Anthony L. Rice

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.

The IOSDL DEEPSEAS programme: introduction and photographic evidence for the presence and absence of a seasonal input of phytodetritus at contrasting abyssal sites in the northeastern atlantic

Abstract This paper introduces the IOSDL DEEPSEAS programme. Two abyssal sites in the northeast Atlantic with presumed contrasting regimes of organic carbon supply have been studied. One of these sites, on the Porcupine Abyssal Plain (PAP), has an overlying water column with a winter mixed layer in excess of 500 m and was forecast to receive a highly seasonal organic input, a significant portion arriving in the form of rapidly sinking phytodetritus derived from the spring bloom. The winter mixed layer over the second site, on the Madeira Abyssal Plain (MAP), is much shallower, and the resulting flux to the benthos was expected to be quantitatively less and not in the form of aggregated phytodetritus. Recently published sediment-trap results from nearby localities indicate relatively similar total fluxes and widespread seasonality at depth, contrary to our expectations. However, benthic photographic data from the two stations seem to support the original hypothesis, at least in part. Transect photographs (and multiple-corer samples) at the PAP site in August 1989 and May 1991 revealed the presence of phytodetritus on the seafloor, relatively flocculent and evenly distributed in May and more granular and patchily distributed in August. Time-lapse photographs obtained between May 1991 and April 1992 recorded the sudden arrival of phytodetritus on 16 May and a further deposition at the beginning of June. In contrast, at the MAP site neither transect photographs in August 1990 nor time-lapse photographs obtained between August 1990 and July 1991 show evidence of the arrival of aggregated phytodetritus.

Variations in the invertebrate abyssal megafauna in the North Atlantic Ocean

Epibenthic sledge and otter trawl samples of invertebrate megabenthos were collected from the abyss of the northeastern Atlantic Ocean. Stations were selected to have contrasting overlying water column structure. Strong seasonal pulses of phytodetritus on the sea floor have been recorded on the Porcupine Abyssal Plain (PAP) but not on the Madeira Abyssal Plain (MAP). Sledge-and trawl-derived abundances of 13.73 and 8.13 individuals 103 m−2 were found at PAP, with corresponding wet biomass values of 169.4 and 189.2 g 103 m−2. Data from sledge hauls taken at MAP gave an abundance of 7.48 individuals and 5.15g 103 m−2. Sledge and trawl abundances at Great Meteor East (GME) were 2.91 and 2.19 individuals 103 m−2 and 4.31 and 11.30 g 103 m−2. Holothurians dominated the invertebrate biomass at PAP while Astoroidea and Decapoda Natantia were important taxa at MAP and GME. Otter trawl samples demonstrated fish to be the major component of total megadaunal biomass and suggest that small fixed-frame trawls give gross underestimates of fish abundance and biomass. Size spectra based on abundance and biomass data indicated the megafauna to be a functional group at PAP stationl only. Here peak invertebrate biomass occurred in the 40–80 g wet wt size class. In contrast, no large invertebrates (>20 g wet wt) were found at MAP of GME. Concurrent photographic records indicate higher faunal densities than do net catches. Faunal differences between stations were examined, and a weak relationship between surface productivity and megafaunal abundance was established. Data are inadequate to detect a similar relationship with biomass. Biomass values at PAP of 16–39 times those of MAP and GME appear to be explained by the deposition of phytodetritus at PAP but not at MAP or, probably, at GME. Among invertebrates, detritivores form the dominant feeding guild at all localities, but carnivores are important at MAP and GME. Phytodetritus appears to support the abundant surface-grazing holothurians at PAP.

Organic matter in deep-sea sediments from the Porcupine Abyssal Plain in the north-east Atlantic Ocean. I—Lipids

Abstract The extractable lipids of four undisturbed sediment cores from the Porcupine Abyssal Plain, in the north-eastern Atlantic Ocean, have been analysed. Several compound classes including n-alkanes, n-alcohols, alkanoic and alkenoic acids, sterols, triterpenoids and alkenones were present in most of the samples. Surficial sediments showed evidence of mixed marine and terrigenous organic matter, the latter presumably being higher plant-derived. There was, however, considerable variability in the distributions of lipids in the surficial sediments; this is consistent with photographic evidence of the patchiness of the Porcupine Abyssal Plain sediments, which are strongly influenced by the benthic faunal community. In general, the concentrations of most compounds (with respect to total organic carbon) are attenuated with depth in the sediments, although the “higher plant” signal is apparently more recalcitrant thanthe “marine” signal. There is also evidence of significant down-core variability in some of the cores.

The BENGAL programme : introduction and overview

BENGAL (High-resolution temporal and spatial study of the BENthic biology and Geochemistry of a north-eastern Atlantic abyssal Locality) was a three-year multidisciplinary study of the abyssal benthic boundary layer in the northeast Atlantic. The aim of BENGAL was to determine how the seabed community and the geochemistry of the sediments change seasonally in response to a highly seasonal input of organic matter from the overlying water column. It did this by organising an intensive sampling programme on 14 research cruises over a two-year period. This introductory paper sets the scene for the subsequent scientific contributions. It describes the study area, sampling strategy and techniques, and provides a brief overview of the contributions to the BENGAL Special Volume.

Material supply to the European continental slope : a budget based on benthic oxygen demand and organic supply

Two sites were selected adjacent to the Northeast Atlantic continental shelf to examine material supply to and loss from the benthos. At both a bathyal mid-slope (2000 m water depth) and an abyssal (4800 m water depth) site an in situ technique was used to measure sediment community oxygen consumption. At the mid-slope site a sediment trap mooring provided a measure of the annual vertical flux of material, whereas at the abyssal site, vertical flux was deduced from data obtained nearby. At the mid-slope site, vertical flux was similar to values determined elsewhere in the mesotrophic Northeast Atlantic, but oxygen demand was somewhat higher than values reported for other comparable regions. Vertical flux was, however, sufficient to fuel only 20% of the benthic oxygen demand. A strong seasonal signal was seen from time lapse photographs of the seabed in terms of the quantity of phytodetritus lying on it, but no significant seasonal variation in oxygen consumption was detected. Similarly, on fine spatial scales (between cores) the existence of a detrital layer appeared to have no effect on oxygen demand. It is concluded that substantial quantities of material are transported down slope to this mid-slope region, to provide the organic carbon required by the sediment and that this may be assumed to be the case for the entire particulate pool, including the inorganic fraction. In contrast to the bathyal station, at the deep site on the Porcupine Abyssal Plain, the organic carbon required to fuel benthic oxygen demand was similar to the estimated vertical supply.