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.

Offshore Wind Energy and Marine Biodiversity in the North Sea: Life Cycle Impact Assessment for Benthic Communities.

Large-scale offshore wind energy developments represent a major player in the energy transition but are likely to have (negative or positive) impacts on marine biodiversity. Wind turbine foundations and sour protection often replace soft sediment with hard substrates, creating artificial reefs for sessile dwellers. Offshore wind farm (OWF) furthermore leads to a decrease in (and even a cessation of) bottom trawling, as this activity is prohibited in many OWFs. The long-term cumulative impacts of these changes on marine biodiversity remain largely unknown. This study integrates such impacts into characterization factors for life cycle assessment based on the North Sea and illustrates its application. Our results suggest that there are no net adverse impacts during OWF operation on benthic communities inhabiting the original sand bottom within OWFs. Artificial reefs could lead to a doubling of species richness and a two-order-of-magnitude increase of species abundance. Seabed occupation will also incur in minor biodiversity losses in the soft sediment. Our results were not conclusive concerning the trawling avoidance benefits. The developed characterization factors quantifying biodiversity-related impacts from OWF operation provide a stepping stone toward a better representation of biodiversity in life cycle assessment.

Faunal and environmental drivers of carbon and nitrogen cycling along a permeability gradient in shallow North Sea sediments.

Ecosystem functions are driven by abiotic and biotic factors, but due to high collinearity of both, it is often difficult to disentangle the drivers of these ecosystem functions. We studied sedimentological and faunal controls of benthic organic matter mineralization, a crucial ecosystem process provided for by sediments of shelf seas. Subtidal benthic habitats representative of the wide permeability gradient found in the Belgian Part of the North Sea (Northeast Atlantic Shelf) were characterized in terms of sediment descriptors, macrofauna, and sediment biogeochemistry was estimated. Our results confirmed a strong correlation between sediment characteristics and macrofauna, and estimated sediment biogeochemical process rates were clearly linked to both. Results of variance partitioning and statistical modelling showed that oxic mineralization and nitrification were mainly regulated by faunal activities whereas anoxic mineralization was regulated by sediment properties, with permeability as a decisive factor. Both biotic and abiotic factors were needed to explain variability in oxygen consumption and total mineralization estimates, suggesting that macrofaunal activities have different effects across habitats. The statistical models were a useful tool to interpret the impact of anthropogenic activities in the study area and represent a step towards predicting the effects of human activities on crucial ecosystem functions.

Glacial melt disturbance shifts community metabolism of an Antarctic seafloor ecosystem from net autotrophy to heterotrophy.

Climate change-induced glacial melt affects benthic ecosystems along the West Antarctic Peninsula, but current understanding of the effects on benthic primary production and respiration is limited. Here we demonstrate with a series of in situ community metabolism measurements that climate-related glacial melt disturbance shifts benthic communities from net autotrophy to heterotrophy. With little glacial melt disturbance (during cold El Niño spring 2015), clear waters enabled high benthic microalgal production, resulting in net autotrophic benthic communities. In contrast, water column turbidity caused by increased glacial melt run-off (summer 2015 and warm La Niña spring 2016) limited benthic microalgal production and turned the benthic communities net heterotrophic. Ongoing accelerations in glacial melt and run-off may steer shallow Antarctic seafloor ecosystems towards net heterotrophy, altering the metabolic balance of benthic communities and potentially impacting the carbon balance and food webs at the Antarctic seafloor.

Spatial variability of biogeochemistry in shallow coastal benthic communities of Potter Cove (Antarctica) and the impact of a melting glacier.

Measurements of biogeochemical fluxes at the sediment-water interface are essential to investigate organic matter mineralization processes but are rarely performed in shallow coastal areas of the Antarctic. We investigated biogeochemical fluxes across the sediment-water interface in Potter Cove (King George Island/Isla 25 de Mayo) at water depths between 6-9 m. Total fluxes of oxygen and inorganic nutrients were quantified in situ. Diffusive oxygen fluxes were also quantified in situ, while diffusive inorganic nutrient fluxes were calculated from pore water profiles. Biogenic sediment compounds (concentration of pigments, total organic and inorganic carbon and total nitrogen), and benthic prokaryotic, meio-, and macrofauna density and biomass were determined along with abiotic parameters (sediment granulometry and porosity). The measurements were performed at three locations in Potter Cove, which differ in terms of sedimentary influence due to glacial melt. In this study, we aim to assess secondary effects of glacial melting such as ice scouring and particle release on the benthic community and the biogeochemical cycles they mediate. Furthermore, we discuss small-scale spatial variability of biogeochemical fluxes in shallow water depth and the required food supply to cover the carbon demand of Potter Cove's shallow benthic communities. We found enhanced mineralization in soft sediments at one location intermediately affected by glacial melt-related effects, while a reduced mineralization was observed at a location influenced by glacial melting. The benthic macrofauna assemblage constituted the major benthic carbon stock (>87% of total benthic biomass) and was responsible for most benthic organic matter mineralization. However, biomass of the dominant Antarctic bivalve Laternula elliptica, which contributed 39-69% to the total macrofauna biomass, increased with enhanced glacial melt-related influence. This is contrary to the pattern observed for the remaining macrofauna. Our results further indicated that pelagic primary production is able to fully supply Potter Cove's benthic carbon demand. Therefore, Potter Cove seems to be an autotrophic ecosystem in the summer season.

The Link between Microbial Diversity and Nitrogen Cycling in Marine Sediments Is Modulated by Macrofaunal Bioturbation.

OBJECTIVES: The marine benthic nitrogen cycle is affected by both the presence and activity of macrofauna and the diversity of N-cycling microbes. However, integrated research simultaneously investigating macrofauna, microbes and N-cycling is lacking. We investigated spatio-temporal patterns in microbial community composition and diversity, macrofaunal abundance and their sediment reworking activity, and N-cycling in seven subtidal stations in the Southern North Sea. SPATIO-TEMPORAL PATTERNS OF THE MICROBIAL COMMUNITIES: Our results indicated that bacteria (total and ß-AOB) showed more spatio-temporal variation than archaea (total and AOA) as sedimentation of organic matter and the subsequent changes in the environment had a stronger impact on their community composition and diversity indices in our study area. However, spatio-temporal patterns of total bacterial and ß-AOB communities were different and related to the availability of ammonium for the autotrophic ß-AOB. Highest bacterial richness and diversity were observed in June at the timing of the phytoplankton bloom deposition, while richness of ß-AOB as well as AOA peaked in September. Total archaeal community showed no temporal variation in diversity indices. MACROFAUNA, MICROBES AND THE BENTHIC N-CYCLE: Distance based linear models revealed that, independent from the effect of grain size and the quality and quantity of sediment organic matter, nitrification and N-mineralization were affected by respectively the diversity of metabolically active ß-AOB and AOA, and the total bacteria, near the sediment-water interface. Separate models demonstrated a significant and independent effect of macrofaunal activities on community composition and richness of total bacteria, and diversity indices of metabolically active AOA. Diversity of ß-AOB was significantly affected by macrofaunal abundance. Our results support the link between microbial biodiversity and ecosystem functioning in marine sediments, and provided broad correlative support for the hypothesis that this relationship is modulated by macrofaunal activity. We hypothesized that the latter effect can be explained by their bioturbating and bio-irrigating activities, increasing the spatial complexity of the biogeochemical environment.

Temporal dynamics in a shallow coastal benthic food web: Insights from fatty acid biomarkers and their stable isotopes.

We investigated the temporal variation of pelagic and benthic food sources in the diet of benthic taxa at a depositional site in the Southern Bight of the North Sea by means of fatty acid (FA) biomarkers and compound-specific stable isotope analysis (CSIA). The taxa were the non-selective deposit feeding nematodes (Sabatieria spp. and 'other nematodes'), and three dominant macrobenthic species: two true suspension-deposit feeders (the bivalve Abra alba and the tube dwelling polychaete Owenia fusiformis) and the suspected predatory mud-dwelling anemone Sagartia sp. These species make up on average 16% (Abra alba), 17% (Sagartia sp.) and 20% (Owenia fusiformis) of the biomass in the Abra alba-Kurtiella bidentata community in this area. Phytoplankton dynamics in the suspended particulate matter of the water column as inferred from cell counts, chlorophyll-a and organic carbon content were clearly visible in sediment and animal FA abundance as well, whereas phytodetritus dynamics in the sediment FA composition were less clear, probably due to patchy distribution or stripping of FA by macrofauna. Nematodes appeared to assimilate mainly Polyunsaturated Fatty Acids (PUFAs) from their sedimentary environment and were further non-selectively accumulating more (Sabatieria spp.) or less ('other nematodes') FA from the deposited phytodetritus. In contrast, Abra alba FA composition was consistent with a diatom-dominated diet and consumption of Phaeocystis was observed in Owenia fusiformis, whereas Sagartia sp. showed evidence of a predatory behaviour. While the total FA content in Owenia fusiformis remained constant throughout the year, Sagartia sp. doubled and Abra alba increased its FA level more than 10-fold in response to the organic matter deposition from the phytoplankton bloom. This leads to the conclusion that there is no resource partitioning between non-selective deposit feeding nematodes and the suspension-deposit feeding macrobenthic organisms, suggesting they belong to separate parts of the benthic food web.

Empirical evidence reveals seasonally dependent reduction in nitrification in coastal sediments subjected to near future ocean acidification.

Research so far has provided little evidence that benthic biogeochemical cycling is affected by ocean acidification under realistic climate change scenarios. We measured nutrient exchange and sediment community oxygen consumption (SCOC) rates to estimate nitrification in natural coastal permeable and fine sandy sediments under pre-phytoplankton bloom and bloom conditions. Ocean acidification, as mimicked in the laboratory by a realistic pH decrease of 0.3, significantly reduced SCOC on average by 60% and benthic nitrification rates on average by 94% in both sediment types in February (pre-bloom period), but not in April (bloom period). No changes in macrofauna functional community (density, structural and functional diversity) were observed between ambient and acidified conditions, suggesting that changes in benthic biogeochemical cycling were predominantly mediated by changes in the activity of the microbial community during the short-term incubations (14 days), rather than by changes in engineering effects of bioturbating and bio-irrigating macrofauna. As benthic nitrification makes up the gross of ocean nitrification, a slowdown of this nitrogen cycling pathway in both permeable and fine sediments in winter, could therefore have global impacts on coupled nitrification-denitrification and hence eventually on pelagic nutrient availability.

Effect of short-term hypoxia on marine nematode community structure and vertical distribution pattern in three different sediment types of the North Sea.

The responses of nematode communities to short-term hypoxia (1 and 7 days) were investigated in three North Sea stations with different sediment types (coarse silt, fine sand and medium sand). In the field, nematode density, diversity, vertical distribution and community structure differ among the stations. In the laboratory, oxic and hypoxic treatments were established for 1 and 7 days for all sediment types. Comparison between field control and oxic day 1 treatments showed that experimental sediment handling did not affect nematode characteristics. Our results revealed that short-term hypoxia did not affect total density, diversity, community composition, vertical density profiles (except in the fine sand) and densities of five dominant species in all sediment types.

Meiofauna metabolism in suboxic sediments: currently overestimated.

Oxygen is recognized as a structuring factor of metazoan communities in marine sediments. The importance of oxygen as a controlling factor on meiofauna (32 µm-1 mm in size) respiration rates is however less clear. Typically, respiration rates are measured under oxic conditions, after which these rates are used in food web studies to quantify the role of meiofauna in sediment carbon turnover. Sediment oxygen concentration ([O(2)]) is generally far from saturated, implying that (1) current estimates of the role of meiofauna in carbon cycling may be biased and (2) meiofaunal organisms need strategies to survive in oxygen-stressed environments. Two main survival strategies are often hypothesized: 1) frequent migration to oxic layers and 2) morphological adaptation. To evaluate these hypotheses, we (1) used a model of oxygen turnover in the meiofauna body as a function of ambient [O(2)], and (2) performed respiration measurements at a range of [O(2)] conditions. The oxygen turnover model predicts a tight coupling between ambient [O(2)] and meiofauna body [O(2)] with oxygen within the body being consumed in seconds. This fast turnover favors long and slender organisms in sediments with low ambient [O(2)] but even then frequent migration between suboxic and oxic layers is for most organisms not a viable strategy to alleviate oxygen limitation. Respiration rates of all measured meiofauna organisms slowed down in response to decreasing ambient [O(2)], with Nematoda displaying the highest metabolic sensitivity for declining [O(2)] followed by Foraminifera and juvenile Gastropoda. Ostracoda showed a behavioral stress response when ambient [O(2)] reached a critical level. Reduced respiration at low ambient [O(2)] implies that meiofauna in natural, i.e. suboxic, sediments must have a lower metabolism than inferred from earlier respiration rates conducted under oxic conditions. The implications of these findings are discussed for the contribution of meiofauna to carbon cycling in marine sediments.

The early life history of the clam Macoma balthica in a high CO2 world.

This study investigated the effects of experimentally manipulated seawater carbonate chemistry on several early life history processes of the Baltic tellin (Macoma balthica), a widely distributed bivalve that plays a critical role in the functioning of many coastal habitats. We demonstrate that ocean acidification significantly depresses fertilization, embryogenesis, larval development and survival during the pelagic phase. Fertilization and the formation of a D-shaped shell during embryogenesis were severely diminished: successful fertilization was reduced by 11% at a 0.6 pH unit decrease from present (pH 8.1) conditions, while hatching success was depressed by 34 and 87%, respectively at a 0.3 and 0.6 pH unit decrease. Under acidified conditions, larvae were still able to develop a shell during the post-embryonic phase, but higher larval mortality rates indicate that fewer larvae may metamorphose and settle in an acidified ocean. The cumulative impact of decreasing seawater pH on fertilization, embryogenesis and survival to the benthic stage is estimated to reduce the number of competent settlers by 38% for a 0.3 pH unit decrease, and by 89% for a 0.6 pH unit decrease from present conditions. Additionally, slower growth rates and a delayed metamorphosis at a smaller size were indicative for larvae developed under acidified conditions. This may further decline the recruit population size due to a longer subjection to perturbations, such as predation, during the pelagic phase. In general, early life history processes were most severely compromised at ~pH 7.5, which corresponds to seawater undersaturated with respect to aragonite. Since recent models predict a comparable decrease in pH in coastal waters in the near future, this study indicates that future populations of Macoma balthica are likely to decline as a consequence of ongoing ocean acidification.

Biological vs. physical mixing effects on benthic food web dynamics.

Biological particle mixing (bioturbation) and solute transfer (bio-irrigation) contribute extensively to ecosystem functioning in sediments where physical mixing is low. Macrobenthos transports oxygen and organic matter deeper into the sediment, thereby likely providing favourable niches to lower trophic levels (i.e., smaller benthic animals such as meiofauna and bacteria) and thus stimulating mineralisation. Whether this biological transport facilitates fresh organic matter assimilation by the metazoan lower part of the food web through niche establishment (i.e., ecosystem engineering) or rather deprives them from food sources, is so far unclear. We investigated the effects of the ecosystem engineers Lanice conchilega (bio-irrigator) and Abra alba (bioturbator) compared to abiotic physical mixing events on survival and food uptake of nematodes after a simulated phytoplankton bloom. The (13)C labelled diatom Skeletonema costatum was added to 4 treatments: (1) microcosms containing the bioturbator, (2) microcosms containing the bio-irrigator, (3) control microcosms and (4) microcosms with abiotic manual surface mixing. Nematode survival and subsurface peaks in nematode density profiles were most pronounced in the bio-irrigator treatment. However, nematode specific uptake (Δδ(13)C) of the added diatoms was highest in the physical mixing treatment, where macrobenthos was absent and the diatom (13)C was homogenised. Overall, nematodes fed preferentially on bulk sedimentary organic material rather than the added diatoms. The total C budget (µg C m(-2)), which included TO(13)C remaining in the sediment, respiration, nematode and macrobenthic uptake, highlighted the limited assimilation by the metazoan benthos and the major role of bacterial respiration. In summary, bioturbation and especially bio-irrigation facilitated the lower trophic levels mainly over the long-term through niche establishment. Since the freshly added diatoms represented only a limited food source for nematodes, the macrobenthic effect was more pronounced in niche establishment than the negative structuring effects such as competition.