Thyssen M, Ferreyra G, Moreau S, Schloss I, Denis M, Demers S
Flow cytometry, global warming, mesocosm, phytoplankton dynamics, temperature, Ultraviolet radiation, UVB, 2 m3, Rimouski, Quebec, Canada
Temperature and ultraviolet radiation B (UVB) are expected to increase in the next few decades and will mostlyaffect mid and high latitudes. In order to study the combined effect of temperature and UVB increase, on thephytoplankton community in the Saint Lawrence Estuary, duplicates of four different treatmentswere applied to2m3 mesocosms to simulate an overall 3 °C and a 77.8% UVB increase, and combined. Samples were collectedevery 6 h over 10 days and the phytoplankton community was then analysed using a conventional flowcytometer and a Cytosense flow cytometer. Flow cytometry distinguished 9 clusters (Pico, Nano I, C3, C4,CHAINS, C6, C7 and C8) of cells sharing similar optical properties with average sizes varying from 1.3 ?m up to101 ?m for chain forming cells. Compared to untreated enclosures, the high UVB treatment induced lower cellabundances (up to ?40%) for clusters Pico, Nano I, C4, CHAINS and C7, followed by an unexpected cellabundance increase in all the clusters during the last 3 days of the experiment (up to 46%). This increase wassustained by faster calculated periodicities of the cell optical characteristics and abundances, linked to a shortercell cycle. In the samples fromthe high temperature treatmentmesocosms, a positive delaywas observed for thecell abundance increase in clusters CHAINS, C6 and C8, combined with higher average abundance values (up to67% with respect to untreated mesocosms). During the last 3 days of the experiment, abundances decreasedcompared to the values observed in the high UVB mesocosms, with a slower trend in the periodicities,suggesting that high temperature inhibits the cell cycle. The combined temperature and UVB treatmentemphasized the effects observed under high temperature treatments, maintaining temperature positive effects(i.e. higher abundances) on clusters C3, CHAINS, C6 and C8 suggesting a compensation from the positivetemperature effects over the negative (i.e. lower abundances) UVB effects. Increasing temperature induced anegative effect on the abundance of clusters C4 and C7. In this case, trends in C6 and C8 cell periodicities werefaster than under normal conditions while Nano I, C4 and C7 cycles were slower. Cellsb3 ?m were negativelyaffected by the combined exposure (up to ?55% compared to untreated mesocosms) while most of the largercells were positively affected (up to 75% compared to untreated mesocosms), suggesting a shift to anherbivorous food web (sensu Legendre and Rassoulzadegan, 1995). Results suggest that changes in cell cyclesdue to increases in temperature or UVB exposure may play an important role in controlling abundance.