of measurements and 
numerical models to determine levels and trends of ambient noise and 
encourage international collaboration within sub regions to guarantee 
consistent monitoring (Dekeling et al., 2014; Van der Graaf et al., 2012; 
Snoek et al., 2015). Such collaboration has been established for all Eu- 
ropean seas within the past decade in the form of regional projects 
(Merchant et al., 2022). 
The EU project “Joint Monitoring Programme for Ambient Noise in 
che North Sea” (JOMOPANS), commissioned by the EU Interreg North 
Sea Region programme in 2018, was one of these projects. It aimed at 
developing a framework for joint monitoring of ambient noise to aid 
oolicy makers in evaluating the status of the North Sea in relation to 
URN. Data from 19 stations across the North Sea (Fig. 1) throughout the 
years 2019 and 2020 were shared between twelve institutions (NL*?* 
SE,* DK,° DE,° UK”&9 BE.1° NO1L12) to address the transnational issue 
of continuous underwater noise pollution in the North Sea region. The 
primary aim of the project was to -produce maps of ambient sound in the 
North Sea (taking into account shipping and wind as sound sources), 
rom which URN was then assessed and coordinated measures devel- 
oped (Kinneging et al., 2021). 
The monthly statistical noise maps were produced for 2019 and 2020 
by using models. These maps were validated with the in-situ measure- 
ments to assess the performance of the acoustic model (Putland et al., 
2022). The modelled sound maps showed that shipping noise dominates 
‘he lower frequencies (up to 2 kHz) of the North Sea underwater ambient 
20ise (de Jong et al., 2022). In southern parts and along major shipping 
lanes, the shipping noise exceeds the broadband natural sound of wind 
by >20 dB for >50 % of the time (Kinneging et al., 2023). 
To enable policy makers to assess the impacts of URN, the JOMO- 
PANS project further developed a management tool, in which the sound 
maps can be overlaid with distribution maps of sensitive species 
(Jomopans-Interreg North Sea Region, 2021). 
Highlighting the model results of the JOMOPANS project we present 
the Dominance Noise Map for the whole year 2019 (Fig. 1), representing 
‘he areas of the North Sea that are mainly impacted by anthropogenic 
noise. The map shows over which percentage of the time anthropogenic 
noise dominates over the natural ambient noise by >20 dB (in the 
* Rijkswaterstaat Ministerie van Infrastructuur en Waterstaat 
* Netherlands Organization for Applied Scientific Research (TNO) 
* WaterProof Marine Consultancy & Services B.V. 
Swedish Defence Research Agency (FOI) 
* Department of Ecoscience - Aarhus Universitet 
* Federal Maritime and Hydrographic Agency (BSH) 
” Marine Scotland 
* National Physical Laboratory (NPL) 
? Centre for Environment, Fisheries and Aquaculture Science (CEFAS) 
vo Royal Belgian Institute of Natural Science (RBINS) 
'1 Institute of Marine Research (IMR) 
3 Norwegian Defence Research Establishment (FFD 
Marine Pollution Bulletin 198 (2024) 115891 
broadband frequency range from 10 Hz to 20 kHz). 
Most stations are located in the relatively shallow waters of the North 
Sea (below 50 m) - only the Scottish and Norwegian stations are in 
deeper waters (max. 340 m at the 13-NO-LOV station). The relatively 
shallow depths in the North Sea are an important feature for the sound 
propagation in the area. The depths determine the dominant frequency 
that propagates through the water (Jensen and Kuper, 1983) and due to 
the shallow depths; propagation loss with range is greater compared to 
deeper waters (see Table 2 and Fig. A for depths at the measurement 
stations and an overview of the bathymetry in the North Sea). For more 
zontext and further orientation maps of the three main variables treated 
in this study (currents, wind and shipping) are provided in the supple- 
mentary material (see supplementary Figs. B-D). 
For the North Sea region, the JOMOPANS measurements are unique 
in terms of international collaboration, the number of stations, their 
spatial extent and the extended time period over which data are avail- 
able. However, similar large-scale monitoring efforts have preceded 
JOMOPANS in other regions — e.g. the BIAS project in the Baltic Sea 
‘Mustonen et al., 2019) and the national monitoring of the United States 
‚Haver et al., 2018). 
The measurements provide an opportunity to evaluate the overall 
ambient noise and not only quantify sounds from specific animals or 
specific anthropogenic activities, which is often the aim of passive 
acoustic monitoring. In this study, the overall objective was to explore 
the collected measurement data in more detail to answer the following 
three questions: 
L. What are the acoustic characteristics at the individual stations and 
what are similarities across the stations? 
Which frequency bands are affected by, and how are sound pressure 
levels related to, wind and shipping? 
3. What is the impact of flow noise on the measurement data? 
A 
Wind-generated sound is the predominant natural component of the 
marine underwater ambient soundscape (Knudsen et al., 1948; Wenz, 
962). Waves are created and grow by the wind blowing over the water 
surface. During periods of higher wind speeds, waves break and create 
sound underwater by oscillation of bubbles from entrained air (Banner 
and Cato, 1988; Medwin and Beaky, 1989). Between 500 Hz and 50 kHz, 
wind-generated sound often dominates, depending on the location, 
water depth, wind conditions and the presence of other sound sources 
‘see Fig. 3 as an example). In accordance with the wind noise model that 
was used in JOMOPANS (de Jong et al., 2021) a clear correlation and a 
linear scaling is expected between logarithmic wind speeds and 
measured acoustic sound levels (Ainslie, 2010; de Jong et al., 2022; 
Hildebrand et al., 2021). Importantly, as the prevailing wind and sea 
state conditions are closely connected, the wind field may indirectly 
influence other sound sources (e.g. decrease in ship traffic or offshore 
activities due to high sea state). Increased wind speed also affects the 
propagation of ship noise through increased attenuation and scattering 
when sound interacts with the rougher sea surface. 
The significant energy of underwater sound emitted by vessels has 
often been described to be between 10 Hz and 1000 Hz (e.g. Richardson 
at al., 1995; Hildebrand, 2009; Merchant et al., 2012). However, Her- 
mannsen et al. (2014) also showed considerable energy above ambient 
sound levels up to 150 kHz at distances to the ship between 60 and 1000 
m. Small boats with proportionally smaller engines and faster revolving 
propellers generate less low frequency noise (Au and Green, 2000; 
Hermannsen et al., 2019), which is particularly relevant for more coastal 
stations such as 01-SE-VIN or 11-SC-HEL, where close passages of small 
boats are likely. Ships operating in DP (dynamic positioning) mode to 
stay at a certain position using bow and stern thrusters also emit sound 
n higher frequencies - up to 16 kHz (Fischer, 2000). This might affect 
neasurements near offshore wind farms or oil rigs, such as 06-DE-FN1 or 
18-DK-EDA where service vessels operate in DP-mode on a regular basis. 
Flow of water around the surface of a hydrophone induces local