Prochlorococcus and Synechococcus growth rates and contributions to production in the Arabian Sea during the 1995 Southwest and Northeast Monsoons

We investigated the growth rates of Prochlorococcus and Synechococcus spp. and their relative contributions to carbon production at five stations in t he Arabian Sea during the late Southwest and early Northeast Monsoon season s in 1995. Estimates of Prochlorococcus growth rates were based on diel cel l cycle analysis. Accurate determination of the duration of the cell cycle terminal event, e.g., t(S +G2) was not possible for Synechococcus because o f its highly variable cell cycle patterns (e.g., imperfect phasing, multipl e DNA-replication peaks, and dark-arrested division). Consequently, growth and mortality rates of Synechococcus were estimated from diel variations in population abundance. The assumptions of this approach were validated by o bservations that Synechococcus cell division occurred only during the dayti me as well as good agreement between growth rate estimates for Prochlorococ cus compared to cell cycle analyses. Prochlorococcus growth rates were typi cally less than 1 doubling per day, although higher rates occurred in the s urface waters at an offshore site (Stn. N7) during the SW Monsoon and at a coastal station (S2) during the NE Monsoon. For Synechococcus, maximum grow th rates >2 d(-1) were observed at mesotrophic (nitrate concentration 0.1-3 mu m) onshore stations during both seasons. Synechococcus spp. grew much f aster than Prochlorococcus in the upper water column at almost every statio n during both seasons, but the depth range of its maximum growth rate was s hallower and its growth and abundance decreased sharply in deeper waters. I n addition, growth rates of Synechococcus increased with nutrient availabil ity whereas Prochlorococcus growth rates did not vary dramatically with nut rients. Although there was no significant difference in Synechococcus growt h rates between the late SW and early NE Monsoon seasons, the estimated car bon production and relative contribution to primary production were greater during the early NE Monsoon due to the larger biomass of Synechococcus and lower total primary production. Maximum Prochlorococcus production was fou nd only in the most oligotrophic regions, and Prochlorococcus was not a maj or contributor of primary production for the most part of the Arabian Sea d uring the SW and NE Monsoons. Overall, Prochlorococcus and Synechococcus we re inversely related in terms of their relative contributions to primary pr oduction. (C) 1998 Elsevier Science Ltd. All rights reserved.