H. Lil et al., ALKALINE-PHOSPHATASE ACTIVITIES AMONG PLANKTONIC COMMUNITIES IN THE NORTHERN RED-SEA, Marine ecology. Progress series, 173, 1998, pp. 107-115
During a June 1996 cruise in the northern Red Sea, we studied spatial
distributions of alkaline phosphatase (AP) activities in different pla
nkton size fractions and in the aqueous fraction which passes through
0.2 mu m 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 par
ticulate alkaline phosphatase activities were highest in surface water
s. The contribution of the aqueous fraction to total alkaline phosphat
ase activity varied between 42 and 74%. The bulk of particulate alkali
ne phosphatase activity was associated with picoplankton (50 to 71%) a
t all sampling sites. A good correlation was found between depth distr
ibutions of alkaline phosphatase activities and Synechococcus, suggest
ing 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 hig
hest for plankton communities in the surface layers of the northern ha
lf of the Gulf. Alkaline phosphatase activities increased in the Gulf
of Aqaba in water bodies traveling north, further away from the Strait
s of Tiran. At this narrow and shallow entrance to the Gulf, local tur
bulent conditions and coral reef flats inject phosphate into the produ
ctive layer. Plankton communities immediately north of the Straits sho
wed basal levels of alkaline phosphatase activity, and phosphate suppl
y presumably satisfied their P-demands. Based on the regulation of alk
aline 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 i
ndicated that inorganic phosphate availability in the waters of the no
rthern Gulf of Aqaba was persistently low during the period prior to s
ampling.