Distribution and Elemental Composition of Picoplankton in the North Pacific Ocean
Marine picoplankton account for a considerable portion of primary production in the Ocean, particularly in the oligotrophic regions. Picoplankton are dominated by three major groups: Prochlorococcus, Synechococcus and picoeukaryotes, whose populations are controlled by many complex interacting factors. We analyze a rich dataset from a cruise in the North Pacific Ocean to model the distribution and elemental composition of each picoplankton group using only environmental data as predictors. Linear regression, generalized additive models, and random forests were used to make models of phytoplankton abundances and carbon biomasses. Elemental composition for each phytoplankton group was modeled using Bayesian linear regression by regressing elemental C, N, and P concentrations on picoplankton biovolumes. Our species distribution models show temperature and salinity are consistently the most important predictors to explain variation in abundance and biomass. Along the full transect, nutrient concentrations (PO4, Fe, Mn, Cu) provide useful insights on sharp population shifts over short distances and our results support the claim that iron and phosphorus are limiting nutrients in the North Pacific Ocean’s oligotrophic gyre. Our elemental quota model show that the composition and cellular C:P, N:P and C:N ratios varies substantially among the three picoplankton groups. Average carbon content for Prochlorococcus, Synechococcus and picoeukaryotes were 167 fg C/um (95% CR: 6:89-457:8), 538 fg C/um (95% CR: 307:2- 771:5), and 297 fg C/um ( CR: 13:74-804:6) respectively. This model provides a method for estimating individual elemental content of each phytoplankton group, which is otherwise unmeasurable directly from field samples.