| Summary: | Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1984 The distribution and feeding behavior of bacterivorous micro flagellates (2-20 μm protozoa) and their ingestion by copepods were examined in an attempt to assess the importance of these protozoa as a trophic link between planktonic bacteria and zooplankton. The abundance of microflagellates relative to other picoplankton (0.2-2.0 μm) and nanoplankton (2-20 μm) populations in water samples in the North Atlantic and in Lake Ontario and on macroaggregates in the North Atlantic was determined using direct microscopical and culture estimation techniques. Seasonal, vertical and geographical changes in the density of microflagellates were generally not greater than one order of magnitude. Microscopical counts of heterotrophic nanoplankton (presumably microflagellates) typically ranged from a few hundred to a few thousand m1-1 for a variety of planktonic environments. They constituted approximately 1/3 to 1/2 of the nanoplankton in the euphotic zone and dominated the nanoplankton in the aphotic zone. Most Probable Number (MPN) estimation of the density of bacterivorous protozoa indicated that microflagellates were, on average, an order of magnitude more abundant than bacterivorous ciliates and amoebae. MPN and direct microscopical counts of microflagellates differed by as much as 104. This discrepancy was smaller in eutrophic environments (e.g. Continental Shelf and Lake Ontario) and on macroscopic detrital aggregates. All microbial populations enumerated were highly concentrated on macroscopic detrital aggregates relative to their abundance in the water surrounding the aggregates. Enrichment factors (the ratio of abundance of a population on a macroaggregate to its abundance in the surrounding water) increased along a eutrophic-to-oligotrophic gradient because of the combined effects of an increased abundance of microorganisms ...
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