Introduction As part of a study investigating phytoplankton diversity and physiology in the Western Pacific Warm Pool, we measured group-specific rates of picoplankton growth and protozoan grazing. The Western Pacific Warm Pool is of interest not only in terms of global carbon cycling due its large area, but also because waters are highly stratified with extreme surface temperatures. Photosynthetic picoplankton dominate this region and are expected to be uniquely adapted to such conditions. Towards the overall goal of understanding factors shaping both the magnitude and diversity of these populations, depth- specific growth and mortality rates were estimated at stations along a transect from Hawaii to Australia in January-February of During this time, sea surface temperatures ranged from °C across the study area. Picoplankton community dynamics across the Western Pacific Warm Pool (#1027) Brown, S.L., K.E. Selph, R.R. Bidigare – SOEST, University of Hawaii at Manoa Materials and Methods Two Bottle Dilution Method Growth and grazing rate estimates were derived from modified seawater dilution experiments coupled with microscopy, HPLC and flow cytometry (FCM). Changes in diagnostic pigments are assumed to represent changes in corresponding taxa. Experiments were incubated in on-deck incubators at light levels 73, 50, 25, 18, 6, 1% surface PAR, corresponding to sampling depths. Pigment-based rates have not been corrected for potential photo-adaptation effects. Rates were estimated by solving two equations for two unknowns. Pigment concentrations were determined by High-Pressure Liquid Chromatography (HPLC). DVa = Divinyl Chlorophyll a (Prochlorococcus spp.) ZEAX = Zeaxanthin (Synechococcus spp.) Numerical abundances were determined by flow cytometry (FCM). PRO = Prochlorococcus spp. (cells/ml) SYN = Synechococcus spp. (cells/ml) net rate = k = 1/t*ln (Pt/Po) k = growth (μ) – mortality due to grazing (m) k’ = μ - Dm D = 0.34 m = (k’- k)/(1-D) μ = k + m Results FSW WSW FSW WSW + nutrients x 2 D = 0.34 Station Locations of Dilution Experiments Sea Surface Temperature Station 7: 7ºN T = 28.4 ºC surface [N + N] = ~ 0.01 surface [P04] ~ 0.11 Estimates demonstrate excellent agreement between cell (Pro) and pigment (Dva) based growth rates (left figure) and mortality rates due to grazing (middle figure). Growth rates increased substantially in response to added ammonia -- indicating N limitation. Both growth and grazing rates were highest at the 25% light level. Averaged pigment and cell-based rates (right figure) show balanced growth and grazing at all depths except the 25% light level, where grazing exceeded growth. However, nutrient amended growth balanced grazing. Syn cell abundance and zeaxanthin also showed good agreement in terms of rate estimates (left and middle figure). Rates were highest at the 25% light level and growth rates responded strongly to the addition of ammonia. Averaged estimates of cell and pigment-based rates showed a similar trend of balanced growth and grazing with the exception of the 25% light level, where only nutrient amended growth matched grazing (right figure). Station 10: 0 º EqT = 29.1 ºC surface [N + N] = ~ 1.4 surface [P04] ~ 0.32 Station 14: 9 º S, Warm Pool T = 30.6 ºCsurface [N + N] = ~ 0.10surface [P04] ~ 0.17 Preliminary Conclusions An initial analysis of dilution experiments showed generally good agreement between pigment and cell-based rates. Further analyses are necessary to account for the effect of photo-adaptation of cells on pigment-based rates, as determined by changes in flow cytometric fluorescence per cell. Nevertheless, several patterns emerge with regards to growth and grazing. In low nutrient waters at Station 7, both picoplankton populations appeared to be severely nutrient limited, allowing grazing to exceed growth at the 25% light level. Growth and mortality rates for both picoplankton populations were similar. In the Warm Pool and at the Equator, N+N concentrations were 1 and 2 orders of magnitude higher respectively, and neither population appeared to be nutrient-limited. Prochlorococcus growth rates were higher than Synechococcus at the Equator, and substantially higher in the Warm Pool, which points to the possibility of temperature as a controlling factor in shaping picoplankton populations. Synechococcus growth was tightly coupled to protozoan grazing, whereas Prochlorococcus spp. growth exceeded grazing in the Warm Pool, consistent with its numerical dominance in this region. Analyses of additional stations in varying temperature and nutrient regimes will help to further distinguish the interplay between temperature, nutrients and grazing in shaping picoplankton populations. Replicate dilution experiments were conducted back to back over a 48 hour period at Station 14 in the Warm Pool. Ammonia (0.5 µM), phosphate (0.03 µM) and iron (1.0 nM) were added to assess nutrient limited growth. Figures represent the averages of replicate experiments, replicate treatments, and averaged cell-based and pigment-based rates. Prochlorococcus spp. growth appeared to be inhibited at the surface, possibly due to high light. At the 50% light level, growth rates reached 1 doubling per day and growth exceeded grazing throughout most of the water column. Growth rates did not appear to be nutrient-limited. Synechococcous spp. growth was relatively uniform throughout the water column, tightly coupled to grazing rates, and did not appear to be nutrient-limited. Growth rates were substantially lower than that of Prochlorococcus spp. no nutrients Both pigment and cell-based rates for Prochlorococcus spp. showed higher growth rates at the surface, decreasing with depth. Cell-based rates of both growth and grazing were higher at the surface than pigment-based rates, possible due to photo-bleaching of pigments. Populations did not show a positive response to ammonia additions. Averaged rate estimates point to slightly higher growth than mortality for Prochlorococcus populations. Synechococcus spp. growth and mortality rates were notably lower than that of Prochlorococcus. Similarly, pigment-based rates at the surface were lower than cell-based rates and populations did not respond in response to ammonia additions. On average, Synechococcus spp. growth and mortality due to grazing were tightly coupled.