Impact of ocean acidification on Arctic phytoplankton blooms and dimethyl sulfide concentration under simulated ice-free and under-ice conditions

Abstract

Abstract. In an experimental assessment of the potential impact of Arctic Ocean acidification on seasonal phytoplankton blooms and associated dimethyl sulfide (DMS) dynamics, we incubated water from Baffin Bay under conditions representing an acidified Arctic Ocean. Using two light regimes simulating under-ice or 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 −1 took place in all experimental bags. Total dimethylsulfoniopropionate (DMSP T ) and DMS concentrations reached 155 and 19 nmol L −1 , respectively. The sharp increase in DMSP T and DMS concentrations coincided with the exhaustion of NO − 3 in most microcosms, suggesting that nutrient stress stimulated DMS(P) synthesis by the diatom community. Under both light regimes, chlorophyll a and DMS concentrations decreased linearly with increasing proton concentration at all pH levels tested. Concentrations of DMSP T also decreased but only under high light and over a smaller pH range (from 8.1 to 7.6). In contrast to nanophytoplankton (2-20 µm), pico-phytoplankton (≤ 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 and taxonomy, and DMSP and DMS net concentrations. These results show that ocean acidification 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.

Resistance of Arctic phytoplankton to ocean acidification and enhanced irradiance

“…In line with our results, Hussherr et al (2017) found C. socialis/gelidus to dominate their experimental assemblages over the full range of the pH treatments (8.1-7.2; achieved by acid and bicarbonate addition) from an experiment initiated at the same location. Our results also agree well with previous laboratory experiments, which have shown that Antarctic Chaetoceros isolates show little to no stimulation by increasing pCO 2 levels (Boelen et al 2011;Trimborn et al 2013;Hoppe et al 2015).…”

“…C. socialis/gelidus was also dominating the [5 lm fraction of the initial and final assemblages of another experiment initiated with waters from within the DCM at the same time as our experiment (Hussherr et al 2017). As C. socialis/ gelidus has been shown to be the most abundant diatom species in the DCM throughout Canadian Arctic waters (Martin et al 2010) and to commonly form blooms in Baffin Bay in summer (Booth et al 2002), we conclude that its dominance can be held representative for in situ dynamics and that it is not an artifact from our experimental set-up.…”

“…Changes in light availability due to sea-ice decline and snow dynamics, as well as ongoing ocean acidification may exert a lesser effect on Arctic primary production than previously thought (see also Hussherr et al 2017). It has been argued that sea-ice retreat and enhanced stratification will increase Arctic primary production through enhanced light availability (Arrigo and van Dijken 2011).…”

“…Our results from this C. socialis/gelidus-dominated assemblage suggest that Arctic phytoplankon can have the capacity to buffer the effects of changing CO 2 concentrations and pH on various levels (stoichiometry, photophysiology, productivity, species composition). Please note that other studies observed negative effects of OA on biomass build-up only at pH levels below 7.6 (Thoisen et al 2015;Hussherr et al 2017), i.e., more extreme than the predicted changes for the next century. The environmental conditions prevailing in the Arctic could promote high physiological plasticity towards varying pCO 2 levels.…”

Dimethylsulfide (DMS) production in polar oceans may be resilient to ocean acidification

“…Svalbard (Archer et al 2013;Riebesell et al 2013) and one shipboard microcosm experiment with seawater collected from Baffin Bay (Hussherr et al 2017). Both studies reported significant reductions in DMS concentrations with increasing levels of pCO 2 during seasonal phytoplankton blooms.…”