Oxygen in the Atmosphere and Surface Ocean

The Bender Group has continued to study of the O2/N2 ratio of air. The primary objective of this work is to use the atmospheric O2 balance to constrain the partitioning of fossil fuel CO2 sequestration between the land biosphere and the oceans. Atmospheric O2/N2 measurements constrain this partitioning because CO2 uptake by the land biosphere adds O2 to air, while dissolution in the oceans does not. The researchers are currently in the process of updating mass balance calculations based on their atmospheric record, which extends back to 1991. The length of record now allows the group to begin examining whether there is evidence in the O2/N2 data for acceleration of ocean CO2 uptake.

The other objective of this work is to constrain rates of ocean fertility on the scale of the ocean basins. To this purpose, the group recently installed air samplers at Hateruma (Japan) and Cape Point, South Africa. Hateruma will allow intercalibration of results from their global collection network with those from the Japanese network in the western Pacific. Cape Point will help constrain productivity of the large region of high-chlorophyll waters east of South America.

A second project involves studies of local fertility of waters of the Southern Ocean. In this work, the concentration and isotopic composition of dissolved O2 at the sea surface are measured, together with the Ar concentration. The researchers use the results to constrain gross primary production and net community production in the mixed layer which, in the Southern Ocean, generally comprises most of the euphotic zone. The work also includes continuous measurements of the ratio of dissolved O2 to dissolved Ar along cruise tracks, enabling them to continuously constrain net community production (and export production) on oceanographic transits. The work is providing unparalleled detail about variations in upper ocean carbon fluxes in the Southern Ocean, and results generally support the view that light (which depends on mixed layer depth) and iron availability are key variables promoting higher production rates in the Southern Ocean. At the same time, they contradict expectations that silica availability (which enables diatom growth) enhances productivity. The group has added an analysis/modeling component to this work; one early result is to demonstrate that ocean biogeochemistry GCMs correctly simulate productivity in some broad areas of the Southern Ocean, but do a poor job at local scales.


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 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.

In about a third of the world oceans, primary production is limited by Fe availability. 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 decrease in the uptake rate of Fe by phytoplankton (Figure 19). The same result should obtain in the field where Fe is bound to organic chelators of unknown structure. Preliminary field experiments, conducted in the Fe-limited North Atlantic Ocean during summer 2008, support this prediction.

While the effect of acidification in decreasing Fe availability is clear, the net effect on the growth of the phytoplankton is complicated by possible changes in the Fe requirements of the organisms. In particular, an increase in CO2 concentration could alleviate the need to concentrate it intracellularly and make photosynthesis more efficient. This idea is supported by the researchers’ laboratory experiments showing a large decrease in Fe-use efficiency (defined as the rate of photosynthesis per mole of cellular Fe) at high pH/low CO2. The team has not observed an increase in Fe-use efficiency when CO2 is increased above ambient values (pH is decreased). Analyses of Fe-containing photosynthetic proteins in the group’s field samples are consistent with the laboratory observations.

Figure 19. Impact of pH on iron uptake. Steady state iron uptake rates, ρ, of two diatoms (A) Thalasssiosira weissflogii and (B) Thalassiosira oceanica as a function of total iron concentration, FeT, and (C) the unchelated iron concentration, Fe’, at three different pH values.