Water column anoxia: vertical zonation of planktonic protozoa

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Abstract
The vertical zonation of planktonic protozoa was studied in 2 Danish eutrophic fjords at localities with an anoxic layer of bottom-water. Three distinct assemblages of protozoa are found. The fully oxygenated surface waters harbour a typical ciliate-plankton assemblage dominated by tintinnids and other oligotrichs in addition to various gymnostome ciliates. Heterotrophic flagellates totally dominate as bacterivores. In the oxycline a quite different group of ciliates occur; it is dominated by bacterivorous scuticociliates. Beneath the oxycline, in strictly anoxic and sulphide containing water, an assemblage of specialised anaerobic ciliates occurs. The high bacterial productivity in the oxycline is reflected by a biomass maximum of ciliates and to a lesser degree of heterotrophic flagellates. In anoxic microbial communities the biomass ratio between phagotrophs and their prey is much lower than in aerobic communities due to the low net growth efficiency of anaerobic eukaryotes. The potential significance of methane production by methanogenic symbionts of anaerobic protozoa is discussed.