Abstract
Population densities of filter feeding ciliates in the water of the three marine protectorates of Ras Mohammed, Nabq and Abu Galoum in the Gulf of Aqaba at the northern Red Sea, were estimated during the period from November 2006 to November 2007. Also, autotrophic nanoflagellates, heterotrophic nanoflagellates and heterotrophic bacteria were characterised in order to gain some indication of the food resources for ciliates. The abundance of ciliates in the waters of the three protectorates were found to vary according to an annual cycle, with the highest ciliate numbers of 2.5x10(4) cells L-1 occurring in the spring and the lowest numbers of 0.2x10(4) cells L-1 ccurring in the summer. Abundances were at times eight-fold higher than those found in comparable studies of nutrient-poor pelagic systems and approached those observed in coastal waters and in more productive open ocean systems. Nanoflagellates that could provide a food supply for the filter feeding ciliates were especially numerous during spring, and it is confirmed that the production of bacteria is an important component at the base of this food chain in the waters of the three marine protectorates. The study explored these food chain relationships by calculating the potential rate of capture of prey and the clearance rate of heterotrophic nanoflagellates and filter feeding ciliates. The study also compared the filtration rates of heterotrophic flagellates and of filter feeding ciliates.