SPATIAL VARIATIONS OF SIZE-FRACTIONATED CHLOROPHYLL, CYANOBACTERIA AND HETEROTROPHIC BACTERIA IN THE CENTRAL AND WESTERN PACIFIC

Geographic and vertical variations of size-fractionated (0.2-1 mu m, 1 -10 mu m, and >10 mu m) Chlorophyll a (Chl.a) concentration, cyanobact eria abundance and heterotrophic bacteria abundance were investigated at 13 stations from 4 degrees S, 160 degrees W to 30 degrees N, 140 de grees E in November 1993. The results indicated a geographic distribut ion pattern of these parameters with instances of high values occurrin g in the equatorial region and offshore areas, and with instance of lo w values occurring in the oligotrophic regions where nutrients were al most undetectable. Cyanobacteria showed the highest geographic variati on (ranging from 27x10(3) to 16,582x10(3) cell l(-1)), followed by Chl .a (ranging from 0.048 to 0.178 mu g l(-1)), and heterotrophic bacteri a (ranging from 2.84x10(3) to 6.50 x 10(5) cell l(-1)). Positive corre lations were observed between nutrients and Chl.a abundance. Correspon dences of cyanobacteria and heterotrophic bacteria abundances to nutri ents were less significant than that of Chl.a. The total Chl.a was acc ounted for 1.0-30.9%, 35.9-53.7%, and 28.1-57.3% by the >10 mu m, 1-10 mu m and 0.2-1 mu m fractions respectively. Correlation between size- fractionated Chl.a and nutrients suggest that the larger the cell size , the more nutrient-dependent growth and production of the organism. T he ratio of pheophytin to chlorophyll implys that more than half of th e > 10 mu m and about one third of the 1-10 mu m pigment-containing pa rticles in the oligotrophic region were non-living fragments, while mo st of the 1-10 mu m fraction was living cells. In the depth profiles, cyanobacteria were distributed mainly in the surface layer, whereas he terotrophic bacteria were abundant from surface to below the euphotic zone. Chl.a peaked at the surface layer (0-20 m) in the equatorial are a and at the nitracline (75-100 m) in the oligotrophic regions. Cyanob acteria were not the principle component of the picoplankton. The carb on biomass ratio of heterotroph to phytoplankton was greater than 1 in the eutrophic area and lower than 1 in oligotrophic waters.