Marine planktonic ecosystem dynamics in an artificial upwelling area of Japan: Phytoplankton production and biomass fate

Abstract Marine phytoplankton production as well as biomass is globally significant. This study hypothesized that a mild increase in nutrients concentrations (by a factor 1–2 higher) from artificial upwelling in oligotrophic water can alter phytoplankton dynamics (biomass, composition and size), and consequently, phytoplankton-derived particulate organic carbon flux. Primary production, carbon biomass of different planktonic groups, and organic carbon remineralization were examined by incubating seawater collected from two oceanographic stations in the vicinities of Ikitsuki Island, Japan, in 2004 and 2005. Station C, located near to and downstream of an artificial seamount that generates upwelling, had higher nutrient concentrations and biomass (Chl. a and carbon) than Station O-2, which was upstream and far from the seamount. Phytoplankton biomass and primary production rates remained higher during incubation for station C than for station O-2 in both 2004 and 2005. Phytoplankton composition also differed between the two stations. Picophytoplankton contributions to total biomass were smaller at station C than at station O-2 while those of micro- and nanophytoplankton were higher at station C. The results of this study indicate that even a small increase in nutrient availability in oligotrophic waters can alter productivity, biomass, and phytoplankton composition. Additionally, around 4.0% of net primary production was estimated to escape from grazers and remineralization, instead sinking into deep ocean layers in the form of less easily degraded particles. The difference in net primary productivity between Stns. C and O-2 gave an estimation of excess production at Stn. C resulting from upwelled nutrients. Owing to this 1.3 to 1.5 mg more carbon m − 2  d − 1 is likely to be exported at Stn. C compared to Stn. O-2.