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RESEARCH PRODUCT

Are there steady-state phytoplankton assemblages in the field?

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The difficulty in advancing in Ecology is due, in part, to the fact that this science uses a mainly qualitative language instead of a more formal or mathematical one. Therefore, many ecologists' efforts are expended in controversies resulting from the vagueness of ecological concepts, for example: stability, equilibrium, ecosystem, community, and so on. When approaching the study of steady-state phytoplankton assemblage, the different interpretations of these concepts can paralyse fruitful discussion. In the following pages, there is an endeavour to both restrict and precise the meaning of some of the concepts related to this topic and to broaden the range of possibilities of steady-state in the field. It is argued here that, in order to test whether or not there is a steady-state assemblage, first of all a variable or descriptor of such assemblage should be chosen. It is also argued that a steady-state does not necessarily occur as the result of a competition process with a stable equilibrium end. Moreover, we suggest that some other processes and mechanisms could control the assemblage as a steady-state. Examples of steady-state phytoplankton assemblage observed in the field (perturbed and unperturbed situations), but probably not related to a competition system equilibrium, are shown: the alternate dominance of two species (Cryptomonas erosa and Limnothrix redekei) during seven unperturbed consecutive weeks in El Porcal Lake (a gravel pit in Central Spain); the co-dominance of five species (Planktothrix agardhii, Limnothrix redekei, Dictytosphaerium sp., Cyclotella meneghiniana and Cryptomonas erosa) over nine unperturbed weeks in the same lake; the dominance of different species in thirty one fluctuating sites of a wetland (La Safor, Mediterranean Spanish coast) and the persistence of some non-dominant species (Peridinium willei and Planktonema lauterbornii) over more than three weeks in the water column mixing period in Las Madres Lake (Central Spain).