Can silicate and turbulence regulate the vertical flux of biogenic matter? A mesocosm study

Abstract

The effects of silicate and turbulence on the vertical flux of biogenic matter were stud- ied in mesocosms. The experiment consisted of eight 27 m 3 enclosures all fertilised with nitrate and phosphate (NP), while 4 of the enclosures were supplied with silicate as well (NPS). A 2-layer density gradient was created, and turbulence was generated at 2 intensity levels in the upper layer of the enclosures by a vertically moving grid. We tested the hypotheses that: (1) dissolved silicate (DSi) has a strong regulating effect on biogenic sedimentation by favouring the growth of diatoms instead of flagellates; (2) there is a positive linear relationship between DSi consumption and carbon export; and (3) elevated levels of turbulence would further increase the loss rates of diatoms through aggre- gate formation. Addition of DSi caused higher primary production and a shift from a flagellate to diatom-dominated phytoplankton community. However, contrary to expectations, sedimentation of chl a was lower (<15 mg m -2 d -1 ) where diatoms dominated than where flagellates prevailed (≤ 80 mg m -2 d -1 ). The hypothesised linear relationship between addition of DSi and vertical export was thus not found in this experiment. The 2 levels of turbulence caused no statistically significant differences in the suspended concentrations or sedimentation rates of phytoplankton groups. In conclusion, DSi triggered a diatom bloom with stable sedimentation rates in the NPS replicates, while comparatively higher loss rates were found in the flagellate-dominated NP enclosures. Turbulence had little effect on the phytoplankton community and sedimentation of biogenic matter.