Evidence for the sustainable levels of human activities

Based on the findings of the case studies in this report and the recent scientific studies,

Laamanen et al. (2021) Impacts on seabed: Approaches for assessment as step towards successful measures. HELCOM ACTION report. Available at: https://helcom.fi/helcom-at-work/projects/action/

Virtanen et al. (2018). Task 4.2.1 Definition of adversely affected habitats. HELCOM SPICE. Available at: https://helcom.fi/helcom-at-work/projects/spice/

Korpinen et al. (2018) Estimating physical disturbance on seabed – Baltic Sea Environment Proceedings No. 164.

we summarize the existing knowledge of the change points which may represent the level where sustainability can be achieved.

Four observations of capital dredging impacts: (1) the core zone impacts are lethal, (2) the pressure increase (i.e. sedimentation and turbidity) is not linear but logarithmic (i.e. high pressures occur already at low activity levels and an increase in activity only marginally effects the pressure), (3) the turbidity pressure decreases away from the ‘core zone’, and (4) the turbidity pressure is mostly limited to within 2 km distance. A macrofauna community index (BBI) indicates that already low levels of dredging (7–9 mg L^-1 suspended solids or turbidity of 5–8 NTU) decrease the state of the community, resulting in ecological status less than good. In enclosed and semi-enclosed bays charophyte meadows and number of sensitive species start declining in turbidity >2.5 NTU or >2–3 mg CDOM L^-1. Charophyte-dominated enclosed bays exhibit a change-point even at 1–2 NTU. In open coasts, there can be more (5–6 mg L^-1) suspended solids in water. In general, it can be stated that dredging leads to loss of sensitive species in enclosed and semi-enclosed bays.

A lethal pressure for all biota which are buried, but some thresholds can be presented for sedimentation and turbidity to the nearby impact areas. On naturally hard surfaces even 1–2 cm of sediment cover will kill the sessile macrofauna and 2.4 g dw sediment per dm² (3 mm layer) prevents Fucus serratus recruitment (healthy F. serratus stands had sediment <0.25 mm or 0.2 g dw dm^-2). The effects of sedimentation are seen as mortality and changes in the population structure of benthic organisms, e.g. resulting in a Macoma balthica population with only large individuals, reduced herring spawning (50% mortality at 1 km distance from disposal site) and reduced coverage and lacking colonization in bladderwrack at a distance of 2 km from the disposal site. Estimates of the time to re-establish the macrozoobenthic community can be a few years or at least 5 years depending on whether condition is determined, respectively, univariate indices or by multivariate analyses of species composition. The magnitude of change in the macrozoobenthic community will depend on how closely the dumped material mounds resemble the natural seafloor in terms of e.g. grain size, organic content and consistency.

The mechanism of impact is similar to dredging, but the resuspended material is often heavier and deposits at shorter distances. Half of the macrofauna dies at a distance of 0.4–1 km from the extraction site, but it is also assessed that ecological status of macrofauna community is not impaired over 0.5 km from the sand extraction site. Full recovery of biota takes more than 10 years whilst the topography is permanently impacted.

Impacts of ships and ferries depend on both speed of vessels and frequency of shipping. In shallow areas and inside archipelagos, abrasion stirs up sediment causing concentrations of suspended solids over 8 mg L^-1. Impacts of 10 ferries per day can be up to 55% increase in turbidity, circa 31% decrease in plant species richness, 29% decrease in vegetation cover, 38–100% decrease of sensitive plant species coverage and 38–39% increase in coverage of plant species indicating eutrophication. Sensitive macrophyte species are impacted up to 700 m from ferry routes.

Marinas of recreational boats cause, on average, a 135% increase in turbidity, 31% decline in vegetation cover, 37% decline in plant species richness, 10–82% decline in coverage of sensitive macrophyte species and 25–29 % increase of plant species promoted by eutrophication. A marina also negatively affects pike recruitment (89% decrease) and increases catches of fish typically observed under eutrophic conditions, such as bleak.

The primary impact of motor boating is sediment resuspension and the consequent turbidity. This has been reported to cause loss of vegetation along the busiest boating routes. A 10 hp engine causes resuspension at 1.5 m depth, while a 50 hp engine affects the seabed at 4.5 m depth. Sensitive macrophyte species start disappearing if there are 2–5 actively used piers per hectare in a bay. The best mitigation measures are (1) to establish boating routes away from shallow-water areas and (2) set speed limits.

The impact is lethal where the turbine and its base are located on the natural seabed (circa 30 m diameter). Similar impacts are expected when a turbine is deconstructed after its use.

Cables are dug into a trench on a seabed which is then covered by the sediment again. On hard surfaces, the cables can be protected by concrete casings. Pipeline placements follow the similar principles on a larger scale. The impacts are highest at the construction period.

Bottom-trawling gears cause significant negative impacts on seabed.

Hiddink et al. (2017) Global analysis of depletion and recovery of seabed biota after bottom trawling disturbance. PNAS 114 (31) 8301-8306, https://doi.org/10.1073/pnas.161885811

Hinz et al. (2009) Trawl disturbance on benthic communities: chronic effects and experimental predictions. Ecol Appl 19:761–773.

The impacts of pressures decline with distance from the core zone. As shown above, some impacts decline sharply after a change-point. Table 2 shows review results for impact distances beyond which impacts can be assumed negligible. Note, however, that these are maximum (safe) distances and good status of marine ecosystem can be found closer to a pressure core zone.

Table 2. Impact distances from a pressure core zone. Over the impact distance, impacts on the mentioned species or parameters can be estimated as negligible.