Fluxes of phytopigments and labile organic matter to the deep ocean in theNE Atlantic Ocean

Downward fluxes of labile organic matter (phytopigments, proteins and carbo hydrates) were measured between September 1996 and August 1998 at three dep ths 1000 m, 3000 in and 4700 m (c. 100 mab) over the Porcupine Abyssal Plai n (PAP, NE Atlantic), to provide detailed information on the biochemical ch aracteristics of organic inputs to the deep sea. Temporal changes in the ca rbohydrate and protein fluxes were compared to carbohydrate and protein con tents of the surficial sediment on the seabed beneath the traps at 4850 m d epth. Fluxes of carbohydrate, protein and phytopigments (chlorophylls-a and -b, and phaeophytins-a and -b) displayed strong seasonal variations, but l imited interannual variability between the two years of measurement. Fluxes : of labile organic matter were characterised by strong pulses which occurr ed in spring and early summer, suggesting that the deep PAP area experience s relatively predictable patterns of vertical fluxes. No major quantitative differences in organic matter fluxes were observed between traps at differ ent depths, but highest carbohydrate fluxes (time-weighted mean 2.4 mg m(-2 ) d (1)) were observed at 4700 m, whereas highest protein fluxes were obser ved at 1000 m (time-weighted mean 2.1 mg m(-2) d(-1)). Carbohydrate, protei n and phytopigment fluxes were correlated significantly, suggesting that se ttling material was associated with primary organic matter (i.e., phytodetr itus) inputs from the photic layer. The contributions of chlorophyll-a and -b, and of phaeophytin-a and -b did not change significantly with increasin g depth. Nor did the ratio of total phaeopigments to total chlorophylls did change greatly with depth (0.3-0.4 at both 3000 m and 4700 m depth) sugges ting that degradation rates in the sinking particles were low. Protein and carbohydrate concentrations in the sediments at 4850 m depth (collected dur ing 6 cruises between 1996 and 1998) and vertical fluxes at 3000 m depth fo llowed inverse temporal patterns; peak concentrations of protein in the sed iment corresponded to low vertical fluxes of particulate proteins. These da ta suggest that there is a decoupling between pelagic input and benthic acc umulation. However, bacterial secondary production and sedimentary RNA conc entrations displayed temporal patterns similar to those of the vertical flu xes, suggesting that increases in the metabolism of the smallest-sized biot a was associated with maxima in the organic matter supply. Our results also suggest that benthic utilisation Could exceed the organic matter being sup plied by the vertical fluxes. (C) 2001 Published by Elsevier Science Ltd.