| dc.description.abstract | The effects of silicate and turbulence on the vertical flux of biogenic matter were studied
in mesocosms. The experiment consisted of eight 27 m3 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 aggregate
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
chla 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. | en |
