Ciliate community in the oligotrophic Gulf of Aqaba, Red Sea

Abstract

Ciliate data were gathered along a 600 m deepwater column in central waters of the Gulf of Aqaba, Israel. Samples were taken during winter mixing, the onset of stratification in spring and during summer stratification (2003 to 2005). The phytoplankton community strongly differed in these 3 periods, from highest abundances of eukaryotic algae during mixing, followed by a Syne- chococcus bloom right after the onset of stratification and a Prochlorococcus-dominated community in summer. Ciliate abundance and biomass were high compared to chlorophyll standing stock, with maximal values of 3534 cells l -1 and 3554 ng C l -1 , respectively. Abundances were at times 5-fold higher than those found in comparable studies on nutrient-poor pelagic systems and approached those observed in coastal waters. Ciliate carbon:chlorophyll ratios of up to 26 were astonishingly high, as compared to values of 2 to 5 normally observed. This indicates a high efficiency of the cili- ates in utilizing the available food. The phytoplankton community in the Gulf is dominated by pico- cyanobacteria, and by repacking the algal prey into accessible particles, protozoans might be an important intermediate link in the pathway between phytoplankton and metazoan grazers. Olig- otrichs and nanociliates contributed up to 98 and 88% of ciliate total abundance, respectively. Small oligotrich species appeared to be the superior competitors in this oligotrophic marine system, being able to utilize the dominant picoautotrophs efficiently.