The role of nutricline depth in regulating the ocean carbon cycle

Carbon uptake by marine phytoplankton, and its export as organic matter to the ocean interior (i.e., the “biological pump”), lowers the partial pressure of carbon dioxide (pCO2) in the upper ocean and facilitates the diffusive drawdown of atmospheric CO2. Conversely, precipitation of calcium carbonate by marine planktonic calcifiers such as coccolithophorids increases pCO2 and promotes its outgassing (i.e., the “alkalinity pump”). Over the past ≈100 million years, these two carbon fluxes have been modulated by the relative abundance of diatoms and coccolithophores, resulting in biological feedback on atmospheric CO2 and Earth's climate; yet, the processes determining the relative distribution of these two phytoplankton taxa remain poorly understood. We analyzed phytoplankton community composition in the Atlantic Ocean and show that the distribution of diatoms and coccolithophorids is correlated with the nutricline depth, a proxy of nutrient supply to the upper mixed layer of the ocean. Using this analysis in conjunction with a coupled atmosphere–ocean intermediate complexity model, we predict a dramatic reduction in the nutrient supply to the euphotic layer in the coming century as a result of increased thermal stratification. Our findings indicate that, by altering phytoplankton community composition, this causal relationship may lead to a decreased efficiency of the biological pump in sequestering atmospheric CO2, implying a positive feedback in the climate system. These results provide a mechanistic basis for understanding the connection between upper ocean dynamics, the calcium carbonate-to-organic C production ratio and atmospheric pCO2 variations on time scales ranging from seasonal cycles to geological transitions.

[1]  Ann E. Gargett,et al.  Time and space scales of vertical mixing and advection of phytoplankton in the upper ocean , 1983 .

[2]  P. Gent,et al.  Isopycnal mixing in ocean circulation models , 1990 .

[3]  Andrei P. Sokolov,et al.  A flexible climate model for use in integrated assessments , 1998 .

[4]  P. Holligan,et al.  Significance of ocean carbonate budgets for the global carbon cycle , 1996 .

[5]  P. Falkowski,et al.  Representing key phytoplankton functional groups in ocean carbon cycle models: Coccolithophorids , 2002 .

[6]  M. Brzezinski,et al.  A time series study of silica production and flux in an eastern boundary region: Santa Barbara Basin, California , 2001 .

[7]  F. Wilkerson,et al.  Silicate regulation of new production in the equatorial Pacific upwelling , 1998, Nature.

[8]  Richard T. Barber,et al.  A rising tide lifts all phytoplankton: Growth response of other phytoplankton taxa in diatom‐dominated blooms , 2006 .

[9]  W. Koeve,et al.  Basin‐wide particulate carbon flux in the Atlantic Ocean: Regional export patterns and potential for atmospheric CO2 sequestration , 2001 .

[10]  S. Doney,et al.  Modelling regional responses by marine pelagic ecosystems to global climate change , 2002 .

[11]  Paul G. Falkowski,et al.  The Evolution of Modern Eukaryotic Phytoplankton , 2004, Science.

[12]  Ulf Riebesell,et al.  Reduced calcification of marine plankton in response to increased atmospheric CO2 , 2000, Nature.

[13]  L. Perelman,et al.  A finite-volume, incompressible Navier Stokes model for studies of the ocean on parallel computers , 1997 .

[14]  Scott C. Doney,et al.  Response of ocean ecosystems to climate warming , 2004 .

[15]  M. Denis,et al.  Response of coccolithophorid Emiliania huxleyi to elevated partial pressure of CO2 under nitrogen limitation , 2003 .

[16]  C. Brown,et al.  Coccolithophorid blooms in the global ocean , 1994 .

[17]  Ulf Riebesell,et al.  Decreasing marine biogenic calcification: A negative feedback on rising atmospheric pCO2 , 2001 .

[18]  P. Holligan,et al.  Vertical distribution and partitioning of organic carbon in mixed frontal and stratified waters of the English Channel , 1984 .

[19]  V. Smetácek,et al.  Diatoms and the ocean carbon cycle. , 1999, Protist.

[20]  Peter H. Stone,et al.  Efficient Three-Dimensional Global Models for Climate Studies: Models I and II , 1983 .

[21]  Watson W. Gregg,et al.  Ocean primary production and climate: Global decadal changes , 2003 .

[22]  P. Falkowski,et al.  Historical climate change and ocean turbulence as selective agents for two key phytoplankton functional groups , 2004 .

[23]  David Tilman,et al.  Phytoplankton Community Ecology: The Role of Limiting Nutrients , 1982 .

[24]  W. Broecker,et al.  The Carbon Cycle and Atmospheric CO2 : Natural Variations Archean to Present: Sundquist/The Carbon Cycle and Atmospheric CO2 : Natural Variations Archean to Present , 1985 .

[25]  Elena Litchman,et al.  The role of functional traits and trade-offs in structuring phytoplankton communities: scaling from cellular to ecosystem level. , 2007, Ecology letters.

[26]  David A. Siegel,et al.  Climate-driven trends in contemporary ocean productivity , 2006, Nature.

[27]  D. J. Franklin,et al.  Growth rates of six coccolithophorid strains as a function of temperature , 2008 .

[28]  R. Margalef.  Life-forms of phytoplankton as survival alternatives in an unstable environment , 1978 .

[29]  Wallace S. Broecker,et al.  The carbon cycle and atmospheric CO2 , 1986 .

[30]  Richard Betts,et al.  The science of climate change , 2010 .

[31]  Jean-Pierre Gattuso,et al.  Marine calcification as a source of carbon dioxide : positive feedback of increasing atmospheric CO2 , 1994 .

[32]  P. Falkowski,et al.  Biogeochemical Controls and Feedbacks on Ocean Primary Production , 1998, Science.

[33]  Ulf Riebesell,et al.  Effect of CO2 concentration on C:N:P ratio in marine phytoplankton: A species comparison , 1999 .

[34]  P. Falkowski,et al.  Mix and match: how climate selects phytoplankton , 2007, Nature Reviews Microbiology.

[35]  L. Perelman,et al.  Hydrostatic, quasi‐hydrostatic, and nonhydrostatic ocean modeling , 1997 .

[36]  Melanie Abecassis,et al.  Ocean's least productive waters are expanding , 2008 .

[37]  Olivier Aumont,et al.  Response of diatoms distribution to global warming and potential implications: A global model study , 2005 .

[38]  Toby Tyrrell,et al.  Phytoplankton Calcification in a High-CO2 World , 2008, Science.

[39]  Fei-xue Fu,et al.  Interactive effects of increased pCO2, temperature and irradiance on the marine coccolithophore Emiliania huxleyi (Prymnesiophyceae) , 2008 .

[40]  T. Tyrrell.  Calcium carbonate cycling in future oceans and its influence on future climates , 2007 .

[41]  Gurvan Madec,et al.  Potential impact of climate change on marine export production , 2001 .

[42]  T. Platt,et al.  An estimate of global primary production in the ocean from satellite radiometer data , 1995 .

[43]  David Archer,et al.  What caused the glacial/interglacial atmospheric pCO2 cycles? , 2000 .

[44]  U. Riebesell,et al.  Enhanced biological carbon consumption in a high CO2 ocean , 2006, Nature.