The response of Oneirophanta mutabilis (Holothuroidea) to the seasonal deposition of phytopigments at the Porcupine Abyssal Plain in the Northeast Atlantic

The impact of seasonal pulses of phytodetritus on the grazing behaviour of Oneirophanta mutabilis was assessed on the Porcupine Abyssal Plain (PAP) in the NE Atlantic. Sediment and sediment trap samples were analysed by HPLC to estimate the quantity and quality of the organic material in terms of phytopigments and nucleic acids. Food selection by Oneirophanta was estimated by analysing these constituents in the gut contents.The study area is characterised by large interannual variations in the deposition of fresh organic material. The mass fluxes at 10 m above bottom (mab) varied from 0.25 g DW m⁻² d⁻¹ in September 1996 to <0.1 g DW m⁻² d⁻¹ in March 1997. The material caught in the sediment trap in September 1996 had a relative fresh signature with a chlorophyll-a:phaeophorbide ratio of 1.33. During the other seasons (March 1997, July 1997 and October 1997) the chlorophyll-a:phaeophorbide ratio remained low. In sediment cores this ratio showed a similar seasonal and inter-annual pattern, and again September 1996 was the period of maximum abundance of fresh organic material in the surficial sediment. The analyses of the gut contents of Oneirophanta mirrored exactly the seasonal variation of the phytopigments in both the sediment and the sediment trap material. Concentrations of pigments in the fore-gut were 5 to 15 times higher than in the sediment and the nucleic acid concentrations were up to 80 times higher. This discrepancy between pigments and nucleic acids concentrations suggests that the latter are “indigenous” to the gut of Oneirophanta, either because the gut contains high numbers of actively-dividing bacteria or as a result of cell lysis of the gut epithelium. The seasonal differences in the pigment concentration factor suggest that Oneirophanta does not actively search for hotspots where pigment concentrations are enriched. By using the degradation rate of chlorophyll-a in the PAP sediments, the minimum residence time of chlorophyll in the sediment within the gut of Oneirophanta was calculated. In combination with gut volume and density data it was estimated that each year the Oneirophanta population skims a third of the sediment surface at the PAP site.     

A comparison between the megafauna communities on the N.W. Iberian and Celtic continental margins—effects of coastal upwelling?

Megafauna biomass and feeding guilds were studied on the NW Iberian upwelling Continental Margin in order to determine the presence of enriched zones pointing to enhanced particle input. We compare these findings with similar data obtained from a transect across the Celtic Continental Margin that represents a regime without coastal upwelling. Additionally sediment concentrations of phytopigments (chlorophyll-a, phaeophorbides) representing recent inputs of algal production and of nucleic acids (DNA, RNA) are used as proxies for microbial biomass, to assess if there was a relation between these parameters and the megafauna distribution. The sediment on the upper slope (<1600 m) of the Iberian Margin was found to be inhabited by filter-feeding megafauna (26–73% of total invertebrate density, and 1–35% of biomass), and contained relatively low levels of phytopigments (3–6 ng/cm³ chlorophyll-a) and nucleic acids (12–16 μg⁻¹ DNA, 1.5–3.5 μg⁻¹ RNA). In contrast, on the upper slope of the Celtic Margin the dominant component of the megafauna were deposit-feeders (57–92% of total invertebrate density, and 23–90% of biomass) and the sediments contained higher concentrations of phytopigments and nucleic acid. These observations, supplemented by video records revealing the presence of current ripples on the Iberian upper slope, show that these upper slope regions are non-depositional, high energy environments. Conditions at the lower slope and the abyssal plain on the Iberian transect were more quiescent with large deposit-feeding holothurians dominating the megafauna (72–94% of invertebrate biomass), and with relatively high sediment concentrations of phytopigments (7–9 ng/cm³ chlorophyll-a, 157–170 ng/cm³ phaeophorbides) and nucleic acids (21–38 μg⁻¹ DNA, 2.4–5.5 μg⁻¹ RNA). On the basis of our data we argue that the benthic food for the deepest stations on the Iberian transect does not consist of shelf derived organic matter. More likely, fast sinking offshore blooms, possibly associated with filaments of upwelling water, form the major contribution to the annual food supply of the deep living megafauna.     

Deep-sea macrourid fishes scavenge on plant material: Evidence from in situ observations

Deep-sea benthic communities primarily rely on an allochthonous food source. This may be in the form of phytodetritus or as food falls e.g. sinking carcasses of nekton or debris of marine macrophyte algae. Deep-sea macrourids are the most abundant demersal fish in the deep ocean. Macrourids are generally considered to be the apex predators/scavengers in deep-sea communities. Baited camera experiments and stable isotope analyses have demonstrated that animal carrion derived from the surface waters is an important component in the diets of macrourids; some macrourid stomachs also contained vegetable/plant material e.g. onion peels, oranges, algae. The latter observations led us to the question: is plant material an attractive food source for deep-sea scavenging fish? We simulated a plant food fall using in situ benthic lander systems equipped with a baited time-lapse camera. Abyssal macrourids and cusk-eels were attracted to the bait, both feeding vigorously on the bait, and the majority of the bait was consumed in <30 h. These observations indicate (1) plant material can produce an odour plume similar to that of animal carrion and attracts deep-sea fish, and (2) deep-sea fish readily eat plant material. This represents to our knowledge the first in situ documentation of deep-sea fish ingesting plant material and highlights the variability in the scavenging nature of deep-sea fishes. This may have implications for food webs in areas where macrophyte/seagrass detritus is abundant at the seafloor e.g. canyon systems and continental shelves close to seagrass meadows (Bahamas and Mediterranean).     

Do abyssal scavengers use phytodetritus as a food resource? Video and biochemical evidence from the Atlantic and Mediterranean

Deep-sea benthic communities derive their energetic requirements from overlying surface water production, which is deposited at the seafloor as phytodetritus. Benthic invertebrates are the primary consumers of this food source, with deep-sea fish at the top of the trophic hierarchy. Recently, we demonstrated with the use of baited cameras that macrourid fish rapidly respond to and feed vigorously on large plant food falls mimicked by spinach (Jeffreys et al., 2010). Since higher plant remains are scarce in the deep-sea, with the exception of canyons, where terrestrial material has been observed, these results led us to ask if a more commonly documented plant material i.e. phytodetritus might form a food source for deep-sea fish and mobile scavenging megafauna. We simulated a phytodetritus dump at the seafloor in two contrasting environments (1) the NE Atlantic where carpets of phytodetritus have been previously observed and (2) the oligotrophic western Mediterranean, where the deposition of phytodetritus at the seafloor is a rare occurrence. We recorded the response of the scavenging fauna using an in situ benthic lander equipped with baited time-lapse cameras. In the NE Atlantic at 3000 m, abyssal macrourids and cusk-eels were observed ingesting the phytodetritus. The phytodetrital patch was significantly diminished within 2 h. Abundance estimates calculated from first arrival times of macrourids at the phytodetrital patch in the Atlantic corresponded with abundance estimates from video-transect indicating that fish were attracted to the scent of phytodetrital bait. In contrast to this, in the western Mediterranean at 2800 m a single macrourid was observed investigating the phytodetrital patch but did not feed from it. The phytodetrital patch was significantly diminished within 6.5 h as a result of mainly invertebrate activity. At 1900 m, Lepidion lepidion was observed near the lander and the bait, but did not feed. The phytodetrital patch remained intact until the end of the experiment. In the deployments in the Mediterranean abundance estimates from first arrival times at the bait, corrected for their body size, were lower than estimates obtained from video-transects and trawl catches. This suggests that the Mediterranean fish were not readily attracted to this food source. In contrast, invertebrates in the Balearic Sea were observed ingesting the phytodetritus bait despite the rare occurrence of phytodetritus dumps in the Mediterranean. Stable isotope values of the fish at both study sites, set within the context of the benthic food web, did not demonstrate a strong trophic link to phytodetritus. Fatty acid profiles of these fish indicated a strong link between their lipid pool and primary producers i.e. phytoplankton, which may be attributed to trophic transfer. The usefulness of fatty acid biomarkers in ascertaining deep-sea fish diets is discussed. Our study suggests that the abyssal grenadier C. armatus on the Atlantic Iberian margin is attracted to phytodetritus. However the exact contribution of this food source to the diet of macrourids in this area remains unresolved.     

The benthic silica cycle in the Northeast Atlantic: annual mass balance, seasonality, and importance of non-steady-state processes for the early diagenesis of biogenic opal in deep-sea sediments

Within the framework of the EU-funded BENGAL programme, the effects of seasonality on biogenic silica early diagenesis have been studied at the Porcupine Abyssal Plain (PAP), an abyssal locality located in the northeast Atlantic Ocean. Nine cruises were carried out between August 1996 and August 1998. Silicic acid (DSi) increased downward from 46.2 to 213 μM (mean of 27 profiles). Biogenic silica (BSi) decreased from ca. 2% near the sediment–water interface to <1% at depth. Benthic silicic acid fluxes as measured from benthic chambers were close to those estimated from non-linear DSi porewater gradients. Some 90% of the dissolution occurred within the top 5.5 cm of the sediment column, rather than at the sediment–water interface and the annual DSi efflux was close to 0.057 mol Si m⁻² yr⁻¹. Biogenic silica accumulation was close to 0.008 mol Si m⁻² yr⁻¹ and the annual opal delivery reconstructed from sedimentary fluxes, assuming steady state, was 0.065 mol Si m⁻² yr⁻¹. This is in good agreement with the mean annual opal flux determined from sediment trap samples, averaged over the last decade (0.062 mol Si m⁻² yr⁻¹). Thus ca. 12% of the opal flux delivered to the seafloor get preserved in the sediments. A simple comparison between the sedimentation rate and the dissolution rate in the uppermost 5.5 cm of the sediment column suggests that there should be no accumulation of opal in PAP sediments. However, by combining the BENGAL high sampling frequency with our experimental results on BSi dissolution, we conclude that non-steady state processes associated with the seasonal deposition of fresh biogenic particles may well play a fundamental role in the preservation of BSi in these sediments. This comes about though the way seasonal variability affects the quality of the biogenic matter reaching the seafloor. Hence it influences the intrinsic dissolution properties of the opal at the seafloor and also the part played by non-local mixing events by ensuring the rapid transport of BSi particles deep into the sediment to where saturation is reached.