Biological Pump Working Group Summary

Chair: Dave Karl
Rapporteur: Debbie Steinberg

Working Group Members: Mark Abbott, Bob Anderson, David Archer, Rob Armstrong, William Balch, Dick Barber, Jack Barth, Michael Bender, Ron Benner, Will Berelson, Jim Bishop, Ed Boyle, Ken Brink, Deborah Bronk, Robert Byrne, Mark Brzezinski, Ken Buesseler, Mary-Elena Carr, Francisco Chavez, Jon Cole, Ellen Druffel, Hugh Ducklow, Steve Emerson, Christopher Field, Anand Gnanadesikan, Burke Hales, Dennis Hansell, Rick Jahnke, Ken Johnson, Cindy Lee, Tony Michaels, James Murray, Mercedes Pascual, Mike Roman, Jorge Sarmiento

 

Biological processes affect transport of organic carbon into the oceans' interior which in turn affects atmospheric CO2. The annual uptake of CO2 by the surface ocean varies between 1-3 Gt carbon (Battle et al., 2000); how much of the interannual variability in the uptake rate can be attributed to the ocean's biological pump? Where is export by the biological pump most significant, and what components of the pump are most important? In OCTET we must determine the contribution of the biological pump to interannual variability of atmospheric CO2, and provide greatly improved projections of the response of ocean biogeochemistry to future environmental change and its impact on future CO2 concentrations.

The biological pump is the process by which CO2 fixed in photosynthesis is transferred to the interior of the ocean resulting in a temporary or permanent sequestration (storage) of carbon. A simplified diagram of the principle components of the biological pump is presented in Figure 1.


Figure 1. Atmospheric CO2 (or N2 gas) fixed by autotrophs in the upper ocean is transported to deep waters (i.e. below the mixed layer) by various processes. Phytoplankton become senescent and sink out as aggregates, or are consumed by herbivores that produce sinking fecal pellets. Aggregates may then be decomposed by bacteria or consumed by animals. Diel vertical migration is a mechanism by which zooplankton (or nekton) feeding in the surface waters at night actively transport dissolved or particulate material to depth by metabolizing the ingested food at their daytime residence depths. Vertical migration of some phytoplankton species may bring nutrients from the nutricline into the euphotic zone. Dissolved organic carbon produced by phytoplankton or by animal excretion in surface waters can be transported downward during deep mixing events. The biological pump also includes the sinking of particulate inorganic carbon (PIC) of biological origin (calcite and aragonite - the "carbonate pump").


Past research has highlighted many of the major processes involved in the biological pump. However, many key questions remain:

  1. What is the strength of the biological pump and how does it differ between biogeographical provinces? How do we most accurately measure its strength?

  2. How does the structure and composition of the biological pump change in space and time? How might community structure affect it, and what is the importance of selected functional groups (e.g., nitrifiers, calcifiers, large grazers)? What are the relative roles of the microbial and zooplankton communities?

  3. What is the sensitivity of the biological pump to perturbations in forcing (upwelling, dust and Fe deposition, North Atlantic Oscillation, El Niño)? How do we quantify this variability (e.g., time series).

  4. How will the biota respond to warming, chemical changes (DIC, pH), and physical changes to the habitat such as enhanced stratification?

  5. What are the important processes (N2 fixation, Fe limitation, etc.) that prevent a simple relationship between net or total production of ecosystems and the nutrient concentrations of the ambient waters?

  6. What processes cause the C/N/P of organic matter produced in the euphotic zone to differ from the metabolic C/N/P ratio of waters in the underlying twilight zone?

  7. How does the ratio of net/gross production in the euphotic zone depend on sea surface temperature?

  8. What are the time and space varying processes in the mesopelagic zone (100 to 1000 m) that control the recycling and gravitational flux of carbon?

 

Several areas need particular consideration in OCTET:

 

Effect of climate change on the biological pump

Biological processes will almost certainly be significantly modified by the chemical and physical changes that will accompany future increases of atmospheric CO2 and associated global warming (e.g., Sarmiento et al. [1998]; Matear and Hirst [1999]). A summary of what some of those changes might be and how they might affect the biological pump is presented below as a set of hypotheses regarding production and remineralization of organic matter. An important goal of research over the next decade must be to refine and test these hypotheses and to put into place an observational system that will allow us to detect these changes as they occur.

Response of production to global warming

  1. Ocean simulations of global warming show increased stratification in low latitudes due to the increased temperature. To the extent that insufficient nutrient supply from below limits new production in low latitude regions, global warming would be expected to decrease production.

  2. Ocean simulations of global warming show increased high latitude stratification due primarily to increased rainfall as the hydrological cycle intensifies. In regions where low light supply due to deep mixing may be presently limiting new production (cf. Mitchell and Holm-Hansen [1991]), such an increase in stratification might actually result in an increase in new production.

  3. The changes in temperature, stratification and chemistry that will occur over this and future centuries will lead to a change in the biogeography of functional groups. Among the possible modifications are a retreat of diatom production toward the Antarctic in the Southern Ocean due to reduced silicate supply; a decrease in calcification due to reduced CO32- ion concentration (e.g., Kleypas et al. [1999]); and changes in the range of N2 fixation as a result of increased stratification and modifications in the nutrient supply and iron supply by dust.

  4. Our present understanding of the sensitivity of the f-ratio to temperature suggested the hypothesis that warming may lead to increased efficiency of recycling of nutrients in the euphotic zone (decreased f-ratio/increased production for a given nutrient supply) in eutrophic and mesotrophic systems (Laws et al., 2000). The contrary may occur in oligotrophic regions, where warming may lead to an increased f-ratio.

Response of remineralization to global warming

  1. Warmer temperatures will increase the efficiency of remineralization in the water column.

  2. Reduction of new production and the shift away from diatoms and calcifiers will reduce export to the deep sea.

Impact on the biological pump

The efficiency of the biological pump will increase (nutrient drawdown will increase) and atmospheric CO2 will be taken up. This will occur at the same time that new production drops. Both the increased efficiency of the biological pump and reduced export production result primarily from modifications in the stratification. In model simulations, the increase in the efficiency of the biological pump largely compensates for the decrease in the efficiency of the solubility pump, which slows down because of the increased stratification.

 

The "twilight zone"

The mesopelagic zone, between ~100 - 1000 meters (incorporating mode waters and the main thermocline), is an important focus area for OCTET, and one not addressed by JGOFS, which largely focused on the euphotic zone. The significant decomposition, recycling and repackaging of particles (and DOM) in the ~100-1000 meter depth zone, as seen by transmissometer and other data, is critical to the biological pump. Evidence from bomb C-14 inventories indicates that sinking particulate matter at depth is relatively young whereas deep water dissolved organic carbon is old (mean age ~ 6,000 yrs). There is also evidence, based on modeling studies, that particles in the deep waters may have distant sources and thus particles caught in traps are not representative of processes occurring directly above. (This is not a problem, however, if the objective is to integrate a large horizontal space). Another approach is to look at the products of particle decomposition (i.e., Apparent Oxygen Utilization). The distribution of bioactive tracers in the twilight zone constrains rates of respiration and the composition of metabolized organic matter. The study of these properties provides an essential complement to studies of POC and DOC. Recent discoveries such as Archaea in deep waters, non-Redfield stoichiometry in remineralization patterns at depth, and our lack of knowledge of much of the biological community in general below the euphotic zone all point to the need for more intense future research in this zone.

 

What controls the structure and efficiency of the biological pump?

Is the intensity and efficiency of the biological pump a function of sea surface concentration of chlorophyll alone, or are heterotrophic processes also important? What are the components of the pump that cycle carbon with respect to processes such as ocean-atmospheric exchange? This would include changes in N2 fixation, stoichiometry of organic matter production and remineralization, and twilight zone processes. At the present time we lack a detailed mechanistic understanding of the biological pump. It is not appropriate to use empirical models, no matter how statistically sound, to make future predictions.

What are the critical modeling steps that need to be taken to quantify the role of the biological pump? For example, do we need to have complicated food webs in our models or can we simplify them? It is the same with physical processes; how do we start to parameterize them?

 

Time scales

The biological pump must be studied on longer time scales than previously done to understand the oceanic response on centennial time scales. This would allow OCTET studies to encompass natural perturbations in the climate cycle (e.g., an El Niño or North Atlantic Oscillation cycle) and to assess how episodic surface processes (e.g., dust deposition) might lead to long term sequestration of carbon. The exact time scales needed are subject to debate and must be determined.

Paleoclimate work currently does focus on decade to century time scales, but the focus is almost entirely on the physical climate, not the ocean carbon cycle. Where OCTET can make significant advances is in the calibration of proxies used to understand paleo-records (for example- the effect of temperature on nutrient proxies). OCTET could also benefit from retrospective analyses like those in the GLOBEC program.

 

Regions of interest

One way to focus studies of the biological pump is to consider processes within biogeographical provinces; these are distributional boundaries mostly defined by water masses and current boundaries with distinct planktonic assemblages. For example, the ocean margin province is important for deposition and sequestration of carbon, and is sensitive to natural perturbation. A number of coastal time-series sites exist that may be helpful to OCTET objectives. We need to know how the pump may operate differently between ocean margin and open ocean environments. Provinces of interest to OCTET will determine to some extent the processes studied; for example, if the northwest Atlantic is a focus, then N fixation may not be a major consideration. Future OCTET studies must be three dimensional; we can not consider vertical transport processes alone, but must also consider lateral advection and interactions of open ocean waters with the ocean margin. A control volume approach (characterization of all the internal transformations of carbon while constraining the inputs and outputs to the appropriate physically or biologically relevant scale) was suggested as a way to assess the role of advection.

 

Methodological issues

A number of issues with respect to methodology were raised. For example, if the biological pump did change dramatically as a result of warming, could we detect it? We need to develop a strategy for monitoring the ocean to detect global change and put this monitoring network into place. In addition, part of the reason the "twilight zone" is poorly known is that we do not have the tools to study all the important processes. Some methods do already exist that can be applied to measure transformation rates experimentally in this zone (e.g., respiratory ETS activity, thymidine incorporation) and should be tested. An aggressive program to develop methodologies and compare methods should be implemented.

What properties can we sense remotely? Methods that would allow coverage of large areas (e.g., global drifter program and satellites) should be used. However, caution should be exercised in deriving parameters of the pump from satellite data (e.g., primary production or export flux) as algorithms still are in their infancy. It is clear we need other sensors to better measure parameters such as nutrients or organism stocks, but some of these technologies are not available yet, thus technology development is needed.

 

References

Battle, M., M.L. Bender, P.P. Tans, J.W.C. White, J.T. Ellis, T. Conway and R.J. Francey. 2000. Global carbon sinks and their variability inferred from atmospheric O2 and delta 13C. Science 287: 2467-2470.

Kleypas, J.A., R.W. Buddemeier, D. Archer, J.P. Gattuso, C. Langdon and B.N. Opdyke. 1999. Geochemical consequences of increased atmospheric carbon dioxide on coral reefs, Science, 284, 118-120.

Laws, E.A., P.G. Falkowski, W.O. Smith, Jr., H. Ducklow, and J.J. McCarthy. 2000. Temperature effects on export production in the open ocean, Submitted

Matear, R.J. and A.C. Hirst. 1999. Climate change feedback on the future oceanic CO2 uptake, Tellus, 51B, 722-733.

Mitchell, B.G. and O. Holm-Hansen. 1991. Observations and modeling of the Antarctic phytoplankton crop in relation to mixing depth, Deep Sea Res., 38: 981-1007.

Sarmiento, J.L., T.M.C. Hughes, R.J. Stouffer and S. Manabe. 1998. Simulated response of the ocean carbon cycle to anthropogenic climate warming, Nature, 393, 245-249.