On the trophic fate of Phaeocystis pouchetii. VII. Sterols and fatty acidsreveal sedimentation of P-pouchetii-derived organic matter via krill fecalstrings

As part of a joint project on the fate of phytoplankton in Balsfjorden in N orthern Norway, we investigated the trophic fate and sedimentation potentia l of Phaeocystis pouchetii by tracing the transition of biomarker patterns from a phytoplankton bloom to sediment traps and during a gut-passage exper iment. Thephytoplankton biomass during the spring bloom 1996 was dominated by colonial P. pouchetii (ca 85%) and 4 members of the diatom family Thalas siosiraceae (ca 10%). Particulate organic carbon in sediment traps largely consisted of fecal material from the Arctic krill Thysanoessa sp. Sterol an d fatty acid biomarker patterns in the phytoplankton bloom could be reprodu ced by combining the individual biomarker patterns of the isolated phytopla nkters P. pouchetii and Thalassiosira decipiens a ratio of ca 75:25. In a l aboratory experiment, Arctic krill (Thysanoessa raschii) fed with similar e fficiency on P. pouchetii colonies and the Thalassiosiraceae. During gut pa ssage, the abundance of Thalassiosiraceae biomarkers in fecal strings incre ased relative to P. pouchetii biomarkers, while biomarkers from krill becam e dominant. This transition of biomarker patterns due to gut passage in T. raschii closely resembled the biomarker transition from the surface bloom t o material in sediment traps at 40 to 170 m depth. which was mainly compose d of krill fecal strings. We conclude that krill grazed efficiently on P. p ouchetii colonies in Balsfjorden and caused sedimentation of P, pouchetii-d erived organic matter below the euphotic zone via fecal strings. Hence, bot h transfer to higher trophic levels and sedimentation of Phaeocystis spp.-d erived organic matter can be more effective than is commonly believed.