Impact of ocean acidification on Arctic phytoplankton blooms and dimethylsulfide production under simulated ice-free and under-ice conditions

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Hussherr, Rachel ; Levasseur, Maurice ; Lizotte, Martine ; Tremblay, Jean-Éric ; Mol, Jacoba ; Thomas, Hemuth ; Gosselin, Michel ; Starr, Michel ; Miller, Lisa A. ; Jarniková, Tereza ; Schuback, Nina ; Mucci, Alfonso (2016)

In an experimental assessment of the potential impact of Arctic Ocean acidification on seasonal phytoplankton blooms and associated dimethylsulfide (DMS) dynamics, we incubated water from Baffin Bay under conditions representing an acidified Arctic Ocean. Using two light regimes simulating under-ice/ subsurface chlorophyll maxima (low light; Low PAR and no UVB) and ice-free (high light; High PAR + UVA + UVB) conditions, water collected at 38 m was exposed over 9 days to 6 levels of decreasing pH from 8.1 to 7.2. A phytoplankton bloom dominated by the centric diatoms Chaetoceros spp. reaching up to 7.5 µg chlorophyll a L<sup>&minus;1</sup> took place in all experimental bags. Total dimethylsulfoniopropionate (DMSPT) and DMS concentrations reached 155 nmol L<sup>&minus;1</sup> and 19 nmol L<sup>&minus;1</sup>, respectively. Under both light regimes, chlorophyll a and DMS concentrations decreased linearly with increasing proton concentration at all pH tested. Concentrations of DMSPT also decreased but only under high light and over a smaller pH range (from 8.1 to 7.6). In contrast to nanophytoplankton (2&ndash;20 µm), picophytoplankton (≤ 2 µm) was stimulated by the decreasing pH. We furthermore observed no significant difference between the two light regimes tested in term of chlorophyll a, phytoplankton abundance/ taxonomy, and DMSP/ DMS net concentrations. These results show that OA could significantly decrease the algal biomass and inhibit DMS production during the seasonal phytoplankton bloom in the Arctic, with possible consequences for the regional climate.
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