Influence of dissolved silicate on vertical flux of particulate biogenic matter

Abstract

The influence of dissolved silicate (DSi) addition on primary production, phytoplankton development and subsequent vertical export of particulate matter was studied in enclosures. Blooms of different phytoplankton communities were initiated in the upper part of 10 m deep enclosures supplied with nitrate and phosphate (NP) and nitrate, phosphate and silicate (NPS). Primary production was 31% higher in the NPS enclosure as compared to the NP enclosure over the experimental period of 27 days. Increased phytoplankton growth was mainly caused by mass development of diatoms in the NPS enclosure. Enhanced growth was accompanied by an increased vertical flux of organic matter (86, 15.9 and 16.9% in terms of chlorophyll, particulate nitrogen and particulate carbon, respectively) and was dominated by diatoms. The present study indicates that 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 turn-over time in the euphotic zone by increasing vertical export. This may improve water quality of the surface layer of eutrophicated environments, but can lead to oxygen depletion of bottom waters.