Latitudinal variation in invertebrate megafaunal abundance and biomass in the North Atlantic Ocean Abyss

Abstract

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 pulses of phytodetritus deposition, whereas the southern site, on the Madeira Abyssal Plain (OLIGO, 4500–4650 m), showed no indication of such 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 biomass of invertebrate megafauna at PAP were 72.6 individuals ha -1 and 1974 g ha -1 respectively. The corresponding values for OLIGO were 10.2 individuals ha -1 and 63.4 g ha -1 and for GME 21.7 individuals ha -1 and 112.9 g ha -1. Size-spectral curves of abundance and biomass based on PAP samples showed peaks in the 40–80 g wet weight class, thus confirming the megafauna as a functional entity. No evidence for seasonal variation of abundance or biomass was found. At OLIGO, abundance declined more or less regularly over the organism size range sampled, and biomass was spread fairly evenly across the larger size classes. Major differences in trophic structure among the three sites were 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 particle-selective detritivorous holothurians. The contribution of invertebrates to overall megafaunal biomass at OLIGO (8%) and GME (30%) was lower than at PAP (48%), but a high proportion of fish biomass at OLIGO and GME, and almost all at PAP, belonged to macrophagous species trophically independent of benthic production.