D. Van Gansbeke

Nematode communities from the North Sea: environmental controls on species diversity and vertical distribution within the sediment

Nematode assemblages were sampled seasonally at three subtidal stations along the Belgian coast. The stations were characterized by muddy sediments (station 115), fine sand (station 702) and fine to coarse sand (station 790). The forces structuring vertical distribution were investigated by evaluating abundance, species composition, diversity and trophic composition, and relating these to sediment composition, redox state and food sources. The nematode assemblages at the two finer grained stations (115, 702) were dominated by Daptonema tenuispiculum and Sabatieria punctata . For both species, the vertical distribution in the sediment seemed not dependent on the redoxchemistry, as former believed for S. punctata , but primarily influenced by food availability. This feature could also be recognized for Ixonema sordidum and Viscosia langrunensis , the most abundant nematodes at the coarse sandy station (790). In general, nematode diversity was regulated primarily by sediment granulometry. Coarser sediments (station 790) yielded more diverse communities compared to the fine sediments (station 115, 702), however seasonal fluctuations and variations with depth into the sediment were not obvious. At the silty stations, when the sediment column was more oxidized in March, overall diversity was higher and showed a positive relationship to the mud content which varied with depth into the sediment. This positive relation is probably explained by an enhanced deposition of organic matter associated with the accumulation of fine particles near the river-mouths. Furthermore, the higher abundance, the lower diversity and the higher dominance found at the two silty stations of the eastern and the western part of the Belgian coast, pointed to a stressed, organically enriched environment. The results demonstrate that controls on nematode community structure are complex and that information at both species and community level are required to properly evaluate the effects of natural and anthropogenic impacts.

Tight coupling between enrichment of iron and manganese in North Sea suspended matter and sedimentary redox processes: Evidence for seasonal variability

Abstract Suspended matter and sediments from the North Sea were analysed for Al, Fe and Mn. A seasonal variation of Mn and Fe over Al ratios is observed. For most of the year suspended matter is enriched in Mn and Fe relative to average soil composition. No enrichment, and even depletion, occurs during a short period of the year (April–May) coinciding with the occurrence of the spring phytoplankton bloom. Surface sediments from the Southern Bight show the inverse image, with Mn and Fe enrichment occurring only in April–May and no enrichment during the rest of the year. These observations suggest a tight coupling between: (1) the change of redox potential in surface sediments, as triggered by the advection of organic matter from the spring bloom; (2) the outflow of reduced iron and manganese from sediments to the watercolumn; and (3) the enrichment of suspended matter in iron and manganese due to the oxidation-precipitation of reduced iron and manganese. The role of the coastal sediments investigated here as an important site of organic matter mineralization is stressed by the dissolved oxygen profiles indicating a transition from 100

Linking estuarine nematodes to their suspected food. A case study from the Westerschelde Estuary (south-west Netherlands).

The present study investigates correlations between abundances of nematodes (at the genus level) and benthic microalgae on an intertidal mudflat in the Westerschelde Estuary (south-west Netherlands), using both multi- and univariate methods. Two sample series, covering surface areas of 10 cm 2 (meioscale) and 1.25 cm 2 (microscale) per sample were analysed. Trophic type analysis indicated that an average of 31% of the nematode community were candidate grazers of microalgae. Multivariate data analysis indicated that only a limited part of the variation in the nematode data could be explained in relation to pigments. Total nematodes did not show any correlation with the pigment data. On the meioscale, the genera Tripyloides and Calyptronema correlated negatively with chlorophyll concentration (chl- a and chl- c , respectively), while Prochromadorella correlated positively with the ratio of fucoxanthin to chl- a , a ratio which at the present sampling site can be considered to be a measure of the proportion of diatoms in the total microalgal standing stock. On the microscale, up to ten genera, comprising 76% of total nematode numbers, were correlated with pigments. A majority (74%) correlated specifically with the ratio of fucoxanthin to chl- a , while much fewer nematodes showed a direct correlation to pigment concentrations. Whereas many of these correlations could be explained in terms of direct trophic links, several others probably represented indirect relationships, trophic or other. Food densities may be less important structuring factors of nematode communities on tidal flats than relative abundances of particular food sources. It is suggested that nematodes actively migrate towards ‘optimal’ food patches, and that this dynamic aspect of nematode–microalgae correlations is best revealed at a spatial-scale small enough to allow a rapid response of nematodes to changes in adjacent patches. Apparently, the microscale used in the present study is more adequate for the study of such intricate interactions than the meioscale.

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.

Macrobenthic infauna of mangroves and surrounding beaches at Gazi Bay, Kenya

Twelve sites around Gazi Bay, Kenya, were examined for macrobenthic infauna. Stations differed in human disturbance, vegetation, and sediment type (sandflat, beach, denuded and virgin mangrove sites, with Sonneratia, Rhizophora, Avicennia, Ceriops or Bruguiera vegetation).Sixteen higher taxa were counted; Isopoda, Amphipoda, Polychaeta, Cumacea and Tanaidacea were determined to family level (sometimes to genus or species). Total densities of infauna ranged from 265 to 6025 ind m−2.Gazi mangals had higher macrobenthos densities than other mangrove sediments described in literature and than the Gazi sandflats. The densities found in Gazi sandflats were comparable to similar habitats elsewhere.Virgin mangrove sediments were rich in mud and organic material, and were characterized by high densities of macro-Oligochaeta and Mollusca. Sandflats, beaches, exploited (denuded) and less dense mangroves or mangroves higher in the tidal zone (Bruguiera) were much more sandy and had a high abundance of Polychaeta and Nemertini.Structural patterns in the macrobenthos were only vaguely associated with vegetation type. Human impact (cutting) has resulted in a drastic decrease in densities of macro-infauna, possibly related to a reduction of organic material in the sediment.

The metazoan meiofauna in its biogeochemical environment : The case of an Antarctic coastal sediment

The metazoan meiobenthos was investigated in an Antarctic coastal sediment (Factory Cove, Signy Island, Antarctica). The fine sands contained much higher abundances compared to major sublittoral sediments worldwide. Classified second after Narrangansett Bay (North Atlantic) they reached numbers of 13 × 106 ind m-2. The meiofauna was highly abundant in the surface layers, but densities decreased sharply below 2 cm. Vertical profiles mirrored steep gradients of microbiota, chloropigments and organic matter and were coincident with chemical stratification. Spatial patchiness manifested especially in the surface layer. Nematodes dominated (up to 90%), and Aponema, Chromctdorita, Diplolaimella, Daptonema, Microlaimus and Neochromadora constituted almost the entire community. Overall, the nematode fauna showed a strong similarity with fine sand communities elsewhere. The dominant trophic strategies were epistrarum and non-selective deposit feeding, but the applied classification for feeding guild structure of the nematodes of Factory Cove is discussed. High standing stock, low diversity and shallow depth distribution may have occurred because of the high nutritive (chlorophyll exceeded lOOOmgm-2 and constituted almost 50% of the organic pool) and reductive character of the benthic environment. These observations must have originated from the substantial input of fresh organic matter from phytoplankton and microphytobenthic production, typical for an Antarctic coastal ecosystem during the austral summer.

The meiobenthos of five mangrove vegetation types in Gazi Bay, Kenya

The vertical distribution of meiofauna in the sediments of Avicennia marina, Bruguiera gymnorrhiza, Ceriops tagal, Rhizophora mucronata and Sonneratia alba at Gazi Bay (Kenya), is described. Seventeen taxa were observed, with highest densities in the sediments of Bruguiera (6707 ind. 10 cm−2), followed by Rhizophora (3998 ind. 10 cm −2), Avicennia (3442 ind. 10 cm −2), Sonneratia (2889 ind. 10 cm−2) and Ceriops (1976 ind. 10 cm−2). Nematodes accounted for up to 95%, of total densities; other common taxa were copepods, turbellarians, oligochaetes, polychaetes, ostracods and rotifers. High densities occurred to about 20 cm depth in the sediment. Especially Ceriops sediments show still high densities of nematodes (342 ind. 10 cm−2) and copepods (11 ind. 10 cm−2) in the deepest layer (15–22 cm). Particle size and oxygen conditions were major factors influencing meiobenthic distribution; Uca burrows had a major impact on distribution and abundance of meiofauna.