Effects of ocean acidification & warming on organic matter production : Possible changes in biological carbon pump efficiency Ja-Myung Kim PICES 2015 Annual Meeting October 14-25 Qingdao China
Atmosphere Surface ocean CO 2 CO 2 (µm) 0 20 40 60 500 C:N ratio Carbon overconsumption? Phytoplankton Org C CO 2 1000 DOC 1500 POC:DOC ratio Carbon sequestration enhancement? Biological pump POC 2000 Sediments Depth (m)
Controlled in-situ mesocosm experiment
Experimental approaches Natural env. Big bags Bottles Open ocean monitoring Coastal in-situ perturbation Lab cultures Molecular mechanisms
Experimental approaches Similarity Natural env. Big bags Bottles in-situ perturbation expt. Mesocosm Control Observation on multidisciplinary parameters
Mesocosm Experiments Adjacent areas of the Arctic Ocean North Pacific
Mesocosm System Jangmok, Geoje island, Korea
Experimental conditions Combined effects of seawater pco 2 concentration & temperature 400 ppm 950 ppm 950 ppm Ambient temp. Ambient temp. 3 o C higher temp. Control Acidification Greenhouse
Experimental set up: pco 2 adjustment 2 m 950 ppm pco 2 2000 L
Homogeneous mixing
Temperature adjustment Nutrient addition N ~33 µm P ~ 2.5 µm Si ~50 µm Water bath + 3 o C
Total organic carbon production TOC production (μmol kg -1 ) 250 200 150 100 50 0 Control Acidification Greenhouse N depleted 0 2 4 6 8 10 12 14 16 18 20 Day
% ratio of DOC to TOC production % (DOC dayn / TOC day20 ) 100 80 60 40 20 0 Control Acidification Greenhouse N depleted 0 2 4 6 8 10 12 14 16 18 20 Day
DOC production process in marine system Production Consumption DIC Phytoplankton Virus Microzooplankton DOC Bacteria Sinking particles Macrozooplankton
The most likely mechanism of enhanced DOC production Production Virus DIC Phytoplankton Grazing-induced production Microzooplankton DOC Bacteria Sinking particles Macrozooplankton
The most likely mechanism of enhanced DOC production 1.2 Growth rate (d -1 ) 0.8 0.4 0 1.2 0.8 0.4 950 ppm 750 ppm Skeletonema costatum major prey DOC Kim et al. 2006 Limnol. Oceanogr. Grazing-induced production Heterotrophic dinoflagellates ~ 90% of the total carbon biomass of microzooplankton 0
The most likely mechanism of enhanced DOC production Control Acidification Greenhouse Grazing-induced production Grazing rate (d -1 ) 1.1 0.9 0.7 0.5 0.3 3 9 12 17 Day Abundance (x10 4 cells L -1 ) 20 15 10 5 0 Heterotrophic dinoflagellates 0 4 8 12 16 20 Day
DOC production (μmol kg -1 ) Another possible mechanism 150 120 90 60 30 0 Extracellular DOC release Control Literature Acidification Greenhouse Extracellular release of photosynthetic products was enhanced in warm ocean conditions : Zlotnik and Dubinsky 1989,Morán et al. 2006 Photosynthetic activity increase N depleted 0 2 4 6 8 10 12 14 16 18 20 Day POC production (μmol kg -1 )
Enhanced DOC production under warm ocean condition ΔDOC = ΔTOC - ΔPOC Wohlers et al. 2009 Indoor-mesocosm Baltic seawater (winter/spring) Diatom (S. costatum) dominated Ctrl TOC build-up POC build-up Net build-up of TOC was NOT markedly affected by changes in temperature. Yet, after the biomass peak, an enhanced accumulation of DOC occurred at high Temp. + 6 o C * Underlying Mechanism High respiratory consumption of organic C relative to autotrophic production
Atmosphere Surface ocean CO 2 DIC:DIN ratio Carbon overconsumption? vs. POC:DOC ratio Carbon sequestration enhancement? Phytoplankton DOC Org C Biological pump POC Strengthening of biological carbon removal efficiency CO 2 Sediments
The molar ratio of C to N 120 6.6 ± 0.3 This study ΔC (μmol kg -1 ) 90 60 30 0 0 3 6 9 12 15 18 ΔN (µmol kg -1 ) Riebesell et al. 2007 6.6 350 ppmv 7.1 700 ppmv 8.0 1050 ppmv
CO2 effect Temp. effect Partitioning Elemental ratio ΔTOC ΔPOC ΔDOC ΔTOC:N (or ΔDIC) (or ΔDIC:N) This study Engel et al. 2004 Riebesell et al. 2007 Wohlers et al. 2009 Taucher et al. 2012 Engel/Czerny et al. 2013 Biermann et al. 2015
CO 2 Photosynthesis Respiration Microbial pump Weakening of biological carbon removal efficiency? Org C microbes Biological pump CO 2 RDOM slow cycle up to millennia POC Jiao et al. 2010 Nature reviews
Conclusion 1. Elevated seawater pco 2 concentration and temperature stimulated two main processes responsible for enhancing DOC production (release by grazers and the extracellular release by phytoplankton). 2. An increase in the DOC:POC production ratio implies a shift in the organic carbon flow from particulate to dissolved forms under future ocean condition. 3. Excess DOC production may act as a positive feed back to increase the atmospheric CO 2 (if the produced DOC is labile and rapidly transformed into inorganic carbon by microbial degradation).