Turbulence effects on cold ocean microbial communities: an enclosure study

There have been numerous studies on turbulence effects on macro- and zooplankton during the last decade, but studies on microplankton have been fewer, sometimes contradictory, and conducted largely under temperate conditions. In the present study, an enclosure experiment was carried out with natural seawater of Logy Bay, Newfoundland to determine the effects of turbulence on heterotrophic and autotrophic growth in the presence and absence of micrograzers. The experiment was conducted in February with water temperatures at 0 °C. We found no difference in bacterial growth between the static and turbulence enclosures in either the presence or absence of micrograzers. Heterotrophic nanoflagellate growth, however, was found to be significantly higher in the presence of turbulence but only when micrograzers were absent. A similar pattern was found with the small autotrophic community, determined as chlorophyll a passing through a 5-μm filter, in which growth was significantly higher with turbulence but only when micrograzers were absent. Turbulence significantly enhanced growth rates of the large autotrophic community (chlorophyll a retained by a 5-μm filter), which was largely composed of diatoms, both in the presence and absence of grazers. These results suggest that turbulence effects are small under cold ocean conditions, but increase with the size of the organism. It thus appears that turbulence could affect patterns of biogenic carbon export differentially, being dependent upon the conditions of the marine environment.