Nematodes and meiofauna in sediments obtained from RRS Charles Darwin cruise CD179, Portuguese continental margin

Samples collected at two different depths (ca. 3200 and ca. 4200 m) in the Setúbal and Cascais canyons off the Portuguese coast, during the HERMES RRS Charles Darwin cruise CD179, were analysed for (1) sediment biogeochemistry (TOC, TN) and (2) composition, and structural and trophic diversity of nematode communities. Multivariate PERMANOVA analysis on the nematode community data revealed differences between sediment layers that were greater than differences between canyons, water depths, and stations. This suggests that biogeochemical gradients along the vertical sediment profile are crucial in determining nematode community structure. The interaction between canyon conditions and the nematode community is illustrated by biogeochemical patterns in the sediment and the prevalence of nematode genera that are able to persist in disturbed sediments. Trophic analysis of the nematode community indicated that non-selective deposit feeders are dominant, presumably because of their non-selective feeding behaviour compared to other feeding types, which gives them a competitive advantage in exploiting lower-quality food resources. This study presents a preliminary conceptual scheme for interactions between canyon conditions and the resident fauna.

Nematodes and meiofauna in sediments obtained from RRS Discovery cruise D297, Portuguese continental margin

Samples collected in the deep Nazaré Canyon and at the adjacent slope, during the HERMES RRS Discovery D297 cruise (2005), were analysed for metazoan meiofauna, nematode structure and diversity and its relation to quality and quantity of sedimentary organic material. The amount and quality of organic matter available for direct consumption was much higher in the canyon compared to the slope and positively correlated with high nematode abundances (795-1171 ind. 10 cm**-2) and biomass (93.2-343.5 µg dry weight 10 cm**-2), thus leading to higher standing stocks. Canyon nematode assemblages also showed particular adaptations (e.g. higher trophic complexity, variability of nematode morphology, and presence of opportunistic genera) to canyon conditions, particularly in the deeper sediment layers. The Nazaré Canyon's nematode diversity was slightly lower than that of the adjacent slope and its assemblages were characterised by a higher dominance of certain genera. Still, the canyon contributes considerably to total Western Iberian Margin diversity due to different assemblages present compared to the slope. Furthermore, the harsh conditions in terms of hydrodynamic disturbance and the high organic matter flux are likely to have a negative impact on the establishment of species rich meiobenthic communities, especially in the canyon axis.

Nematodes and meiofauna in sediments obtained during Belgica cruise BG06/13, north-east Atlantic

The Gollum Channels and Whittard Canyon (NE Atlantic) are two areas that receive high input of organic matter and phytodetritus from euphotic layers, but they are typified by different trophic and hydrodynamic conditions. Sediment biogeochemistry was analysed in conjunction with structure and diversity of the nematode community and differences were tested between study areas, water depths (700 m vs 1000 m), stations, and sediment layers. The Gollum Channels and Whittard Canyon harboured high meiofauna abundances (1054-1426 ind. 10 cm**-2) and high nematode diversity (total of 181 genera). Next to enhanced meiofauna abundance and nematode biomass, there were signs of high levels of organic matter deposition leading to reduced sedimentary conditions, which in turn structured the nematode community. Striking in this respect was the presence of large numbers of 'chemosynthetic' Astomonema nematodes (Astomonema southwardorum, Order Monhysterida, Family Siphonolaimidae). This genus lacks a mouth, buccal cavity and pharynx and possesses a rudimentary gut containing internal, symbiotic prokaryotes which have been recognised as sulphur-oxidising bacteria. Dominance of Astomonema may indicate the presence of reduced environments in the study areas, which is partially confirmed by the local biogeochemical environment. The nematode communities were mostly affected by sediment layer differences and concomitant trophic conditions rather than other spatial gradients related to study area, water depth or station differences, pointing to small-scale heterogeneity as the main source of variation in nematode structure and function. Furthermore, the positive relation between nematode standing stocks, and quantity and quality of the organic matter was stronger when hydrodynamic disturbance was greater. Analogically, this study also suggests that structural diversity can be positively correlated with trophic conditions and that this relation is tighter when hydrodynamic disturbance is greater.

Macroalgal communities of gas vents near Ischia island in the Medterranean sea, 2011

There are high levels of uncertainty about how coastal ecosystems will be affected by rapid ocean acidification caused by anthropogenic CO2, due to a lack of data. The few experiments to date have been short-term (< 1 year) and reveal mixed responses depending on the species examined and the culture conditions used. It is difficult to carry out long-term manipulations of CO2 levels, therefore areas with naturally high CO2 levels are being used to help understand which species, habitats and processes are resilient to the effects of ocean acidification, and which are adversely affected. Here we describe the effects of increasing CO2 levels on macroalgal communities along a pH gradient caused by volcanic vents. Macroalgal habitat differed at taxonomic and morphological group levels along a pH gradient. The vast majority of the 101 macroalgal species studied were able to grow with only a 5% decrease in species richness as the mean pH fell from 8.1 to 7.8. However, this small fall in species richness was associated with shifts in community structure as the cover of turf algae decreased disproportionately. Calcitic species were significantly reduced in cover and species richness whereas a few non-calcified species became dominant. At mean pH 6.7, where carbonate saturation levels were < 1, calcareous species were absent and there was a 72% fall in species richness. Under these extremely high CO2 conditions a few species dominated the simplified macroalgal assemblage and a very few exhibited enhanced reproduction, although high CO2 levels seemed to inhibit reproduction in others. Our data show that many macroalgal species are tolerant of long-term elevations in CO2 levels but that macroalgal habitats are altered significantly as pH drops, contributing to a scant but growing body of evidence concerning the long-term effects of CO2 emissions in vegetated marine systems. Further study is now needed to investigate whether the observed response of macroalgal communities can be replicated in different seasons and from a range of geographical regions for incorporation into global modelling studies to predict effects of CO2 emissions on Earth's ecosystems.

Sediment biogeochemistry at CO2 vents off Papua-New Guinea (2013, 2014)

Measurement of biogeochemical parameters in coral reef sediments at carbon dioxide vents off Upa-Upasina (Normandy Island, Papua-New Guinea). The data includes in-situ micro/minisensor profiles, sediment characteristics, microbial and meiofauna abundances of vent sediments and reference sites without vent influence.

Fatty acid composition, and bacteria and meiofauna abundance in sediment cores PS71/013 and PS71/085 during POLARSTERN cruise ANT-XXIV/2

During the RV Polarstern ANT XXIV-2 cruise to the Southern Ocean and the Weddell Sea in 2007/2008, sediment samples were taken during and after a phytoplankton bloom at 52°S 0°E. The station, located at 2960 m water depth, was sampled for the first time at the beginning of December 2007 and revisited at the end of January 2008. Fresh phytodetritus originating from the phytoplankton bloom first observed in the water column had reached the sea floor by the time of the second visit. Absolute abundances of bacteria and most major meiofauna taxa did not change between the two sampling dates. In the copepods, the second most abundant meiofauna taxon after the nematodes, the enhanced input of organic material did not lead to an observable increase of reproductive effort. However, significantly higher relative abundances of meiofauna could be observed at the sediment surface after the remains of the phytoplankton bloom reached the sea floor. Vertical shifts in meiofauna distribution between December and January may be related to changing pore-water oxygen concentration, total sediment fatty acid content, and pigment profiles measured during our study. Higher oxygen consumption after the phytoplankton bloom may have resulted from an enhanced respiratory activity of the living benthic component, as neither meiofauna nor bacteria reacted with an increase in individual numbers to the food input from the water column. Based on our results, we infer that low temperatures and ecological strategies are the underlying factors for the delayed response of benthic deep-sea copepods, in terms of egg and larval production, to the modified environmental situation.