THE IOSDL DEEPSEAS PROGRAM - INTRODUCTION AND PHOTOGRAPHIC EVIDENCE FOR THE PRESENCE AND ABSENCE OF A SEASONAL INPUT OF PHYTODETRITUS AT CONTRASTING ABYSSAL SITES IN THE NORTHEASTERN ATLANTIC

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 mixe d layer in excess of 500 m and was forecast to receive a highly season al organic input, a significant portion arriving in the form of rapidl y sinking phytodetritus derived from the spring bloom. The winter mixe d layer over the second site, on the Madeira Abyssal Plain (MAP), is m uch shallower, and the resulting flux to the benthos was expected to b e quantitatively less and not in the form of aggregated phytodetritus. Recently published sediment-trap results from nearby localities indic ate relatively similar total fluxes and widespread seasonality at dept h, contrary to our expectations. However, benthic photographic data fr om 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 phytodetritu s on the seafloor, relatively flocculent and evenly distributed in May and more granular and patchily distributed in August. Time-lapse phot ographs obtained between May 1991 and April 1992 recorded the sudden a rrival of phytodetritus on 16 May and a further deposition at the begi nning of June. In contrast, at the MAP site neither transect photograp s in August 1990 nor time-lapse photographs obtained between August 19 90 and July 1991 show evidence of the arrival of aggregated phytodetri tus.