*5.4. Analysis of the Project's Impact on Marine Circulation and Nutrient Distribution*

Once the offshore wind farm is in operation, the operation of the wind turbines could generate changes in the atmospheric and oceanic dynamics due to the capture and modification of wind energy. On the other hand, the floating platforms, with a draft of −15 m, could generate a certain wave reduction effect or local alteration of the currents.

The main expected effect could be the reduction in wind energy in the area of the wind farm, associated to the wake effect, which consists of the reduction of wind speed and kinetic energy downwind of the wind farm.

It is considered that the capacity of the underwater structures to alter the hydrodynamic regime of the Gulf of Roses is very low, considering the depth at which the wind farm is located (>100 m) and that the installed structures (floating platform, chains, anchors, and inter-array cables) are not large enough to produce a significant reduction in current flows, stratification, or vertical transport phenomena in the water column.

Although these effects may happen, they will be very localized in the immediate surroundings of the structures and of little entity at the level of the Gulf of Roses (where they will probably be imperceptible), which does not confer the ability to become cumulative or synergistic downstream. The effect of this potential reduction in the wind and hydrodynamic regime on the development of species and habitats is assumed to be low, due to the expected low incidence of the reduction in physical effects on the column and seabed of the installed wind turbines, as well as the capacity of biological adaptation of the species potentially affected to small changes in the circulation.

This impact has first been assessed on the basis of a critical analysis of the existing literature, and a rigorous study by IHCantabria about the impact of the project on wind fields and marine currents has also been undertaken, by means of numerical modeling. This study, whose methodology has been presented to the Spanish National Research Council (CSIC) experts for their approval, and no objections have been received from them, will provide quantitative information on the foreseeable variation in current speeds in the area of the Gulf of Roses.

As a result of the bibliographic analysis, some scientific publications that analyze and document wind field alteration effects were identified, mainly based on wind farms located in the North Sea [37]. In some cases, it is suggested that these alterations could lead to increases in precipitation, although the predicted changes are so small in magnitude that it is difficult to distinguish them from natural variability. According to Platis et al. (2018) [38], the wake effect is greater under stable atmospheric conditions than under turbulent conditions. When conditions are turbulent, such as those associated with the Tramuntana winds that will feed the wind farm, the wake effect is restricted to the local area inside the wind farm and the immediate surroundings.

It should also be noted that in general, regarding wind disturbance, the numerical models give overestimated values with respect to the field measurements. This divergence between modeled and measured values indicates deficiencies in the knowledge of the effects. Measurements made in the "Horn Rev" and "Nysted" fields in the Baltic Sea show reductions of 8–9% immediately after the field, and 2% at distances between 5 and 20 km [39]. As for the regional effect, it is estimated to be small, the energy loss in the first kilometer of the atmosphere being 0.007%. The conclusion is that floating wind farms may have a minor to moderate impact on atmospheric and oceanic dynamics (depending on the location and size of the wind farm), although there is insufficient specific knowledge of the cascading effects of large-scale atmospheric and oceanic processes to reduce the current uncertainty.

In relation to the possible effect of the alteration of currents on the distribution of nutrients, Van Berkel et al. (2020) [40] point to the potential upwelling effect associated with the wake, causing the upwelling of deep-water mass to the surface. However, this effect depends on the relative direction between the wind and the coast, being a frequent phenomenon on the Atlantic coast but not so in the western Mediterranean.

It should be noted that the bibliographic references in which appreciable variations in the current regime have been observed as an indirect consequence of the presence of a wind farm correspond to cases in shallow areas, with fixed foundation wind farms, and with a high concentration of turbines, being scenarios not very comparable with the project scenario.

The results of the hydrodynamical study by means of numerical modeling developed by IHCantabria show that the effect of the wind farm over the surface currents and those averaged over the water column in the coastal area is very small, with an averaged difference in the currents magnitude below 1 cm/s and maximum values of 1 cm/s during 98% of the time.

In the proximities of the wind farm in the open sea, the differences obtained, in both surface and averaged water column, remain below 1 cm/s at the north, east, and west from the farm, while at the south this difference is up to 2 cm/s, which represents an averaged variation inferior to 3.5% of the average speed registered in the study area by the buoy of *Puertos del Estado*.

In any case, to confirm the potential impact, the mitigation measures and Environmental Monitoring Plan propose monitoring the wind and current regime of the involved area, as well as monitoring the thermohaline structure of the main indicators of the water masses (temperature, salinity, turbidity), nutrients in the water column, and monitoring the evolution of the main pelagic and benthic communities in the affected area, whose evolution will be analyzed with a Before-After Control-Impact (BACI) study approach.

The impact is considered significant, permanent (during the life of the wind farm), and synergistic. Due to the uncertainty of the effect and the fact that mitigation measures cannot be developed, a medium magnitude is assigned to it. The resulting classification is shown at Table 8:


**Table 8.** Assessment of the impact on marine hydrodynamics.

#### *5.5. Analysis of the Impact of the Project on the Proliferation of Invasive Alien Species*

Another aspect that has been identified by scientists as a possible threat to marine biodiversity, in association with offshore wind farms, is their potential to promote the proliferation of invasive alien species.

According to the available knowledge by the scientific community on the proliferation of allochthonous species in the Mediterranean, either introduced intentionally or by chance by different ways, it is currently considered that this phenomenon could be one of the main causes of biodiversity loss in the Mediterranean. The number of invasive alien species inventoried in this sea in 2012 reached almost a thousand (more than 5% of the species present in this sea), with more than 300 identified in the western basin [41].

There is a wide variety of species with the potential to become invasive, although organisms with the ability to attach to hard surfaces (biofouling), including mollusks, are generally the most notorious. The known introduction ways are numerous, one of the most relevant being the opening of the Suez Canal in 1869, which involved communication between the Red Sea and the Mediterranean Sea, favoring the migration of numerous species from the Red Sea to the Eastern Mediterranean, some of which have come to spread throughout the Mediterranean basin [42].

Other known ways of anthropogenic introductions are the transport of sessile species attached to the hull of merchant ships or other structures moving from one place to another, the discharge of ballast water, which carries planktonic and nektonic organisms (some of which are larval stages of invasive species), the introduction of species of interest for mariculture and fish farming (which often carry parasitic organisms), the intentional or accidental release of specimens by aquariums, or even the recent proliferation of microplastics in the seas [43].

Regarding this threat, it is also worth noting the effect that climate change is likely to have on this phenomenon, since it is expected to affect, by modifying water temperature, the structure of marine communities, providing more opportunities for exotic species to disperse and compete with and displace native species [42].

Based on current knowledge, and in relation to the structures and actions foreseen in the project, the following possible routes of entry of allochthonous species into the project area are contemplated:

• Through adherence to the structures of the foundation floaters, during their towing from the manufacturing or assembly port:

The structures are expected to be manufactured either in western Mediterranean ports, with a low probability of introduction of new species, or from the Atlantic ports of the Iberian Peninsula. The rest of the elements that will remain on the seabed (submarine cables, anchors, and chains) are not expected to be exposed to marine colonization outside the park, since they will be transported aboard ships.

In both cases, the main means of transport will be semi-submersible vessels between the manufacturing port and the wind turbine assembly port (e.g., in Tarragona), so possible adhesions will only occur during transport between the port of Tarragona and the wind farm site. The probability of introduction of allochthonous species is therefore very low, and comparatively lower than that which may be due to the transit of merchant ships coming from the Strait of Gibraltar or the Suez Canal to ports in the western Mediterranean.

• By attachment to working vessels during construction (cable-layers, tugboats, etc.): These vessels are highly specialized and generally work in all the world's oceans, so there is a possibility that they may carry allochthonous species attached to their hulls, with potential for invasion and displacement of native species. However, this is not a significant threat, in relation to the number of vessels with which it is associated, compared to the intense maritime traffic of merchant ships, pleasure craft, fishing boats, or cruise ships that sail the waters of the western Mediterranean from other seas.

• By releasing ballast water from the foundation platforms:

The floating turbine foundation platforms have ballast systems that involve filling or emptying tanks integrated into their structure to contribute to the stability of the whole. The partial filling of these tanks will be carried out at the assembly port (Tarragona), ending at the location of the wind farm.

It is therefore not foreseeable, as has been argued above in the case of the adhesion entry route, that these ballast waters will constitute a significant entry route for invasive non-native species.

Finally, the potential effect of the floating structures themselves, as well as the anchoring system and the inter-array cables, which will be the only elements exposed to marine colonization, should be evaluated as possible elements of attraction and settlement of invasive alien marine species. This effect, far from introducing new species, could only contribute to a greater growth of the species present in the area. Its effect would be equivalent to that provided by any structure capable of providing a substrate for the settlement of sessile organisms, especially if it is located within the photic zone, where photosynthesis occurs and with it the greatest growth of biomass. That is, it would be equivalent to that associated with boat hulls, buoys, fishing gear, artificial reefs, or shallow rocky bottoms.

It should also be noted, in the case of platforms, that these structures are protected against corrosion and biofouling by means of cathodic protection systems and/or impressed current, which significantly minimizes biological adhesion. Therefore, only the surface of the chains and inter-array cables would be exposed to colonization at a relevant level.

It is possible that the presence of the floating structures may generate slight changes in the behavior of pelagic species, which may be attracted or repelled in their immediate environment depending on their feeding, predatory, shelter, or reproduction habits. However, these effects are not considered to be relevant in terms of compromising the conservation of these species, especially when compared to other pressures on these species in the project area, such as trawling or maritime traffic.

For all of the above reasons, the effect of the project on the proliferation of invasive alien species was considered not significant for the purposes of impact assessment.

#### **6. Conclusions**

The social controversy related to the introduction of offshore wind power in Catalonia has scaled to a scientific debate about its environmental compatibility, and specifically in relation to the Parc Tramuntana project. Recent publications have contributed to sustaining this debate, without being adequately based on rigorous analysis of both the characteristics of the project and its specific location within the Gulf of Roses, so it has been considered appropriate to provide an exhaustive analysis of the main impacts identified by the scientific community in relation to offshore wind farms.

The aim of this report was therefore to dispel some of the uncertainty that motivates this opposition, based on detailed information and quantitative data, in order to place these impacts in a realistic context and determine their true magnitude.

This is possible because a series of diverse and detailed studies have been developed over the last two years to assess the environmental impact of the project, which are relatively advanced and ready to provide some well-founded conclusions. In the meantime, the study continues to be prepared and, once the relevant studies are completed, it will be submitted as part of the documentation required by current legislation for the environmental processing of the project. Although this procedure contemplates a 30-day public information phase, it is desired to maintain a less limited channel of dialogue and prior communication, for which reason the promoter commits to advance as far as possible the conclusions that emerge from the environmental studies and to make them available to any interested person through the project's web page.

It should be noted that both the project and the corresponding environmental impact study are being developed in accordance with current environmental processing regulations and based on the Scope Document submitted by the General Directorate for Environmental Quality and Assessment of the Ministry of the Environment. Likewise, the design of the wind farm's infrastructure is in accordance with the conditions established by urban and sectorial regulations.

Based on the studies carried out, which are based on a standardized methodology, the impact of the project is objectively classified as MODERATE on vectors such as turbidity and sedimentation, underwater noise, hydrodynamic circulation, or the alteration of electromagnetic fields, and NOT SIGNIFICANT on aspects such as the proliferation of invasive exotic species (Table 9).


**Table 9.** Summary of analyzed environmental impact ratings and characterization.

Impacts on turbidity and sedimentation are expected along the evacuation cables track during construction stage and only in the close vicinity of turbine moorings during the operational stage.

Submarine noise is expected also during both stages, mainly associated to construction equipment and ships during construction and to both maintenance ships and turbine structure vibrations during operation, although emission levels are not expected to be higher than current background noise in the area or to cause severe damage to sensitive species present in the area.

EMF are only expected in operational stage, being significant mainly in the close vicinity of the floating inter-array cables. Generated fields are expected to be detectable by sensitive species, but not strong enough to cause severe alterations in their behavior.

The impact on marine hydrodynamics, based on numerical modeling, is expected to be low, limited to changes in currents magnitude of 1–2 cm/s in the vicinity of the wind farm during operational stage.

Finally, the effect of the project on the potential proliferation of invasive species was considered not significant, as there are small chances for the introduction of alien species associated to project structures, being irrelevant when compared with other identified introduction vectors in the area.

In general, the studies carried out allow to anticipate a low impact of the floating offshore wind farm project on marine biodiversity in the area of implementation, which makes it compatible with the conservation of such biodiversity.

In addition, most of the significant impacts identified allow the application of measures to mitigate their effects, as well as their follow-up during the environmental monitoring derived from the environmental processing, which will make it possible to know the evolution of the wind farm's effects on the environment, as well as the adoption of additional measures if necessary.

These initial conclusions address only certain environmental aspects of the project that have been identified as being of greatest concern to some members of the scientific community. As the environmental impact study progresses, it is expected that conclusions regarding other potential environmental or socioeconomic impacts will also be published.

It is clear that any project, of whatever nature, introduces certain changes in the environment and therefore has an associated impact. However, it is equally true that it is imperative to slow down climate change by all available means, among which the transformation of the energy generation model to a renewable model is a priority.

It should be remembered that the effects of climate change predicted by the IPCC for the coming decades (many of which are already occurring today, including in the Mediterranean) include an increase in the temperature of the planet and its oceans, the extinction and anomalous migration of species, colonization by invasive species, the modification of marine currents and acidification of the seas, droughts, and catastrophic climatic phenomena.

Therefore, the suitability or otherwise of offshore wind power in Catalonia should not be evaluated solely from the perspective of its effect on biodiversity, especially considering that climate change itself will certainly introduce much more drastic changes in Mediterranean ecosystems than those that can be attributed to any individual project, resulting in more severe, lasting, and irreversible impacts on marine biodiversity.

Thus, from an equanimous and global vision, the assessment of a project such as Parc Tramuntana should be carried out with scientific rigor and without falling into the tunnel vision that only focuses on the local negative impacts, without placing them in context. Because, according to environmentalists [44], even if it is accepted that, like any other renewable energy project or any other type of project, it may have a certain level of impact on local biodiversity, the relationship between this impact and the protection of biodiversity throughout the Mediterranean is largely in favor of the latter.

Finally, in response to the arguments calling for a halt to marine renewables under a precautionary principle that only greater scientific knowledge can overcome, it is necessary to indicate that unfortunately the time available for the study is limited, since the climate emergency is already a reality and requires an urgent response. It is therefore a priority to join efforts to ensure the optimal response to the climate challenge, as is being done in neighboring countries.

**Author Contributions:** Conceptualization, K.D.-C. and S.T.; methodology, K.D.-C.; software, J.G.- A.; formal analysis, J.G.-A.; investigation, K.D.-C. and S.T.; supervision, S.A.; writing—original draft preparation, S.T. and K.D.-C.; writing—review and editing, S.A., R.J., M.G. and J.R.V.; project administration, M.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** Mr. Koldo Diez-Caballero and Mr. Juan Ramón Vidal are employees of Tecnoambiente S.L., an environmental consultant that has developed the hydrographic, geophysical and environmental surveys of the project area, and is developing the Environmental Impact Assessment studies of the offshore wind farm project. Ms. Silvia Troiteiro, Ms. Marta González, Mr. Sergi Ametller and Ms. Raquel Juan are employees of SENER Ingeniería y Sistemas, S.A., a company of the SENER Group. The offshore wind farm mentioned in the article is being developed by BlueFloat Energy International, S.L.U. together with SENER Renewable Investments, S.L. (a company of the SENER Group) as minority partner. Ms. Silvia Troiteiro, Ms. Marta González, Mr. Sergi Ametller and Ms. Raquel Juan have participated in the elaboration of the study referred to in the article and have had access to all information produced by such study. Mr. Javier García-Alba is an employee of the IHCantabria, a non-profit research organization that has developed the specialized hydrodynamic studies to determine the impact of the wind farm on marine dynamics.
