Loss and disturbance to the seabed is caused by human activities that inflict permanent changes or temporary disruptions to the physical habitat. Examples of such activities include extraction of seabed sand and gravel, modification of the seabed for installations, maintenance of open waterways by dredging, and bottom trawling. Based on the data available for the assessment period (2011-2016) and current knowledge, less than 1 % of the Baltic Sea seabed is potentially lost due to human activities while roughly 40 % of the seabed area is potentially disturbed. There is currently no regionally agreed method for assessing how loss and disturbance is causing adverse effects on the marine environment.
Several human activities may cause damage to the seabed, and hence to benthic habitats and species. Some activities may affect the seabed directly, but activities may also cause indirect effects, for example by increasing the level of turbidity or dispersal of sediments. Whether an activity leads to a permanent loss or a temporary disturbance of the seabed depends on many factors, such as the duration and intensity of the activity, the technique used, and the sensitivity of the area affected. The loss of a natural habitat may in some cases lead to a new artificial type of habitat, for example when a construction gives rise to hard substrates in a naturally sand-dominated habitat. Such alterations may also lead to ecological changes that are undesirable (Tyrrell and Byers 2007).
Many activities at sea may contribute to both permanent loss and disturbance of the seabed (Figure 4.7.1). Estimating physical loss and disturbance at a regional and sub-basin scale requires a generalised approach which links together different types of activities with potential loss and disturbance of the seabed, and thereby simplifies the complex reality (Box 4.7.1). There is currently no regionally agreed method for assessing how loss and disturbance is causing adverse effects on the marine environment.
Human activities potentially attributed to seabed loss and disturbance
Off-shore wind farms, harbours, underwater cables and pipelines are examples of constructions that cause a local but permanent loss of habitat. In addition, disturbance to the seabed may occur during the period of construction and installation. The pressures exerted during the construction phase have similarities with those during seabed extraction or dredging (see below). Installation of off-shore construction may also encompass drilling, pile driving, or the relocation of substrate for use as scour protection. The area lost by scour protection around the foundation of a wind farm turbine has been estimated to be in the order of tens of metres from the wind turbine (van der Wal & Tamis 2014). The scour protection will give rise to a new man-made habitat.
Pipelines may be placed in a trench and then covered with sediment extracted elsewhere, so that the sediment composition differs from surrounding habitat (Schwarzer et al. 2014). On hard substrates, cables are often covered with a protective layer of steel or concrete casings. The loss of habitats by smothering and sealing from cables may occur up to a couple of metres from the cable (OSPAR 2008).
Open systems of mariculture affect the seabed habitat through sedimentation of excrements under the fish and shellfish farms, as the accumulated material changes the seabed substrate. However, the extent of the effects in terms of loss and disturbance of the seabed depends on the hydrological conditions and on the properties of the mariculture, and currently limited information exists on the recovery rate when the pressure is removed (but see Kraufvelin et al. 2001).
Dredging activities are usually divided into capital dredging and maintenance dredging. Capital dredging is carried out when building new constructions, increasing the depth in existing waterways, or making new waterways, while maintenance dredging is done in order to maintain existing waterways.
Dredging causes different types of pressure on the seabed; removal of substrate alters physical conditions through changes in the seabed topography, increased turbidity caused by re-suspended fine sediments, and smothering and siltation of nearby areas due to settling of suspended load. Physical loss occurs during capital dredging, which usually occurs once at a specific location. It may also be connected to maintenance dredging when performed repeatedly at regular intervals. The physical loss is limited to the dredging site, whilst physical disturbance through sedimentation may have a wider spatial extent.
Disturbance through sedimentation may affect animals and vegetation even farther away from the dredging activity, on the scale of hundreds of metres (LaSalle et al. 1990, Boyd et al. 2003, Orviku et al. 2008). In addition, remobilisation of polluted deposited sediments may contribute to contamination and eutrophication effects.
During sand and gravel extraction sediment is removed from the seabed, for use in construction, coastal protection, beach nourishment and land-fills, for example.
Sand and gravel extraction can be performed using either static dredging or trailer dredging. When static dredging is used, the exerted pressures are of similar type as during dredging, potentially leading to partial or complete physical loss of habitat (depending on the extraction technique and on how much sand or gravel is removed) and altered physical conditions (through changes in the seabed topography, increased turbidity caused by re-suspended fine sediments, smothering or siltation on nearby areas). When performing trailer dredging, the pressure exerted to the seabed is more limited compared to static dredging, although the dredged area is greater. The intensity of the pressure is also dependent on the site. In areas where sediment mobility and dynamics are naturally high, the impacts of sand and gravel extraction are typically lower than in areas with more stable sediment types.
There is high mortality of benthic organisms at the site of sand and gravel extraction, as the species are removed together with their habitat (Boyd et al. 2000, 2003, Barrio Frojan et al. 2008). Since the extracted material is sieved at sea (to the required grain size) and the unwanted matter is discharged, the extraction may also result in changed grain size of the local sediment on the seabed. Adjacent areas are also affected by the activity albeit less severely (Vatanen et al. 2010).
Importantly, there are modern techniques and concepts which, if applied, can help to reduce the extent and intensity of physical disturbance of benthic organisms. Recolonization by sand- and gravel dwelling organisms is for example facilitated if the substrate is not completely removed. Precautionary measures are also recommended in HELCOM Recommendation 19/1 on ‘Marine Sediment Extraction in the Baltic Sea Area’.
Deposit of dredged material may cause covering of the seabed, smothering of benthic organisms, and lead to loss of habitat if the sediment characteristics are permanently changed. In addition, increased turbidity during the activity causes increased siltation on the site and in its adjacent areas. In some cases, deposited material may contain elevated concentrations of hazardous substances or nutrients.
The impacts on the species depends mainly on the seabed habitat type, and the type and amount of deposited material. Burial of benthic organisms may cause mortality, but some species have the ability to re-surface (Olenin 1992, Powilleit et al. 2009). The probability of survival is higher on unvegetated soft bottoms, whereas vegetation and fauna on hard substrates die when covered by a few centimetres of sediment (Powilleit et al. 2009, Essink 1999). The spatial extent of the disturbance is similar to that during dredging (Syväranta and Leinikki 2015, Vatanen et al. 2015).
Ship traffic can cause disturbance to the seabed in several ways; propeller induced currents may cause abrasion, resuspension and siltation of sediments, ship-bow waves may cause stress to littoral habitats, and dragging of anchors may cause direct physical disturbance to the seabed.
Disturbances to the seabed from shipping mainly occur in shallow areas. The effects are often local, concentrated to shipping lanes, and in the vicinity of harbours. For larger vessels, the effect on turbidity has been observed down to depths of thirty metres (Vatanen et al. 2010). Mid-sized ferry traffic has been estimated to increase turbidity by 55 % in small inlets (Eriksson et al. 2004). Erosion of the sea-floor can be substantial along heavy shipping lanes, and has been observed to cause up to one metre of sediment loss due to abrasion (Rytkönen et al. 2001).
Estimation of physical loss
The level of long term physical loss of seabed in the Baltic Sea was estimated to be less than 1 % on the regional scale (up to the year 2016). Highest estimates of potential loss at the level of sub-basins were found in the more densely populated southern Baltic Sea and ranged between 1 and 5 % in the Sound, the great Belt, the Arkona Basin and the Bay of Mecklenburg. In the majority of the sub-basins, less than 1 % of the seabed area was estimated to be potentially lost (Figure 4.7.2).
The human activities mainly connected with seabed loss were sand extraction, dredging and deposit of dredged material, harbours and marinas, and to a lesser extent offshore installations and mariculture. In terms of broad benthic habitat types, the highest proportion of area potentially lost was ‘infralittoral sand’, but the highest total area potentially lost was estimated for ‘infralittoral mixed’ substrate’ (Figure 4.7.3).
Estimated physical disturbance
Around 40 % of the Baltic seabed was estimated to have been potentially disturbed (180 000 km2) during 2011–2016. The spatial extent of potential physical disturbance to the seabed varied between 8 and 95 % per sub-basin (from around 900 to 35,500 km2; Figure 4.7.4). However, the estimation does not reflect whether these areas are associated with adverse effects to the benthic habitats, since the intensity of the disturbance is unknown. The intensity or severity of the disturbance is an important aspect which is intended to be covered in future indicator-based assessments.
The activities connected to the widest potential physical disturbance are bottom-trawling, which is common in the southern parts of the Baltic Sea, shipping, and recreational boating. At a local scale, physical disturbance may be caused by dredging and the deposit of dredged material. The largest areas of potentially disturbed seabed were estimated in the Bornholm Basin and the Eastern Gotland Basin, which are also both comparatively large sub-basins (Figures 4.7.4 and 4.7.5). The sub-basins with highest proportion of potentially disturbed seabed were found in the southern Baltic Sea, between the Kattegat and the Bornholm Basin.
Importantly, these estimates are based on best available data about the extent of the activities concerned. In some cases, due to limited data, areas licensed for an activity, such as dredging, deposit of dredged material and extraction of sand and gravel, were used in the calculations. This type of information does not necessarily reflect the extent of the exerted pressure, as the activity may be undertaken only in parts of the licensed area. These limitations in data add to the uncertainties of the estimate.