INFLUENCE OF DISSOLVED SILICATE ON VERTICAL FLUX OF PARTICULATE BIOGENIC MATTER

The influence of dissolved silicate (DSi) addition on primary producti on, phytoplankton development and subsequent vertical export of partic ulate matter was studied in enclosures. Blooms of different phytoplank ton communities were initiated in the upper part of 10 m deep enclosur es supplied with nitrate and phosphate (NP) and nitrate, phosphate and silicate (NPS). Primary production was 31% higher in the NPS enclosur e as compared to the NP enclosure over the experimental period of 27 d ays. Increased phytoplankton growth was mainly caused by mass developm ent of diatoms in the NPS enclosure. Enhanced growth was accompanied b y an increased vertical flux of organic matter (86, 15.9 and 16.9% in terms of chlorophyll, particulate nitrogen and particulate carbon, res pectively) and was dominated by diatoms. The present study indicates t hat for each gram of DSi added, vertical flux was enhanced by 3.6 g C, implying that the ratio of DSi added/carbon exported was close to the Redfield ratio. Thus DSi presence appears to decrease the nutrient tu rn-over time in the euphotic zone by increasing vertical export. This may improve water quality of the surface layer of eutrophicated enviro nments, but can lead to oxygen depletion of bottom waters. (C) 1997 El sevier Science Ltd.