The onset and evolution of a spring bloom on the Scotian Shelf

Abstract

The spring bloom on the Scotian Shelf is examined using a mooring array deployed from 18 March 2002 to 18 April 2002 to provide physical, chemical, and biological measurements with high temporal and vertical resolution. These measurements are complemented by the Atlantic Zone Monitoring Program (AZMP) biweekly occupations of a station near the mooring site (HL2). Results from AZMP sampling and Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) ocean color imagery in early March show that coastal upwelling played an important role in the initiation of the spring bloom near the coast. This was a period of very strong horizontal gradients in surface chlorophyll. The main bloom at HL2 was sustained for most of the mooring period with peak chlorophyll levels reaching 6 mg m‐3. Following the drawdown of nutrients in the upper 20 m, the bloom continued for 9 d and then disappeared at the surface but remained at the depth of the nutricline (30‐50 m). The onset and evolution of the spring bloom on the inner Scotian Shelf is a complex process in which nutrient inventory, vertical mixing, and coastal upwelling play roles of varying importance throughout its lifetime. Mesozooplankton biomass does not change significantly until the very end of the mooring period indicating the grazing by this component of the zooplankton did not have as important a role in the termination of the bloom as the exhaustion of near‐surface nutrients.