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 and current knowledge, less than 1 % of the Baltic Sea seabed is potentially lost due to human activities while over 50% of the seabed area is potentially disturbed during the assessment period (2011-2015). 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 severe damage to benthic habitats and species, some by direct contact with the seabed and others through indirect effects caused by the increased turbidity or sedimentation, for example. Whether an activity leads to a permanent loss or a temporary disturbance of benthic habitats 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 give rise to a new artificial habitat, for example when a construction creates rocky bottoms on sand. This may also lead to ecological changes that are undesirable.
Many activities may contribute to both permanent loss and disturbance of the seabed (Figure 4.7.1). Estimating seabed loss and physical 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).
Human activities potentially attributed to seabed loss and disturbance
Off-shore wind farms, harbours and 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 are in some instances similar to those during sea-bed extraction or dredging into the seabed (see below).
Installation of off-shore construction may in some cases also encompass drilling 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 20 meters from the wind turbine (OSPAR 2008). The scour protection will give rise to a new man-made habitat.
Cables and pipelines may be placed in a trench and then covered with sediment extracted elsewhere. Most often the sediment composition then differs from surrounding habitats (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 has been generalised to a 2 meters distance for the assessment purposes (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 depends on the hydrological conditions and on the properties of the mariculture, and currently no information exists on the recovery rate when the pressure is removed.
Dredging activities are usually divided into capital dredging, which is carried out when building new constructions, and maintenance dredging, which is done in order to maintain existing waterways.
Dredging causes different types of pressure on the sea bed; 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. Loss of habitat occurs during capital dredging which usually is a pressure occurring once at a specific location. But loss of habitat also occurs during maintenance dredging which is performed repeatedly, often at regular intervals. The loss is limited to the dredging site, whilst disturbance through sedimentation may have a wider spatial extent.
Some studies have estimated that disturbance through sedimentation may affect animals and vegetation up to a couple of kilometres from the core activity (Lassalle et al. 1990, Boyd et al. 2003, Orviku et al. 2008). In addition, remobilisation of sediments with deposited substances 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 using static dredging, the pressures exerted by sand and gravel extraction are comparable to those during dredging; potential physical loss of habitat (which may be partial or complete depending on how much sand or gravel is removed and which extraction technique is used), altered physical conditions through changes in seabed topography, increased turbidity caused by fine sediments that are mobilised into the water, or smothering or siltation on nearby areas. When performing trailer dredging the pressures exerted are more limited. In addition, in areas where the sediment mobility and dynamics are naturally high, the effects of sand and gravel extraction may be less significant.
Since the extracted material is sieved at sea to the wanted grain size, the unwanted matter is discharged and may result in a changed grain size of the local sediment on the seabed. Sedimentation levels are more restricted during sand and gravel extraction than during dredging, and may occur a few hundred metres from the core activity (Newell et al. 1998). There is more or less full mortality of benthic organisms at the site of sand and gravel extraction as they are removed together with their habitat (Boyd et al. 2000, 2003, Barrio Frojan et al. 2008), whereas the extent of the impact on adjacent areas is smaller (Vatanen et al. 2010).
Importantly, there are modern techniques and concepts which, if applied, can help to reduce the negative impact. 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’.
Disposal of dredged matter may cause covering of the seabed, smothering of benthic organisms, and lead to loss of habitat if the sediment characteristics are changed. In addition, increased turbidity during the disposal cause increased siltation on the site itself and in the areas around it. Disposed material may contain higher concentrations of hazardous substances and nutrients than the disposal site and may cause accumulation of these pollutants at the disposal site and adjacent areas.
The impacts on the species depend mainly on the seabed habitat type, the type and amount of disposed material, and distance to the disposal site. 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 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 impacts is similar to that of dredging a couple of kilometres from the core zone of the activity (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, shipbow 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 to the vicinity of harbours. For larger vessels, increased turbidity has been observed down to 30 m depth (Vatanen et al. 2010), and 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 1 m 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 until the year 2015. 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 Bay of Mecklenburg and the Great Belts. 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 disposal of dredged matter and to a lesser extent offshore and coastal 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 half of the Baltic seabed was estimated to have been potentially disturbed (236 000 km2) during 2011–2015. The spatial extent of potential physical disturbance to the seabed varied between 20 and almost 100% per sub-basin (1 200 to 39 000 km2; Figure 4.7.4). However, the estimation does not reflect whether these areas are associated with adverse effects to the benthic habitats as 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 fishing, which is common in the southern parts of the Baltic Sea, and shipping. At a more local scale, however, more severe physical disturbance may be caused by dredging and the disposal of dredged material. The largest area of potentially disturbed seabed were estimated in the Eastern Gotland Basin and the Bornholm Basin, which are also both comparatively large sub-basins in the Baltic (Figures 4.7.4 and 4.7.5). The sub-basins with highest proportion of potential disturbed seabed were found in the southern Baltic Sea, between the Kattegat and the Arkona Basin.
Importantly, these estimates are based on best available data on the extent of the activities concerned. In some cases, areas licensed for an activity, such as dredging, disposal of dredged matter and extraction of sand and gravel, do 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.