Alkaline phosphatase activities among planktonic communities in the northern Red Sea

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 pm 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 Oh. The bulk of particulate alkaline phosphatase activity was associated wlth picoplankton (50 to 71 1; ") 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 aspecific alkaline phosphatase activities were highest for plankton communities in the surface layers of the northern half of the Gulf. Alkahne phosphatase activities increased in the Gulf of Aqaba in water bodies traveling north, further away from the Straits of Tlran. At t h ~s 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 regulat~on of alkaline phosphatase synthesis, its secretion and stabhty, 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 durlng the period prior to sampling.