Lack of pattern among phytoplankton assemblages. Or, what does the exception to the rule mean?

In Science we cannot say that 'the exception proves the rule'. We have been looking to define patterns in phytoplankton occurrence across trophic spec tra where conspicuous covariations between algae and trophic states have be en reported. We consider quite different phytoplankton communities observed under similar trophic conditions: we illustrate this point by considering five different phytoplankton communities living in five water bodies in the same wetland, along a TP gradient and over a period of 2 years. This syste m showed a remarkable dissimilarity of species representation, implying com munities of uncorrelated species vary considerably over time. Despite the p resence of some characteristic species, communities were not related to a g iven trophic state. However, coarser community attributes, such as clusters of taxonomic classes, appeared to be more useful in identifying patterns a nd assembly rules related to trophic spectra. Some ecological concepts can be related to this lack of pattern, e.g., nonconvergence, trajectories far from equilibrium and assembly rules of communities.