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 scanning electron microscope techniques together with evidence from sediment trap studies have contributed to a reappraisal of the annual cycle of diatom production and export flux. We propose that where there is a strong seasonal thermocline and nutricline, a number of diatoms, hitherto regarded as typical sparse summer flora, characteristic of oligotrophic waters, are able to generate substantial production at depth. These species, including Rhizosolenia spp., Stephanopyxis palmeriana, Thalassiothrix spp. and some Coscinodiscus 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 over the (several-month) period of summer stratification may be as significant as the “spring bloom” or greater. The term fall or autumn bloom (as a counterpart of the “spring bloom”) is therefore a misnomer. Whereas the “spring bloom” involves a rapid burst of reproduction and sedimentation, the “fall dump” is the sedimentation of a long-lived episode of production (lasting the duration of the seasonal thermocline) and triggered by the fall/winter mixing that breaks down stratification. The “fall dump” may produce as much, or in some cases more, export production than the “spring bloom”. The results of this study suggest that a reorientation of thinking on diatom ecology and palaeoecology may be required.