Ninth Year Annual Report:
Carbon Science: Controls on Ocean Productivity
The Sarmiento, Morel, and Bender Groups conduct research on the impacts of changing atmospheric and ocean chemistry on the marine biota.
Sarmiento's Group members Sarmiento, Slater and Hiscock, along with GFDL scientists Dunne and Gnanadesikan, have performed several simulations to examine the sensitivity of atmospheric CO2 drawdown to patch iron fertilization in High Nutrient Low Chlorophyll (HNLC) regions in the North and Equatorial Pacific Ocean and Southern Ocean (Sarmiento et al, 2009b). The group used a prognostic global ocean biogeochemical general circulation model developed at GFDL and Princeton.
The researchers found the greatest response to fertilization occurred in the Ross Sea site of the Southern Ocean, with the smallest response in the North Pacific due to its lower initial nutrients and hence a lesser efficiency. When the model included a well-mixed atmospheric reservoir of CO2, the drawdown efficiency was reduced significantly due to a back flux of CO2 to the atmosphere. The back flux is caused by the fertilization-induced drawdown lowering the atmospheric CO2 globally, thereby reducing the CO2 gradient between the atmosphere and the ocean and lowering the uptake of CO2 by the ocean everywhere outside of the fertilization region. This effect is often left out of model simulations of iron fertilization, but these results show that the overall drawdown can be reduced by as much as 70% over a century time scale.
Continuous Measurement of Dissolved Inorganic Carbon in Surface Seawater
The dissolved inorganic carbon concentration of surface seawater is of interest for two reasons. First, fossil fuel CO2 enters the ocean at the surface, and its entry is registered as an increase in the dissolved inorganic carbon concentration (DIC, including dissolved CO2, bicarbonate, and carbonate). Comprehensive data on the concentration of DIC in ocean surface waters are one basic component of an ocean CO2 observing system. Local observations concentrated in surface regions where there is intense mixing with the subsurface can give information about the uptake of fossil fuel CO2 and its transport into the ocean interior.
Second, ocean productivity leads to changes in the DIC concentration of surface waters over seasonal timescales. DIC concentrations in the upper ~100 m of the water column fall during spring and summer, as organisms assimilate DIC near the surface and remove it by export (sinking). The magnitude of the DIC decrease is a measure of the fertility of the ecosystems, and has been used to constrain ocean productivity over large scales.
Michael Bender's group has begun the development of an instrument to make continuous measurements of DIC along oceanographic cruise tracks. Their goal is to make an instrument with an accuracy of 0.1% or better (about twice the annual increase in the surface ocean DIC burden) that will operate reliably with very little attention underway.
The basic approach involves adding the rare heavy stable isotope of carbon (13C) to seawater at a precisely metered rate, and measuring the ratio of natural 12C to added 13C. This ratio rises with the DIC concentration of the seawater. At the present time, the researchers have set up a fluid flow system for the work, and are achieving a precision of ±0.2% or better averaged over tens of minutes. They are working through several approaches for improving this number and hope to deploy the instrument for selected cruises beginning sometime this year. Eventually the plan is to replicate the instrument and integrate it into the global seagoing carbon observing system. The initial focus in this respect will probably be the Southern Ocean.
Net Community Production and Carbon Export in the Southern Ocean
The rate of net organic carbon production by the upper ocean biological community, and the associated "export" (sinking) of organic carbon to deeper, dark waters, is a keystone in our understanding of the ocean carbon cycle. This flux supplies essentially all the food for organisms living in the dark ocean, below about 100 m depth. It is the process that removes carbon and nutrients from the surface ocean and supplies them to the deep; it thereby lowers the DIC and nutrient concentration of the surface ocean, raises the concentrations of DIC and nutrients in the deep ocean, and modulates the CO2 concentration of air.
This past year, the Bender Group continued a decade-old program to measure net community production in the upper ocean mixed layer by making measurements of dissolved gas concentrations on ships of opportunity operating in the Southern Ocean during spring and summertime. A special point of emphasis this past year was to synthesize data from discrete samples and continuous measurements made on over 50 cruises.
The results support assumptions about several factors controlling productivity and show that they obtain over very large scales (Figure 18). Waters with shallow mixed layers tend to be more fertile as phytoplankton are concentrated in well-lit waters near the sea surface. Productivity decreases going southward towards Antarctica because phytoplankton grow less rapidly in ironpoor regions south of Australia, New Zealand, Africa, and South America. Productivity is enhanced in some areas where shallow or rough topography accelerates vertical mixing.
At the same time, two widely held views were not supported. First, contrary to expectations, there was no indication of a spring bloom on repeat cruises south of Australia. Actually, productivity is higher in the summer. Second, again contrary to expectations, there was no indication that diatoms were required for rapid export of organic carbon from the euphotic zone. In fact, some regions of the oceans depleted in SiO2 (which diatoms need to make their skeletons) were found to be highly productive.
Impacts of Ocean Acidification on Phytoplankton
The most certain effect of the ongoing increase in atmospheric CO2 is change in the chemistry of the surface ocean, including an increase in dissolved CO2 concentration, a decrease in pH, and a decrease in the saturation state of calcium carbonate, CaCO3(s). These changes, designated collectively as "ocean acidification," will have a host of effects on the ocean biota -- some subtle, some perhaps dramatic. The work of the Morel Group concerns the effects of ocean acidification on the growth of marine phytoplankton, the microscopic plants that form the basis of the marine food chain and are responsible for nearly half of primary production on Earth.
Marine primary production is limited by a variety of major and trace nutrients such as nitrogen (N), phosphorus (P) and iron (Fe). Iron, which is limiting in large regions of the world's oceans, has a chemistry that is particularly sensitive to acidification. Morel and colleagues have found that decreasing seawater pH decreases the concentration of free Fe, Fe', at equilibrium with organic chelating agents to an extent that depends on the acid-base chemistry of the chelator. In laboratory cultures, the net result of this decrease in Fe' is a proportional decrease in the uptake rate of Fe by phytoplankton. A qualitatively similar effect was observed in experiments carried out during 2009 with natural seawater from the Atlantic Ocean, where Fe is bound to chelators of unknown structure.
In another field study in the Gulf of Alaska, incubation experiments were conducted to assess the effect of CO2 on the growth and photosynthetic physiology of natural phytoplankton assemblages under iron-limited and iron-replete conditions. In several incubations under iron-limited conditions, key photosynthetic proteins were down-regulated at elevated CO2 relative to low CO2. Comparisons of primary productivity with net growth rate indicate growth efficiency was increased at elevated CO2 in several experiments, possibly due to down-regulation of the carbon concentrating mechanism. Variability in the response of phytoplankton to CO2 was observed among the experiments. This did not appear to be caused by differences in phytoplankton community structure and may reflect the sensitivity of the net response of phytoplankton to antagonistic effects of the several parameters that co-vary with CO2.
In regions where the available inorganic P is very low, some phytoplankton can acquire this nutrient from organic compounds. This requires that phosphate first be cleaved from organic molecules using an external alkaline phosphatase enzyme. The activity of this enzyme decreases rapidly with pH, however (Figure 19). As a result, phytoplankton grow more slowly at lower pH when grown on organic phosphate. A field study of this effect in the Sargasso Sea is planned for Spring 2010. Other ongoing laboratory studies concern the effect of ocean acidification on phytoplankton growing under nitrogen limiting conditions.