Plankton metabolism in surface waters of the tropical and subtropical Pacific Ocean

We measured plankton metabolism and evaluated changes in plankton community structure within the central Pacific Ocean during a trans-equatorial cruise from the South Pacific Subtropical Gyre (SPSG) to the North Pacific Subtropical Gyre (NPSG). Rates of net community production (NCP) and respiration (R) were determined in near-surface ocean waters, based on light-dark bottle measurements of oxygen production and consumption. Gross primary production (GPP) was computed, where GPP = NCP + R. Rates of GPP and R in the oligotrophic SPSG and NPSG were tightly coupled, resulting in NCP that was either zero or slightly negative. These subtropical waters were characterized by very low concentrations of inorganic nutrients (e.g. nitrate+nitrite range: 0.004 to 0.033 μmol l^-1). Within the relatively nutrient-enriched equatorial waters (e.g. nitrate+nitrite concentrations range: 0.67 to 5.7 μmol l^-1), GPP exceeded R by up to 5-fold, resulting in positive NCP. GPP varied ̃16-fold (0.25 to 4.05 μmol O₂ produced l^-1 d^-1) while R varied ̃5-fold (0.31 to 1.56 μmol O₂ consumed l ^-1 d^-1). Picoplankton cell abundances and diagnostic pigment biomarkers were used to determine regional differences in phytoplankton community structure. In the near-equatorial waters, abundances of Synechococcus and photosynthetic picoeukaryotes increased up to 100- and 10-fold, respectively, and diagnostic pigment biomarkers for eukaryotic phytoplankton (including diatoms, pelagophytes, and haptophytes) also increased. These results indicate that variability in nutrient availability constitutes an important regulator of net productivity and plankton community structure in large regions of the open ocean.