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 phosph orus (PP) in the euphotic zone (EZ) at Sta. ALOHA (22 degrees 45'N, 158 deg reesW) in the North Pacific Subtropical Gyre (NPSG) reveal a two-layered sy stem with distinct upper (0-75 m) and lower (75-175 m) EZ dynamics. Particu late matter mean concentrations in the upper EZ were relatively constant wi th depth, and those in the lower EZ decreased significantly with increasing water depth. The vertical partitioning of particulate matter was approxima tely 60% in the upper EZ and 40% in the lower EZ. Significant temporal vari ability, both seasonal and interannual, was observed within both regions. P C and PN inventories in the upper EZ displayed a distinct annual cycle with variable interannual amplitude. The annual cycle was characterized by PC a nd PN maxima in summer and fall, and minima in winter. PP exhibited a small er variation with season but also had a distinct wintertime minimum. These variations in particulate matter concentrations were accompanied by seasona l changes in elemental composition; summer and fall conditions were charact erized by high C:P and N:P ratios exceeding 140:1 and 20:1, respectively. I t is hypothesized that these concentration and composition patterns result from a net seasonal accumulation of non-living particulate matter throughou t the summer and fall periods, and a rapid export during transition to wint er conditions. Data also suggest that PN inventories in the NPSG have incre ased during the past three decades in response to changes in habitat, commu nity 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, (C) 2001 Elsev ier Science Ltd. All rights reserved.