MEPS 217:67-80 (2001)  -  doi:10.3354/meps217067

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

Camilla Svensen1,*, Jorun K. Egge2, Jan Erik Stiansen3

1Norwegian College of Fishery Science, University of Tromsø, Tromsø 9037, Norway
2Department of Fisheries and Marine Biology, University of Bergen, HIB, Bergen 5020, Norway
3Institute for Marine Research, PO Box 1870, Nordnes 5817, Norway

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 (¾80 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.

KEY WORDS: Vertical flux · Silicate · Turbulence · Diatoms · Phaeocystis pouchetii · Mesocosm

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