Composition of benthic protozoan communities along a depth transect in the eastern Mediterranean Sea

The benthic protozoans were investigated along a transect in the eastern Mediterranean Sea from the Ionian to the Levantine Sea. Sediment samples were taken during Meteor cruise 25/1 in May and June 1993 at water depths of between 156 and 4617 m with a box corer and a multiple corer. A semi-quantitative cultivation technique was applied by placing sediment in petri dishes and diluting with sterile biotope water. Microscopic analyses of the protozoan taxa were conducted immediately after sampling, and then daily after enrichment with organic substrate. A total of 134 protozoan morphospecies were recorded, including 87 flagellates, 12 naked amoebae and 35 ciliates. 58% of the species could be attributed to genera known from shallow waters, demonstrating the wide distribution of these taxa. The number of recorded species per location ranged from 0 to 35 and decreased with increasing water depth. At stations deeper 1300 m very low species numbers as well as low potential abundances were recorded. Qualitative changes in the taxonomic composition with increasing depth were recorded as a decrease in the proportion of amoebae and euglenid flagellates and as an increase in the proportion of dinoflagellates. Ciliates were found down to a depth of 4260 m. The number of species was also dependent on the chloroplastic pigment equivalent (CPE), indicating a dependency on sedimented phytodetritus. The observed species included picophagous species, which feed mostly on bacteria, as well as nano- and microphagous species, which generally feed on protists, suggesting the existence of several trophic levels within the deep sea microbial food web.