Bioavailability of organic matter in the sediments of the Porcupine Abyssal Plain, northeastern Atlantic

We investigated spatial and temporal changes in quantity, quality and bioav ailability of organic matter in abyssal sediments of the northeastern Atlan tic. Sediment samples were collected in the Porcupine Abyssal Plain (PAP, 4 800 M depth) during 6 oceanographic cruises from September 1996 to October 1998 down to a depth of 15 cm. Sedimentary proteins, carbohydrates and lipi ds, and their enzymatically hydrolysable fractions showed significant tempo ral changes, but different biochemical classes displayed different temporal patterns. Total proteins, carbohydrates and lipids displayed high concentr ations, whereas the potentially hydrolysable fractions accounted for only a bout 10% of their total pools. From September 1996 to October 1998, bioavai lable organic carbon concentration in the sediments decreased about 10 gC m (-2) indicating that this benthic system was not steady state. Hydrolysed p roteins and carbohydrates were characterised by different vertical patterns . Carbohydrates increased their relative significance with depth in the sed iment indicating a shift of organic matter bioavailability with important t rophodynamic implications for subsurface consumers, Vertical profiles of 'r eactive' and refractory organic carbon in PAP sediments indicate that organ ic matter bioavailability in deeper sediment layers is higher than expected from previous theoretical models.