BG Biogeosciences BG Biogeosciences 1726-4189 Copernicus Publications Göttingen, Germany 10.5194/bg-10-3109-2013 Implications of elevated CO<sub>2</sub> on pelagic carbon fluxes in an Arctic mesocosm study – an elemental mass balance approach Czerny J. 1 Schulz K. G. 1 6 Boxhammer T. https://orcid.org/0000-0002-9632-5947 1 Bellerby R. G. J. 2 3 4 Büdenbender J. 1 Engel A. https://orcid.org/0000-0002-1042-1955 1 Krug S. A. 1 Ludwig A. 1 Nachtigall K. 1 Nondal G. 4 Niehoff B. 5 Silyakova A. https://orcid.org/0000-0002-1992-1868 2 Riebesell U. https://orcid.org/0000-0002-9442-452X 1 GEOMAR Helmholtz Centre for Ocean Research Kiel, 24105 Kiel, Germany Bjerknes Centre for Climate Research, 5007 Bergen, Norway Geophysical Institute, University of Bergen, 5007 Bergen, Norway Norwegian Institute for Water Research (NIVA), 5006 Bergen, Norway Alfred Wegener Institute for Polar and Marine Research, 27515 Bremerhaven, Germany Centre for Coastal Biogeochemistry, Southern Cross University, Lismore, NSW 2480, Australia 08 05 2013 10 5 3109 3125 Copyright: © 2013 J. Czerny et al. 2013 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://bg.copernicus.org/articles/10/3109/2013/bg-10-3109-2013.html The full text article is available as a PDF file from https://bg.copernicus.org/articles/10/3109/2013/bg-10-3109-2013.pdf

Recent studies on the impacts of ocean acidification on pelagic communities have identified changes in carbon to nutrient dynamics with related shifts in elemental stoichiometry. In principle, mesocosm experiments provide the opportunity of determining temporal dynamics of all relevant carbon and nutrient pools and, thus, calculating elemental budgets. In practice, attempts to budget mesocosm enclosures are often hampered by uncertainties in some of the measured pools and fluxes, in particular due to uncertainties in constraining air–sea gas exchange, particle sinking, and wall growth. In an Arctic mesocosm study on ocean acidification applying KOSMOS (Kiel Off-Shore Mesocosms for future Ocean Simulation), all relevant element pools and fluxes of carbon, nitrogen and phosphorus were measured, using an improved experimental design intended to narrow down the mentioned uncertainties. Water-column concentrations of particulate and dissolved organic and inorganic matter were determined daily. New approaches for quantitative estimates of material sinking to the bottom of the mesocosms and gas exchange in 48 h temporal resolution as well as estimates of wall growth were developed to close the gaps in element budgets. However, losses elements from the budgets into a sum of insufficiently determined pools were detected, and are principally unavoidable in mesocosm investigation. The comparison of variability patterns of all single measured datasets revealed analytic precision to be the main issue in determination of budgets. Uncertainties in dissolved organic carbon (DOC), nitrogen (DON) and particulate organic phosphorus (POP) were much higher than the summed error in determination of the same elements in all other pools. With estimates provided for all other major elemental pools, mass balance calculations could be used to infer the temporal development of DOC, DON and POP pools. <br><br> Future elevated <i>p</i>CO<sub>2</sub> was found to enhance net autotrophic community carbon uptake in two of the three experimental phases but did not significantly affect particle elemental composition. Enhanced carbon consumption appears to result in accumulation of dissolved organic carbon under nutrient-recycling summer conditions. This carbon over-consumption effect becomes evident from mass balance calculations, but was too small to be resolved by direct measurements of dissolved organic matter. Faster nutrient uptake by comparatively small algae at high CO<sub>2</sub> after nutrient addition resulted in reduced production rates under future ocean CO<sub>2</sub> conditions at the end of the experiment. This CO<sub>2</sub> mediated shift towards smaller phytoplankton and enhanced cycling of dissolved matter restricted the development of larger phytoplankton, thus pushing the system towards a retention type food chain with overall negative effects on export potential.

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