SPATIAL MESOSCALE PATTERNS OF WEST PACIFIC PICOPHYTOPLANKTON AS ANALYZED BY FLOW-CYTOMETRY - THEIR CONTRIBUTION TO SUBSURFACE CHLOROPHYLL MAXIMA

Vertical distributions of picophytoplankton (ppp) (< 2 mum) were studied by ship-board flow cytometry during two cruises in Western Pacific waters to Palau and to Australia in 1990. Weak red-fluorescing small ''ppp'', supposed to be free-living prochlorophytes (Chisholm et al. 1988), were abundant in the area surveyed. These ppp, designated ''the prochlorophytes'', were abundant in the surface waters (> 10(4) cells ml-1) at the northern region (27-degrees 03'N; 7-degrees 11'N) in November, whereas in December at the southern tropical stations (0-degrees 23.54'S; 9-degrees 20.30'S; 13-degrees 50.6'S), they formed subsurface maximum layers (> 10(5) cells ml-1) on a nitracline at a depth of 3.5 to 5.4% surface irradiation. Their fluorescence intensity increased with depth below 10% surface irradiation. The prochlorophytes at a depth of 1% surface irradiation had ten times higher fluorescence than those at the surface layer. The total fluorescence intensity of the prochlorophytes accounted for 32 to 63% of the sum of the total fluorescence intensity of all fluorescing phytoplankton detected at subsurface chlorophyll maxima in the tropical area. These results suggest that distribution of the prochlorophytes is greatly affected by nitracline and by light intensity and that their chlorophyll is a major contributor to the subsurface chlorophyll maximum in the pelagic West Pacific Ocean.