PHYTODETRITUS ENTERING THE BENTHIC BOUNDARY-LAYER AND AGGREGATED ON THE SEA-FLOOR IN THE ABYSSAL NE PACIFIC - MACRO-SCOPIC AND MICROSCOPIC COMPOSITION

Phytodetritus, which can originate in surface waters following seasona l phytoplankton blooms, is considered to be an important food resource for abyssal fauna. We analyzed the composition of phytodetritus sampl ed discretely in situ at an abyssal station in the NE Pacific. The phy todetritus was collected during two time periods straddling a seasonal maximum in the flux of particulate matter into a sediment trap moored within the benthic boundary layer. Using the sediment trap record and concurrent time-lapse photographs of the sea floor in addition to the results for the composition of sea floor phytodetritus, we created a scenario for the origin, accumulation, and degradation of phytodetritu s at the study site. It appeared that the sediment trap record did not entirely account for the accumulation and composition of phytodetritu s on the sea floor, likely due to the undersampling of large sinking a ggregates by the sediment trap. Within each sea-floor-sampling period, two types of aggregated phytodetritus were identified, based on macro scopic appearance: loosely or strongly cohesive. The loosely cohesive phytodetritus collected just before the maximum sinking flux was flocc ulent and contained large numbers of chain-forming diatoms. The bulk o f the loosely cohesive phytodetritus collected one month later consist ed of phaeodarians that appeared to have entrained both older and new ''diatom floc'' material. The strongly cohesive, discrete aggregates c ollected during both sampling periods appeared to be derived from zoop lankton mucous webs. Although samples within each time period were het erogeneous, microscopic composition of phytodetritus, in terms of seve ral phytoplankton and protozooplankton components, differed significan tly between sampling periods. These results suggest that phytodetritus can change dramatically over a short time period; this may have impli cations for the potential food resource and feeding behavior of abyssa l fauna. (C) 1998 Elsevier Science Ltd. All rights reserved.