Joëlle Galéron

Long-term change in the megabenthos of the Porcupine Abyssal Plain (NE Atlantic)

A radical change in the abundance of invertebrate megafauna on the Porcupine Abyssal Plain is reported over a period of 10 years (1989–1999). Actiniarians, annelids, pycnogonids, tunicates, ophiuroids and holothurians increased significantly in abundance. However, there was no significant change in wet weight biomass. Two holothurian species, Amperima rosea and Ellipinion molle, increased in abundance by more than two orders of magnitude. Samples from the Porcupine Abyssal Plain over a longer period (1977–1999) show that prior to 1996 these holothurian species were always a minor component of the megafauna. From 1996 to 1999 A. rosea was abundant over a wide area of the Porcupine Abyssal Plain indicating that the phenomenon was not a localised event. Several dominant holothurian species show a distinct trend in decreasing body size over the study period. The changes in megafauna abundance may be related to environmental forcing (food supply) rather than to localised stochastic population variations. Inter-annual variability and long-term trends in organic matter supply to the seabed may be responsible for the observed changes in abundance, species dominance and size distributions.

Temporal patterns among meiofauna and macrofauna taxa related to changes in sediment geochemistry at an abyssal NE Atlantic site

Abstract Two major size classes of the sediment community, meiofauna and macrofauna, and four classes of lipid compounds, fatty acids, alkanes, alcohols and sterols, were investigated using multicorer and USNEL boxcorer samples, collected during six cruises over a two year period (September 1996 to September–October 1998), at the Porcupine Abyssal Plain (∼ 48° 50′N 16° 30′W, 4850 m depth) within the framework of the MAST 3 BENGAL project. This site was known to be subject to seasonality in the input of organic matter to the seafloor. Results are given for each faunal size class in terms of taxonomic structure at the level of phylum, class or order, depending on the taxon, and for the dominant faunal components in terms of density and vertical distribution. For each lipid compound class, results are given in concentration and vertical distribution. The taxonomic structure of each size class did not change within the study period. Total meiofaunal and macrofaunal densities were particularly high, probably reflecting the high quantity and quality of organic matter inputs to the site. The dominant components of the two size classes presented different temporal patterns in their responses to changes in their environment. Populations of meiofaunal species, a foraminiferan and an opheliid polychaete, which inhabit the surface or sub-surface of sediment and feed on phytodetritus, responded with a rapid increase in abundance to a pulse of organic input in summer 1996. The macrofaunal polychaetes showed a lagged response to the same event by slowly increasing in density. Other components of the sediment community, that can live deeper in the sediment, moved down the sediment column, in response to 1) the impoverishment and bioturbation of the surface layer, and 2) the downward mixing of organic matter in the sediment by larger organisms. In this study, different temporal patterns were demonstrated for the first time in different size classes of the sediment community, and in the biological and environmental parameters that were studied simultaneously.

Community structure and spatial heterogeneity of the deep-sea macrofauna at three contrasting stations in the tropical northeast Atlantic

A study of macrofauna community structure and spatial patterns was undertaken at three deep-sea stations during three EUMELI cruises in the tropical northeast Atlantic Ocean. The benthic response to contrasting regimes of organic carbon supply at the eutrophic (E), mesotrophic (M) and oligotrophic (O) stations (1700 m, 3 100 m and 4700 m depth, respectively) was evaluated through qualitative and quantitative analysis of the composition and structure of benthic macrofauna (> 250 μm) and discussed with regard to the biological and physical conditions. The mean densities of the total macrofauna, dominated by the Polychaeta, Tanaidacea, Isopoda and Bivalvia, decreased with increasing depth from 5403 ind. m−2 at station E to 231.5 ind. m−2 at station O. From the 16 taxonomic groups represented at the E and M sites, only eight could be sampled at the O site. The density gradient can be related to the decreasing surface production, food supply and hydrographic conditions between the three stations. The comparison of the high densities encountered at station E with other studies underlines the particular feature of this station subjected to intense upwelling activity resulting in high organic carbon input. The patchy distribution of the macrofauna at stations E and M suggests that both physical and biologically-mediated disturbances create environmental heterogeneity favouring the aggregation of organisms. This process occurs at different observation scales. At a large scale, bottom current and upwelling activity induce high densities and faunal aggregations, displaying a degree of density-dependence. At the local scale, the complexity of the food web induces biologically-mediated disturbances, and the activity of major deposit-feeding taxa, like polychaetes, create sediment microhabitats favouring patchy distribution. Faunal patch sizes are variable among the taxa considered. Most are distributed in patches less than 1 m z. Our work contributes to the understanding of how the deep benthic communities are structured, particularly under the influence of environmental conditions in tropical latitudes.

Deep-sea habitat heterogeneity influence on meiofaunal communities in the Gulf of Guinea

To estimate the degree of spatial heterogeneity of benthic deep-sea communities, we carried out a multiple-scale (from ms to 200 km) investigation in the Congo-Angola margins (Equatorial West African margin, 3150–4800 m) in which we examined the metazoan meiofauna at a variety of habitats along the Congo Channel system and in the associated cold seep. We investigate the structure, density, vertical distribution patterns in the sediment and biomass of meiofaunal communities in the Gulf of Guinea and how they are controlled by hydrologic and biogeochemical processes. The meiofaunal communities in the Gulf of Guinea were shaped by heterogeneous conditions on the margin, and reflect the multiple-scale spatial variability that corresponds with the different identified habitats. The two control sites, located at >100 km away from the canyon, were inhabited by very dense and the most diverse meiobenthic communities. Similar meiobenthic communities inhabited the transition zone between the canyon and the cold seep. Sites located along the Congo Channel were obviously affected by the local high-velocity bottom currents and unstable sedimentary conditions in this active submarine system. Extremely low meiobenthic densities and very low proportions in the most surficial sediment layers provided evidence for recently highly disturbed sediments at these sites. The remote operated vehicle (ROV) Victor 6000 provided images of the cold seep, showing a patchy distribution of several types of patchy distributed megafaunal communities dominated by three key symbiotic taxa (Mytilidae, Vesicomyidae and Siboglinidae). These cold seep sediments were colonised by a unique meiobenthic community, characterised by a high small-scale (ms) patchiness, low species richness and the prominent dominance of two large-sized nematode species: Sabatieria mortenseni, which is a cosmopolitan nematode known from littoral habitats, and an undescribed Desmodora species. The high individual body weight of S. mortenseni and its dominance at the cold seep site resulted in a significantly higher nematode biomass at the seep compared to the surrounding sites. In addition, the vertical nematode profiles, with maximum proportions in subsurficial layers, points to a chemosynthesis-based meiobenthic community in this cold seep, in contrast to the phytodetritus-based communities at the control sites and at the transition zone.

Evidence for episodic recruitment in a small opheliid polychaete species from the abyssal NE Atlantic

Abstract The abundance and size spectra of an infaunal opheliid polychaete species was followed over a two year period (September 1996–October 1998) in meiofaunal—(32 μm–1 mm) and macrofaunal —(>250 μm) samples collected at an abyssal site (4850 m depth) in the NE Atlantic. The site, situated on the Porcupine Abyssal Plain (PAP), is characterised by the episodic deposition of aggregated phytodetritus. The response of the fauna to this seasonal food supply was addressed by time-series sampling within the MAST-III BENGAL programme. In autumn 1996, small opheliid juveniles (mean length: 281 μm in September and 254 μm in October) were sampled only in the meiofauna samples. In March 1997, juvenile specimens of the opheliid, which were on average nearly twice as large (mean length: 480 μm) as those collected in 1996 were sampled in both both meio- and macrofaunal samples. The occurrence of only small juvenile individuals in 1996 suggests that a synchronous recruitment event had taken place earlier during that year. Small juveniles (mean length: 252 μm) were also abundant in a sample collected at the PAP site in May 1991, immediately following the deposition of a pulse of phytodetritus. The opheliid population structure in 1997 and 1998 indicates the slow progression of the settled cohort, possibly supplemented by a further, but relatively minor recruitment event in March 1998. Size spectra analysis implies that either growth was slow or that immigration of larger juveniles had augmented the population. The PAP opheliid may be an opportunist, which waits for optimal conditions before converting its slowly accumulated energy into reproduction. In addition, this species can apparently maintain a stable pool of developing juveniles if the organic pulse fails to materialise. The present study also shows that a more holistic approach is necessary to investigate the life cycles of some organisms, which lie close to the boundary between the meiofauna and macrofauna.

Deep-sea Environment

The deep sea, defined as the part of the ocean over 1,000 m deep, covers 60 % of the Earth’s surface, making it the planet’s largest biome (homogeneous ecological formation). The geological, geochemical and physical conditions of the seabed and water column define varied habitats, sheltering specific biological communities. The ocean floor is composed of various distinct environments, including continental margins, abyssal plains, oceanic trenches, mid-ocean ridges and seamounts. Over 90 % of the deep seabed is covered with fine sediments composed of particles of biogenic (i.e. produced by living organisms), terrigenous, volcanic and authigenic (i.e. originating from the rock where it is found) origin: they are found on abyssal plains, but also on continental slopes. The seabed is however rockier on mid-ocean ridges and seamounts, as well as in some isolated areas of continental slopes, into which submarine canyons with abrupt cliffs may be cut. The marine environment is globally characterised by an absence of sunlight, high pressure, low and relatively constant temperatures, low currents and an oxygen content generally sufficient for animal life to develop. In the case of the total absence of dissolved oxygen, in specific situations and in small areas, only anaerobic bacteria can develop.