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 10(3) m-2 were found at PAP, with corresponding wet biomass values of 169.4 and 189.2 g 10(3) m-2. Data from sledge hauls taken at MAP gave an abundance of 7.48 individuals and 5.15g 10(3) m-2. Sledge and trawl abundances at Great Meteor East (GME) were 2.91 and 2.19 individuals 10(3) m-2 and 4.31 and 11.30 g 10(3) m-2. Holothurians dominated the invertebrate biomass at PAP while Asteroidea and Decapoda Natantia were important taxa at MAP and GME. Otter trawl samples demonstrated fish to be the major component of total megafaunal 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 station 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 or 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.