Abstract
In order to understand how ocean acidification (OA) and enhanced irradiance levels might alter phytoplankton eco-physiology, productivity and species composition, we conducted an incubation experiment with a natural plankton assemblage from subsurface Subarctic waters (Davis Strait, 63°N). The phytoplankton assemblage was exposed to 380 and 1000 µatm pCO2 at both 15% and 35% surface irradiance over two weeks. The incubations were monitored and characterized in terms of their photo-physiology, biomass stoichiometry, primary production and dominant phytoplankton species. We found that the phytoplankton assemblage exhibited pronounced high-light stress in the first days of the experiment (20-30% reduction in photosynthetic efficiency, Fv/Fm). This stress signal was more pronounced under OA and high light, indicating interactive effects of these environmental variables. Primary production in the high light treatments was reduced by 20% under OA compared to ambient pCO2 levels. Over the course of the experiment, the assemblage fully acclimated to the applied treatments, achieving similar bulk characteristics (e.g. net primary production and elemental stoichiometry) under all conditions. We did, however, observe a pCO2-dependent shift in the dominant diatom species, with Pseudonitzschia sp. dominating under low and Fragilariopsis sp. under high pCO2 levels. Our results indicate an unexpectedly high level of resilience of Subarctic phytoplankton to OA and enhanced irradiance levels. The co-occurring shift in dominant species suggests functional redundancy to be an important, but so-far largely overlooked mechanism for resilience towards climate change.
Highlights
Climate change imposes strong alterations in the environmental conditions of marine ecosystems (Pörtner et al, 2014)
Sampling was conducted at the bottom of the chlorophyll a (Chla) maximum at 50–55m depth, where the water temperature was −1.5◦C, salinity was 32.44 and seawater pCO2 was slightly below atmospheric values (Table SI1)
Such high physiological plasticity toward varying irradiance and pCO2 levels could be caused by the fact that the initial assemblage was taken from a coastal system
Summary
Climate change imposes strong alterations in the environmental conditions of marine ecosystems (Pörtner et al, 2014). Beneficial effects of OA have been shown to be more pronounced under low light levels (Kranz et al, 2010; Rokitta and Rost, 2012), while OA appears to inhibit phytoplankton growth under high or variable light (Gao et al, 2012; McCarthy et al, 2012; Jin et al, 2013). This latter result was attributed to an enhanced susceptibility toward high-light stress under OA (Hoppe et al, 2015)
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