Seasonal and interannual variability of phytoplankton biomass and productivity in the sub-arctic Pacific using multi-sensor satellite data

摘要

This study documents the results of a multi-sensor satellite investigation aimed at comparing the seasonality and interannual variability of phytoplankton biomass and primary productivity (PP) in the western and eastern sub-arctic Pacific. Remotely sensed data from multi-sensors, including ocean color (OCTS and SeaWiFS), sea surface temperature (SST, AVHRR), wind (SSM/I) and photosynthetically available radiation (PAR, SeaWiFS) datasets were utilized for the purpose of this study. Calculation was made for primary productivity using VGPM Model (Behrenfeld and Falkowski, 1997), and sea surface nitrate using SST and chl-a data (Goes et al,. 1999,2000). Satellite data helped discern several features, most importantly the existence of significant east-west gradients in the supply of nitrate in winter, in the consumption of nitrate by phytoplankton and in phytoplankton production and biomass accumulation over the growth season. In the western sub-arctic gyre many of these features appear to be regulated by the strength of sea surface winds through increased iron and nitrate inputs. Multiple regression analysis of data extracted from 12 boxes spanning different hydrographic regimes in the sub-arctic Pacific, showed that over 65% of the variations in PP in the sub-arctic Pacific could be explained solely on the basis of changes in the strength of sea surface winds and the intensity of incident irradiance (PAR). The dependence of PP on sea surface wind stress was far greater in the western sub-arctic Pacific Gyre (WSG), than in the Alaskan Gyre (ALG) due to diminishing impact of surface winds towards the east. Spring accumulation of phytoplankton biomass was greater in the WSG than in the ALG despite the higher rates of PP in the latter. This study assumes particular significance because it helps ascertain the existence of several sub-regions within the two broader domains of the WSG and the ALG. In addition, large interannual variations in phytoplankton biomass and PP were observed in the sub-arctic Pacific following the onset of the El-Nino event of 1997 and the transition to La-Nina conditions in 1999. These variations were largely the result of differences in meteorological and oceanographic conditions across the sub-arctic Pacific following the development of the El-Nino.