CO<sub>2</sub> perturbation experiments: similarities and differences between dissolved inorganic carbon and total alkalinity manipulations

Abstract

Abstract. Increasing atmospheric carbon dioxide (CO 2 ) through human activities and invasion of anthropogenic CO 2 into the surface ocean alters the seawater carbonate chemistry, increasing CO 2 and bicarbonate (HCO − 3 ) at the expense of carbonate ion (CO 2− 3 ) concentrations. This redistribution in the dissolved inorganic carbon (DIC) pool decreases pH and carbonate saturation state ( ). Several components of the carbonate system are considered potential key variables influencing for instance calcium carbonate precipitation in marine calcifiers such as coccolithophores, foraminifera, corals, mollusks and echinoderms. Unravelling the sensitivities of marine organisms and ecosystems to CO 2 induced ocean acidification (OA) requires well-controlled experimental setups and accurate carbonate system manipulations. Here we describe and analyse the chemical changes involved in the two basic approaches for carbonate chemistry manipulation, i.e. changing DIC at constant total alkalinity (TA) and changing TA at constant DIC. Furthermore, we briefly introduce several methods to experimentally manipulate DIC and TA. Finally, we examine responses obtained with both approaches using published results for the coccolithophore Emiliania huxleyi. We conclude that under most experimental conditions in the context of ocean acidification DIC and TA manipulations yield similar changes in all parameters of the carbonate system, which implies direct comparability of data obtained with the two basic approaches for CO 2 perturbation.

Technical Note: Controlled experimental aquarium system for multi-stressor investigation of carbonate chemistry, oxygen saturation, and temperature

“…Equilibration by bubbling can be quite slow depending on the volume of seawater needed (Schulz et al, 2009). Table 2.…”

“…One published system bubbles a custom gas mixture while monitoring p(CO 2 ) (Fangue et al, 2010); yet another uses acid additions to create constant pH seawater as determined by spectrophotometric measurements (McGraw et al, 2010). Some discussion has surrounded the various accepted methods of manipulation, and the differences in carbonate chemistry have been evaluated and found to be small (Gattuso and Lavigne, 2009;Schulz et al, 2009). Nevertheless, bubbling is often recommended as the "first choice" because it "exactly mimics carbonate chemistry changes occurring in the years to come" .…”

Effects of elevated CO<sub>2</sub> and temperature on phytoplankton community biomass, species composition and photosynthesis during an experimentally induced autumn bloom in the western English Channel

“…Following 48 h acclimation in batch culture, semi-continuous daily dilution rates were maintained at between 10 and 13 % of the incubation bottle volume throughout the experiment. CO 2 -enriched seawater was added to the high-CO 2 treatment replicates every 24 h, acclimating the natural phytoplankton population to increments of elevated pCO 2 from ambient to ∼ 800 µatm over 8 days followed by maintenance at ∼ 800 µatm as per the method described by Schulz et al (2009). Adding CO 2 -enriched seawater is the preferred protocol, since some phytoplankton species are inhibited by the mechanical effects of direct bubbling (Riebesell et al, 2010;Shi et al, 2009), which causes a reduction in growth rates and the formation of aggregates (Love et al, 2016).…”

Ocean acidification alters the response of intertidal snails to a key sea star predator

“…In the second step of the pH adjustment employed to create a series of discrete treatment levels, smaller volumes were extracted from the source tank for use in individual 10 l containers that held snails during experimentation. To achieve a particular pH level in a given snail container, these smaller volumes were manipulated by equimolar addition of 1 M sodium bicarbonate (NaHCO 3 ) and 1 M hydrochloric acid (HCl), which increases DIC without changing alkalinity, identical to changes induced by direct entry of CO 2 into seawater [31][32][33]. The precise amounts of NaHCO 3 and HCl required was determined using the carbonate system software CO2SYS [34].…”