Seasonal variability of chlorophyll a in the Mid-Atlantic Bight

For this manuscript we use a 9-year time series of Sea-viewing Wide Field of view Sensor (SeaWiFS), HF radar, and Webb Glider data to assess the physical forcing of the seasonal and inter-annual variability of the spatial distribution in phytoplankton. Using Empirical Orthogonal Function (EOF) analysis, based on 4-day average chlorophyll composites, we characterized the two major periods of enhanced chlorophyll biomass for the MAB in the fall–winter and the spring. Monthly averaged data showed a recurrent chlorophyll biomass in the fall–winter months, which represented 58% of the annual surface chlorophyll for the MAB. The first EOF mode explained ∼33% of the chlorophyll variance and was associated with the enhanced phytoplankton biomass in the fall–winter found between the 20 and 60 m isobaths. Variability in the magnitude of the enhanced chlorophyll in fall–winter was associated with buoyant plumes and the frequency of storms. The second EOF mode accounted for 8% of the variance and was associated with the spring time enhancements in chlorophyll at the shelf-break/slope (water depths greater than 80 m), which was influenced by factors determining the overall water column stability. Therefore the timing and the inter-annual magnitude of both events are regulated by factors influencing the stability of the water column, which determines the degree that phytoplankton are light-limited. Decadal changes observed in atmospheric forcing and ocean conditions on the MAB have the potential to influence these phytoplankton dynamics.

► The largest annual bloom in the Mid-Atlantic occur in the winter.
► Storm frequency regulates magnitude of the winter bloom.
► Storm frequency determines the overall light limitation of winter bloom.