The "Fall dump" - a new perspective on the role of a "shade flora" in the annual cycle of diatom production and export flux

Investigations of diatom fluxes recorded in laminated sediments using scann ing electron microscope techniques together with evidence from sediment tra p studies have contributed to a reappraisal of the annual cycle of diatom p roduction and export flux. We propose that where there is a strong seasonal thermocline: and nutricline, a number of diatoms, hitherto regarded as typ ical sparse summer flora, characteristic of oligotrophic waters, are able t o generate substantial production at depth. These species, including Rhizos olenia spp., Stephanopyxis palmeriana, Thalassiothrix spp. and some Coscino discus spp., may represent a "shade flora" that have adapted to grow in low -light conditions and/or to regulate their buoyancy to move between a deep nutrient source and the euphotic zone. Although rates of growth and primary production are substantially lower than species characteristic of "spring bloom" or "upwelling" conditions, the total primary production integrated o ver the (several-month) period of summer stratification may be as significa nt as the "spring bloom" or greater. The term fall or autumn bloom las a co unterpart of the "spring bloom") is therefore a misnomer. Whereas the "spri ng bloom" involves a rapid burst of reproduction and sedimentation, the "fa ll dump" is the sedimentation of a long-lived episode of production (lastin g the duration of the seasonal thermocline) and triggered by the fall/winte r mixing that breaks down stratification. The "fall dump" may produce as mu ch, or in some cases more, export production than the "spring bloom". The r esults of this study suggest that a reorientation of thinking on diatom eco logy and palaeoecology may be required. (C) 2000 Elsevier Science Ltd. All rights reserved.