On the trophic fate of Phaeocystis pouchetii.: VII.: Sterols and fatty acids reveal sedimentation of P-pouchetii-derived organic matter via krill fecal strings

As part of a joint project on the fate of phytoplankton in Balsfjorden in Northern Norway, we investigated the trophic fate and sedimentation potential of Phaeocystis pouchetii by tracing the transition of biomarker patterns from a phytoplankton bloom to sediment traps and during a gut-passage experiment. Thephytoplankton biomass during the spring bloom 1996 was dominated by colonial P. pouchetii (ca 85%) and 4 members of the diatom family Thalassiosiraceae (ca 10%). Particulate organic carbon in sediment traps largely consisted of fecal material from the Arctic krill Thysanoessa sp. Sterol and fatty acid biomarker patterns in the phytoplankton bloom could be reproduced by combining the individual biomarker patterns of the isolated phytoplankters P. pouchetii and Thalassiosira decipiens a ratio of ca 75:25. In a laboratory experiment, Arctic krill (Thysanoessa raschii) fed with similar efficiency on P. pouchetii colonies and the Thalassiosiraceae. During gut passage, the abundance of Thalassiosiraceae biomarkers in fecal strings increased relative to P. pouchetii biomarkers, while biomarkers from krill became dominant. This transition of biomarker patterns due to gut passage in T. raschii closely resembled the biomarker transition from the surface bloom to material in sediment traps at 40 to 170 m depth. which was mainly composed of krill fecal strings. We conclude that krill grazed efficiently on P. pouchetii colonies in Balsfjorden and caused sedimentation of P, pouchetii-derived organic matter below the euphotic zone via fecal strings. Hence, both transfer to higher trophic levels and sedimentation of Phaeocystis spp.-derived organic matter can be more effective than is commonly believed.