Notes 1813 Limnol. Oceatzogr., 38(8). 1993, 1813-1818 (0 1993, by the Ameruzan Society of Limnology and Oceanography. Inc Chlorophyll a concentrations in the North Pacific: Does a latitudinal gradient exist? Abstract -Chlorophyll a concentrations were tion properties of glass-fiber and membrane measured as a function of depth from 28 to 48”N filters have demonstrated that glass-fiber filters along 152”W in March 199 1 with Whatman GF/F and 0.2~pm Nuclepore filters. Surface Chl a concen- inadequately retain < 1-pm-diameter cells due trations measured with 0.2~ym Nuclepore filters were to their large nominal pore size (0.7 and 1.2 up to fourfold higher than those measured with pm for Whatman GF/F and GF/C filters). Low Whatman GF/F filters. The largest difference be- retention efficiencies of glass-fiber filters result tween the two filter types was found in subtropical waters, where picoplankton were a major constituent when Chl a concentrations are low (<0.5-l .O of the phytoplankton assemblage. Chl a concentra- pg Chl a liter-‘) and when picoplankton are a tions integrated from 0 to 175 m showed a threefold dominant fraction of the phytoplankton as- increase (9-26 mg Chl a m-I) between 28 and 48”N semblage (Phinney and Yentsch 1985; Taguchi when Whatman GF/F filters were used. However, integrated Chl a concentrations based on measure- and Laws 1988). Under such conditions the ments with 0.2~pm Nuclepore filters were nearly con- use of membrane filters with submicron pore stant (25-3 1 mg Chl a mpZ) over the transect. These sizes has been recommended (although not al- results lead us to question the existence of previously ways followed). Many studies have made ex- reported latitudinal gradients in integrated Chl a tensive use of glass-fiber filters due to their low concentrations in the North Pacific Ocean. cost and fast flow rates. Results from previous basinwide studies Attempts to understand the ocean’s role in have suggested the presence of a latitudinal global climate change require accurate esti- gradient in integrated Chl a concentrations be- mates of phytoplankton biomass and produc- tween subtropical and subarctic waters. The tion. Recent work has shown that picoplank- purpose of this study was to compare Chl a ton constitute a major fraction of the concentrations in subtropical, transition, and phytoplankton biomass in tropical, subtropi- subarctic waters and to evaluate the existence cal, and temperate waters. As well, these small of a meridional gradient in integrated Chl a phytoplankters have been found to contribute concentrations across the North Pacific basin. significantly to the primary production in open- Sampling was conducted from 20 to 55”N ocean ecosystems (Li et al. 1983; Platt et al. along 152”W in March 199 1 on the RV Dis- 1983). However, in oligotrophic tropical and coverer as part of the Climate and Global subtropical waters, where picoplankton dom- Change Program of NOAA. Water samples inate, Chl a concentrations and the level of were collected at -20-m intervals from the primary production may have been underes- surface to 100 m and then every 25 m to a timated due to the use of filters with inappro- depth of 175 m with lo-liter Niskin bottles priate pore sizes. attached to a rosette sampler. Duplicate 200- Numerous comparative studies of the reten- ml subsamples of seawater were filtered under low vacuum pressure (5 180 mm of Hg) onto either 25-mm-diameter combusted Whatman GF/F or 0.2~pm Nuclepore filters. Near the Acknowledgments We thank the captain, officers, and crew of the NOAA end of the filtration, 0.5 ml of saturated MgC03 ship Discoverer for their cooperation and assistance during was added to the samples. Chl a concentra- the cruise. We are grateful to Larry Small, Barry and Ev tions were measured with a Turner Designs 10 Sherr, and two anonymous reviewers for their helpful sug- fluorometer after a 24-h extraction in 90% ac- gestions and comments. etone at -20°C in the dark. Before the cruise, This research was supported by a NASA graduate stu- dent fellowship in global change research to M.-L.D. and the fluorometer was calibrated with a pure Chl a NOAA climate and global change grant (NA 16RC009- a standard purchased from Sigma Chemical 01) to P.A.W. Co. In addition, the fluorometer was also stan- 1814 Notes dardized before each use with a secondary Chl Table 1. Comparison of Chl a retained by combusted a standard, also from Sigma Chemical Co., and and noncombusted Whatman GF/F filters and by com- busted filters at vacuum pressures of 80 and 180 mm of with 90% acetone blanks and extracted filter Hg. Duplicate samples were compared for each treatment. blanks (Whatman GF/F and Nuclepore filters) The overall precision (SD) for these measurements was (Venrick 1987). Integrated Chl a concentra- 0.01, n = 6. tions were calculated with the trapezoid meth- GF/F filters od of integration. The vertical profile of the Depth (noncombusted/ Combusted filters Chl a concentration at 32”N, 152”W was not Cm) combusted) (80/ 180 mm of Hg) included in Fig. 1 because some of the samples 15 0.97 0.92 may have been mixed. However, there was 100 1.10 0.93 no significant effect on the integrated Chl a Mean + SD 1.04+0.09 0.93*0.01 concentrations. The mean precision * 1 SD for duplicate Chl a measurements was 0.025+0.017 for GF/Ffiltersand 0.015*0.004 GF/F filters were lowest in the subtropical wa- for 0.2~pm Nuclepore filters. ters (0.023 pg Chl a liter’) and highest in In September 199 1 during a cruise off north- subarctic waters (0.299 pg Chl a liter-l). A ern California, we used the same procedure deep, subsurface Chl a maximum layer (0.250- outlined above to test if a difference could be 0.300 pg Chl a liter-l) was located between detected in the amount of Chl a measured by 100 and 120 m in the subtropical and transi- combusted and noncombusted Whatman tion waters but was absent in the subarctic GF/F filters. We also checked whether lower water at 48”N. The 0.2~pm Nuclepore filters vacuum pressure (80 mm of Hg) increased the retained 3-4 times more Chl a than the What- amount of Chl a measured. The station se- man GF/F filters in subtropical waters and 2- lected for these tests was 650 km off northern 3 times more Chl a in the transition zone. Chl California (42”N, 132”W) in oligotrophic wa- a concentrations measured with 0.2-pm Nu- ters characterized by low ambient nitrate con- clepore filters were not different from those centrations (- 1 ,uM) and a low phytoplankton measured with Whatman GF/F filters below standing stock (-0.080-0.300 pg Chl a li- 100 m in the subarctic (48”N). However, in the ter- l). This site was chosen for its similarity upper 100 m the difference in the Chl a con- to stations in subtropical waters along the centrations between the two filter types was NOAA transect. significantly different from zero (Student’s Comparison of combusted with noncom- t-test, P < 0.001). The relative picoplankton busted Whatman GF/F filters showed no ap- abundance in the Chl a maximum layer was preciable difference in the Chl a concentrations highest in subtropical water (85% of the total measured for two depths at the oligotrophic Chl a), intermediate in transition water (78% site off northern California (Table 1). Chl a of the total Chl a), and lowest in subarctic concentrations were found to be -7% lower water (35-48% of the total Chl a) (Table 2). when the vacuum pressure was 80 mm of Hg Chl a concentrations integrated from 0 to compared to 180 mm of Hg. Chl a concentra- 175 m (Fig. 2) showed a threefold increase (9- tions measured at 15 and 100 m were com- 26 mg Chl a m-*) between 28 and 48”N when parable to those encountered at the southern end of the NOAA transect. The results of these tests indicate that the low Chl a values for the Table 2. Percent abundance of picoplankton Chl a in Whatman GF/F filters during the NOAA tran- the Chl a maximum layer of subtropical, transition, and subarctic waters. The percentage of Chl a in the 0.2-3-pm sect were not due to increased porosity of the fraction was calculated by [( >0.2 pm - > 3 pm)/>0.2 pm] combusted Whatman GF/F filters, inappro- x 100%. ND-No data. priate vacuum pressures, or a combination of the two. Chl a (kg liter ‘) % Chl a (0.2-3-wrn Vertical profiles of Chl a concentrations at N lat Depth (m) >0.2 I.trn >3 I.rrn fraction) four stations extending from the central North 28” 100 0.244 0.037 84.8 Pacific gyre (28”N), through the transition zone 32” 20 0.211 0.047 77.7 (37”N), and into subarctic waters (42 and 48”N) 37” 100 0.293 ND ND showed very different patterns (Fig. 1). Surface 42” 40 0.350 0.228 34.9 Chl a concentrations measured with Whatman 48” 6 0.382 0.200 47.6 Notes 1815 Chl a (1.19liter-‘) Chl a (pg liter-‘) 0 0.10 0.20 0.30 I I I I i 40 - 160 ISO/-\ / o-o GF-F o--o Nuclepore Chl a (pg liter-‘) Chl a (pg liter-‘) 0.10 0.20 , - 120 I I I 42”N 152” W ;I’ o-o GF-F 160 I @ Nuclepore Fig.