LATITUDINAL VARIATION IN INVERTEBRATE MEGAFAUNAL ABUNDANCE AND BIOMASS IN THE NORTH-ATLANTIC OCEAN ABYSS

Megafauna was collected by otter trawl at two widely separated abyssal sites in the eastern North Atlantic Ocean. The northern site, on the Porcupine Abyssal Plain (PAP, 4850 m), is subject to strong seasonal p ulses of phytodetritus deposition, whereas the southern site, on the M adeira Abyssal Plain (OLIGO, 4500-4650 m), showed no indication of suc h deposition. Data from these two sites were compared with those from a third site (GME), also apparently not affected by phytodetritus, but on the Madeira Abyssal Plain 1200 km from OLIGO. Mean abundance and b iomass of invertebrate megafauna at PAP were 72.6 individuals ha(-1) a nd 1974 g ha(-1), respectively. The corresponding values for OLIGO wer e 10.2 individuals ha(-1) and 63.4 g ha(-1) and for GME 21.7 individua ls ha(-1) and 112.9 g ha(-1). Size-spectral curves of abundance and bi omass based on PAP samples showed peaks in the 40-80 g wet weight clas s, thus confirming the megafauna as a functional entity. No evidence f or seasonal variation of abundance or biomass was found. At OLIGO, abu ndance declined more or less regularly over the organism size range sa mpled, and biomass was spread fairly evenly across the larger size cla sses. Major differences in trophic structure among the three sites wer e evident, with OLIGO and GME more similar to one another than either were to PAP, with much of the higher biomass at PAP represented by par ticle-selective detritivorous holothurians. The contribution of invert ebrates to overall megafaunal biomass at OLIGO (8%) and GME (30%) was lower than at PAP (48%), but a high proportion of fish biomass at OLIG O and GME, and almost all at PAP, belonged to macrophagous species tro phically independent of benthic production. (C) 1998 Elsevier Science Ltd. All rights reserved.