Anundance of phytoplankton at time series station DYNAPROC

A general study of biogeochemical processes (DYNAPROC cruise) was conducted in May 1995 at a time-series station in the open northwestern Mediterranean Sea where horizontal advection was weak. Short-term variations of the vertical distributions of pico- and nanophytoplankton were investigated over four 36-h cycles, along with parallel determinations of metabolic CO2 production rates and amino acid-containing colloid (AACC) concentrations at the chlorophyll maximum depth. The vertical (0-1000-m depth) distributions of (i) AACC, (ii) suspended particles and (iii) metabolic CO2 production rate were documented during the initial and final stages of these 36-h cycles. This study was concerned with diel vertical migration (DVM) of zooplankton, which provided periodic perturbations. Accordingly, the time scale of the experimental work varied from a few hours to a few days. Although all distributions exhibited a periodic behaviour, AACC distributions were generally not linked to diel vertical migrations. In the subsurface layer, Synechococcus made the most abundant population and large variations in concentration were observed both at day and at night. The corresponding integrated (over the upper 90 m) losses of Synechococcus during one night pointed to a potential source of exported organic matter amounting to 534 mg C/m**2. This study stresses the potential importance of organic matter export from the euphotic zone through the daily grazing activity of vertically migrating organisms, which would not be accounted for by measurements at longer time scales. The metabolic CO2 production exhibited a peak of activity below 500 m that was shifted downward, apparently in a recurrent way and independently of the vertical distributions of AACC or of suspended particulate material. To account for this phenomenon, a 'sustained wave train» hypothesis is proposed that combines the effect of the diel superficial faecal pellet production by swarming migrators and the repackaging activity of the nonmigrating midwater populations. Our results confirm the recent finding that the particulate compartment is not the major source of the observed instantaneous remineralisation rate and shed a new light on the fate of organic matter in the aphotic zone.

(Table 1) Adenosine triphosphate, urea, dissolved organic carbon, nitrogen and methane concentrations in bottom water of the northwestern Indian Ocean

Concentrations of adenosine triphosphate (ATP), urea, and dissolved organic carbon in bottom water are shown to be considerable, sometimes several times higher than in the photic and surface layers of the ocean. Urea and ATP concentrations are inversely proportional. Identified biochemical characteristics of bottom water are of great importance in determining the status of the aquatic environment. The highest life activity (maximum ATP content) in bottom water appeared in the vicinity of faults in rift zones of the ocean, where high gas concentrations were also found. Population of chemoautotrophic microorganisms was clearly present under these conditions. Biochemical investigations provide additional criteria for identifying oil and gas prospects. They are also of definite interest in combination with gasometric determinations, which will undoubtedly give us deeper understanding of processes of formation of oil and gas and will help in finding them.