157-185 Neritic zooplankton succession studied with statistical entropy measures: The role of space and the hierarchical matching of the environment

Resum

Succession in nentIc zooplankton is studied, and the role of space and hierarchic adaptation to the environment assessed. Four stations located at the vertices of a square with sides of 1.5 miles, off Tarragona (Western Mediterranean), at depths between 20 and 50 m, were sampled from October 1984 to October 1985. Hydrographical data were obtained and zooplankton was caught by oblique hauls; copepoda were classified to the species level and the individuals were counted, as well as those belonging to other groups. Two disturbances (perturbations) were identified in March and June, which gave rise to various successional events. The process has been studied by computing entropies, taking into account the space (conditional probabilities),which allowed us to calculate the contribution of space to total entropy, and the mutual information between space and populations. These calculations were performed separately for the copepoda populations and for the rest of zooplanktonic groups. Evidence is given that during a successional episode initiated by a perturbation, relative mutual information between the space and the populations rises abruptly at the beginning and decreases slowly afterw ard s , until the next disturbance, while the maximum total entropy is obtained approximately two months after the beginning of the process. There is a time lag between the maximum of relative mutual information calculated for cope poda and that calculated for the groups of zooplankton. Taken together, this suggests that during succession, plankton structures itself hierarchically, it being possible to identify two steps in each episode: the first one is directed by external energy, the second is directed from the system itself. During each successional episode, the specific composition of the populations of copepoda changes continuously, but at a decreasing rate, while higher order taxonomic groups remain fixed in the first step but change in the second.