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

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 m23. 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. Phytoplankton biomass increases substantially every spring in the subpolar North Atlantic and contributes significantly to the total annual primary production. The magnitude, timing, duration, and spatial extent of these blooms vary interannually. The effects of this variability extend beyond the bloom period since they influence the population dynamics of higher trophic levels (Head et al. 2000; Platt et al. 2003). A relationship between physical forcing and biological response was initially described by Gran and Braarud (1935). In the critical-depth theory of Sverdrup (1953), the onset of the spring bloom was controlled by the depth of the mixed layer and available light. However, Townsend et al. (1992) confirmed that phytoplankton blooms can occur in the absence of vertical stratification. Ongoing research is attempting to understand more fully the causes of interannual variability of the spring bloom in the open ocean and in coastal sea environments. Waniek (2003) has investigated the role of physical forcing in initiation of northeast Atlantic spring blooms using a coupled biological–physical model with data collected at the Biotrans site (47uN, 20uW). The results indicate that, in the open ocean, springtime shallowing of the mixed layer is not typically a smooth transition since the process is interrupted by the passage of weather systems. Interannual variability in mixed layer development is determined by changes in the frequency, intensity, and track of storms. Computer simulations suggest that such synoptic events are important for phytoplankton growth. Enhanced storm frequency in the late winter and early spring period prior to the spring bloom increases the nutrient inventory at shallow depths and leads to an increase in amplitude and duration of the bloom. Ueyama and Monger (2005) examined the role of wind forcing in the North Atlantic using Sea-viewing Wide Field-of-view Sensor (SeaWiFS) surface chlorophyll imagery and satellite-inferred winds for the period of 1998 through 2004. They concluded that wind-induced mixing during the spring bloom period was the key forcing contributing to interannual variability in the bloom timing, intensity, and duration. Strong mixing in subpolar regions during the bloom period results in deeper mixed layers that limit light and lead to reduced blooms (Dutkiewicz at al. 2001). Moderate mixing, on the other hand, may extend the period of the bloom by providing additional nutrients without compromising light conditions favorable for growth. Tidal mixing may also be important in controlling the biophysical coupling in temperate shelf seas (Sharples et al. 2006). However, on the inner Scotian Shelf tidal mixing does not appear to be as significant a factor as wind-driven vertical mixing and coastal upwelling (Greenan et al. 2004). In spring, when stratification is weak, coastal upwelling can be important in determining the initiation of the bloom and can offset the loss of phytoplankton through sinking (Huisman et al. 2002). High-frequency sampling of physical, chemical, and biological variables is needed to improve our understanding of spring bloom dynamics. In an attempt to address this, a field program was carried out during the spring bloom at a fixed station monitoring site on the inner Scotian Shelf in 2002. The circulation in this area is 1 Corresponding author ([email protected]).