3.2 A Study of Primary Phytoplankton 

 Production 



MIKHAEL N. KORSAK 



Institute of Global Climate and Ecology, State Committee for Hydrometeorology and Academy of Sciences, Moscow, USSR 



Introduction 



The study of the rate of formation of new organic matter 

 by phytoplankton photosynthesis in the tropical Pacific reported 

 in the present paper was carried out during the First Joint 

 US-USSR Central Pacific Expedition aboard the R/V Akademik 

 Korolev in 1988. The ocean areas investigated included 

 portions of the central and western Pacific, which had received 

 little previous attention. Primary production in the area of the 

 first transect, beginning near Caroline Atoll and ending at 

 Tarawa Island (Republic of Kiribati), ranges from values 

 characteristic of oligotrophic parts of the ocean 

 (100 mg C/nr/day and lower) to values corresponding in 

 mesotrophic areas of the Pacific (25 mg c/nr/day) (Sorokin, 

 1976). 



The central portion of the tropical Pacific (stations of the 

 second transect) may be characterized as an oligotrophic 

 productivity zone where a major role in primary production 

 enhancement is played by synoptic phenomena such as cyclones, 

 tornadoes, waterspouts, etcetera (Sorokin, 1976). Situated in 

 the open portion of the northern tropical Pacific (i.e., within the 

 northern tradewind zone), the second transect studied showed 

 a primary rate of organic matter production by phytoplankton 

 of about 1 00 mgC/m7day (Sorokin, 1976). The relatively low 

 rales of primary production in this part of the Pacific are 

 attributable largely to the deficiency of biogenic elements in 

 the photosynthetic layer. These low concentrations are in turn 

 due to anticyclonic circulation, which produces downwelling 

 of nitrogen- and phosphorus-poor surface waters. It is the 

 resulting low nitrogen and phosphorus levels in the 

 photosynthetic layer that limit photosynthesis rates in the 

 phytoplankton community. 



Despite the absence of significant seasonal variations in 

 illumination and water temperature in the tropical ocean, 

 considerable seasonal changes of photosynthesis rates have 

 been reported in spring and autumn (Sorokin, 1976). 



Materials and Methods 



Studies al Stations 1 14-120 in the central tropical Pacific 

 were conducted from 27 September to 7 October 1988. The 

 work al Stations 121-126 along the Marianas transect was 

 performed from L 6 October to 2] October. Primary production 

 was determined by means of a radiocarbon version of the "jars 

 method" proposed by Sorokin (Sorokin el al., 1983). Work al 

 each station included measurements of photosynthesis in a 

 surface-water sample (C ps ) as well as determinations of 

 photosynthesis in the water layer as a function of phytoplankton 

 distribution over (he water column (the coefficients K,). 



The light curves (the coefficients K,) were determined at one 

 station in each transect. The sample incubation was usually 8- 

 10 h, beginning in the morning. The radioactivity of filters with 

 l4 C-labeled phytoplankton and of the working NaH l4 CO, 

 solutions was measured using a Nuclear Chicago "Mark 2" 

 scintillation counter. Sample radioactivities were counted 

 using liquid scintillator cocktails of previously described 

 composition (Sorokin, 1976; Sorokin et al., 1983). 



Primary production was calculated using the standard 

 formula, with a factor of 1.5 to correct for l4 C loss due to 

 phytoplankton sample filtering (Sorokin, 1976). All 

 determinations of primary production were carried out in 

 triplicate. The extent of the photosynthesis zone was taken to 

 equal the white-disk transparency multiplied by three. Samples 

 for determining phytoplankton production were taken using 

 5-1 Niskin bottles at depths of 0.5: 10; 15; 25; 45; 70, and 

 100m. 



Results 



The vertical structure of phytoplankton communities in 

 high-transparency tropical ocean waters is characterized by 

 several phytoplankton growth peaks or maxima in the euphoric 

 zone, whose depth sometimes exceeds 100 m. Two layers with 

 elevated phytoplankton concentrations are usually in evidence. 

 The first of these occurs at a depth of 1 0-30 m and is associated 

 with a photosynthesis-optimal light zone; the second lies at 

 depths of 70-90 m and is related to heightened biogenic 

 element levels in (he vicinity of the pycnocline (Sorokin. 

 1 976). As is evident from Table 1 , the depth of the photosynthesis 

 layer at all stations of both transects was usualh slightly in 

 excess of 100 m. As a rule, only a single photosynthesis peak 

 was observed (the one in the 15-25-m depth range), since the 

 pycnocline lay below the photosynthesis layer boundary (see 

 Table 1 ). However, Stations 1 22 and 1 26 did exhibit a second 

 relatively small primary production peak at 70 m (Table 1). 

 Primary production values for the topmost levels of the water 

 column were usually markedly lower than at depths of 

 10-15 m. which was probably due to photic inhibition of 

 photosynthesis by the high-intensity incident light. Stratification 

 of water masses o\ er the w ater column had no significant effect 

 on primary organic-matter production by phytoplankton. 

 inasmuch as the top 100 m of the water column was 

 homothermal. Thus, the water temperature in the top 100 mof 

 the water column at stations of both transects varied within just 

 1-2°C. Salinity in the same laser varied within the same 

 narrow limits, so that the primary production level at various 

 depths depended largely upon light intensity, amount of 

 phytoplankton present, and biogenic-element availability. 



212 



