Pelagic cold ocean microbial dynamics and small-scale turbulence interactions with special reference to bacteria

The marine microbial food web is a major source of biogenic carbon cycling within the world's oceans. Phytoplankton consume carbon dioxide and produce dissolved organic matter that is absorbed by the bacteria, thus stimulating bacterial growth and providing a valuable food source to micrograzers. It is believed that the energy of large scale turbulent events is propagated to ever smaller scales and can affect and influence the activity of microbial grazers. Little research exists today regarding this effect, and even less is known about how turbulence may affect the microbial food web in cold oceans in particular. Therefore, this investigation was designed to observe the combined effects of low temperature and small-scale turbulence on the microbial food web.;In addition, 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 and April, with water temperatures at 0°C and 5°C, respectively. There was no difference in bacterial growth between the static and turbulent enclosures in either the presence or absence of micrograzers at both experimental temperatures. Heterotrophic nanoflagellate growth was found to be significantly higher at 0°C in the presence of turbulence but only when micrograzers were absent. A similar pattern was found with the small autotrophic community at 0°C, as determined by chlorophyll a <5 μm, in which growth was significantly higher with turbulence but only when micrograzers were absent. Turbulence significantly enhanced growth rates of the large autotrophic community (>5 μm chl a) 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 turbulence could affect patterns of biogenic carbon export differentially, dependent upon the size structure of the plankton community.;The role of bacteria and the microbial food web within a polar region was examined as part of a multi-year (1997-1999) seasonal study of the North Water polynya (NOW). Regions of the polynya were characterized into two distinct water masses: the silicate-rich Arctic water (SRAW) that flowed from the north and down the coast of Ellesmere Island and the Baffin Bay water (BBW) that flowed from the south and up along the coast of Greenland. Bacterial abundances were generally higher in the BBW region than the SRAW region of the polynya. Bacterial growth rates were found to be higher rates in the SRAW region of the NOW. It was also found that bacterial biomass was positively correlated with phytoplankton biomass and negatively correlated with both inorganic nutrients. Bacterial grazing mortality exceeded growth during high turbulence treatments and that bacterial production tended to be higher in the low turbulence treatment relative to the static and high turbulence treatments. These results thus suggest that bacterioplankton are actively growing and are actively grazed within the NOW, that turbulence does have an effect on bacterial grazing mortality even at low (≤1°C) temperatures and that there are regional differences in the pelagic food web structure and the patterns of biogenic carbon export in the NOW as the summer progresses.