Habitat type and dredge dumping intensity determine the sensitivity of different benthic ecosystem components in the Belgian part of the North Sea

Multiple human activities are executed in the Belgian part of the North Sea (BPNS), such as sandextraction, fisheries, offshore wind energy installations and dredging of navigation channels, with theassociated disposal of dredged material at sea. We assessed the impact of the dredge disposal activitieson the benthic ecosystem, i.e. organisms living in, on and near the bottom, to evaluate the sensitivity ofdifferent habitat types and ecosystem components to different dredge disposal intensities.In the BPNS, dredged sediments are dumped with different intensities at five designated disposal sites.Two sites are situated in the muddy Limecola balthica habitat near Ostend (LOO; low intensity) andZeebrugge (LZO; high intensity). Two sites are located in the fine muddy and sandy Abra alba habitat infront of Nieuwpoort (LNP; very low intensity) and on the Sierra Ventana (LS1; high intensities). The fifthdisposal site is located in the sandy Nephtys cirrosa habitat on the Vlakte van de Raan (LS2; moderateintensity).Dredge disposal may influence the benthic ecosytem directly by for example the burial of organismsand indirectly by changing the sediment composition. During the period 2005-2019, macrobenthossamples were taken in autumn using a Van Veen grab at the five dumping sites and at predeterminedcontrol sites, while epibenthos and demersal fish were collected in spring and autumn by means of an8m shrimp trawl at the same sites. The BEQI-tool (Benthic Ecosystem Quality Indicator, www.beqi.eu)was used to assess differences in species richness, species composition, density and biomass, accordingto a control-impact design. The use of a standard indicator approach allows for an objective evaluationof the impact across sites, habitats and ecosystem components. The indicator gives scores between 0and 1, where 1 means high similarity between control and impact. A BEQI score of 0.6 was set as athreshold, where values <0.6 reflect a clear difference between control and impact sites, indicating areal impact of the dredge disposal activity.Despite the high dredge disposal intensity at LZO, high BEQI scores were noted for both LOO and LZOdisposal sites. This indicates that the Limecola balthica habitat is not very sensitive to dredge disposalactivities. Equally high BEQI values were calculated for the LNP disposal site, while the BEQI score forLS1 was <0.6, indicating a low similarity with the control site for the latter. This is probably related to ahigher sensitivity of the fine muddy sand Abra alba habitat in combination with the high dredge disposalintensity at LS1. For the LS2 disposal site, high (>0.6) to very high (>0.8) BEQI scores were found,indicating a relatively low impact of moderate dredge disposal intensity in the sandy Nepthys cirrosahabitat. Part of the higher BEQI score for LS2 may be attributed to a slightly ‘positive’ effect of dredgedisposal, related to the creation of a new habitat by dumping fine sediments in a sandy environment.That way, mud-loving species can co-occur together with typical benthic species already living in thatsandy environment.The observed differences in fauna characteristics were similar across ecosystem components, but morepronounced for the macrobenthos, due to the more sessile mode of life of these animals compared toepibenthos and fish. This mid-term environmental impact assessment proves that the impact of dredgedisposal activities depends on both the sensitivity of the benthic habitat and the intensity of dredgedisposal. This means that multiple factors have to be considered for a sound management of dredgedisposal and other human activities in the BPNS.Keywords: Dredge disposal; BEQI; Impact assessment; Benthic habitat; BPNS