Aggregation, sedimentation and biodegradability of phytoplankton-derived material during spring in the Ross Sea, Antarctica

The sedimentation rates, aggregation and biodegradability of phytoplankton-derived material were investigated in the Ross Sea Polynya during austral spring, 1997. Planktonic organisms and aggregates were collected using a large volume (100 l) sampler. The phytoplankton assemblages were dominated by diatoms and/or Phaeocystis antarctica, a colonial haptophyte. Total community sinking rates of phytoplankton or derived aggregates were low, ranging between 0 and 1 m d −1, except towards mid-December when the sinking rates increased to 3 m d −1. At this time, the co-occurrence of Phaeocystis colonies and diatoms resulted in the enhanced formation of aggregates. Taxon-specific differences in sinking behaviour were observed, in that diatoms and dinoflagellates had higher sinking rates than P. antarctica. The biodegradability of phytoplankton-derived material was estimated from the enzymatic (ectoprotease, ecto- β-glucosidase) activities of free-living vs. particle-attached bacteria. Bacterial production rates were very low until late November, and from mid-December the bacterial activities increased slowly. At the same time, the proportion of attached bacteria increased; nevertheless, their contribution to the total bacterial activity remained low (less than 19% for ectoprotease, 24% for ecto- β-glucosidase and 18% for bacterial production). In contrast, specific (per cell) activities were higher for particle-attached bacteria than for free-living bacteria. Calculations suggest that 1.4–38.5% of the primary production potentially could be removed from the euphotic zone via sinking during spring, and 0.5–11.6% could be remineralized by bacterial metabolism, with a minor contribution of particle-attached bacteria (less than 2% of the primary production). Thus, during the most active period of phytoplankton growth when grazing losses are nearly non-existent, most of the removal of phytoplankton-derived particulate material is by passive sinking, and only a small amount of the particulate organic carbon is heterotrophically oxidized.