Abundance and trophodynamics of surface microbial loop populations in the northern Red Sea

Abstract

The population densities of filter feeding ciliates in the water of the three marine protectorates of Ras Mohammed, Nabq and Abu Galoum in the northern Red Sea, were calculated during the period from November 2006 to November 2007. Also, autotrophic nanoflagellates, heterotrophic nanoflagellates and heterotrophic bacteria were enumerated to gain some indication of the food resources available for ciliates. The abundance of ciliates in the three protectorates were found to follow an annual cycle with the highest ciliate numbers of 25.3 cells ml−1 observed in the spring, and the lowest numbers of 2.1 cells ml−1 in the summer. Abundances were at times 8-fold higher than those found in comparable studies on nutrient-poor pelagic systems and, astonishingly, 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 in the spring, while the production of bacteria was presumably a more important component at the base of this food chain in the three protectorates’ water. The filtration activities of heterotrophic flagellates and filter feeding ciliates were compared with the population densities of bacteria, nanoflagellates and ciliates in spring and summer. Moreover, filtration rates from the literature were used to calculate the potential rate of capture of prey of different categories, as well as to calculate the time required for the whole water body to be filtered by heterotrophic nanoflagellates and filter feeding ciliates.