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 the Arabian Sea during the late Southwest and early Northeast Monsoon seasons in 1995. Estimates of Prochlorococcus growth rates were based on diel cell cycle analysis. Accurate determination of the duration of the cell cycle terminal event, e.g., t(S +G2) was not possible for Synechococcus because of its highly variable cell cycle patterns (e.g., imperfect phasing, multiple 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 observations that Synechococcus cell division occurred only during the daytime as well as good agreement between growth rate estimates for Prochlorococcus compared to cell cycle analyses. Prochlorococcus growth rates were typically less than 1 doubling per day, although higher rates occurred in the surface waters at an offshore site (Stn. N7) during the SW Monsoon and at a coastal station (S2) during the NE Monsoon. For Synechococcus, maximum growth rates >2 d(-1) were observed at mesotrophic (nitrate concentration 0.1-3 mu m) onshore stations during both seasons. Synechococcus spp. grew much faster than Prochlorococcus in the upper water column at almost every station during both seasons, but the depth range of its maximum growth rate was shallower and its growth and abundance decreased sharply in deeper waters. In addition, growth rates of Synechococcus increased with nutrient availability whereas Prochlorococcus growth rates did not vary dramatically with nutrients. Although there was no significant difference in Synechococcus growth rates between the late SW and early NE Monsoon seasons, the estimated carbon 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 found only in the most oligotrophic regions, and Prochlorococcus was not a major contributor of primary production for the most part of the Arabian Sea during the SW and NE Monsoons. Overall, Prochlorococcus and Synechococcus were inversely related in terms of their relative contributions to primary production. (C) 1998 Elsevier Science Ltd. All rights reserved.