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Since the last decades, previous long-term Wadden Sea studies revealed significant changes in the abundance, biomass and spatial distribution of characteristic macrofauna communities in response to environmental changes and anthropogenic stressors. In this study, we performed statistical community analysis for the East-Frisian Wadden Sea (EFWS, southern North Sea) on two reference datasets across a period with severe climatic and environmental changes (1980s-2018). Therefore, historical macrofauna data from the Quantitative Sensitivity Mapping (1980s, SENSI 1) were reanalyzed and compared with data from the Synoptic Intertidal Benthic Survey (SIBES/SENSI 3) collected in 2018. Our results revealed significant quantitative and spatial changes in the characteristic macrofauna communities between the 1980s and 2018, most likely in response to de-eutrophication and sea level rise mediated habitat changes. Since the 1980s, the total number of taxa remained relatively stable (1980s: 90, 2018: 81), but the total abundance decreased by ca. -31% and the total biomass decreased by ca. -45%, particularly in the eastern regions of the study site probably due to de-eutrophication processes. Thereby, the mean abundances/m2 of ≥ -80% (1980s-2018) in the EFWS of several dominant species decreased: e.g. the gastropod Peringia ulvae, the polychaete Lanice conchilega and the bivalve Mya arenaria . In contrast, the mean abundance/m2 of one dominant species increased by ≥ +80% (1980s-2018): the invasive bivalve Ensis leei. The mean biomass [g/m2] of three dominant species decreased by ≥ -80% (1980s-2018): P. ulvae, L. conchilega and theamphipod Corophium volutator. In contrast, the mean biomass [g/m 2] of one dominant species increased by ≥ +75% (1980s-2018): the polychaete Arenicola marina. In the western part of the EFWS, notonly higher abundances of A. marina, but al

DNA metabarcoding methods have been implemented in studies aimed at detecting and quantifying marine benthic biodiversity. In such surveys, universal barcodes are amplified and sequenced from environmental DNA. To quantify biodiversity with DNA metabarcoding, a relation between the number of DNA sequences of a species and its biomass and/or the abundance is required. However, this relationship is complicated by many factors, and it is often unknown. In this study, we validate estimates of biomass and abundance from molecular approaches with those from the traditional morphological approach. Abundance and biomass were quantified from 126 samples of benthic intertidal mudflat using traditional morphological approaches and compared with frequency of occurrence and relative read abundance estimates from a molecular approach. A relationship between biomass and relative read abundance was found for two widely dispersed annelid taxa (Pygospio and Scoloplos). None of the other taxons, however, showed such a relationship. We discuss how quantification of abundance and biomass using molecular approaches are hampered by the ecology of DNA i.e. all the processes that determine the amount of DNA in the environment, including the ecology of the benthic species as well as the compositional nature of sequencing data.

The coastal intertidal ecosystem of the Bijagós Archipelago, Guinea-Bissau, one of the largest and most important in West Africa, sustains a considerable proportion of the migratory shorebird populations of the East Atlantic Flyway and operates as a nursery area for benthic fish in the region. The macrozoobenthos in these mudflats constitute the main food source for both groups so that spatial and temporal variation in their abundance and community composition is likely to influence the abundance and distribution of fish and birds. In this study we described the spatial and temporal dynamics in the density, biomass, and community composition of macrozoobenthos across six intertidal flats in three islands of the Bijagós Archipelago. Overall, the Bijagós Archipelago was characterised by a highly species-rich macrozoobenthic community, with ca. 88 taxa identified across all sites, reaching a mean density of 1871 ± 58.3 ind.m-2 (mean ± SE) and mean biomass of 5.65 ± 0.41 g of AFDM.m-2 (ash-free dry mass per m2), values much lower than what was described for nearby intertidal areas, namely the Band d’Arguin, Mauritania. Density and biomass of major macrozoobenthos classes (Bivalvia, Polychaeta, Malacostraca and Gastropoda) differed across sites and months, displaying an overall increase in density towards the final months of the dry season (March and April). Similarly, community composition also differed significantly between sites and throughout the season. The site with most distinct community composition (Adonga) supported low diversity and high abundance of a few bivalve species, whilst other two sites that hosted the most diverse communities, were also the most similar between them (Anrumai and Abu). These spatial and temporal patterns constitute an important baseline to improve knowledge of this intertidal ecosystem and will contribute towards a better understanding of the spatial and temporal distribution patterns of their cons

DNA metabarcoding methods have been implemented in studies aimed at detecting and quantifying marine benthic biodiversity. In such surveys, universal barcodes are amplified and sequenced from environmental DNA. To quantify biodiversity with DNA metabarcoding, a relation between the number of DNA sequences of a species and its biomass and/or the abundance is required. However, this relationship is complicated by many factors, and it is often unknown. In this study, we validate estimates of biomass and abundance from molecular approaches with those from the traditional morphological approach. Abundance and biomass were quantified from 126 samples of benthic intertidal mudflat using traditional morphological approaches and compared with frequency of occurrence and relative read abundance estimates from a molecular approach. A relationship between biomass and relative read abundance was found for two widely dispersed annelid taxa (Pygospio and Scoloplos). None of the other taxons, however, showed such a relationship. We discuss how quantification of abundance and biomass using molecular approaches are hampered by the ecology of DNA i.e. all the processes that determine the amount of DNA in the environment, including the ecology of the benthic species as well as the compositional nature of sequencing data.

Lugworm Arenicola spp. dredging affects the intertidal benthic community in the Dutch Wadden Sea. Previous studies have found contradicting results regarding the recovery rates of targeted and non-targeted macrozoobenthic species, and meiobenthic communities have been neglected in these studies. The current study explores the short-term effects of dredging on both the macrofaunal and the meiofaunal communities using both a morphological and molecular approach. Benthic samples were collected right before and regularly after dredging for up to 1.5 yr at both control and fished transects. Significant differences between the control and fished transects were found by morphological and molecular approaches. Ordination analysis suggests distinct community compositions between control and fished transects in the first months after dredging. Opportunistic species with short life cycles, typically more than 1 generation yr-1, thrived more in the fished transects during the spring/summer season compared to these species in the control transects in the same season, whereas recovery for long-lived species was slow. Both approaches showed similar results; however, compared to the morphological approach, the molecular approach was more sensitive to the effects due to a larger set of benthic taxa.

Lugworm Arenicola spp. dredging affects the intertidal benthic community in the Dutch Wadden Sea. Previous studies have found contradicting results regarding the recovery rates of targeted and non-targeted macrozoobenthic species, and meiobenthic communities have been neglected in these studies. The current study explores the short-term effects of dredging on both the macrofaunal and the meiofaunal communities using both a morphological and molecular approach. Benthic samples were collected right before and regularly after dredging for up to 1.5 yr at both control and fished transects. Significant differences between the control and fished transects were found by morphological and molecular approaches. Ordination analysis suggests distinct community compositions between control and fished transects in the first months after dredging. Opportunistic species with short life cycles, typically more than 1 generation yr-1, thrived more in the fished transects during the spring/summer season compared to these species in the control transects in the same season, whereas recovery for long-lived species was slow. Both approaches showed similar results; however, compared to the morphological approach, the molecular approach was more sensitive to the effects due to a larger set of benthic taxa.

During the SIBES project, a large-scale monitoring of the marine benthos of the Dutch and western German Wadden Sea, three specimens of the bivalve Mulinia lateralis (Say, 1822) (Bivalvia: Mollusca: Mactridae), a common opportunistic species native in the east coast of North America, were found in August 2017 in the Ems-Dollard estuary. Present records backdate of two months the arrival of this species in Europe and first report it for the German Wadden Sea. Further specimens were then found in 2018, confirming its establishment in the area. An analysis of favourable habitat characteristics for M. lateralis showed that large parts of the Dutch Wadden Sea are a potential suitable habitat for this species.

During the SIBES project, a large-scale monitoring of the marine benthos of the Dutch and western German Wadden Sea, three specimens of the bivalve Mulinia lateralis (Say, 1822) (Bivalvia: Mollusca: Mactridae), a common opportunistic species native in the east coast of North America, were found in August 2017 in the Ems-Dollard estuary. Present records backdate of two months the arrival of this species in Europe and first report it for the German Wadden Sea. Further specimens were then found in 2018, confirming its establishment in the area. An analysis of favourable habitat characteristics for M. lateralis showed that large parts of the Dutch Wadden Sea are a potential suitable habitat for this species.

At a time when there is a growing discussion about the natural state of estuaries, a comparison of macrozoobenthoscommunities from two surveys conducted 30 years apart in the Ems Dollard estuary, in the easternWadden Sea, The Netherlands, provides a unique opportunity to compare changes over time. As expected, ourcomparison revealed a gradient in species composition from land (the Dollard) to sea (the Outer Ems) at bothpoints in time, with brackish species in the Dollard and more marine species in the Outer Ems (Wadden Sea).Total richness increased over time; however, this mainly reflected the immigration of new species and samplingdifferences. In the Dollard, total biomass declined over time, most likely reflecting de-eutrophication in this area.Strikingly, at the meeting point between the sea and the brackish Dollard, i.e. the Inner Ems, the communitycomposition changed from one dominated by bivalves (1970s) to one dominated by worms (since 2009). Thischange involved a reduction in total biomass, mainly of Mya arenaria, and immigration of polychaete worms(Marenzellaria viridis and Alitta succinea). In the Outer Ems, an increase in total biomass was observed, associatedwith the recent successful recruitment of Cerastoderma edule. This comparison highlights that historical dataprovides useful insights at large spatial scales. However, a full understanding of the complex dynamics of estuariesrequires an analysis of continuous long-term monitoring series.

Within the COST action EMBOS (European Marine Biodiversity Observatory System) the degree and variation of the diversity and densities of soft-bottom communities from the lower intertidal or the shallow subtidal was measured at 28 marine sites along the European coastline (Baltic, Atlantic, Mediterranean) using jointly agreed and harmonized protocols, tools and indicators. The hypothesis tested was that the diversity for all taxonomic groups would decrease with increasing latitude. The EMBOS system delivered accurate and comparable data on the diversity and densities of the soft sediment macrozoobenthic community over a large-scale gradient along the European coastline. In contrast to general biogeographic theory, species diversity showed no linear relationship with latitude, yet a bell-shaped relation was found. The diversity and densities of benthos were mostly positively correlated with environmental factors such as temperature, salinity, mud and organic matter content in sediment, or wave height, and related with location characteristics such as system type (lagoons, estuaries, open coast) or stratum (intertidal, subtidal). For some relationships, a maximum (e.g. temperature from 15–20°C; mud content of sediment around 40%) or bimodal curve (e.g. salinity) was found. In lagoons the densities were twice higher than in other locations, and at open coasts the diversity was much lower than in other locations. We conclude that latitudinal trends and regional differences in diversity and densities are strongly influenced by, i.e. merely the result of, particular sets and ranges of environmental factors and location characteristics specific to certain areas, such as the Baltic, with typical salinity clines (favouring insects) and the Mediterranean, with higher temperatures (favouring crustaceans). Therefore, eventual trends with latitude are primarily indirect and so can be overcome by local variation of environmental factors.

At a time when there is a growing discussion about the natural state of estuaries, a comparison of macrozoobenthoscommunities from two surveys conducted 30 years apart in the Ems Dollard estuary, in the easternWadden Sea, The Netherlands, provides a unique opportunity to compare changes over time. As expected, ourcomparison revealed a gradient in species composition from land (the Dollard) to sea (the Outer Ems) at bothpoints in time, with brackish species in the Dollard and more marine species in the Outer Ems (Wadden Sea).Total richness increased over time; however, this mainly reflected the immigration of new species and samplingdifferences. In the Dollard, total biomass declined over time, most likely reflecting de-eutrophication in this area.Strikingly, at the meeting point between the sea and the brackish Dollard, i.e. the Inner Ems, the communitycomposition changed from one dominated by bivalves (1970s) to one dominated by worms (since 2009). Thischange involved a reduction in total biomass, mainly of Mya arenaria, and immigration of polychaete worms(Marenzellaria viridis and Alitta succinea). In the Outer Ems, an increase in total biomass was observed, associatedwith the recent successful recruitment of Cerastoderma edule. This comparison highlights that historical dataprovides useful insights at large spatial scales. However, a full understanding of the complex dynamics of estuariesrequires an analysis of continuous long-term monitoring series.

Within the COST action EMBOS (European Marine Biodiversity Observatory System) the degree and variation of the diversity and densities of soft-bottom communities from the lower intertidal or the shallow subtidal was measured at 28 marine sites along the European coastline (Baltic, Atlantic, Mediterranean) using jointly agreed and harmonized protocols, tools and indicators. The hypothesis tested was that the diversity for all taxonomic groups would decrease with increasing latitude. The EMBOS system delivered accurate and comparable data on the diversity and densities of the soft sediment macrozoobenthic community over a large-scale gradient along the European coastline. In contrast to general biogeographic theory, species diversity showed no linear relationship with latitude, yet a bell-shaped relation was found. The diversity and densities of benthos were mostly positively correlated with environmental factors such as temperature, salinity, mud and organic matter content in sediment, or wave height, and related with location characteristics such as system type (lagoons, estuaries, open coast) or stratum (intertidal, subtidal). For some relationships, a maximum (e.g. temperature from 15–20°C; mud content of sediment around 40%) or bimodal curve (e.g. salinity) was found. In lagoons the densities were twice higher than in other locations, and at open coasts the diversity was much lower than in other locations. We conclude that latitudinal trends and regional differences in diversity and densities are strongly influenced by, i.e. merely the result of, particular sets and ranges of environmental factors and location characteristics specific to certain areas, such as the Baltic, with typical salinity clines (favouring insects) and the Mediterranean, with higher temperatures (favouring crustaceans). Therefore, eventual trends with latitude are primarily indirect and so can be overcome by local variation of environmental factors.