MEPS 173:107-115 (1998)  -  doi:10.3354/meps173107

ABSTRACT: During a June 1996 cruise in the northern Red Sea, we studied spatial distributions of alkaline phosphatase (AP) activities in different plankton size fractions and in the aqueous fraction which passes through 0.2 µm filters. AP proved to be a highly stable compound in the Red Sea and has potential use as an indicator for inorganic P-availability to planktonic communities. Depth profiles showed that aqueous and particulate alkaline phosphatase activities were highest in surface waters. The contribution of the aqueous fraction to total alkaline phosphatase activity varied between 42 and 74%. The bulk of particulate alkaline phosphatase activity was associated with picoplankton (50 to 71%) at all sampling sites. A good correlation was found between depth distributions of alkaline phosphatase activities and Synechococcus, suggesting that this abundant picoplanktonic cyanobacterium is a significant contributor to the enzyme activity in the northern half of the Gulf of Aqaba. Chlorophyll a-specific alkaline phosphatase activities were highest for plankton communities in the surface layers of the northern half of the Gulf. Alkaline phosphatase activities increased in the Gulf of Aqaba in water bodies traveling north, further away from the Straits of Tiran. At this narrow and shallow entrance to the Gulf, local turbulent conditions and coral reef flats inject phosphate into the productive layer. Plankton communities immediately north of the Straits showed basal levels of alkaline phosphatase activity, and phosphate supply presumably satisfied their P-demands. Based on the regulation of alkaline phosphatase synthesis, its secretion and stability, we proposed the use of alkaline phosphatase activities for a (partial) assessment of the P-status of marine planktonic communities. Such an assessment indicated that inorganic phosphate availability in the waters of the northern Gulf of Aqaba was persistently low during the period prior to sampling.

KEY WORDS: Marine picoplankton · Phosphate · P_i-limitation · Alkaline phosphatase activity · Synechococcus