Interannual variability of SeaWiFS-derived chlorophyll in the northeast shelf region

B. C. Monger

In an attempt to consolidate the enormous amount of information contained in extended time series of SeaWiFS data in the Northeast Shelf Region, a segmentation process was established to subdivide large areas of ocean into a relatively small number of subregions that exhibit similar phytoplankton dynamics. Chlorophyll concentrations within each subregion were then averaged separately to yield a relatively small number of individual time series.

The amplitude of the seasonal change in sea-surface temperature (SST), derived from AVHRR Pathfinder imagery, was chosen as the principal criteria for segmenting a region because the focus of the current study is on seasonal bloom dynamics that is driven primarily by seasonal change in thermal stratification (Figure 1 and Figure 2).

To validate the appropriateness of the regional partitioning approach, a time-series of the 1998 SeaWiFS-derived chlorophyll concentration was computed for each subregion to demonstrate that the magnitude of spring phytoplankton blooms increase in a consistent manner with increases in the level of seasonal SST amplitude (Figure 3 and Figure 4).

A manageable number of subregions was obtained using two-degree intervals in seasonal SST amplitude with bathymetric differences as used as a secondary criteria (Figure 5 and Figure 6).

Average chlorophyll concentration was determined for regions specified in Fig. 6 using weekly-composite HRPT SeaWiFS date (second global reprocessing). Average SST over the corresponding region was determined from weekly-composite AVHRR-Pathfinder data provided by the NASA/JPL Physical Oceanography DAAC (Figure 7).

It has been shown that partitioning the Northeast Shelf Region based on seasonal SST amplitude is useful for creating subregions that exhibit distinctly different seasonal phytoplankton dynamics. This conclusion is supported by the relatively strong correlation between the strength of the spring bloom, phytoplankton bloom and the strength of the seasonal SST amplitude within corresponding subregions (Figure 4).

Significant differences in the seasonal pattern of chlorophyll concentration were observed within certain subregions of the Gulf of Maine between 1998 and 1999 (Figure 7). In many regions the 1999 spring phytoplankton bloom began earlier and was larger in magnitude than the 1998 bloom. Moreover, in most regions the spring phytoplankton bloom was closely timed with the increase in SST. However, in the Scotian Shelf region the spring phytoplankton bloom preceded the increase in SST.

The Scotian Shelf and Georges Bank regions exhibited the greatest interannual variability in the timing and magnitude of the spring phytoplankton bloom. It is believed that the high level of interannual variability in the phytoplankton dynamics in the Scotian Shelf and Georges Bank regions was caused by water column stabilization brought about by the enhanced intrusion of low salinity water along the Scotian Shelf that occurred in 1999 and the subsequent cross-over of this water onto Georges Bank. Many regions experienced periods when chlorophyll concentration were below 1.0 mg m-3 implying periods where zooplankton may be food limited.


Acknowledgements: This research was supported the National Aeronautics and Space Administration’s Ocean Biology and Biogeochemistry Program through grant NAG5-6273. SeaWiFS data was produced by the SeaWiFS Project and distributed by the Distributed Active Archive Center at NASA’s Goddard Space Flight Center.