Soft-sediment Communities Research Articles

A comparison of visual and molecular methods for inferring biological communities in aquaculture enriched sediments - Impact assessment and cost-benefit analysis

Nutrients introduced to the environment by finfish aquaculture pose environmental risks, which can be mitigated by robust environmental monitoring. Biological communities in soft sediments are good indicators of aquaculture derived environmental changes. Traditionally, monitoring programs have visually surveyed macrofauna communities. However, DNA metabarcoding is a potentially more efficient alternative. We compared alpha diversity, multivariate dispersion and taxonomic composition of macrofauna communities with metabarcoding derived bacterial and eukaryote communities along an organic enrichment gradient at a salmon farm in Tasmania, Australia. Additionally, we conducted a cost-benefit analysis comparing the approaches. All methods identified indicator taxa that changed in abundance over the enrichment gradient. Macrofauna analysis was the most sensitive method for detecting changes in alpha diversity, while metabarcoding was most sensitive for multivariate dispersion. Taxonomic composition of animal communities derived from the two methods differed drastically. Metabarcoding was cheaper than macrofauna for ≥93 samples and quicker for ≥14 samples.

Ocean acidification alters the benthic biofilm communities in intertidal soft sediments

Microphytobenthos (MPB) and bacterial biofilms play crucial roles in primary and secondary production, nutrient cycling and invertebrate settlement in coastal ecosystems, yet little is known of the effects of ocean acidification (OA) on these communities in intertidal soft sediments. To fill in this gap, a 28-day CO2 enhancement experiment was conducted for the benthic biofilms in soft intertidal sediments (muds and sands) from Qingdao, China. This experiment included three CO2 treatments: 400 ppm CO2 (control), 700 ppm CO2 and 1000 ppm CO2 (IPCC predicted value in 2100), which were established in a three-level CO2 incubator that can adjust the CO2 concentration in the overlying air. The effects of OA on benthic biofilms were assessed in the following three aspects: MPB biomass, biofilm community structure and microbial biogeochemical cycling (e.g., C-cycle, N-cycle and S-cycle). This study found that the 700 ppm CO2 treatment did not significantly affect the benthic biofilms in intertidal soft sediments, but the 1000 ppm CO2 treatment significantly altered the biofilm community composition and potentially their role in microbial biogeochemical cyc\ling in sediments (especially in sandy sediments). For the bacterial community in biofilms, the 1000 ppm CO2 enhancement increased the relative abundance of Alteromonadales and Bacillales but decreased the relative abundance of Rhodobacterales and Flavobacteriales. For microbial biogeochemical cycling, the 1000 ppm CO2 treatment enhanced the potential of chemoheterotrophic activity, nitrate reduction and sulfur respiration in sediments, likely resulting in a more stressful environment (hypoxic and enriched H2S) for most benthic organisms. Even though incubations in this study were only 28 days long and thus couldn’t fully accommodate the range of longer-term adaptions, it still suggests that benthic biofilms in intertidal sandy sediments are likely to change significantly near the end of the century if anthropogenic CO2 emissions unmitigated, with profound implications on local ecosystems and biogeochemical cycling.

Experimental result of a shipwreck influence on the surrounding soft sediment communities in the vicinity of Bushehr port, Persian Gulf

Unplanned artificial reefs like old shipwrecks provide a hard substratum for the colonization of organisms, contributing to increasing local productivity and environmental carrying capacity especially when they are lies in soft sediment-dominated areas. Also, these structures can change the local current regime, sedimentation processes and composition of benthic species in the adjacent sediment. The present study aimed to evaluate the spatial effect of a shipwreck on its surroundings. Experimentally have been examined the relationship between sediment characteristics, abundance and macroinvertebrates richness in distance gradient (20, 50, 100 and 400 m) from a shipwreck that sank in 1983 on a soft bottom floor in the north of the Persian Gulf. Around the shipwreck, a total of 74 taxa were identified. The result indicated that macrobenthic density and richness varied significantly between distances with higher values at 20 and 50 m away from the wreck. Likewise, spatial variations in grain size were observed. A high concentration of gravel (63%) was recorded at about 20 m away from the wreck. Although, the amount of total organic carbon did not differ significantly among distances and does not explain the observed variations in density, richness and diversity of macrobenthos. In consequence, it is suggested that the proximity of the wreck modified the community structure of macrobenthos and sediment characteristics up to a distance of 50 m.

Changes in macrofauna bioturbation during repeated heatwaves mediate changes in biogeochemical cycling of nutrients

The increasing frequency and intensity of marine heatwaves (MHWs) observed worldwide entails changes in the structure and functioning of ecological communities. While severe and extreme heatwaves often have more destructive effects, the more subtle effects of moderate and strong heatwaves may nevertheless affect ecosystem functioning through complex, context-dependent linkages between different processes. Here we conducted a laboratory experiment to study the effects of repeated short-term, strong MHWs on macrofauna bioturbation and associated solute fluxes as a measure of ecosystem functioning using natural soft-sediment communities from the Baltic Sea. Our results showed changes in both bioturbation and biogeochemical cycling of nutrients following short-term, strong heatwaves, which seemed to contribute to an enhanced degradation of organic matter in the seafloor and an enhanced exchange of solutes across the sediment-water interface as well as increased sediment oxygen consumption. Following changes in these processes, the relative contribution of macrofauna and the environmental context to ecosystem functioning was altered. Our results highlight the potential of even shorter-term, strong MHWs of having system-wide impacts due to changes in the mechanistic process of bioturbation underpinning the biogeochemical cycling of nutrients. This study also highlights the need to measure a wide range of variables for a comprehensive understanding of the changes in functioning under disturbances, such as MHWs.

Diversity and distribution of bivalve molluscs in the Central Cantabrian Sea and the Avilés Canyons System (Bay of Biscay)

Bivalve molluscs is one of the dominant benthic groups in soft-sediment communities off the Atlantic Ocean, including the European margin and the Bay of Biscay. The current knowledge of deep-sea Euro-Atlantic bivalves and their distribution is still far from comprehensive and has been the scope of several recent publications. We examined the collected bivalve samples from two oceanographic campaigns (COCACE and BIOCANT) carried out in the central Bay of Biscay, including the Avilés Canyons System (ACS), a Site of Community Importance within the Natura 2000 Network. Bivalve specimens were collected from the continental shelf, slope and bathyal zones, ranging in deep from 25 to 4700 m. This is the first study focused on bivalve diversity from the Cantabrian Sea that covers such a wide bathymetric range. Eighty-four live taxa in 36 families were identified; Cetomya neaeroides is reported for the first time in the area. We provide photographs showing the shell diagnostic features of all recorded species. The conducted Multivariate Analysis showed three species assemblages: i) samples from shallow waters (continental shelf); ii) intermediate depths (shelf and upper slope) and iii) deep water (slope and abyssal plain). Depth was the main structuring factor that influenced the distribution of the recorded species and the higher species richness was found on the continental shelf. The feeding strategy and the substrate preference appeared as not significant factors in the zonation patterns of the studied bivalves.

Generalized changes of benthic communities after construction of wind farms in the southern North Sea

Over the last years, the development of offshore renewable energy installations such as offshore wind farms led to an increasing number of man-made structures in marine environments. Since 2009, benthic impact monitoring programs were carried out in wind farms installed in the southern North Sea. We collated and analyzed data sets from three major monitoring programs. Our analysis considered a total of 2849 sampling points converted to a set of biodiversity response metrics. We analyzed biodiversity changes related to the implementation of offshore wind farms and generalized the correlation of these changes with spatial and temporal patterns. Our results demonstrate that depth, season and distance to structure (soft-bottom community) consistently determined diversity indicators and abundance parameters, whereas the age and the country affiliation were significantly related to some but not all indices. The water depth was the most important structuring factor for fouling communities while seasonal effects were driving most of the observed changes in soft-sediment communities. We demonstrate that a meta-analysis can provide an improved level of understanding of ecological patterns on large-scale effects of anthropogenic structures on marine biodiversity, which were not visible in single monitoring studies. We believe that meta-analyses should become an indispensable tool for management of offshore wind farm effects in the future, particularly in the view of the foreseen development of offshore renewable energies. This might lead to a better picture and more comprehensive view on potential alterations. However, this requires a modern open-source data policy and data management, across institutions and across national borders.

Enhancing multiple scales of seafloor biodiversity with mussel restoration

Restoration projects are underway internationally in response to global declines in shellfish beds. As diverse biological assemblages underpin a variety of ecosystem services, understanding broader changes in biodiversity associated with mussel restoration becomes increasingly valuable to scientists and restoration practitioners. Studies generally show bivalve beds increase species richness and abundance, but results are scale-dependent and conditional on the mobility of specific communities observed. We examined biodiversity at multiple scales to determine how communities with varying levels of mobility are influenced by subtidal mussel restoration. Significant changes in assemblage structure were observed in both mobile fish and epifaunal communities, with enhanced species richness and total abundance of associated individuals. In contrast, we observed site-dependent effects of bivalve restoration on macrofaunal community structure and composition, with sheltered, harbour mussel bed communities numerically dominated by detritivores accustomed to organically enriched, muddy sediments. Sediment organic matter significantly increased within mussel beds, and distance-based linear models showed that sediment organic matter was an important predictor of macrofaunal assemblage structure on mussel beds, highlighting the significance of benthic-pelagic coupling and biodeposition to soft-sediment communities. This study contributes novel methods and ecological insights on the role of species mobility and site selection in structuring restoration outcomes, better informing future mussel restoration efforts aimed at emphasising functionally-driven ecosystem services.

Organic contaminants in imported salmon feed and their effects on reef ecosystems in New Zealand

Organic matter from salmon farms has been shown to be assimilated by soft sediment and rocky reef communities within the ecological footprint of salmon farms. Given these findings, another question arises – What other chemicals in salmon feed may be assimilated into wild communities via organic waste from salmon farms? Here we measured a suite of organic contaminants in salmon feed, in organisms used in a controlled feeding experiment, and in reef species collected within the depositional footprint of salmon farms. Gas Chromatography-Tandem Mass Spectrometry was used to quantify trace concentrations of polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs) and current-use (CPUs) and historic-use pesticides (HUPs) in salmon feed imported to New Zealand. The effect of assimilation of farm-derived organic matter on contaminant profiles differed among species during the controlled feeding experiment and demonstrated that migration of individuals to a farm-associated site has the potential to increase or decrease organic contaminant concentrations. Concentrations of PCBs in Parapercis colias (blue cod), a highly resident, long-lived fish, were significantly higher at farm sites than at reference sites. While these concentrations were relatively low in a global context, this result presents blue cod as an important candidate for future monitoring of organic contaminants around point sources. PCBs and PBDEs measured in wild marine species were all below limits set by the European Union, whereas concentrations of certain HUPs, specifically dichlorodiphenyltrichloroethane (DDT) and its degradation products and endosulfan, may be of concern as a consequence of alternative anthropogenic activities. Overall, feed imported to New Zealand had relatively low levels of most organic contaminants that, at current levels, are unlikely to result in significant ecological effects to wild communities in adjacent habitats.

Deep-sea acorn worms (Enteropneusta) from the Bering Sea with the description of a new genus and a new species of Torquaratoridae dominating soft-bottom communities

Five species of deep-sea acorn worms (Enteropneusta) were found in the Bering Sea during cruises of the RV Akademik M.A. Lavrentyev in 2016, 2018, and 2021. The worms were recorded on slopes of the Volcanologists Massif and in the Komandorsky Graben at depths between 1830 and 4278 m and on the Koryak slope between 419 and 662 m. Four of the species were only photographed but not collected; these were an unidentified Torquaratoridae gen. sp. 1 and three harrimaniids: Saxipendium sp., Harrimaniidae gen. sp. 1 and Harrimaniidae gen. sp. 2. Harrimaniidae gen. sp. 1 and gen. sp. 2 were recorded on the Koryak slope in the vicinity of methane seeps. The fifth species was photographed in situ and collected. It has been examined using morphological and molecular methods. It is described in the present study as Quatuoralisia malakhovi n. gen. n. sp. Ezhova et Lukinykh (fam. Torquaratoridae). The abundance of this species in the soft-sediment community at the depth 1830–2290 m was up to 12 specimens per m2, the highest ever recorded for any deep-sea enteropneusts. Morphologically, Q. malakhovi n. gen. n. sp. is noteworthy for the mesocoel ducts, the heart-kidney complex with a pericardial coelom, the two-plated skeleton supporting the proboscis stalk, and conspicuously elongated lateral wings developed on either side of the trunk. The molecular analysis based on the mitochondrial 16S rRNA showed that Q. malakhovi n. gen. n. sp. is closest to several torquaratorid species that have yet to be formally described morphologically.

Soft-Sediment Communities of the Northern Indian River Lagoon, FL, United States

Understanding the structure and function of infaunal communities is useful in determining the biodiversity and ecosystem function of shallow estuaries. We conducted a survey of infaunal communities within three separate water basins [Mosquito Lagoon (ML), Indian River (IR), and Banana River (BR)] in the larger Northern Indian River Lagoon, FL, United States to establish a database of infaunal community structure and function. Twenty-seven sites were sampled quarterly from 2014 to 2016. Analysis of all samples determined that basin, season, and sediment composition were the primary drivers of macrobenthic community composition. Diversity was highest in the ML, and lower in spring compared to other seasons. The occurrence of a brown tide (Aureoumbra lagunensis) in 2016 allowed a comparison of winter and spring communities before (2015) and during (2016) a bloom event. Community composition and diversity at the BR sites were the most affected by the bloom event with the lowest diversity and abundances during the bloom. Diversity in the IR was also lower during the bloom, while the ML was unaffected by the bloom. Species of all feeding groups were affected by the bloom, with lower abundances found in all groups. In addition, to determine the overall trophic diversity of infaunal communities, we collected infaunal organisms from two of the quarterly sampled sites for isotope analyses. Values of δ13C and δ15N from infaunal tissue were compared to those of potential food sources at each site. Substantial interspecific variation in isotope values of infaunal organisms within a site suggests the presence of diverse nutritional modes that include suspension and deposit feeding and predation. Together, these data suggest that infaunal communities contribute to benthic pelagic coupling and nutrient cycling within the estuarine communities, but the overall function of these communities may be tightly linked to their species composition.

Intertidal zonation of hemichordates in soft sediments

AbstractIntertidal zonation of organisms is well studied on rocky shores but less so in soft sediment communities. On rocky shores, communities are two dimensional, with biotic factors such as competition and predation setting the lower bound of a zone, whereas abiotic factors such as desiccation set the upper bound. In soft sediment communities, these patterns persist, but with a dynamic three‐dimensional ecosystem occupied by mobile infaunal organisms, zonation can be more difficult to quantify and detect. Hemichordate worms, however, deposit fecal casts at the surface, which can be easily identified and counted, making them a potential model system for identifying zonation in soft‐sediment systems. Here, we describe the intertidal zonation of hemichordate worms at two sites in Maine and Virginia. In Virginia,Saccoglossus kowalevskiioccurs in the mid‐intertidal at densities up to 500 individuals per square meter, whereas a tube‐building polychaete,Spiochaetopterus oculatus, dominates the lower intertidal. In Maine, two hemichordate species,Saccoglossus bromophenolosusandProtoglossus graveolens, co‐occur at densities up to 100 individuals per square meter in the mid‐intertidal, whereas hermit crabs and errant polychaetes are numerically dominant in the low intertidal. Despite known chemical defenses, crustaceans readily consumed hemichordates in lab assays, although polychaetes did not. In a field tethering experiment, loss rates of hemichordate‐flavored agarose pellets increased as tidal height decreased, suggesting that the lower bound of the hemichordate zone could be set by biotic forces such as crustacean predation. Additionally, a field transplant experiment found low survivorship of hemichordates transplanted to the upper intertidal, suggesting that desiccation may set their upper bound. These results are broadly consistent with prior studies of zonation in soft sediments, and expand our limited understanding of basic hemichordate ecology.

A quantitative analysis of organic matter inputs to soft sediment communities surrounding salmon farms in the Marlborough Sounds region of New Zealand

Local differences in trophic structure and composition of organic matter subsidies can influence the capacity of soft sediment communities to assimilate recycled organic matter from processes such as salmon farm enrichment. The present study combines biochemical analysis with biomass density information on soft sediment taxa collected within the depositional footprint of salmon farms and at reference sites in the Marlborough Sounds, New Zealand. Distinct biochemical signatures confirmed that the flux of organic matter from salmon farms was an important subsidy for soft sediment communities. Isotopic modelling demonstrated that the proportion of biomass supported by farm-derived organic matter did not change in a consistent pattern along the 300 m gradient from each farm site, whereas the average trophic level of communities decreased with increasing proximity to farms. High variability in both the total biomass and the distribution of biomass across trophic levels occurred among sites downstream of farms and among individual farms. Consequently, estimates of basal organic matter assimilation per unit area by communities differed by several orders of magnitude among sites. Total organic matter assimilation tended to decrease with increasing proximity to farms due to a shift towards a more detrital based community. Differences in basal organic matter assimilation among farms did not appear to be directly related to local flow regime, but instead was closely linked to differences in the soft sediment community composition likely influenced by an array of anthropogenic and environmental factors. The results presented here highlight the importance of considering local variability in basal organic matter source pools, and the potential for synergistic and cumulative effects to drive changes in food web trophodynamics when assessing the impacts of aquaculture on soft sediment communities.

Contrasted impact of two macrofaunal species (Hediste diversicolor and Scrobicularia plana) on microphytobenthos spatial distribution and photosynthetic activity at microscale

Microphytobenthos is most often the primary source of carbon for coastal soft-sediment communities, especially in intertidal and shallow subtidal environments. The influence of benthic macrofaunal organisms on microphytobenthic biomass, spatial distribution and photosynthetic capacities is not only resulting from their feeding intensity but also indirectly from their bioturbation activity, which regulates nutrient fluxes and sediment mixing. This study compares the impact of two species (Hediste diversicolor and Scrobicularia plana) that dominate macrofaunal communities in estuarine intertidal mudflats on microphytobenthic biomass and photosynthetic activity. Imaging-PAM fluorescence was used to non-invasively map the development of microphytobenthic biomass and to assess its spatial extent. Our results showed that, due to intense deposit feeding, Scrobicularia plana quickly limited microphytobenthos growth and photosynthetic activity, even at low density (<250 ind m−2). In contrast, the negative impact of Hediste diversicolor on microphytobenthos development due to direct consumption was very low. Thereby, the stimulation of nutrient fluxes at the sediment-water interface resulting from bioirrigation seems to enhance microphytobenthos growth and photosynthesis.

Moonsnail hatching success, development timing and early feeding behaviour at the high-latitude White Sea

AbstractAs predators, Naticidae (Gastropoda) can shape marine soft-sediment communities. Thus understanding of the reproductive biology and development of moonsnails is of great importance. Most moonsnails lay large egg masses, known as sand or egg collars, which are freely distributed on sand or muddy sediments. Here we report upon the abundance of egg collars of two naticid species, Amauropsis islandica and Euspira pallida, from the high-latitude White Sea, as well as describe the morphology of egg collars and hatching success, with a brief description of juvenile feeding in A. islandica. While in the subtidal zone, the egg collars of E. pallida were 10 times more abundant than of A. islandica, the egg collars of the latter species were the only ones that occur in the intertidal zone. The morphology of an egg collar of E. pallida differed from the literature descriptions by having a plicated basal margin. The number of egg capsules inside the collars was twice as high in A. islandica compared with E. pallida, but they were smaller. Amauropsis islandica hatchlings were larger and hatching success was more than twice that in E. pallida. We suggest that these characteristics promote the high abundance of A. islandica populations observed on some tidal flats of the White Sea. Surprisingly, A. islandica juveniles could perform non-drilling feeding in the first month after hatching. This study fills the gap in the knowledge of naticid reproductive biology at high latitudes.

Characterization of deep-sea benthic invertebrate megafauna of the Galapagos Islands

The deep sea represents the largest and least explored biome on the planet. Despite the iconic status of the Galapagos Islands and being considered one of the most pristine locations on earth, the deep-sea benthic ecosystems of the archipelago are virtually unexplored in comparison to their shallow-water counterparts. In 2015, we embarked on a multi-disciplinary scientific expedition to conduct the first systematic characterization of deep-sea benthic invertebrate communities of the Galapagos, across a range of habitats. We explored seven sites to depths of over 3,300 m using a two-part Remotely Operated Vehicle (ROV) system aboard the E/V Nautilus, and collected 90 biological specimens that were preserved and sent to experts around the world for analysis. Of those, 30 taxa were determined to be undescribed and new to science, including members of five new genera (2 sponges and 3 cnidarians). We also systematically analysed image frame grabs from over 85 h of ROV footage to investigate patterns of species diversity and document the presence of a range of underwater communities between depths of 290 and 3,373 m, including cold-water coral communities, extensive glass sponge and octocoral gardens, and soft-sediment faunal communities. This characterization of Galapagos deep-sea benthic invertebrate megafauna across a range of ecosystems represents a first step to study future changes that may result from anthropogenic impacts to the planet’s climate and oceans, and informed the creation of fully protected deep-water areas in the Galapagos Marine Reserve that may help preserve these unique communities in our changing planet.

Increasing oxygen deficiency changes rare and moderately abundant bacterial communities in coastal soft sediments

Coastal hypoxia is a major environmental problem worldwide. Hypoxia-induced changes in sediment bacterial communities harm marine ecosystems and alter biogeochemical cycles. Nevertheless, the resistance of sediment bacterial communities to hypoxic stress is unknown. We investigated changes in bacterial communities during hypoxic-anoxic disturbance by artificially inducing oxygen deficiency to the seafloor for 0, 3, 7, and 48 days, with subsequent molecular biological analyses. We further investigated relationships between bacterial communities, benthic macrofauna and nutrient effluxes across the sediment-water-interface during hypoxic-anoxic stress, considering differentially abundant operational taxonomic units (OTUs). The composition of the moderately abundant OTUs changed significantly after seven days of oxygen deficiency, while the abundant and rare OTUs first changed after 48 days. High bacterial diversity maintained the resistance of the communities during oxygen deficiency until it dropped after 48 days, likely due to anoxia-induced loss of macrofaunal diversity and bioturbation. Nutrient fluxes, especially ammonium, correlated positively with the moderate and rare OTUs, including potential sulfate reducers. Correlations may reflect bacteria-mediated nutrient effluxes that accelerate eutrophication. The study suggests that even slightly higher bottom-water oxygen concentrations, which could sustain macrofaunal bioturbation, enable bacterial communities to resist large compositional changes and decrease the harmful consequences of hypoxia in marine ecosystems.

Benthic trophic networks of the southern North Sea: contrasting soft-sediment communities share high food web similarity

We examined whether taxonomically distinct benthic communities from contrasting sediments in the German Bight (southern North Sea) also differ in their trophic structure. As a case study, we compared the Amphiura filiformis community (AFC) of silty sands and the Bathyporeia− Tellina community (BTC) of fine sands using a combination of stable isotope analysis and data on trophic interactions. Differences between the food webs were evident in the feeding guild compo� sition of important primary consumers: deposit and interface feeders are the most diverse primary consumer guilds in the AFC, whereas suspension and interface feeders play a major role in the BTC, reflecting differences in physical properties and food availability at the sediment−water interface. While all primary consumer guilds had the same trophic level (TL) in the AFC, deposit feeders of the BTC occupied a trophic position intermediate between other primary and higher� order consumer guilds, likely explained by partially incomplete knowledge of their trophic eco - logy and selective feeding, including the ingestion of meiofauna. Most food web properties, how�ever, were similar between the AFC and BTC: they mainly depend on pelagic primary production, reach TL 4 and are characterized by a prevalence of generalist higher-order consumers. Further� more, both trophic networks had similar linkage densities and high directed connectance, the lat�ter feature suggesting considerable food web robustness. Our findings suggest that although com�munities in the German Bight differ in some aspects of their trophic structure, they share a similar food web topology, indicating a comparable degree of resilience towards natural and anthro�pogenic disturbances.

The Effect of Chironomid Larvae on Nitrogen Cycling and Microbial Communities in Soft Sediments

The combination of biogeochemical methods and molecular techniques has the potential to uncover the black-box of the nitrogen (N) cycle in bioturbated sediments. Advanced biogeochemical methods allow the quantification of the process rates of different microbial processes, whereas molecular tools allow the analysis of microbial diversity (16S rRNA metabarcoding) and activity (marker genes and transcripts) in biogeochemical hot-spots such as the burrow wall or macrofauna guts. By combining biogeochemical and molecular techniques, we analyzed the role of tube-dwelling Chironomus plumosus (Insecta, Diptera) larvae on nitrification and nitrate reduction processes in a laboratory experiment with reconstructed sediments. We hypothesized that chironomid larvae stimulate these processes and host bacteria actively involved in N-cycling. Our results suggest that chironomid larvae significantly enhance the recycling of ammonium (80.5 ± 48.7 µmol m−2 h−1) and the production of dinitrogen (420.2 ± 21.4 µmol m−2 h−1) via coupled nitrification–denitrification and the consumption of water column nitrates. Besides creating oxygen microniches in ammonium-rich subsurface sediments via burrow digging and ventilation, chironomid larvae serve as hot-spots of microbial communities involved in N-cycling. The quantification of functional genes showed a significantly higher potential for microbial denitrification and nitrate ammonification in larvae as compared to surrounding sediments. Future studies may further scrutinize N transformation rates associated with intimate macrofaunal–bacteria associations.

Ecosystem functioning along gradients of increasing hypoxia and changing soft-sediment community types

Marine ecosystems world-wide are threatened by oxygen deficiency, with potential serious consequences for ecosystem functioning and the goods and services they provide. While the effects of hypoxia on benthic species diversity are well documented, the effects on ecosystem function have only rarely been assessed in real-world settings. To better understand the links between structural changes in macro- and meiofaunal communities, hypoxic stress and benthic ecosystem function (benthic nutrient fluxes, community metabolism), we sampled a total of 11 sites in Havstensfjord and Askeröfjord (Swedish west coast) in late summer, coinciding with the largest extent and severity of seasonal hypoxia in the area. The sites spanned oxic to anoxic bottom water, and a corresponding gradient in faunal diversity. Intact sediment cores were incubated to measure fluxes of oxygen and nutrients (NO3−, NO2−, NH4+, PO43−, SiO4) across the sediment-water interface. Sediment profile imaging (SPI) footage was obtained from all sites to assess structural elements and the bioturbation depth, and additional samples were collected to characterise sediment properties and macro- and meiofaunal community composition. Bottom-water O2 concentration was the main driver of macrofauna communities, with highest abundance and biomass, as well as variability, at the sites with intermediate O2 concentration. Meiofauna on the other hand was less sensitive to bottom-water O2 concentration. Oxygen was the main driver of nutrient fluxes too, but macrofauna as well meiofauna were also significant predictors; DistLM analyses indicated that O2 concentration, macrofaunal abundance or biomass, and meiofaunal abundance collectively explained 63%, 30% and 28% of the variation in sediment O2 consumption, NH4+flux and PO43− flux, respectively. The study provides a step towards a more realistic understanding of the link between benthic fauna and ecosystem functioning, and the influence of disturbance on this relationship, which is important for management decisions aimed at protecting the dwindling biodiversity in the coastal zones around the world.

Soft-sediment community stability across years in San Francisco Bay

Macrobenthos is used commonly in disturbance-related studies of coastal ecosystems, including those that evaluate invasions by non-native species (NIS), but still little is known about temporal variation in community characteristics, especially in bays and estuaries. In this study we investigated inter-annual changes in the soft-sediment benthic communities of San Francisco Bay over a period of five years, evaluating the contribution of NIS vs. native species to community attributes (species richness, abundance) and the efficacy of sampling (percent richness detected for each NIS and native species). Benthic macrofauna were collected, identified, and quantified from 10 stations (48–50 replicate samples) per year across the high salinity region of the Bay. A total of 36,872 individuals belonging to 126 morphospecies were collected; 61 species were native, accounting for 22% of total abundance, and 31 species were NIS, which reached 74% of total abundance. The other 34 species were either cryptogenic or unresolved taxa. Soft-sediment communities were mainly comprised of amphipods (Ampelisca abdita, Sinocorophium heteroceratum, Monocorophium acherusicum), polychaetes (Sabaco elongatus, Euchone limnicola) and bivalves (Venerupis philippinarum). Community structure and composition were stable across years during the period of the study, despite a major marine heat wave and a record-breaking drought that raised average salinity levels for several years. The sampling was effective, especially for NIS, detecting a higher proportion (94–100%) of estimated richness for NIS compared to native species (74–89%) across the five year period, suggesting NIS were more evenly distributed in space and time and many native species occurred more patchily and less frequently.