Resolving variations in the timing and intensity of the spring bloom in the central North Sea during the 1990s: a comparison of remote sensing and 2-D modelling approaches

Key to understanding the dynamics of the North Sea ecosystem is resolving the timing, intensity, and duration of spring and fall phytoplankton blooms. In order to do so, we apply a potential energy model of thermal stratification based on the energy equation (for turbulence) coupled to a biochemical model Sharpies and Tett (1994). Using this approach we examine the timing and intensity of the spring and fall phytoplankton blooms for the years 1981, 1982, and 1986 as well as 1997 to 2000. These estimates are then compared with temporal estimates of and biomass from CZCS (1981, 1982, and 1986) and SeaWiFS (1997 to 2000). A relatively high degree of similarity was observed between the modelled and observed timing of the spring bloom, with the model prediction earlier by 6.1 days (s.d. ±8.3). Predictions of the timing of the fall bloom from the model were also earlier than observed by 9.4 days (s.d. ± 15.4). Comparisons of biomass from the two techniques resulted in a ratio of 3.0 and 1.2 (Model vs. CZCS) and 6.4 and 2.7 (Model vs. SeaWiFS) for the spring and fall blooms, respectively. A number of inherent weaknesses are suggested for the lack in coherence between the two approaches. For the modelling approach, these include species-specific variations in nutrient uptake and growth rate, error in biomass esti­mates of phytoplankton seed populations, and lack of temperature effects on phy­toplankton growth. Remotely sensed estimates suffer from a lack of depth penetration (thus missing subsurface biomass), cloud cover, and the patchy distribution of blooms. It is suggested that high resolution 3-D modelling may better address the dynamics of phytoplankton blooms in the North Sea, while 2-D models may be more appropriate for the development of indices for examining the effects of climate change due to their lack of dependence on boundary conditions.

Article from Marine Science Symposia Vol. 219 - "Hydrobiological variability in the ICES Area, 1990-1999", symposium held in Edinburgh, 8-10 August 2001. To access  the remaining articles please click on the keyword "MSS Vol. 219".