Changes in ph at the exterior surface of plankton with ocean acidification

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1 Correction notice Nature Climate Change, (). Changes in ph at the exterior surface of plankton with ocean acidification Kevin J. Flynn, Jerry C. Blackford, Mark E. Baird, John A. Raven, Darren R. Clark, John Beardall, Colin Brownlee, Heiner Fabian & Glen L. Wheeler In the version of this file originally published, the unit on the right-hand y axis of Supplementary Figures Sb, Sb, S7b, S9b and Sb was incorrect. This error has now been corrected in this file 7 September. Macmillan Publishers Limited. All rights reserved.

2 SUPPLEMENTARY INFORMATION DOI:.8/NCLIMATE489 Changes in ph at the exterior surface of plankton with ocean acidification Supplemental Methods The simulations presented here were achieved by combining a description of dissolved inorganic carbon (DIC) chemistry 7, using Ocean Carbon-Cycle Model Intercomparison Project (OCMIP ) recommended constants, together with mechanistic submodels describing phytoplankton and microzooplankton 8,,. The model accounted for changes in total alkalinity, DIC, phosphate, nitrate, ammonium and silicic acid concentrations (factors affecting ph and DIC equilibria) over a vertically-mixed surface layer with gas exchange with the atmosphere at the sea surface, and mixing with water from below the mixed layer. Depthintegrated photosynthesis and grazing were simulated over this mixed layer. Near-cell concentrations of nutrients, including of DIC and alkalinity, were calculated from modifications of established descriptions 7, as a balance of diffusion between the bulk water and cell surface, against transport rates in or out of organisms, of metabolites and. While there have been other modelling investigations of the proximal changes in carbonate chemistry with growth of phytoplankton 9 and foraminifera 4, here we place simulations in a dynamic physical and ecological setting. For simplicity plankton (as individuals or as aggregates) were considered as spherical, and for the phytoplankton as non-motile and with neutral buoyancy (i.e. no motion relative to the immediate surrounding water). plankton were considered as motile (see Supplementary Table ); changes in diffusion due to boundary layer thinning was calculated 6. These calculations did not account for additional impacts of temperature and water pressure. We assumed no physiological effect of external [ ] upon organism growth. It has been assumed 9 that the rate of diffusion greatly exceeds that of the chemical equilibrium between forms of DIC, assuming no carbonic anhydrase (CA) activity. In contrast, in oceanographic modelling it has been assumed that external CA activity is so NATURE CLIMATE CHANGE Macmillan Publishers Limited. All rights reserved.

3 ubiquitous that gas exchange occurs at the sea surface with effectively instantaneous equilibrium between forms of DIC, 6, 7. Given that external CA is (at least effectively) ubiquitous amongst plankton 8, see also 8-4, we have assumed equilibrium within the integration time step of min. Making this assumption actually yields results not dissimilar to those obtained assuming no CA activity 9, with the smallest cells effectively experiencing bulk water conditions. Our model assumes carbon assimilation by phytoplankton occurs via CO uptake in the presence of an external CA, although the results are equally applicable if bicarbonate uptake occurs provided intracellular acid-base regulation processes make the extracellular changes in the inorganic carbon system equivalent for bicarbonate entry and CO entry. Assuming the external CA is indeed able to maintain the CO -HCO - equilibrium, in both instances a significant drawdown of CO at the cell surface is not anticipated. However, for a phytoplankton cell relying on CO uptake in the absence of an external CA the cell surface CO may become limited by the rate of diffusion of CO from the bulk seawater. This greater depletion of CO may then result in greater variability in the change of [ ] at the cell surface. To decrease complexity we have not included this mode of operation in our model. Diffusivity varies with molecular weight and to retain charge balance the faster diffusion of lighter ions (most notably ) is thus moderated by that of slower moving counter ions. Of all of these ions HCO - is numerically (after ) the most important diffusing ion, and also has a molecular weight of a similar order to others. Accordingly and considering all other vagaries (see discussion concerning experimental artefacts in the main text) a common diffusivity constant was used based upon that expected at the stated salinity and temperature for HCO -. The value we have used is. 8 µm d - (.8-9 m s - ), as used in 9. We have identified conditions of atmospheric pco for comparisons. The series assume an atmospheric pco of 8ppm giving an equilibrium bulk water ph under the conditions simulated of 8.6. pco and ph values for and conditions Macmillan Publishers Limited. All rights reserved.

4 were 9, 8., and 7, 7.76, respectively. These are configured to be consistent with predictions in reference. Various scenarios have been trialled, as shown in Supplementary Table, and Supplementary Figs. -. These scenarios are not intended to represent any specific environment, but to give an indication of the range of experiences possible. Thus, oligotrophic conditions (ultra-low residual nutrients) would give a range of near-organism experiences even less variable than those given here under oceanic, while extreme eutrophic would be more variable than those shown under eutrophic. Within the additional plots we also show how the simulation predicts the conditions in a warmer (+4ºC) ocean, together with the magnitude of changes brought about by changes in wind speed (Supplementary Table, Figs.,, ). To give an indication of the variation in proximate conditions that an organism may experience under different conditions, we have plotted the median of the model output together with errors,,, 7 and 9 th percentiles for each of the dynamic simulations over the day period of the plot. The term error does not apply to model error, but is a statistic for the data series (generated by the SigmaPlot (v.9) software employed to generate the plots). For reference in considering data such as those shown in Fig., we draw attention to the following: For phytoplankton, growing within a /hr light/dark cycle at a daily growth rate of d -, a photosynthetic rate during day-light exceeding d - is required. For microzooplankton, there is an inverse relationship between respiration rates and their size, which for the size of microzooplankton considered here ( µm equivalent spherical diameter) would equate to a respiration rate of around. gc (gc) - d - 4. Macmillan Publishers Limited. All rights reserved.

5 Supplemental References. Orr, J., Najjar, J., Sabine, C., & Joos, F. Abiotic-Howto, OCMIP- Project, 9 pp., online available at: (999).. Flynn, K. J. in Algal Cultures, Analogues Of Blooms And Applications (ed. Subba Rao, D. V.) p (Science Publishers, Enfield, U.S.A, 6). Mitra. A. A multi-nutrient model for the description of stoichiometric modulation of predation (SMP) in micro- and mesozooplankton. J. Plank. Res. 8, 97-6 (6) 4. Wolf-Gladrow, D. A., Bijma, J. & Zeebe, R. E. Model simulation of the carbonate chemistry in the microenvironment of symbiont bearing foraminifera. Mar. Chem. 64, 8-98 (999). Liss, P. S. & Merlivat, L. in The role of air-sea exchange in geochemical cycling, (ed. Buat-Menard, D.A.) p (Reidel, Dordrecht 986) 6. Wanninkhof, R. Relationship between wind speed and gas exchange over the ocean. J. Geophys.Res. 97, (99) 7. Nightingale, P. D. et al. In situ evolution of air-sea gas exchange parameterization using novel conservative and volatile tracers. Global Biogeochem. Change 4, 7-87 () 8. Goldman, J. C. & Dennet, M. R. Carbon dioxide exchange between air and seawater: no evidence for rate catalysis. Science, 99- (98) 9. Williams, G. R. The rate of hydration of carbon dioxide in natural waters. Ecol. Bull., 8-89 (98) 4. Wanninkhoff, R. & Knox, M. Chemical enhancement of CO exchange in natural waters. Limnol. Oceanogr. 4, (996) 4. Matthews, B. J. H. The rate of air-sea CO exchange: chemical enhancement and catalysis by marine phytoplankton. PhD Thesis, University of East Anglia, U.K. (999) 4. Fenchel, T. in Respiration in aquatic ecosystems (eds. del Giorgio, P. A. & leb. Williams, P. J.) p47 (Oxford University Press, U.K. ) 4 Macmillan Publishers Limited. All rights reserved.

6 Supplementary Table Conditions for simulations. Simulations were run under various scenarios to consider combinations of different atmospheric pco and water column conditions. Initial surface mixed layer seawater pco (and the fixed sub-mixed layer pco ) was set at the atmospheric value. In addition, some simulations were also run under coupled with an elevated water temperature (+4ºC, taking it to ºC), and with wind speed moderated to. m s -, or elevated to m s -. Both temperature and wind speed affect water chemistry and surface gas exchange, while the effect of elevated temperature was enacted by increasing the organism growth rate according to Q = (increasing it by.). C:N:P stoichiometry were as defined in,. Ocean acidification scenario pco (ppm) ph (seawater scale) (nmol kg - seawater) Water column Default Oceanic Eutrophic Initial dissolved inorganic N (DIN; µm) 6 4 Initial dissolved inorganic P (DIP; µm).6 Temperature (ºC) Salinity Mixing depth (m) 8 Mixing rate between surface and... subsurface mixed layers (d - ) Wind speed (m s - ) Surface PAR with / light/dark (µmol photon m - s - ) Plankton Phytoplankton plankton Maximum daily growth rate (d - ) C:N:P variable stoichiometry fixed stoichiometry Maximum (µm), 7, Motility (µm s - ) ( for Oceanic) Macmillan Publishers Limited. All rights reserved.

7 ph Ω Cal Supplementary Fig. Steady-state values of near organism/aggregate for ph and the saturation constant for calcite (Ω cal ). See legend for Fig. for more details. The organisms simulated here are non-calcifiers. 6 Macmillan Publishers Limited. All rights reserved.

8 non-calcifying - - calcifying - - non-calcifying Ω cal - Ω cal - calcifying Ω cal - Ω cal - Supplementary Fig. Steady-state values of [ and the saturation constant for calcite (Ω cal ) for simulated non-calcifiers vs calcifyers (PIC:POC=.) for (left-hand column) and (right-hand column) conditions. 7 Macmillan Publishers Limited. All rights reserved.

9 Default system Nutrients (µm) Ratio... DIN DIP N-status f-ratio Algal C C (a) (b)... Biomass (gc m - ) Bulk (c) Large Bulk (d) Time (d) Supplementary Fig. Variations in [ for the default scenario (see Supplementary Table ). Part a, changes in nutrient status and f-ratio. Part b, changes in dissolved inorganic N (DIN) and P (DIP), total algal biomass (algal-c) and zooplankton (zoo C). Parts c and d show [ in simulations for phytoplankton (growing alone or in aggregates) of µm (), 7µm () or µm (Large; part d) together with zooplankton, under, or OA conditions (see Supplementary Table ). Also shown are [ H ] + bulk values. See also Supplementary Fig Macmillan Publishers Limited. All rights reserved.

10 Default System Bulk F E H Large Warm Supplementary Fig. 4 Variation in [ over the simulated day event in the default systems (Supplementary Fig., table ) for the bulk water, organisms/aggregates of µm (), 7µm () and µm (Large), and of µm zooplankton. Red arrows on the Y-axis indicate the atmosphere-seawater equilibrium [ values for (H), (E) and (F) scenarios; net C-fixation draws the actual value of [ H ] + bulk below these equilibria. Also shown is the variation seen when the temperature is elevated ( Warm ; see Supplementary Table ). Variability in [ over the d period is shown as the median (centre box line), th and 7 th quartiles (lower and upper extent of box), errors (whiskers) and the th and 9 th quartiles (closed circles). The red lines are aligned with the median value for [ over the d period; distributions centred below this line indicate a more alkaline experience, while distributions above it indicate more acidic experiences. ] bulk ] bulk 9 Macmillan Publishers Limited. All rights reserved.

11 Oceanic system Ratio.. f-ratio N-status (a) Nutrients (µm). DIN Algal C DIP Bulk C (b) (c)... Biomass (gc m - ) Large Bulk (d) Time (d) Supplementary Fig. Variations in [ for the oceanic scenario (see Supplementary Table ). Details as for Supplementary Fig.. See also Supplementary Fig. 6. Macmillan Publishers Limited. All rights reserved.

12 Oceanic System Bulk F E H Large Warm Supplementary Fig. 6 Variation in [ over the simulated day event in the oceanic systems (Supplementary Fig., Table ). Other details as for Supplementary Fig. 4. Macmillan Publishers Limited. All rights reserved.

13 Eutrophic system Nutrients (µm) Ratio... 4 DIN DIP f-ratio Algal C N-status C (a) (b) Biomass (gc m - ) Bulk (c) Bulk Large (d) Time (d) Supplementary Fig. 7 Variations in [ for the eutrophic scenario (see Supplementary Table ). Other details as for Supplementary Fig.. See also Supplementary Fig. 8. Macmillan Publishers Limited. All rights reserved.

14 Eutrophic System Bulk F E H Large Warm Supplementary Fig. 8 Variation in [ over the simulated day event in the eutrophic systems (Supplementary Fig. 7, Table ). Other details as for Supplementary Fig. 4. Macmillan Publishers Limited. All rights reserved.

15 Nutrients (µm) Ratio... 4 Eutrophic system; HAB f-ratio N-status DIN Algal C DIP C (a) (b) 6 4 Biomass (gc m - ) Bulk (c) Bulk Large (d) Time (d) Supplementary Fig. 9 Variations in [ for the eutrophic harmful algal bloom (HAB) scenario running with no zooplankton (this was run with eth eutrophic conditions shown in Supplementary Table, but with an extremely low zooplankton biomass present). Although zooplankton were effectively absent from the simulated biomass, their experience is still shown here. Other details as for Supplementary Fig.. See also Supplementary Fig.. 4 Macmillan Publishers Limited. All rights reserved.

16 Eutrophic System; HAB Bulk F E H Large Warm Supplementary Fig. Variation in [ over the simulated day event in the eutrophic HAB systems with no zooplankton (Supplementary Fig. 9, Table ). Other details as in Supplementary Fig. 4. plankton were effectively absent from the simulated biomass, but their experience is still shown here. Macmillan Publishers Limited. All rights reserved.

17 Nutrients (µm) Ratio... Default system; PIC:POC=. DIN DIP f-ratio Bulk Algal C N-status C (a) (b)... (c) Biomass (gc m - ) Large Bulk (d) Time (d) Supplementary Fig. Variations in [ for the default scenario but assuming plankton deposit CaCO with a PIC:POC of. (see Supplementary Table, CF. Supplementary Fig. ). Other details as for Supplementary Fig.. See also Supplementary Fig.. 6 Macmillan Publishers Limited. All rights reserved.

18 Default System; PIC:POC=. Bulk F E H Large Warm Supplementary Fig. Variation in [ for the default scenario but assuming plankton deposit CaCO with a PIC:POC of. (Supplementary Fig., Table ). Other details as for Supplementary Fig Macmillan Publishers Limited. All rights reserved.

19 Default System Abiotic vs Biotic variation Bulk Large E Default Still Windy Default HAB Supplementary Fig. Variation in [ for the default scenario but assuming different wind speeds (Supplementary Table ). Other details as for Supplementary Fig Macmillan Publishers Limited. All rights reserved.