Eutrophication and vertical flux: A critical evaluation of silicate addition

Abstract

Availability of dissolved silicate (DSi) can affect phytoplankton species composition and the pelagic-benthic coupling. Further, consumption of DSi is assumed to be proportional to export production and sedimentation. The effect of DSi on the vertical flux of biogenic matter was investigated in 4 mesocosm experiments, where all enclosures were fertilised with nitrate and phosphate (NP), while half of the enclosures were given DSi as well (NPS). Sedimentation rates were expected to increase with consumption of DSi according to the 'silicate-sedimentation' hypothesis, as a result of diatom aggregate formation and/or increased faecal pellet production form diatom-grazing copepods. Diatoms and faecal pellet carbon did contribute significantly more to the vertical flux in the NPS than in the NP enclosures. However, sedimentation of particulate organic carbon (POC) was still not significantly different between the NP and NPS treatments. This was due to high sedimentation rates of Phaeocystis pouchetii in the NP enclosures. The results of these experiments indicate that the relationship between DSi and daily carbon export is not necessarily as simple as suggested by the silicate-sedimentation hypothesis. Much of the uncertainties concerning the hypothesis are related to the sedimentation of P. pouchetii, as they contribute not only cell carbon but also mucilagenous matter.