Phytoplankton phenology on the Scotian Shelf

The impact of physical forcing on seasonal and interannual phytoplankton dynamics was examined using SeaWiFS chlorophyll, AVHRR sea surface temperature (SST), nitrate, and other hydrographic measurements for the Scotian Shelf and Scotian Slope. The spring bloom was characterized by a shifted Gaussian function fitted to seasonal chlorophyll time-series. The background chlorophyll (a constant term in the Gaussian function) is a joint property of the stratification and bio-optics of the mixed layer. Rapid shoaling of the mixed-layer depth in spring promoted the early spring bloom on the middle Scotian Shelf and Slope, triggered when averaged light in the mixed layer reached 15 W m(-2). The duration of the spring bloom was prolonged in slope water, resulting in a discontinuity in duration between the shelf and slope water masses. The position of the latitudinal discontinuity in duration was correlated with that of the shelf-slope front in SST. The amplitude of the spring bloom was correlated with the nitrate inventory in the surface layer at the end of winter. The rate of decrease (increase) in chlorophyll after the spring bloom was related to the depletion (resupply) of nitrate in summer. The position of the shelf-slope front influenced the interannual variability of bloom characteristics.