Measurements of biodiversity

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When considering the biodiversity of an aquatic system after a human caused disaster (chemical spill). What would be a adequate measure of biodiversity. In the past I have used Margelef’s index to study species richness Dmg = (S-1)/ln N. This however does not take into account influences or stresses placed upon the population / ecosystem or the reducing of the population due to death and or emigration. Is there such a measurement that can predict future loss or is a before and after study design needed to test impacts?