Ascendency and Seasonal Variations

Baird and Ulanowicz analyzed the seasonal dynamics of the Chesapeake Bay ecosystem. In this system, the level of activity is, not surprisingly, increasing in the spring and summer, and declining in autumn and winter. An analysis of the ascendency shows similar trends. In this case, the result underlines the fact that different levels of activity are acting on an essentially unchanged network. Another measure that was considered in this study, and then reported in several other works, is the fraction of realized ascendency (ascendency/capacity). In this case, they found a more interesting result: the maximum fraction of realized ascendency is in the winter, while the minimum is in the summer. This inverse trend shows that when the system level of activity is on the rise, a greater part of the production is lost (i.e., the system is less efficient), while scarcity of activity is associated with a more efficient management of the resources. This type of analysis can therefore be utilized to explain the seasonal variations in the topology and level of activity of the system.

These findings are confirmed by the analysis of bottlenecks in nutrient dynamics that can be performed using sensitivity analysis: by checking how increasing slightly one particular flow reverberates on ascendency values one can see which flows are likely to be crucial for ecosystem development. Also, the analysis can be performed on several different nutrients, pinpointing which ones are more likely to be the limiting factors.

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