Dynamics of transparent exopolymeric particles (TEP) production by Phaeocystis globosa under N- or P-limitation: a controlling factor of the retention/export balance

The concentration of transparent exopolymeric particles (TEP) was monitored during Phaeocystis globosa blooms that developed in mesocosms under different initial N:P ratios (from N- to P-limited conditions). TEP concentration was measured using the microscopic (TEPmicro, ppm) and the colorimetric (TEPcolor, Xanthan equiv. L−1) methods. TEP concentrations varied from 5 to >75 ppm and from 60 to >1500 μg Xanthan equiv. L−1, and were relatively low until the mesocosms reached nutrient (either N or P) depletion and then increased abruptly. From the TEPmicro versus TEPcolor concentrations comparison and from their relation to chlorophyll a concentrations, two phases for the dynamics of TEP production were identified: (1) production through active release of precursors during the growth phase of P. globosa - defined as TEP1 - and their integration into the TEP pool through coagulation processes; (2) release of large TEP from the mucilaginous matrix of P. globosa colonies subsequent to disruption caused by nutrient depletion - defined as TEP2 - and their direct integration into the TEP pool outside the constraint of coagulation. The formation of a multiorigin TEP pool during P. globosa blooms may have implications for the fate of the blooms, due to difference in TEP bioreactivity according to their source and to difference in timing and intensity of TEP1 versus TEP2 production according to N- or P-depletion. For P. globosa blooms developing under N-limiting conditions, the transition from the first source (i.e. TEP1) to the second one (i.e. TEP2) was a slow and continuous process. In contrast, the P. globosa bloom developing under P-limiting conditions showed the sudden formation of heavy mucous aggregates when P became depleted, that may have been caused by a massive release of TEP2. Our study suggests that the nutrient regime may control the export vs. retention balance during P. globosa blooms, via production of a multiorigin TEP pool.