An investigation of dominance in phytoplankton using the PROTECH model

1. Using a model (PROTECH-C) that simulates the simultaneous daily growth o f eight phytoplankton species, the following hypotheses were tested: (i) fo r each given set of simulated conditions, the species with the most appropr iate trait, as predicted by a functional group classification, should domin ate the community; (ii) with removal of this dominant species, the next bes t-adapted species should dominate and should be from the same, or a close, functional group where available; (iii) a reduction in the inoculum size of the initially dominant species will not prevent its eventual dominance of the community. 2. For clearer insight into the mechanisms underlying these community proce sses, a functional group classification based upon species morphology has b een used to produce a matrix analogous to Grime's CSR (C, competitor; S, st ress tolerator; R, ruderal) paradigm. The effects upon this phytoplankton c ommunity of temperature, grazing, limiting light and nutrients over a simul ated year were recorded. 3. The results supported all three hypotheses. It was found that, for a giv en selective constraint, functional traits provided excellent predictors of the dominant types. Also, under conditions of resource competition, the nu mber of functional groups represented decreased. Competition was greatest w ithin functional groups where niche overlap was high, but one species was a lways clearly the strongest competitor, i.e. its superiority over its neare st functional competitor was regularly expressed even when the difference i n inoculum size was great (1000-fold). These conclusions emphasized the pow er that trait selection can have in the shaping of communities.