Seasonal, interannual and decadal variations in particulate matter concentrations and composition in the subtropical North Pacific Ocean

The mean distributions of particulate carbon (PC), nitrogen (PN) and phosphorus (PP) in the euphotic zone (EZ) at Sta. ALOHA (22°45′N, 158°W) in the North Pacific Subtropical Gyre (NPSG) reveal a two-layered system with distinct upper (0–75 m) and lower (75–175 m) EZ dynamics. Particulate matter mean concentrations in the upper EZ were relatively constant with depth, and those in the lower EZ decreased significantly with increasing water depth. The vertical partitioning of particulate matter was approximately 60% in the upper EZ and 40% in the lower EZ. Significant temporal variability, both seasonal and interannual, was observed within both regions. PC and PN inventories in the upper EZ displayed a distinct annual cycle with variable interannual amplitude. The annual cycle was characterized by PC and PN maxima in summer and fall, and minima in winter. PP exhibited a smaller variation with season but also had a distinct wintertime minimum. These variations in particulate matter concentrations were accompanied by seasonal changes in elemental composition; summer and fall conditions were characterized by high C : P and N : P ratios exceeding 140 : 1 and 20 : 1, respectively. It is hypothesized that these concentration and composition patterns result from a net seasonal accumulation of non-living particulate matter throughout the summer and fall periods, and a rapid export during transition to winter conditions. Data also suggest that PN inventories in the NPSG have increased during the past three decades in response to changes in habitat, community composition or both. These temporally decoupled seasonal, interannual and decadal-scale ecological processes will complicate attempts to achieve mass balance or to derive mechanistic biogeochemical models.