Late-summer mass deposition of gelatinous phytodetritus along the slope ofthe NW European Continental Margin

In the period 1993-1995 the OMEX area has been visited 3 times to address t he question of across-slope transport of suspended matter from the shelf to the deep sea. By analyzing phytopigments and nucleic acids in the sediment and water of the N.W. European Continental slope and rise seasonal pattern s of benthic food input were investigated and relation between input and th e structure and activity of the benthic community were explored. Chloropigm ents in the surface sediments indicated that a spring bloom effect could be traced down to about 2500 m. During late August 1995 heavy deposits of gel atinous matter, characterized by high concentrations of chloropigments and nucleic acids, were detected all over the ocean floor of the outer slope an d continental rise below 3500 m. It is estimated that this mucus layer had a carbon load of 250 mmol C m(-2) over an area of 50,000 km(2). The recent state of the mucus allowed us to search for its origin. Characteristic pigm ent composition and the presence of coccoliths pointed to prymnesiophytes ( coccolithophorids) as a major contributor, but dinoflagellates (peridinin) and green algae (chlorophyll-b, lutein) must have contributed as well. Simu ltaneous observations of the overlying water column, deep chlorophyll maxim um, revealed the presence of a coccolithophorid bloom in a recent stage of disintegration at St. III. An obvious relation with the mucus carpet, howev er, could not be indicated. This, and the significant differences in pigmen t composition and pigment ratios at the various deep stations lead us to un derstand that the extended mucus field at the Celtic slope originates from different, more or less synoptically occurring surface blooms. The presence of large 'vacuum-cleaning' sea-cucumbers is considered indicative of the o ccurrence of phytodetritus pulses. (C) 1998 Elsevier Science Ltd. All right s reserved.