Jan Vanaverbeke

The role of the benthic biota in sedimentary metabolism and sediment-water exchange processes in the Goban Spur area (NE Atlantic)

We provide an overview of the role of biological processes in the Benthic boundary layer (BBL) and in sediments on the cycling of particulate organic material in the Goban Spur area (Northeast Atlantic). The benthic fauna, sediment and BBL characteristics were studied along a transect ranging from 208 to 4460 m water depth in different seasons over 3 years. Near-bottom flow velocities are high at the upper part of the slope (1000–1500 m), and high numbers of filter-feeding taxa are found there such that organic carbon normally passing this area during high flow conditions is probably trapped, accumulated, and/or remineralised by the fauna. Overall metabolism in shelf and upper slope sediments is dominated by the macrofauna. More than half of the organic matter flux is respired by macrofauna, with a lower contribution of metazoan meiofauna (4%) and anoxic and suboxic bacterial mineralisation (21%); the remainder (23%) being channelled through nanobiota and oxic bacteria. By its feeding activity and movement, the macrofauna intensely reworks the sediments on the shelf and upper slope. Mixing intensity of bulk sediment and of organic matter are of comparable magnitude. The benthos of the lower slope and abyssal depth is dominated by the microbiota, both in terms of total biomass (>90%) and carbon respiration (about 80%). The macrofauna (16%), meiofauna (4%) and megafauna (0.5%) only marginally contribute to total carbon respiration at depths below 1400 m. Because large animals have a lower share in total metabolism, mixing of organic matter within the sediments is reduced by a factor of 5, whereas mixing of bulk sediment is one to two orders of magnitude lower than on the shelf. The food quality of organic matter in the sediments in the shallowest part of the Goban Spur transect is significantly higher than in sediments in the deeper parts. The residence time of mineralisable carbon is about 120 d on the shelf and compares well with the residence time of the biota. In the deepest station, the mean residence time of mineralisable carbon is more than 3000 d, an order of magnitude higher than that of biotic biomass.

The importance of fine-scale, vertical profiles in characterising nematode community structure

The spatial heterogeneity of the nematode community on an intertidal flat (the Molenplaat) in the Westerschelde estuary (SW Netherlands) has been investigated. The extent to which macroscale (km) variability was more important than microscale (m) variability was tested. In addition, the importance of vertical distribution profile in the sediment in explaining the horizontal macroscale variability was evaluated. Differences in the structure of the community were analysed at a kilometre scale at three sites that differed in chemico-physical features. The differences in geochemical and physical conditions on a horizontal scale were reflected in species composition and trophic structure of the nematode communities, and to a much lesser extent in their total abundance and species diversity. Detailed investigation of vertical depth profiles showed more pronounced differences between environmentally divergent sites. Sediment granulometry appears to be important in controlling the fauna in the upper sediment layers. At depth, similar faunal assemblages were found irrespective of sediment granulometry, suggesting that other environmental features are more dominant. Vertically, nematode species showed depth distributions that were indicative of sediment characteristics related to the site-specific hydrodynamic regime. Pronounced vertical segregation of nematode species was observed within sandy sediment under strong hydrodynamic and food-stressed conditions. A surface-dwelling nematode community of large predatory enoplids was separated from a deposit feeding xyalid-microlaimid community in deeper sediment layers (beneath 2 cm). Causal factors for this segregation are thought to be species interactions, feeding strategies and/or physical disturbance. In the finest sediments, with high silt content, almost all nematode species were confined to the upper sediment layers (1.5 cm). A sharp decline in density and diversity with depth was observed. Key factors for this distribution pattern are possibly related to the limited oxygen penetration in surface layers and the occurrence of sulphide in deeper sediment layers. At intermediate hydrodynamic and granulometric conditions, a gradual shifting of nematode community was observed with depth, with dominant nematode species maxima present at specific depth layers.

Exploring the Use of Cytochrome Oxidase c Subunit 1 (COI) for DNA Barcoding of Free-Living Marine Nematodes

Background The identification of free-living marine nematodes is difficult because of the paucity of easily scorable diagnostic morphological characters. Consequently, molecular identification tools could solve this problem. Unfortunately, hitherto most of these tools relied on 18S rDNA and 28S rDNA sequences, which often lack sufficient resolution at the species level. In contrast, only a few mitochondrial COI data are available for free-living marine nematodes. Therefore, we investigate the amplification and sequencing success of two partitions of the COI gene, the M1-M6 barcoding region and the I3-M11 partition. Methodology Both partitions were analysed in 41 nematode species from a wide phylogenetic range. The taxon specific primers for the I3-M11 partition outperformed the universal M1-M6 primers in terms of amplification success (87.8% vs. 65.8%, respectively) and produced a higher number of bidirectional COI sequences (65.8% vs 39.0%, respectively). A threshold value of 5% K2P genetic divergence marked a clear DNA barcoding gap separating intra- and interspecific distances: 99.3% of all interspecific comparisons were >0.05, while 99.5% of all intraspecific comparisons were <0.05 K2P distance. Conclusion The I3-M11 partition reliably identifies a wide range of marine nematodes, and our data show the need for a strict scrutiny of the obtained sequences, since contamination, nuclear pseudogenes and endosymbionts may confuse nematode species identification by COI sequences.

The metazoan meiobenthos along the continental slope of the Goban Spur (NE Atlantic)

Abstract The metazoan meiobenthos along the continental slope of the Goban Spur (NE Atlantic) was studied in the framework of the OMEX-programme (EC-MAST II). Meiobenthic densities, the vertical distribution of the meiobenthos in the sediment, and the biomass and generic composition of the dominant group, the nematodes, were investigated at different water depths. The meiobenthic communities along the depth gradient of the continental slope followed the usual trends: the communities were dominated by nematodes, and meiobenthic densities, total nematode biomass, and mean individual nematode weight decreased with increasing water depth. The shelf station had significantly higher densities, nematode biomass and mean individual weight than the deeper stations. The nematode communities along the OMEX slope were divided into a ‘shelf break’ community (206 m) and a ‘down slope’ community (1034–2760 m), with the station at 670 m depth as a transition station. It is suggested that the changes in the nematode community composition along the slope of the Goban Spur are related to the amount of oxygen (and the presence/absence of an oxic mineralisation pathway) in the sediment. The meiobenthos tended to be more concentrated towards the sediment surface with increasing water depth.

The metazoan meiobenthos along a depth gradient in the Arctic Laptev Sea with special attention to nematode communities

Abstract The meiobenthos along a depth transect of oligotrophic sediments in the Arctic Laptev Sea was studied. The meiobenthos followed the general trends reported from other studies: densities decreased with depth in relation to the more limited supply of degradable organic matter at greater depths. Although the sediments along the transect were poor in organic matter in comparison with the NE Atlantic, the densities fitted well with the meiobenthic densities reported from the latter area. It is suggested that the meiobenthos in the cold polar waters is adapted to this extreme environment by a rapid response to short food pulses to the sediments. Nematodes were identified up to genus level and assigned to trophic groups. A total of 32 families comprising 95 genera were found along the transect. The communities were dominated by deposit feeders whose importance increased with depth. Both TWINSPAN and CCA analyses revealed a community shift along the depth transect: a shelf community dominated by Microlaimus and Chromadora could be distinguished from a slope community dominated by Monhystera and Leptolaimus. Generic diversity decreased with depth.

Nematode distribution in ocean margin sediments of the Goban Spur (northeast Atlantic) in relation to sediment geochemistry

Abstract Nematode density along a depth gradient (206–2760 m) in the northeastern Atlantic (Goban Spur) is linearly and positively related to organic matter mineralization in the sediment. It is estimated that nematodes contribute at most 13% to total carbon turnover, and this contribution decreases with water depth. The vertical distribution of nematodes in the sediment closely follows concentration vs depth profiles of total nitrogen in the sediment. Both nematodes and total nitrogen tend to be more concentrated towards the surface with increasing water depth. There is no indication that nematode vertical distribution is directly influenced by the oxygen penetration in the sediment. However, it is hypothesized that the sediment oxygen concentration can explain the relatively greater importance of smaller organisms in benthic communities with increasing water depth. The nitrogen vs sediment depth profiles are consistent with a greater impact of larger organisms on sediments at the shallow stations and decreased sediment mixing with increasing water depth.

Nematode assemblages from subtidal sandbanks in the Southern Bight of the North Sea: effect of small sedimentological differences

Nematode assemblages from four subtidal sandbanks belonging to different sandbank systems on the Belgian Continental Shelf were investigated both in spring and fall. The assemblages were characterised by different species composition patterns on the different sandbanks. This is in contrast to results of earlier studies which showed that neither meiobenthic nor macrobenthic taxa differed among these sandbanks. Although the sediments on these sandbanks could all be classified as medium sands, the use of Multiple Discriminant Analysis (MDA) suggested that median grain size and the proportions of median sand and very fine sand were the variables explaining the difference in nematode community composition. These findings emphasise the strong relationship between the relative abundance of nematode species and sediment composition. The influence of sand extraction on these sandbanks resulted in coarsening of the sediment, which had a direct effect on the nematode species composition. Diversity was not affected, indicating that nematodes inhabiting highly dynamic environments are well adapted to physical disturbance. The diversity at sandbanks is not necessarily very different from the surrounding areas, since in more offshore parts of the Belgian Continental Shelf, clean and rather coarse sands prevail and the differences in sediment composition are not sufficient to induce large differences in diversity.

Sediment-related distribution patterns of nematodes and macrofauna: Two sides of the benthic coin?

We investigated the sediment-related distribution of both nematodes and macrofauna on the Belgian part of the North Sea (Southern Bight of the North Sea) in order to evaluate whether both faunal groups reflect similar patterns in community composition and diversity. Fine-grained sediments (median grain size <200μm) were inhabited by nematode communities characterised by a low diversity and dominated by non-selective deposit-feeding nematodes. Nematode communities from coarser sediments were significantly different in terms of community composition and diversity. Moreover, all nematode feeding types were present in coarser sediments. These differences were explained by the contrasting biogeochemical processes prevailing in both sediment types, rather than granulometry and food availability per se. Macrofaunal distribution patterns were different from those of the nematode communities and seem to be related to water column processes (SPM loading, food availability, hydrodynamic stress) that promote the establishment of diverse communities in the coarser sediments but not in the finest sediments. This suggests that data on nematodes and macrofauna reveal different, complementary aspects of the factors structuring the benthic ecosystem that can be of importance in assessing the ecological status of the seafloor.

Variable importance of macrofaunal functional biodiversity for biogeochemical cycling in temperate coastal sediments

Coastal marine systems are currently subject to a variety of anthropogenic and climate-change-induced pressures. An important challenge is to predict how marine sediment communities and benthic biogeochemical cycling will be affected by these ongoing changes. To this end, it is of paramount importance to first better understand the natural variability in coastal benthic biogeochemical cycling and how this is influenced by local environmental conditions and faunal biodiversity. Here, we studied sedimentary biogeochemical cycling at ten coastal stations in the Southern North Sea on a monthly basis from February to October 2011. We explored the spatio-temporal variability in oxygen consumption, dissolved inorganic nitrogen and alkalinity fluxes, and estimated rates of nitrification and denitrification from a mass budget. In a next step, we statistically modeled their relation with environmental variables and structural and functional macrobenthic community characteristics. Our results show that the cohesive, muddy sediments were poor in functional macrobenthic diversity and displayed intermediate oxygen consumption rates, but the highest ammonium effluxes. These muddy sites also showed an elevated alkalinity release from the sediment, which can be explained by the elevated rate of anaerobic processes taking place. Fine sandy sediments were rich in functional macrobenthic diversity and had the maximum oxygen consumption and estimated denitrification rates. Permeable sediments were also poor in macrobenthic functional diversity and showed the lowest oxygen consumption rates and only small fluxes of ammonium and alkalinity. Macrobenthic functional biodiversity as estimated from bioturbation potential appeared a better variable than macrobenthic density in explaining oxygen consumption, ammonium and alkalinity fluxes, and estimated denitrification. However, this importance of functional biodiversity was manifested particularly in fine sandy sediments, to a lesser account in permeable sediments, but not in muddy sediments. The strong relationship between macrobenthic functional biodiversity and biogeochemical cycling in fine sandy sediments implies that a future loss of macrobenthic functional diversity will have important repercussions for benthic ecosystem functioning.

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 13C 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 (Δδ13C) of the added diatoms was highest in the physical mixing treatment, where macrobenthos was absent and the diatom 13C 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 TO13C 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.