Nematodes alter the taxonomic and functional profiles of benthic bacterial communities: A metatranscriptomic approach.

Marine sediments cover 70% of the Earth's surface, and harbour diverse bacterial communities critical for marine biogeochemical processes, which affect climate change, biodiversity and ecosystem functioning. Nematodes, the most abundant and species-rich metazoan organisms in marine sediments, in turn, affect benthic bacterial communities and bacterial-mediated ecological processes, but the underlying mechanisms by which they affect biogeochemical cycles remain poorly understood. Here, we demonstrate using a metatranscriptomic approach that nematodes alter the taxonomic and functional profiles of benthic bacterial communities. We found particularly strong stimulation of nitrogen-fixing and methane-oxidizing bacteria in the presence of nematodes, as well as increased functional activity associated with methane metabolism and degradation of various carbon compounds. This study provides empirical evidence that the presence of nematodes results in taxonomic and functional shifts in active bacterial communities, indicating that nematodes may play an important role in benthic ecosystem processes.

Tidal and circadian patterns of European eel during their spawning migration in the North Sea and the English Channel.

Technological advances in tracking methods enable the mapping of anguillid eel migration routes from continental habitats to their spawning sites in the ocean. However, the behaviour and orientation abilities of anguillids are still poorly understood, and have only rarely been studied on the continental shelf. Here we present the results of a study into the vertical and horizontal movement behaviour of 42 European eels (Anguilla anguilla L.) tagged with electronic tags that migrated through the North Sea and English Channel towards and into the Atlantic Ocean during their spawning migration. We used actograms, periodograms and linear mixed effects models to determine the periodicity and significance of the timing and pattern of vertical movement and activity. Overall, eels had a complex behavioural repertoire that included classical diel vertical migration (DVM), reverse DVM and vertical movement behaviours that synchronized with tidal patterns. All of the eels that were tracked showed one or more of these behaviours during their time at liberty, and many exhibited all of them. We also observed that the eels had a higher horizontal migration speed when the current in the favourable direction was stronger. This, together with the vertical movement synchronized with the tides, suggests the eels adopt selective tidal stream transport. Finally, tracked eels had a higher vertical movement range at night compared to daytime. We hypothesize that these behaviours are driven by bio-energetic efficient movement, navigation and predator avoidance.

A wind of change for soft-sediment infauna within operational offshore windfarms.

In this study, "artificial reef" (AR) impacts of offshore windfarms (OWFs) on the surrounding soft-sediments were investigated. Benthic grab samples were collected at nearby (37.5 m) and distant (500 or 350 m) positions from turbines of two Belgian OWFs (Belwind: monopiles and C-Power: jackets). Higher macrobenthos abundance and species richness were found nearby jacket foundations of C-Power compared to distant positions and differences were most pronounced within deeper sediments (i.e., gullies between sandbanks) at intermediate levels of fine sand fractions (10-20%) and total organic matter (0.5-0.9%). Strong benthic enrichment (>1000 ind. m-2, >20 spp. sample-1) was also linked with higher fine sand fractions (>20%) near the jackets. Moreover, nearby sediments showed higher occurrences of coastal species and habitat diversification was promoted by Mytilus edulis shell debris and alive organisms ("biofouling drop-offs"). The lack of similar results around monopiles (Belwind) confirms that the extent of detectable AR-effects depends on site- and turbine specific factors.

Physiological response to seawater pH of the bivalve Abra alba, a benthic ecosystem engineer, is modulated by low pH.

The presence and behaviour of bivalves can affect the functioning of seafloor sediments through the irrigation of deeper strata by feeding and respiring through siphonal channels. Here, we investigated the physiological response and consecutive impact on functioning and body condition of the white furrow shell Abra alba in three pH treatments (pH = 8.2, pH = 7.9 and pH = 7.7). Although no pH effect on survival was found, lowered respiration and calcification rates, decreased energy intake (lower absorption rate) and increased metabolic losses (increased excretion rates) occurred at pH âˆ¼ 7.7. These physiological responses resulted in a negative Scope for Growth and a decreased condition index at this pH. This suggests that the physiological changes may not be sufficient to sustain survival in the long term, which would undoubtedly translate into consequences for ecosystem functioning.

Sampling errors and variability in video transects for assessment of reef fish assemblage structure and diversity.

Video monitoring is a rapidly evolving tool in aquatic ecological research because of its non-destructive ability to assess fish assemblages. Nevertheless, methodological considerations of video monitoring techniques are often overlooked, especially in more complex sampling designs, causing inefficient data collection, processing, and interpretation. In this study, we discuss how video transect sampling designs could be assessed and how the inter-observer variability, design errors and sampling variability should be quantified and accounted for. The study took place in the coastal areas of the Galapagos archipelago and consisted of a hierarchical repeated-observations sampling design with multiple observers. Although observer bias was negligible for the assessment of fish assemblage structure, diversity and counts of individual species, sampling variability caused by simple counting/detection errors, observer effects and instantaneous fish displacement was often important. Especially for the counts of individual species, sampling variability most often exceeded the variability of the transects and sites. An extensive part of the variability in the fish assemblage structure was explained by the different transects (13%), suggesting that a sufficiently high number of transects is required to account for the within-location variability. Longer transect lengths allowed a better representation of the fish assemblages as sampling variability decreased by 33% if transect length was increased from 10 to 50 meters. However, to increase precision, including more repeats was typically more efficient than using longer transect lengths. The results confirm the suitability of the technique to study reef fish assemblages, but also highlight the importance of a sound methodological assessment since different biological responses and sampling designs are associated with different levels of sampling variability, precision and ecological relevance. Therefore, besides the direct usefulness of the results, the procedures to establish them may be just as valuable for researchers aiming to optimize their own sampling technique and design.

Effects of pile driving sound on local movement of free-ranging Atlantic cod in the Belgian North Sea.

Offshore energy acquisition through the construction of wind farms is rapidly becoming one of the major sources of green energy all over the world. The construction of offshore wind farms contributes to the ocean soundscape as steel monopile foundations are commonly hammered into the seabed to anchor wind turbines. This pile driving activity causes repeated, impulsive, low-frequency sounds, reaching far into the environment, which may have an impact on the surrounding marine life. In this study, we investigated the effect of the construction of 50 wind turbine foundations, over a time span of four months, on the presence and movement behaviour of free-swimming, individually tagged Atlantic cod. The turbine foundations were constructed at a distance ranging between 2.3 and 7.1 km from the cod, which resided in a nearby, existing wind farm in the southern North Sea. Our results indicated that local fish remained in the exposed area during and in-between pile-driving activities, but showed some modest changes in movement patterns. The tagged cod did not increase their net movement activity, but moved closer to the scour-bed (i.e. hard substrate), surrounding their nearest turbine, during and after each piling event. Additionally, fish moved further away from the sound source, which was mainly due to the fact that they were positioned closer to a piling event before its start. We found no effect of the time since the last piling event. Long-term changes in movement behaviour can result in energy budget changes, and thereby in individual growth and maturation, eventually determining growth rate of populations. Consequently, although behavioural changes to pile driving in the current study seem modest, we believe that the potential for cumulative effects, and species-specific variation in impact, warrant more tagging studies in the future, with an emphasis on quantification of energy budgets.

Mapping silver eel migration routes in the North Sea.

Recent developments in tracking technology resulted in the mapping of various marine spawning migration routes of the European eel (Anguilla anguilla). However, migration routes in the North Sea have rarely been studied, despite many large European rivers and hence potential eel growing habitat discharge into the North Sea. In this study, we present the most comprehensive map to date with migration routes by silver European eels in the North Sea and document for the first time successful eel migration through the English Channel. Migration tracks were reconstructed for 42 eels tagged in Belgium and 12 in Germany. Additionally, some eels moved up north to exit the North Sea over the British Isles, confirming the existence of two different routes, even for eels exiting from a single river catchment. Furthermore, we observed a wide range in migration speeds (6.8-45.2 km day-1). We hypothesize that these are likely attributed to water currents, with eels migrating through the English Channel being significantly faster than eels migrating northward.

Colonization of macroalgal deposits by estuarine nematodes through air and potential for rafting inside algal structures.

Dispersal is an important life-history trait. In marine meiofauna, and particularly in nematodes, dispersal is generally considered to be mainly passive, i.e. through transport with water currents and bedload transport. Because nematodes have no larval dispersal stage and have a poor swimming ability, their per capita dispersal capacity is expected to be limited. Nevertheless, many marine nematode genera and even species have near-cosmopolitan distributions, and at much smaller spatial scales, can rapidly colonise new habitat patches. Here we demonstrate that certain marine nematodes, like the morphospecies Litoditis marina, can live inside macroalgal structures such as receptacula and-to a lesser extent-floating bladders, which may allow them to raft over large distances with drifting macroalgae. We also demonstrate for the first time that these nematodes can colonize new habitat patches, such as newly deposited macroalgal wrack in the intertidal, not only through seawater but also through air. Our experimental set-up demonstrates that this aerial transport is probably the result of hitchhiking on vectors such as insects, which visit, and move between, the patches of deposited algae. Transport by wind, which has been observed for terrestrial nematodes and freshwater zooplankton, could not be demonstrated. These results can be important for our understanding of both large-scale geographic distribution patterns and of the small-scale colonization dynamics of habitat patches by marine nematodes.

The biological plastic pump: Evidence from a local case study using blue mussel and infaunal benthic communities.

The distinct spatial variability in microplastic concentrations between marine regions and habitats calls for a better understanding about the transport pathways of this omnipresent pollutant in the marine environment. This study provides empirical evidence that a sessile filter feeder, the Blue mussel M. edulis, accelerates microplastic deposition by aggregating them into sinking particulate faeces and pseudofaeces. After settling to the seafloor, the bioturbation of benthic fauna quickly buries these microplastics. Collectively, these results suggest that if such biologically-mediated benthic-pelagic coupling would be integrated into hydrodynamic transport models, the spatial variability and source-sink dynamics of microplastics would be better understood. It is proposed that microplastic pollution is monitored through sampling that takes into account faeces and pseudofaeces underneath filter feeders. The implications of this detrital pathway for microplastic transfer to the seafloor, and the role of shellfish mariculture in this process, are discussed. Studies that consider filter feeders and benthic communities from other regions, and during different seasons, are needed to validate the proposed biological pump mechanism across space and time.

Response of life-history traits of estuarine nematodes to the surfactant sodium dodecyl sulfate.

Species responses to stress are expected to be dependent on their life-history strategy. In this study, we compare the responses of two free-living marine nematodes, Litoditis marina and Diplolaimella dievengatensis, both considered opportunistic, fast-growing, and stress-tolerant species, to the exposure to sublethal concentrations of sodium dodecyl sulfate (SDS) surfactant. Specifically, we evaluated the growth and reproduction rates, as well as the survival of individuals exposed from eggs and/or juveniles (J1) onwards. Exposure to SDS significantly affected the growth and reproduction rates of both species. However, whereas growth and reproduction rates of D. dievengatensis were significantly enhanced at low and intermediate concentrations of SDS (0.001% and 0.003%), for L. marina both parameters were significantly reduced by all SDS concentrations tested (0.001%, 0.003% and 0.006%). Exposure to SDS did not affect the survival of adult nematodes of D. dievengatensis, while for L. marina, survival of males exposed to 0.006% SDS was significantly reduced compared to the control. Responses of the life-history traits growth, fecundity and survival did not exhibit clear trade-offs. The contrasting responses of D. dievengatensis and L. marina indicate that biologically and ecologically similar species can have remarkably distinct tolerances to stress, and that, in agreement with recent studies, rhabditid nematodes cannot a priori be considered very stress tolerant. Consequently, single species traits and phylogenetic relatedness are poor predictors of nematode responses to toxic stress posed by anthropogenic activities.

Seasonal variability in ecosystem functioning across estuarine gradients: The role of sediment communities and ecosystem processes.

Functional trait approaches advance the understanding of biodiversity-ecosystem function (BDEF) relationships and its control by the environmental context. Application of these insights into management remains constrained due to lack of evidence from real-world ecosystems that capture the natural spatial and temporal gradients at which biodiversity and environmental conditions operate. In this study we measured macrofauna community traits, ecosystem processes and abiotic properties at 9 locations during 4 months, spanning a wide gradient in sedimentary habitats and salinity in the Scheldt estuary, and quantified the (a)biotic contribution to sediment community oxygen consumption, as a measure of ecosystem function. We found that functional attributes of the macrofauna community and its effect on bio-irrigation can predict ecosystem function, but especially during the colder period of the year. This result highlights that generalizations about BDEF relationships, and biodiversity loss on ecosystem functions, are limited whenever this temporal component is not acknowledged.

Functional differences in trophic structure of offshore wind farm communities: A stable isotope study.

The proliferation of offshore wind energy installations causes a local change in biodiversity because these structures become heavily colonised by large quantities of fouling fauna, attract large mobile crustaceans and fish, and alter the macrofaunal communities in the soft sediments surrounding the wind turbines. Here, we analysed the stable isotope signals (δ13C and δ15N) of the faunal communities associated with a wind turbine, its scour protection layer (SPL) and the surrounding soft sediments. We hypothesised that structural differences in community composition would be reflected in food web complexity and that resource partitioning could be one of the mechanisms contributing to the co-existence of such dense communities. Sampling was conducted at a gravity-based foundation in the Belgian part of the North Sea, where both sessile and mobile organisms were collected along the depth gradient of the turbine, the SPL and the surrounding soft substrate. The results indicated that the structural differences of the communities are reflected in the food web complexity as indicated by the trophic niche size, the trophic diversity and the redundancy of the communities along the depth gradient. Higher food web complexity was associated with zones where high accumulation of organic matter occurs (soft substrate and SPL). Low food web complexity was observed in depth zones that are dominated by sessile suspension-feeding organisms (intertidal and Mytilus zone). The high trophic diversity and low redundancy observed within the trophic clusters of the soft substrate and the Metridium zone indicated that resource partitioning can be a mechanism allowing the co-existence of large densities of a wide variety of species.

Do different habits affect microplastics contents in organisms? A trait-based analysis on salt marsh species.

Salt marshes in urban watersheds are prone to microplastics (MP) pollution due to their hydrological characteristics and exposure to urban runoff, but little is known about MP distributions in species from these habitats. In the current study, MP occurrence was determined in six benthic invertebrate species from salt marshes along the North Adriatic lagoons (Italy) and the Schelde estuary (Netherlands). The species represented different feeding modes and sediment localisation. 96% of the analysed specimens (330) did not contain any MP, which was consistent across different regions and sites. Suspension and facultative deposit-feeding bivalves exhibited a lower MP occurrence (0.5-3%) relative to omnivores (95%) but contained a much more variable distribution of MP sizes, shapes and polymers. The study provides indications that MP physicochemical properties and species' ecological traits could all influence MP exposure, uptake and retention in benthic organisms inhabiting European salt marsh ecosystems.

Does microplastic ingestion by zooplankton affect predator-prey interactions? An experimental study on larviphagy.

Litter is omnipresent in the ocean where it can be ingested by marine biota. Although ingestion of microplastics (MPs) is abundantly reported, insights into how MP can influence predator-prey interactions currently limits our understanding of the ecological impact of MPs. Here we demonstrate trophic transfer of MPs from zooplankton to benthic filter feeders, through consumption of contaminated prey (i.e. prey with ingested MP). However, predation rates of contaminated prey were significantly lower as compared to predation rates of prey that had no MPs ingested. As filter feeder clearance rates were not affected by consumption of MPs, the lower predation rates of contaminated prey appear to be primarily explained by disruption in zooplankton swimming behaviour that reduces their filtration risk. This is the first study that shows how MPs can change predator-prey interactions that are involved in the coupling between the pelagic and seabed habitat.

Natural stable isotope ratios and fatty acid profiles of estuarine tidal flat nematodes reveal very limited niche overlap among co-occurring species.

The high local-scale species diversity of marine meiofauna, and of nematodes in particular, has puzzled ecologists for decades. Both pronounced niche differentiation and neutral dynamics have been suggested as mechanisms underlying that high diversity. Differential resource use is the most plausible basis for niche differentiation, yet the vast majority of studies demonstrating that this is prominent in marine nematodes are based on laboratory experiments on single species or highly simplified assemblages. Only a small number of studies have investigated resource differentiation under natural conditions. Here we use natural stable-isotope ratios of carbon and nitrogen, as well as fatty-acid profiles, to assess differential resource use and trophic structure in nine abundant estuarine tidal flat nematode species, comprising different presumed feeding modes (deposit feeders, epistratum feeders, predators) and resource guilds (herbivores, carnivores) based on buccal cavity morphology. Nematodes comprise up to three different trophic levels (from primary to tertiary consumers), yet with the exception of some herbivores, omnivory is prominent. Bivariate isotopic niche spaces were of similar size among most species, irrespective of their trophic level. Herbivory not only contributed importantly to the nutrition of suspected herbivores, but also to that of species that were previously considered carnivores based on the morphology of their buccal cavity. Herbivory mainly targets diatoms in some nematode species, yet includes dinoflagellates in others. Bacteria, in contrast, appear to be of limited nutritional importance. Odontophora setosus is identified as a predator/omnivore (possibly of heterotrophic protists) with a trophic level in between that of secondary and tertiary consumers. Our study thus demonstrates that resource differentiation is pronounced among as well as within nematode feeding modes and resource guilds. However, this study included only the most abundant species of the in situ community, hence it remains to be established whether and to what extent its conclusions can be extrapolated to entire, often highly species-rich communities.

Effects of Mercury (Hg) on Soil Nematodes: A Microcosm Approach.

Mercury (Hg), one of the most toxic heavy metals, is commonly used in the gold extraction process in small-scale mining operations in many countries. Our previous field work on the impact of mining on soil nematode assemblages in a small-scale mining area in Sibutad, the Philippines, revealed no significant negative effects despite sometimes strongly elevated Hg concentrations. Using a microcosm approach, we now applied similar Hg concentrations as commonly found in these field sites (2.5, 5, and 10 mg/kg Hg) and determined their impact on nematode assemblages from a different soil with different physicochemical soil attributes. Our results demonstrate (a) limited "bottling" effects (incubation effects) after a 45-day incubation period: a nematode abundance decrease of up to 37%, but absence of significant differences in diversity and nematode assemblage composition; (b) that total nematode abundance already decreased at Hg concentrations (2.5 mg/kg), which did not yield significant impacts on other nematode assemblage descriptors, such as assemblage composition and different diversity indices; and (c) that the Hg concentrations found in the Sibutad field sites can be detrimental to soil nematode assemblages. The discrepancy between our microcosm and the field-based results is probably related to differences in physicochemical soil attributes (e.g., OM contents, soil pH), which suggests that nematode-based environmental assessments should be interpreted in a context-dependent manner.

Effects of the water-soluble fraction of a crude oil on estuarine meiofauna: A microcosm approach.

Although soluble hydrocarbons persist for only hours to days in aquatic environments, they potentially pose a high toxicity to aquatic biota. Here we investigate effects of the water-soluble fraction (WSF) of crude oil on estuarine meiofauna, with special focus on nematodes. Copepods and amphipods were the most sensitive taxa, disappearing almost completely within 3 weeks after exposure. In nematodes, we observed mostly sublethal short-term effects of WSF, such as changes in feeding-type composition and age structure. In the longer term (months), total nematode density and species composition were significantly affected, whereas different diversity indices did not exhibit significant responses. Deposit-feeding and predatory nematodes were the most affected feeding types. Nevertheless, sensitivity was species-specific, with sometimes opposing responses between even congeneric species. Our results demonstrate the need to assess WSF effects on communities at the species level and over time periods well exceeding the residence time of the WSF in the environment.

Effects of the water-soluble fraction of a crude oil on freshwater meiofauna and nematode assemblages.

Oil water-soluble fractions (WSFs) compounds have low persistence in water; evaporating in a matter of hours to days. Nonetheless, they pose high toxicity to exposed biota. Their effects may be instantaneous or delayed, affecting, respectively, mainly survival vs growth and reproduction. We investigated the effects of crude oil WSFs on freshwater meiobenthos, with a focus on nematode assemblages, in community microcosm experiments lasting 15 weeks. Treatments consisted of the application of different concentrations of oil WSFs, i.e. high (100%) medium (50%) and low (10%), and effects were assessed one, three, nine and 15 weeks after contamination, allowing us to detect both short-term and lasting effects of oil-WSF. Additionally, we compared the effects of a single contamination event with those of a so-called 'constant' oil-WSF contamination where we replenished evaporated water with water containing the medium concentration of oil WSF. Next to nematodes, the most abundant meiofaunal taxa were rotifers, gastrotrichs, oligochaetes and tardigrades. Total abundance, different diversity indices, the composition of feeding-types and the age structure were investigated in the assessment of direct oil effects on the structure of nematode assemblages. Limited immediate effects were observed, except for a significant decrease of the index of taxonomic distinctness, which already appeared in the first week. Significant impacts on total nematode abundance, diversity and species composition only became apparent after 9-15 weeks of incubation, indicating that delayed effects of a single exposure are far more pronounced than instantaneous effects. Moreover, for most response variables, the strongest impacts were not observed in the highest-concentration treatment, but in a medium-concentration treatment with regular replenishment of oil WSF, suggesting that internal exposure may be important in generating effects. Furthermore, the predictability of the sensitivity of individual species was sometimes poor, which may not only result from these species' sensitivities, but also from alterations in interspecific interactions in polluted communities. Further toxicity tests should be carried out in order to unravel the main modes of action of crude oil WSF which lead to the observed long-term sublethal effects on nematode communities.

Unimodal head-width distribution of the European eel (Anguilla anguilla L.) from the Zeeschelde does not support disruptive selection.

Since the early 20th century, European eels (Anguilla anguilla L.) have been dichotomously classified into 'narrow' and 'broad' heads. These morphs are mainly considered the result of a differential food choice, with narrow heads feeding primarily on small/soft prey and broad heads on large/hard prey. Yet, such a classification implies that head-width variation follows a bimodal distribution, leading to the assumption of disruptive selection. We investigated the head morphology of 272 eels, caught over three consecutive years (2015-2017) at a single location in the Zeeschelde (Belgium). Based on our results, BIC favored a unimodal distribution, while AIC provided equal support for a unimodal and a bimodal distribution. Notably, visualization of the distributions revealed a strong overlap between the two normal distributions under the bimodal model, likely explaining the ambiguity under AIC. Consequently, it is more likely that head-width variation followed a unimodal distribution, indicating there are no disruptive selection pressures for bimodality in the Zeeschelde. As such, eels could not be divided in two distinct head-width groups. Instead, their head widths showed a continuum of narrow to broad with a normal distribution. This pattern was consistent across all maturation stages studied here.

Global warming-induced temperature effects to intertidal tropical and temperate meiobenthic communities.

Global climate change and the related temperature rise strongly impact marine life and have long been in the center of scientific attention. This experimental work investigates thermal-stress effects on intertidal meiofauna from tropical and temperate coasts, focusing on community responses. Natural communities were exposed for a month to ambient, elevated constant temperatures and diurnal fluctuating temperature regimes with elevated peak maxima, to mimic realistic future climate conditions. Abundance, biodiversity, community composition and functional diversity were assessed. Differential responses between a tropical and a temperate community were revealed. The tropical nematode assemblage was more tolerant to the elevated constant than to the fluctuating temperature regime, whereas the temperate assemblage was equally affected by both. Shifts in dominance of temperature-tolerant species in elevated constant and fluctuating temperature treatments (due to temperature variations) were observed and explained by a combination of differential tolerances and shifts in species interactions. Overall, global warming-induced temperature was found to alter species dynamics within meiobenthic communities, which may have further implications for the ecosystem.