Response of the microbial food web to manipulation of nutrients and grazers in the oligotrophic Gulf of Aqaba and northern Red Sea

The control mechanisms within the pelagic microbial food web of the oligotrophic Gulf of Aqaba and the northern Red Sea were investigated in the spring of 1999. Nutrient conditions and potential grazer impact were manipulated in a series of dilution experiments. Ambient nutrient concentrations and autotrophic biomass were very low (0.23-1.21 mu mol NO3 l(-1), 0.06-0.98 mu mol NH4 l(-1), 1.08-1.17 mu mol Si l(-1), 0.08-0.12 mu mol P l(-1), 0.15-0.36 mu g chlorophyll a l(-1)). The planktonic community was characterized by low abundances [3.0-5.5 x 10(5) heterotrophic bacteria ml(-1), 0.58-7.2 x 10(3) ultraphytoplankton < 8 mu m ml(-1) (small eukaryotic photoautotrophs and Prochlorococcus sp., excluding Synechococcus sp.), 0.45-4.4 x 10(4) Synechococcus sp. ml(-1), 0.32-1.2 x 10(3) heterotrophic nanoflagellates ml(-1), 1.3-3.8 x 10(3) phytoplankton > 8 mu m l(-1), 0.93-5.4 x 10(2) microzooplankton l(-1)] and dominated by small forms (0.2-8 mu m). Dinoflagellates and oligotrichous ciliates were the most common groups in initial samples among the phytoplankton > 8 mu m and microzooplankton, respectively. Results show that bottom-up and top-down control mechanisms operated simultaneously. Small organisms were vulnerable to grazing, with maximum grazing rates of 1.1 day(-1) on heterotrophic bacteria and 1.3 day(-1) on ultraphytoplankton. In contrast, algae > 8 mu m showed stronger signs of nutrient limitation, especially when the final assemblages were dominated by diatoms. Synechococcus sp. were not grazed and only showed moderate to no response to nutrient additions. The high spatial and temporal variation of our results indicates that the composition of the planktonic community determines the prevailing control mechanisms. It further implies that, at this transitional time of the year ( onset of summer strati. cation), the populations fluctuate about an equilibrium between growth and grazing.