Primary production during nutrient-induced blooms at elevated CO<sub>2</sub> concentrations

Abstract

Abstract.A CO 2 enrichment experiment (PeECE III) was carried out in 9 mesocosms in which the seawater carbonate system was manipulated to achieve three different levels of pCO 2 . At the onset of the experimental period, nutrients were added to all mesocosms in order to initiate phytoplankton blooms. Primary production rates were measured by in-vitro incubations based on 14 C-incorporation and oxygen production/consumption. Size fractionated particulate primary production was also determined by 14 C incubation and is discussed in relation to phytoplankton composition. Primary production rates increased in response to nutrient addition and a net autotrophic phase with 14 C-fixation rates up to 4 times higher than initial was observed midway through the 24 days experiment before net community production (NCP) returned to near-zero and 14 C-fixation rates dropped below initial values. No clear heterotrophic phase was observed during the experiment. Based on the 14 C-measurements we found higher cumulative primary production at higher pCO 2 towards the end of the experiment. CO 2 related differences were also found in size fractionated primary production. The most noticeable responses to CO 2 treatments with respect to primary production rates occurred in the second half of the experiment when phytoplankton growth had become nutrient limited, and the phytoplankton community changed from diatom to flagellate dominance. This opens for two alternative hypotheses that the effects are either associated with mineral nutrient limited growth, and/or with a change in phytoplankton species composition. The lack of a clear net heterotrophic phase in the last part of the experiment supports the idea that

Ocean acidification of a coastal Antarctic marine microbial community reveals a critical threshold for CO<sub>2</sub> tolerance in phytoplankton productivity

“…Tolerance of CO 2 levels up to ∼ 1000 µatm has often been observed in natural phytoplankton communities in regions exposed to fluctuating CO 2 levels. In these communities, increasing CO 2 often had no effect on primary productivity (Tortell et al, 2000;Tortell and Morel, 2002;Tortell et al, 2008b;Hopkinson et al, 2010;Tanaka et al, 2013;Sommer et al, 2015;Young et al, 2015;Spilling et al, 2016) or growth (Tortell et al, 2008b;Schulz et al, 2013), although an increase in primary production has been observed in some instances (Riebesell, 2004;Tortell et al, 2008b;Egge et al, 2009;Tortell et al, 2010;Hoppe et al, 2013;Holding et al, 2015). These differing responses may be due to differences in community composition, nutrient supply, or ecological adaptations of the phytoplankton community in the region studied.…”

“…Kim et al, 2006;Hopkinson et al, 2010;Riebesell et al, 2013;Paul et al, 2015;Bach et al, 2016;Bunse et al, 2016). Studies in the Arctic reported increases in phytoplankton primary productivity, growth, and organic matter concentration at CO 2 levels ≥ 800 µatm under nutrient-replete conditions (Bellerby et al, 2008;Egge et al, 2009;Engel et al, 2013;Schulz et al, 2013), whilst the bacterial community was unaffected (Grossart et al, 2006;Allgaier et al, 2008;Paulino et al, 2008;Baragi et al, 2015). These studies also highlight the importance of nutrient availability in the community response to elevated CO 2 , with substantial differences in primary and bacterial productivity, chlorophyll a (Chl a), and elemental stoichiometry observed between nutrient-replete and nutrient-limited conditions Schulz et al, 2013;Sperling et al, 2013;Bach et al, 2016).…”

Dissolved organic matter (DOM) release by phytoplankton in the contemporary and future ocean

“…Riebesell et al speculated that TEP may have played a significant role in the transformation of DOC to POC and the sinking of POC from the upper layer of the mesocosms with high pCO 2 . However, analysis of the TEP concentrations from the same experiment by Egge et al (2009) showed no difference in TEP production between the different pCO 2 treatments. Recently, experiments using mesocosms in Arctic waters showed enhanced production of DOC by phytoplankton grown under elevated CO 2 (Engel et al, 2013).…”

Will ocean acidification affect marine microbes?

“…Increasing photosynthesis with elevated CO 2 is observed for some cyanobacteria (Synechococcus) but not others (Prochlorococcus) , and many eukaryotic phytoplankton species, most notably diatoms, have carbon concentrating mechanisms that diminish almost entirely the sensitivity of photosynthesis to CO 2 variations (Tortell et al, 1997). Bottle incubations and mesocosm experiments with natural plankton communities indicate only a weak sensitivity of primary production to CO 2 , although limited CO 2 fertilization is observed in some cases (Tortell et al, 2008;Egge et al, 2009). There are suggestions that the carbon content of phytoplankton cells may increase under high CO 2 conditions (Riebesell et al, 2007), but any physiological changes appear to be quite subtle, and there is conflicting evidence from different studies on how plankton carbon/nitrogen stoichiometry varies with CO 2 .…”