CAUSES AND CONSEQUENCES OF VARIABILITY IN THE TIMING OF SPRING PHYTOPLANKTON BLOOMS

Established conceptual models of the initiation and progression of spr ing phytoplankton blooms are reconsidered in light of recent observati ons. We use biological simulation modelling as a tool for the analysis of spring plankton blooms in shallow, coastal waters in temperate lat itudes of the North Atlantic. The model shows that interannual variabi lity in the timing of bloom initiation arises from year-to-year differ ences in incident irradiation, as determined by weather (cloudiness). This variability in timing results in some years when the spring bloom occurs in cold water temperatures near 0-degrees-C. Model results sug gest that due to low temperature inhibition of heterotrophic consumpti on, more fresh organic material is delivered to the benthos in these c old-water blooms than when the bloom occurs in waters only 3-degrees-C warmer. Thus we suggest that variable bloom timing can be important t o the trophodynamic fate of bloom products. We suggest that variabilit y in timing of spring phytoplankton blooms in offshore and open ocean waters is also related to weather, through controls on the light field and wind mixing. Our analyses of wind-driven vertical mixing demonstr ate such blooms can begin following the winter period of deep convecti on, and prior to the vernal development of stratification, provided th at wind speed is below a certain, predictable threshold, which we esti mate. In such cases, there may be several spring bloom pulses, each in terrupted by self-shading light limitation or vertical mixing events. Eventually the seasonal thermocline develops and nutrient exhaustion c urtails bloom production. This means that the spring phytoplankton blo om in offshore and open ocean areas may be significantly more producti ve, result in more export production, and be more important to the car bon cycle, than has been previously assumed. Furthermore, these featur es of temperate marine planktonic ecosystems are not only sensitive to annual variations in weather, but also any trends that might result f rom greenhouse warming or other factors that affect the climate system .