In Poland, environmental research in areas designated for offshore wind farms (OWF) started in 2012. At present, comprehensive knowledge is available on 6 areas of the OWF located in the Polish exclusive economic zone, precisely in the vicinity of the Słupsk Bank and areas to the east of it. Altogether they cover a region of considerable area, as much as 1890 km2. Due to the fact that the methodology and scope of conducted research in particular areas were almost identical, it was possible to compare environmental conditions in the OWF areas. For this purpose, the results of the inventory studies performed in 2012-2021 and published in the EIA reports were analyzed in detail. Compilations of information on marine organisms such as phytobenthos, macrozoobenthos, ichthyofauna, avifauna (seabirds and migratory birds), marine mammals, and bats are presented below, paying particular attention to similarities and differences in the qualitative and quantitative structure of particular groups of organisms between the OWF areas. In the EIA procedure it is the biological elements of the environment that constitute the main subject of consideration in terms of the significance of the OWF impact and, what follows, the legitimacy of the implementation of measures minimizing the negative impact of the OWF on the environment.
Phytobenthos are aquatic plants that include vascular plants rooted in the soft bottom (e.g., seagrass) and macroalgae that grow on hard surfaces such as boulders or wrecks (green algae, brown algae, and red algae). No vascular plants are found in the OWF areas, as they are only found in sheltered areas of the coastal zone, down to a depth of 9 m. In open sea areas, only macroalgae can be expected.
In the areas designated for the construction of the OWF, the presence of macroalgae was recorded only in one small area located near the northern boundary of Słupsk Bank, at depths of 21 to 26 m. The cover of the bottom by macroalgae there was less than 1%, which means that the rocky bottom was overgrown by one to several specimens per research transect. Macroalgae occurred as single specimens of small size and very sparsely distributed on the bottom. They were represented by 4 species: the brown algae Pylaiella littoralis and red algae Rhodomela confervoides, Coccotylus truncatus, Furcellaria lumbricalis (strictly protected species). Such a poor occurrence of phytobenthos is typical for the areas of the open waters of the Baltic Sea with depths of more than 20 m, also those in which OWF is located. Therefore, it can be assumed that the areas of future OWF are not valuable in terms of phytobenthos
Macrozoobenthos are invertebrates larger than 1 mm that live on the bottom or in the sediment. These organisms constitute an important food source for fish and birds. Between 21 and 33 taxa (species or higher taxonomic units) of macrozoobenthos were found in pre-investment surveys in the areas of planned offshore wind farms. Based on similarity of abundance and taxonomic composition, two assemblages of benthic fauna were distinguished in all studied areas except one (located north of Słupsk Bank). The first assemblage is associated with soft bottom, covered by sandy sediments (fine- and coarse-grained), the second – with hard substrate, i.e., boulders and pebbles. These assemblages, irrespective of the examined OWF area, differed from each other in species diversity and abundance of particular taxa – in the soft bottom from 21 to 33 taxa were recorded, while in the hard bottom – from 15 to 31. In the soft bottom assemblage, in most analyzed areas, 3 species of polychaetes – Pygospio elegans, Marenzelleria neglecta and Baltic Goldfinch (Bylgides sarsi) were observed most frequently and in greatest numbers. Only in the OWF area located to the north of Słupsk Bank, apart from P. elegans, representatives of the Oligochaeta and the crustacean – the common middens (Diastylis rathkei) were recorded most frequently. On the other hand, the rocky bottom community, irrespective of the OWF basin, was dominated in terms of abundance and biomass by the blue mussel (Mytilus edulis trossulus). Its density, mainly due to the presence of younger individuals, reached a maximum of 152,000 individuals per m2 (east of the Słupsk Bank). In contrast, the soft bottom was dominated in terms of biomass by the Baltic clam (Limecola balthica) – reaching a maximum of 294 individuals/m2 (east of Shoal). It is worth mentioning that the average macrozoobenthos abundance varied in space – the OWF areas at the northern boundary of Słupsk Bank generally showed lower values than the areas located farthest to the east from Słupsk Bank. It is important that in none of the examined OWF areas the presence of rare, protected, or endangered species was found. Species composition, abundance, and biomass of benthic fauna in the studied areas were found to be typical for shallow and medium deep seabed of the open zone of the southern Baltic Sea.
Ichthyofauna. The areas of the future OWF studied so far are like each other in terms of fish taxonomic composition. The most abundant species in all studied areas were cod (Gadus morhua), flounder (Platichthys flesus) herring (Clupea harengus), and sprat (Sprattus sprattus). The larvae of species under partial protection were also found striped seasnail and gobies. The analyzed areas of the OWF may be potential feeding grounds for the above-mentioned fishery species (cod, flounder, herring, and sprat). In all areas of the OWF, there are potential spawning grounds for sprat and herring, however, only in a small part of the area located at the north-eastern border of Słupsk Bank have potential spawning grounds for cod and sand eel have been found – based on the presence of larvae of the species. It should be noted that the presence of fish larvae does not indicate that the areas are permanently inhabited by adults. The larvae may originate from breeding areas in coastal zones or from more favorable areas such as the Słupsk Bank. To sum up, it should be stated that so far examined areas of the future OWF do not stand out against the background of the southern Baltic in terms of ichthyofauna species composition, but also in terms of the occurrence of feeding or spawning grounds, which allows concluding that the OWF areas are not characterized by exceptional natural values in terms of ichthyofauna.
Seabirds. Pre-investment surveys of seabirds showed high variation in bird abundance both spatially (depending on the study area) and temporally (due to the phenological period). Considering birds perched on the water, the highest number of individuals was recorded during the wintering period (from 607 to 46350 individuals) and the lowest during the summer (from 147 to 626 individuals). In terms of spatial variability, both the highest and lowest numbers of individuals sitting on the water were found in areas located north of Słupsk Bank. The analysis of the results of the research on birds flying over the OWF areas allows to conclude that the highest number of birds in flight occurs most frequently during autumn or spring migration. Similarly, as in the case of birds sitting on the water, the smallest number of individuals was observed during summer. During spring migration, when the highest total number of birds was observed, between 931 and 17,232 individuals were recorded, while during summer migration the lowest number of individuals was observed, between 193 and 596 individuals. The highest number of flying birds was observed in the area east of Słupsk Bank and the lowest in the OWF area north of the mentioned Natura 2000 site.
The most abundant species – taking into account birds sitting on the water and flying through the OWF areas and the total number of individuals observed in all phenological periods – was in most areas the long-tailed duck (Clangula hyemalis). The abundance of long-tailed ducks is linked to the fact that the Baltic Sea is the most important wintering ground for this species, so the species is widespread and occurs practically throughout the Baltic Sea. A high proportion of herring gull (Larus argentatus) was also recorded (in the area north of Słupsk Bank and the area furthest to the east it was the most abundant species). The presence of this species is highly dependent on fishing activity. In the OWF areas also fish-eating species (ichthyophagous) were present in large numbers, namely common guillemot (Uria aalge) and razorbill (Alca torda). Despite the dominant share of the species presented above, a relatively broad composition of bird species was observed in each of the OWF areas. Indeed, seabirds, waterbirds rarely seen at sea away from the coast, and birds of terrestrial habitats were recorded. In most cases, the number of observed flying bird species was higher than the number of birds perched on the water. A total of 12 to 18 species of seabirds were observed in individual areas of the OWF during the annual survey cycle. All of the observed species are under total or partial species protection in Poland. The identified species also include those listed in the Annex I of the Birds Directive. In each of the research areas such species as little gull (Hydrocoloeus minutus), black-throated diver (Gavia arctica) and red-throated diver (Gavia stellata) were observed.
The pre-investment surveys also showed that most of the OWF areas do not host high concentrations of seabirds. In most cases the average density was low, amounting to 15-25 individuals per km2. Only in the period of wintering and spring migration in the area located north of Słupsk Bank higher concentrations of birds were observed, reaching or even exceeding 50 individuals per km2. It was also proved that the vast majority of birds fly through the OWF areas at heights below 20 m above the water surface. The flights at heights from 20 to 250 m, concerned mainly numerous observed herring gulls, which often fly over the sea at a higher altitude than other seabird species.
Migratory birds. Pre-investment surveys of migratory birds showed a high species diversity of birds flying through the areas of future OWFs. At least several dozen bird species – both marine and terrestrial – were recorded in each area. In all areas of the OWF, species from the Annex I of the Birds Directive were recorded, such as: little gull (Hydrocoloeus minutus), black-throated diver (Gavia arctica), red-throated diver (Gavia stellata), common crane (Grus grus), whooper swan (Cygnus cygnus), short-eared owl (Asio flammeus). The results of the study were also consistent in terms of the most frequently observed species in the different migration seasons. The autumn migration was dominated by sea ducks (the most numerous species were long-tailed duck (Clangula hyemalis), common scoter (Melanitta nigra), velvet scoter (Melanitta fusca), geese (greylag goose (Anser anser), bean goose (Anser fabalis), white-fronted goose (Anser albifrons)), in some cases also passerine birds (eurasian skylark (Alauda arvensis), common starling (Sturnus vulgaris), siskin (Carduelis spinus), common chaffinch (Fringilla coelebs)), while during the more intensive spring migration season, sea ducks (long-tailed duck, common scoter, velvet scoter) were observed in the highest numbers, and in some study areas also high intensity of migration of auks (razorbill (Alca torda), common guillemot (Uria aalge), black guillemot (Cepphus grylle)) was recorded. However, there were significant differences in terms of the number of individuals recorded. When comparing the results of the surveys from particular areas of the OWF, one can notice a wide range of the total number of observed individuals of particular species (e.g., long-tailed duck, whose number of visual observations ranged from 444 to 3121 individuals). This aspect is largely related to whether the area lies on a major migration route. Where on the basis of the results of the pre-investment research it was found that zones of important migration corridors exist over the OWF area, the Regional Director for Environmental Protection imposed in the decision on environmental conditions the obligation to designate a development-free area. The migration corridors so far designated are shown on the map below.
In most OWF areas, during the autumn season, most birds migrated in September and October, while during the spring season, the highest number of flights was recorded in April. Migration directions were typical for the respective seasons and consistent with the expected migratory purpose, with autumn migration dominated by flights to the west and south-west, while spring migration was mostly in the eastern and north-eastern direction. In terms of flight height, the predominant number of recorded individuals flew through OWF areas during the day at heights of up to 20 m. During night migrations, which were characterised by higher intensity than during the day, flights were recorded at much higher altitudes, in many cases relatively intense migrations were observed at heights of up to 500 m.
Marine mammals. Harbour porpoise (Phocoena phocoena) as well as grey seals (Halichoerus grypus) and harbor seals (Phoca vitulina) were found in all OWF areas. In general, the areas of the future OWF were characterized by negligible marine mammal activity. It is not excluded that the OWF areas may be a place of occasional feeding of harbour porpoises and grey seals (considering the presence of their food base – herring and cod in the given areas), as well as migration areas, through which these animals migrate to places more abundant in food. This is not the case for common seals, which often stay close to their resting places on land. The observed individuals of harbour porpoises were adults and therefore it can be concluded that the analyzed areas of the OWF are probably not breeding sites for them, and thus are not significant for them.
Bats. Ten taxa of bats have been observed in the southern Baltic region. Due to the fact that their migration corridors run mainly along the Baltic coast, only sporadic activity of these mammals has been observed in the areas of planned offshore wind farms so far. During spring and autumn migration, 0 to 5 bat taxa were found in the OWF areas. The species that were recorded most frequently were the great aurochs (Nyctalus noctula), the greater carlik (Pipistrellus nathusii) and the small carlik (Pipistrellus pygmaeus) (all protected). Due to such scarce activity and migration intensity of bats, the areas studied under the OWF are considered to be of low importance for this group of mammals.
In conclusion, the marine environment characterized by marine organisms is largely homogeneous and the OWF areas are not distinguished by special natural features in comparison to the Polish Baltic area. An exception is avifauna, especially the occurrence of migration corridors of birds, which thanks to the research in the OWF areas began to be identified for the first time in Polish maritime areas. At this point, the question arises whether the scope of the planned pre-investment environmental research in the areas of the OWF from the second phase of offshore wind farm development in Poland should not be verified? Is there still a need to conduct such extensive and thus very expensive research on all biological parameters? These questions seem justified in view of the tight schedule (according to EPP2040, the installed capacity of offshore wind farms is expected to reach about 5.9 GW in 2030 and 11 GW in 2040) and the limited resources necessary to perform environmental research, both of specialists, research units and specialized research apparatus and equipment. These aspects apply not only to Poland but also to other Baltic Sea countries [LINK], where offshore wind energy development has recently strongly accelerated.