Phytodetritus entering the benthic boundary layer and aggregated on the sea floor in the abyssal NE Pacific: macro- and microscopic composition

Phytodetritus, which can originate in surface waters following seasonal phytoplankton blooms, is considered to be an important food resource for abyssal fauna. We analyzed the composition of phytodetritus sampled discretely in situ at an abyssal station in the NE Pacific. The phytodetritus 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 phytodetritus at the study site. It appeared that the sediment trap record did not entirely account for the accumulation and composition of phytodetritus on the sea floor, likely due to the undersampling of large sinking aggregates by the sediment trap. Within each sea-floor-sampling period, two types of aggregated phytodetritus were identified, based on macroscopic appearance: loosely or strongly cohesive. The loosely cohesive phyto-detritus collected just before the maximum sinking flux was flocculent and contained large numbers of chain-forming diatoms. The bulk of the loosely cohesive phytodetritus collected one month later consisted of phaeodarians that appeared to have entrained both older and new “diatom floc” material. The strongly cohesive, discrete aggregates collected during both sampling periods appeared to be derived from zooplankton mucous webs. Although samples within each time period were heterogeneous, microscopic composition of phytodetritus, in terms of several phytoplankton and protozooplankton components, differed significantly between sampling periods. These results suggest that phytodetritus can change dramatically over a short time period; this may have implications for the potential food resource and feeding behavior of abyssal fauna.