The 'particle' flux: Origins and biological components

Sedimentation of organic matter from the ocean's surface layers is intimately tied to biological events within the euphotic zone. Although the sedimentation rate is usually found to be positively correlated with primary production, the biological basis for the correlation is not well understood. In eutrophic environments, algal populations or large fecal pellets can account for high mass fluxes. The types of particles leaving oligotrophic systems, in contrast, are less clear. Similarly, various mechanisms have been invoked to explain the loss of particles with depth, but little direct evidence is available to distinguish among the possibilities. The fundamental causes of differences in export rates and of the rapid loss of particles at depth, however, will be reflected in the changing composition of particles across a productivity gradient and down through the water column. We have attempted to investigate the biological processes leading to the productivity-flux relationship and the depth-related losses, using Vertex samples from particle interceptor traps deployed in the Northeastern Pacific between 1980 and 1984. We found the previously reported correlation between production and flux in the Vertex data set ( Pace, Knauer, Karl and Martin , 1987) to result from related changes in intact and presumably living organisms in trap samples, not from non-living particles. The production-flux correlation depended, however, entirely on results from an upwelling station: when data from this station were excluded, no pattern was observed among the remaining 6 stations. Phytoplankton flux at the base of the euphotic zone was likewise significantly correlated with primary production, and the relationship persisted even after data from the upwelling station were removed. Important live contributors to traps also included sarcodine protozoans (foraminiferans and radiolarians), eggs of invertebrates, and invertebrate larvae too small to be removed as “swimmers”. (The present study focuses on smaller or unicellular organisms; another study considers also the role of larger, but also cryptic, “swimmers”: Michaels, Silver, Gowing and Knauer , in press). The flux of non-living materials, including fecal pellets and amorphous aggregates, was not related to primary production, nor was there a correlation between obvious parameters of fecal pellets (i.e. average size or numbers) and production. These results raise questions about the meaning of the term “particle flux”. An average of at least 35%, with a range of 11–80% (minimal values, because larger organisms were not included), of the carbon flux leaving the euphotic zone consisted of algae, protozoans, and small metazoans that may have been executing vertical movements as part of a life history strategy. Furthermore, these organisms continued to be important in the mesopelagic zone, where the sarcodines were sometimes substantive contributors (average of 12%, range up to 49%) to the carbon flux. The declining contribution of these living organisms within traps at depth can account for some of the decreases in carbon flux that have been interpreted as “regeneration”. Clearly, reasonable inferences from traps require a better understanding of the nature of the “particles”, and particularly, the contribution of living organisms to trap collections.