Thresholds and points of no return for oceanic phytoplanktoneffects of pollution on phytoplankton communities

Résumé

Since the Industrial Revolution, significant amounts of natural and synthetic compounds have been released to the environment, interacting with the natural biogeochemical cycles. In oceans, these cycles are partly controlled by phytoplankton. Phytoplankton is the primary producer in the ocean, responsible of inorganic carbon fixation and its transformation and incorporation to the ecosystem as organic carbon. The high mortality observed in the last times by phytoplankton implies that the carbon incorporated cannot be transferred either to the trophic chain nor sedimented with the cells to the ocean bottom, highly influencing the carbon fluxes in the ocean. Among the different factors inducing phytoplankton mortality, exposition to toxic pollutants has been observed as one of the factors causing it. Through multiple experiments with metals and Persistent Organic Pollutants (POPs), several pollution thresholds to marine phytoplankton have been determined during this doctoral thesis. Thresholds are the critical values around which the ecosystem flips from one stable state to another, becoming points of no return when crossing again the same value in the opposite direction does not restore the initial state. Thus, the objectives of this doctoral thesis were to quantify those thresholds, to determine the parameters controlling their sensitivity to marine pollution and to analyze the joint action of mixtures of pollutants and the joint action of these pollutants with environmental factors. This doctoral thesis is the result of these experiments and their findings