NSP2 route with V2 - Line A
7.8 Benthic flora and fauna
Zoobenthos (benthic fauna) and phytobenthos (benthic flora) are important components of the marine food chain and of the ecosystem of the Baltic Sea, often playing the role of “habitat build-ers”. Therefore, although no species that are listed as near threatened, endangered or vulnerable in the HELCOM Red List are expected along the proposed NSP2 route, the NSP2 route V1 or the NSP2 route V2, benthic flora and fauna are considered an important receptor.
Benthic flora
7.8.1.1 Benthic flora in the Baltic Sea
The benthic flora in the Baltic Sea encompasses species-rich seagrass meadows and macroalgae in shallow areas.
Benthic flora is primarily dependant on the availability of light at the seabed. The photic zone is defined as the depth at which 1% of the surface irradiance remains, and typically reaches down to
a maximum depth of 20 m in the Baltic Sea. At depths greater than 20 m, the absence of light prevents phytobenthos from growing on the seabed, and there will thus be no benthic flora /150/.
In the places where light permits benthic flora, a number of other factors influence the biomass and composition, e.g. substrate type, salinity and oxygen concentrations /150/. The marine flora in the Baltic Sea comprise primarily macroalgae and a few species of sea grass and water moss species. Salinity influences the species richness, and the number of marine macroalgae decreases from south-west to north-east in the Baltic Sea with decreasing salinity level.
Hard substrates such a rocks are dominated by brown and red seaweeds, while shallow sandy bottoms can be inhabited by seagrass.
7.8.1.2 Benthic flora in the Danish section
The photic zone is shown in Figure 7-33. It can be seen that the potential for benthic flora in Danish waters exists nearshore around Bornholm and on Rønne Banke south-west of Bornholm.
Figure 7-33 Map showing photic zone, and thus the potential for benthic flora.
Hard-bottom communities consist mainly of macroalgae: typically green, brown and red algae.
Green algae tend to be the most abundant in shallower waters, brown algae (e.g. Fucus spp.
Laminaria spp.), are found in both shallow and deeper parts and red algae (e.g. Furcellaria, Cera-mium) in deeper parts. Soft bottom communities in shallow waters are often dominated by vascular plants or water moss (charophytes). Vascular plants rarely occur at depths greater than 6-8 m.
Typical species include eelgrass (e.g. Zostera marina) and pondweeds (e.g. Potamogeton spp.).
7.8.1.3 Benthic flora along the NSP2 route, the NSP2 route V1 and the NSP2 route V2
The Danish section of the proposed NSP2 route, as well as the NSP2 route V1 and the NSP2 route V2, are situated at water depths below 20 m, which is below the photic zone; hence, no benthic flora is expected.
7.8.1.4 Biodiversity
In 2017, HELCOM assessed the integrated biodiversity status for benthic flora, but this assessment did not cover the Arkona Basin or the Bornholm Basin /111/.
7.8.1.5 Conservation status
The Danish Red List /166/ does not include benthic marine flora.
The HELCOM Red List for the Baltic Sea /167/ includes seven benthic flora species which are con-sidered threatened (Critically Endangered, Endangered or Vulnerable). Three species were assigned to the category Endangered: one charophyte, Lamprothamium papulosum, and two vascular plants, Persicaria foliosa and Hippuris tetraphylla. Four species were categorised as Vulnerable:
charophytes Chara braunii and Nitella hyalina and the vascular plants Alisma wahlenbergii and Zostera noltii. Four species were assessed as Near Threatened: two charophytes Chara horrida and Nitellopsis obtusa and two vascular plants Crassula aquatica and Potamogeton friesii. All Threat-ened and Near ThreatThreat-ened species are characteristic for soft-bottom, sheltered environments. No species identified along the proposed NSP2 route, the NSP2 route V1 or the NSP2 route V2 are listed as Near Threatened, Endangered or Vulnerable on the HELCOM Red List.
Benthic fauna
7.8.2.1 Benthic fauna in the Baltic Sea
The benthic fauna in the open Baltic Sea are primarily affected by salinity and oxygen supply.
Salinity (see sections 7.4 and 7.5) has an impact on the biodiversity of benthic fauna /171/. As illustrated in Figure 7-34, species richness in the open waters in the Baltic Sea decreases from over 1,600 marine benthic species in the open Skagerrak to about 500 in the western part of the Baltic Sea (west of Bornholm), approximately 80 in the western regions (east of Bornholm) and fewer than 20 in the eastern regions of the Gulf of Finland. The species richness of marine species such as polychaetes, molluscs and echinoderms is thus dramatically reduced from west to east /150/.
Conversely, the diversity of freshwater benthic species increases towards the inner reaches of the Gulf of Finland and the Gulf of Bothnia /150//171/. The geographical trend in species richness in the Baltic Sea largely holds true for the open and deeper waters in the Baltic Sea. However, the trend is less distinct closer to the coasts and in shallow waters, with these areas demonstrating a consistently high species richness due to habitat complexity and variable substrates /150/.
Figure 7-34 Number (arbitrary scale) of marine, brackish and freshwater species in open waters, corre-lated with salinity. The range of salinity in the Baltic Sea is indicated as the mean surface water salinity between the Bothnian Bay and the Kattegat. PSU stands for practical salinity units.
Oxygen conditions are crucial for benthic fauna, and benthic habitats in the Baltic Proper are, in general, strongly affected by the prevailing low-oxygen concentrations (see sections 7.4 and 7.5).
Even occasional oxygen depletion will inhibit the usual successional pattern and prevent the devel-opment of a mature benthic community. Recurring low-oxygen concentrations lead to a diminished benthic diversity and dominance by opportunistic species. Tolerance to low-oxygen concentrations is, in general, species-specific, but also depends on the rate of oxygen decline, the duration of low-oxygen concentrations and temperature /172/. Mobile species will show a behavioural response at oxygen concentrations below 4 mg O2/l, while concentrations below 2 mg O2/l are critical for or-ganisms and typically result in the deaths of a number of oror-ganisms. The response is highly species-specific, with some species able to survive complete anoxia for weeks to months. The development of permanent anoxic conditions and subsequent release of toxic hydrogen sulphide will directly impact the survival of zoobenthos.
In the deep basins, the concentration of dissolved oxygen in the bottom water is the most critical factor influencing species richness and the presence/absence of soft-bottom zoobenthos along the proposed NSP2 route, the NSP2 route V1 and the NSP2 route V2 /150//171/. HELCOM and the International Council for the Exploration of the Sea (ICES) have reported that approximately one-third of the total area of the seabed in the Baltic Sea is without benthic fauna /173/.
Furthermore, the composition of the benthic community and the species abundance are subject to a number of other factors, including light, seabed substrate conditions, water movement, water quality, food supply, trophic competition with invasive species, etc.
Generally, the benthic communities in the Baltic Sea all belong to the so-called Macoma community and are characterised by the bivalve Limecola balthica (formerly known as Macoma balthica) and a few other species, e.g. the common mussel Mytilus edulis. The small, brackish amphipod crusta-cean Pontoporeia (Monoporeia) affinis, the isopod crustacean Saduria entomon and the invasive polychaete Marenzellaria are likewise characteristic species in the Baltic Sea. In the basins of the
open part af the Baltic Proper, benthic communities are often characterised by the amphipod crus-tacean Pontoporeia femorata and the Polynoidae Bylgides sarsi /171/. The aforementioned crusta-ceans are all considered ice age relicts of the Baltic Sea.
7.8.2.2 Benthic fauna in the Danish section
The benthic fauna communities in the Danish section are shown in Figure 7-35. In the benthic areas east of Bornholm, recurring low oxygen concentrations in the deeper waters lead to a benthic community dominated by opportunistic species, e.g. the Polynoidae Bylgides sarsi. South and south-west of Bornholm, towards Rønne Banke, the benthic fauna are characterised by the bivalve Limecola balthica, the small crustacean Pontoporeia (Monoporeia) affinis, and the polychaete Ma-renzellaria /171/.
Figure 7-35 Regional distribution of benthic fauna in the Danish section of the Baltic Sea /171/.
7.8.2.3 Benthic fauna along the proposed NSP2 route, the NSP2 route V1 and the NSP2 route V2
As described in section 7.4, poor oxygen conditions prevail along most of the proposed NSP2 route, the NSP2 route V1 and the NSP2 route V2, which limit the presence of higher trophic levels. The depth profiles along each route alternative in Danish waters and general sediment types are shown in Figure 7-36.
Figure 7-36 Depth profile and overall substrate type along the pipeline transect through the Danish section based on bathymetry data and general sediment data from GEUS /104/ (see section 7.3.2, Figure 7-17) . The combination of the proposed NSP2 route with V1 is shown at the top and the combination of the proposed NSP2 route with V2 is shown at the bottom. Mud consists mainly of clay and silt (<0.1 mm diameter), while sand mainly consists of mineral particles between 0.1 and 2 mm diameter. The depth curve is based on the bathymetry sea chart. The halocline layer is at a water depth of between 40 and 70 m, see section 7.4.
The water depth is 60-90 m along most of the combination of the proposed NSP2 route with V1 and along the combination of the proposed NSP2 route with V2. In these deep parts, the seabed consists of fine sediments, mainly silt and clay (<0.1 mm), and the water has a salinity of 15-20 psu (see sections 7.3.2 and 7.4). It is within the depth range of the halocline and therefore this habitat type experiences regular hypoxia/anoxia. The southern part of the proposed NSP2 route has a water depth of approximately 40-60 m, with fine sediment consisting mainly of sand (0.06-0.2 mm diameter) and with a salinity of 8-15 psu. This habitat experiences semi-frequent occur-rences of low-oxygen or hypoxic conditions. The southern-most 5-km stretch of the proposed NSP2 route, closest to the German EEZ, crosses a shallower area with a depth of between 25 and 40 m.
This habitat is above the halocline.
Due to the great water depths along most of the route, the soft muddy sediments and recurring hypoxia, the numbers of benthic fauna species along the route are small /88/.
Baseline surveys of benthic fauna were performed along the proposed NSP2 route and the NSP2 route V2 in August-September 2018 and along the NSP2 route V1 in January 2019, as described in section 7.1.2. The results of the 2018 surveys are presented below and the results of the 2019 surveys will be presented in separate reports.
During the August-September 2018 baseline surveys, sampling was undertaken along a transect that follows the proposed NSP2 route and the NSP2 route V2 /87/. Infauna were identified in nine of the 19 stations sampled. The results are shown in Table 7-27. The positions of the stations are shown in Figure 7-8.
Table 7-27 Depth, oxygen and occurrence of infauna along the proposed NSP2 route and the NSP2 route V2 in August-September 2018 /87//88/.
Station Depth (m) Oxygen (mg/l) Infauna species Abundance (N/m2)
MB-17 84 0.5 1 83
The results show that the highest diversity and abundance of benthic fauna are found at the two stations with the shallowest depth and the highest oxygen content. Specifically, at station MB-16, 13 species were identified at an abundance of 887 ind./m2, and at station MB-15, 9 species were identified at an abundance of 1,247 ind./m2. At all other stations, three or fewer species were identified at abundances of 83 ind./m2 or lower. The results indicate a low overall diversity in the surveyed area, with dominance by a few species including the polychaete Pygospio elegans and the Priapulid Halicryptus spinulosus, both of which are known for having a very wide ecological range and an ability to withstand low oxygen conditions. Overall, the results indicate that very few species can survive at depths greater than 70 m.
Sampling of benthic fauna was also previously carried out further north in the Bornholm Basin during baseline studies for NSP in 2008, monitoring surveys for NSP in 2010-2014 and baseline studies for the NSP2 base case route in 2015 /90//174/. Data from these earlier studies show that
at depths greater than 60 m, zoobenthos are present in very low numbers, and consist mainly of opportunistic and H2S tolerant polychaete species Trochocaeta nultisetosa and Scoloplos armiger.
At depths between 40 and 60 m, biodiversity is higher, and the biomass is dominated by mussels such as Limecola balthica, Astarte borealis, Astarte montagui, and Mytilus edulis. Polychaetes (e.g.
Pygospio elegans, Scoloplos armiger, Terebellides stroemi and Bylgides sarsi), Crustacea (e.g. Pon-toporeia femorata and Diastylis rathkei) and Priapulids (Halicryptus spinulosus and Priapulus cau-datus) are also relatively abundant.
7.8.2.4 Biodiversity
In 2017, HELCOM assessed the integrated biodiversity status for benthic habitats using the core indicator “State of the soft-bottom macrofauna community”, but this assessment did not cover the Arkona Basin or the Bornholm Basin /111/.
Of relevance to benthic fauna is the eutrophication indicator “Oxygen debt”, which is assessed as
“not good” in the Bornholm Basin /111/.
The percentage of disturbed benthic habitat has been assessed by HELCOM. The disturbed area is estimated based on spatial information of the distribution of human activities connected to the pressures. In the Arkona Basin and the Bornholm Basin, the disturbed area is 80-100% /111/.
7.8.2.5 Conservation status
The Danish Red List /166/ does not include benthic marine fauna.
The HELCOM Red List Assessment for the Baltic Sea provides information on the status of benthic species. The Red List includes 19 species of macrofauna categorised as Threatened. One species, the amphipod Haploops tenuis, was categorised as Endangered (EN) and 18 species were catego-rised as Vulnerable (VU). The majority of these occur in the Kattegat or the western-most part of the Baltic Sea, some of them at the border of their distribution area with respect to salinity /167/. None of the benthic species along the proposed NSP2 route, NSP2 route V1 or the NSP2 route V2 are listed as Near Threatened, Endangered or Vulnerable on the HELCOM Red List. Two species that could occur at water depths between 25 and 40 m have a status of Least Concern (Monoporeia affinis and Pontoporeia femorata) /167/.
A HELCOM threat assessment for the Baltic Sea has also been prepared for the characteristic liv-ing environments for species, so-called biotopes and biotope complexes. The Red List includes 17 biotopes evaluated as threatened and one which is critically endangered /175/. None of the ben-thic habitats along the proposed NSP2 route, the NSP2 route V1 or the NSP2 route V2 are listed as Near Threatened, Endangered or Vulnerable on the HELCOM Red List.
Ten biotope complexes recognised in HELCOM HUB /175/ are also listed in the EU Habitats Di-rective. A description of the habitat types as part of Natura 2000 is presented in section 7.13.
7.9 Fish
Fish are an important component of the marine food chain and the ecosystem of the Baltic Sea;
they are also a valuable component of the Danish economy (commercial fishery and the value of fish are described in section 7.16). Given this, in combination with the fact that a number of fish species present along the proposed NSP2 route, the NSP2 route V1 and the NSP2 route V2 have protected status under national/international legislation, fish are considered an important receptor.
Fish in the Baltic Sea
Fish are a human food source, but are also prey for marine mammals and seabirds. Fish themselves feed on benthic species, zooplankton, and smaller fish, and are thereby a link between various
parts of the food web. When migrating, they also play an ecological role in connecting different sea areas /111//177/.
Fish include both bony fish, a diverse taxonomic group of fish that have skeletons primarily com-posed of bone tissue, and cartilaginous fish, which have a skeleton made from cartilage (e.g.
sharks).
The distribution of fish species is mainly determined by salinity levels, and marine fish species dominate the Baltic Proper, while freshwater species occur in the coastal areas and innermost parts of the Baltic Sea. Furthermore, the composition of fish communities varies between different re-gions of the sea in relation to different habitat characteristics and differences in salinity, water temperature, oxygen content and nutrient availability. The Baltic Sea fish populations are also affected by fishing, eutrophication, oxygen depletion, high levels of hazardous substances, as well as natural factors such as cold winters and varying salinity levels /111//177//178/.
Fish communities, especially in the coastal areas of the Baltic Sea, underwent dramatic changes during the late twentieth century as a result of both human activities and natural factors /111//176//177//178/. Fish are subject to a number of anthropogenic impacts, as described above, such as enhanced nutrient loads (eutrophication); contamination by heavy metals, organic con-taminants and hormone-like substances; destruction of recruitment habitats; introduction of non-indigenous species and increased fishing pressure. Climate-driven changes in the salinity, temper-ature and oxygen content of the water can also affect recruitment and growth. Hydrophysical-climatic variability (i.e. low frequency of inflows of saline oxygenated water from the North Sea and increasing temperatures) in combination with heavy fishing over the last 10-15 years has led to a shift in the fish community from cod to clupeids (i.e. herring, sprat) /176/. This shift is ex-plained by a weakened cod recruitment and subsequent favourable recruitment conditions for sprat /176/.
Approximately 230 fish species are known in the Baltic Sea, of which 70 are marine species (in-cluding lampreys) /111//177/. The number of marine species is low compared with more saline waters.
Marine species are well-adapted to the Baltic Sea conditions and occur in high population densities.
Cod (Gadus morhua), Baltic herring (Clupea harengus), and sprat (Sprattus sprattus) comprise the large majority of the Baltic Sea fish communities in terms of biomass and numbers. These three species are also the most important commercially-exploited species, and comprise the majority of the commercial catches in the Baltic Sea. Marine fish species such as cod, sprat, flounder (Platich-thys flesus), plaice (Pleuronectes platessa), dab (Limanda limanda), turbot (Psetta maxima) and brill (Scophthalmus rhombus) prefer more saline areas and are therefore more abundant in the southern Baltic Sea and/or the Baltic Proper. Other marine species migrate from time to time from the North Sea into the Baltic Sea. Such species include whiting (Merlangus merlangus), European anchovy (Engraulis encrasicolus), mackerel (Scomber scombrus) and grey mullet (Liza ramada).
Due to unfavourably low salinity conditions, these marine species are unable to form self-sustaining populations in the Baltic Sea.
Demersal marine fish species, such as flounder, plaice and turbot, live in the central and south-western parts of the Baltic Sea. In the deeper waters, demersal fish are not common due to the low oxygen content and limited presence of benthic fauna. Conversely, shallower waters with high levels of oxygen encourage a more diverse and abundant community of benthic invertebrates as well as small- and medium-sized bottom-dwelling fish species (i.e. gobies, juvenile cod and flat-fish). Top predators such as cod and salmon strongly depend on this food chain.
Freshwater species inhabiting the Baltic Sea include e.g. perch (Perca fluviatilis), pike (Esox lucius), pikeperch (Sander lucioperca), bream (Abramis brama), roach (Rutilus rutilus) and burbot (Lota lota). These freshwater species naturally prefer the less saline areas and so they colonize mostly the coastal areas, especially in the northern Baltic Sea, where the salinity level is lower.
Migratory fish are species that undergo periodical migrations. Fish that migrate to spawn can be divided to anadromous and catadromous species. Anadromous species live and feed mostly in the sea and migrate to freshwater to breed, whereas catadromous species live mostly in lakes or rivers and migrate to sea to breed. In the Baltic Sea, anadromous fish include Atlantic salmon (Salmo
Migratory fish are species that undergo periodical migrations. Fish that migrate to spawn can be divided to anadromous and catadromous species. Anadromous species live and feed mostly in the sea and migrate to freshwater to breed, whereas catadromous species live mostly in lakes or rivers and migrate to sea to breed. In the Baltic Sea, anadromous fish include Atlantic salmon (Salmo