Nanoflagellates (mixotrophs, heterotrophs and autotrophs) in the oligotrophic eastern Mediterranean: standing stocks, bacterivory and relationships with bacterial production

The vertical distribution (0 to 100 m) and abundance of nanoflagellates wer e examined in the oligotrophic Aegean Sea (east Mediterranean) in early spr ing (south basin) and late summer (north and south basins) of 1997 in the f ramework of the MATER project (Mass Transfer and Ecosystem Response). Diffe rent trophic types of nanoflagellates (mixotrophic, heterotrophic, and phot otrophic) were identified based on the possession of chloroplasts and the c onsumption of Fluorescently Labelled Minicells (FLM). Bacterial production (leucine method) was compared with bacterivory estimated from FLM consumpti on. We found that mixotrophic nanoflagellates played a small role as bacter ivores relative to heterotrophic nanoflagellates and total bacterivory roug hly balanced bacterial production. In early spring with cool (14.2 degrees C) well-mixed water columns, flagellate concentrations were lowest, phototr ophic flagellates were the dominant group and concentrations varied little with depth. Average concentrations of mixotrophs, heterotrophs and autotrop hs were 0.07, 0.34, and 0.64 x 10(3) cells ml(-1) respectively. Bacterial p roduction in the 0 to 100 m layer averaged about 0.74 mu g C l(-1) d(-1). E stimated nanoflagellate bacterivory from FLM ingestion accounted for 40% of bacterial production with mixotrophic nanoflagellates consuming 5% of bact erial production. In late summer, total nanoflagellate concentrations were higher. Average concentrations of mixotrophs, heterotrophs and autotrophs w ere 0.09, 1.14, and 0.66 x 10(3) cells ml(-1), respectively in the southern basin and 0.09, 1.1, and 0.98 x 10(3) cells ml(-1), respectively, in the n orthern basin. In September, bacterial production for both basins roughly b alanced estimated nanoflagellate consumption. Similar to the March estimate s, mixotrophic nanoflagellates accounted for about 5% of nanoflagellate bac terivory. In a nutrient enrichment experiment in March, treatments includin g phosphorus resulted in increased bacterial production and reductions in i dentifiable mixotrophs.