Phytoplankton pigment in the Gulf of St. Lawrence, Canada, as determined by the Coastal Zone Color Scanner—Part II: multivariate analysis

We report here on a statistical study of physical-biological interactions in the Gulf of St. Lawrence that uses for the first time ocean color images (phytoplankton pigments) and data on runoff and wind. Based on a Monte Carlo test for statistical significance, we extracted four orthogonal (independent) spatial patterns (Empirical Orthogonal Functions, EOF) in pigments that explain 64% of the total variance. We also computed four EOFs from the wind data that explain 90% of the total variance. Based on multiple correlations among these EOFs and runoff anomalies, we derived two modes of physical-biological variability. The first mode is the dual regulation of production by runoff in the Lower St. Lawrence Estuary (LSLE) and the western Gulf and by alongshore wind stress in the northeastern Gulf. The second mode incorporates mesoscale circulation features (eddies, meanders) that respond to low frequency fluctuations in runoff (in the lower estuary) or wind (Gaspé Current, northwestern Gulf, northern Esquiman Channel). The third mode reflects the impact of seasonal wind regimes on pigment levels in the LSLE, the Gaspé Current, and the southwestern Gulf. The fourth mode is dominated by one coccolithophore bloom event in August 1979. The analysis also gives some insight into sources of interannual variability in pigment levels and distributions. The spring bloom does not dominate the seasonal pigment cycle; only the third pigment EOF (5% of the variation) displays a spring peak stronger than the fall peak. The seasonal cycle of pigments is in part linked to that of runoff, driven mostly by year to year differences in the spring freshet. However, year to year differences in the summer wind and runoff regimes also play a role. This suggests that late summer and fall blooms and the physical factors that regulate them deserve more study.