The fundamental biological rate processes in ocean ecosystems are photosynthesis and respiration. We can measure these rates by measuring two natural tracers in seawater: the triple isotope composition of dissolved O2, and the O2 supersaturation in excess of Ar supersaturation. By measuring these properties, we can characterize basic properties of ocean ecosystems and gain insights into how they work. A major goal of our studies is to collect far larger amounts of rate data than can be done with traditional studies.
Work of Mark Thiemens, Konrad Mauersberger, Boaz Luz and their collaborators has shown that atmospheric O2 is mass-independently fractionated. Normally 17O is fractionated half as much as 18O. Mass-independent fractionation is the anomaly from this normal pattern, expressed as Δ17O:
Δ17O = δ17O - 0.5 δ18O.
Δ17O equals zero when a sample is normally fractioned.
The isotopic composition of stratospheric O3 is highly anomalous in this respect: 17O is fractionated about 1.7 times as much as 18O rather than 0.5 times as much. This anomaly is transferred from O2 to O3 and, by a series of reactions described by Yuk Yung (California Institute of Technology) on to CO2. The result is that O in atmospheric O2 and CO2 is mass-independently fractionated, in opposite directions (Δ17O < 0 for O2; Δ17O > 0 for CO2).
The Δ17O anomaly of O2 in air allows us to determine the rate of photosynthesis in aquatic ecosystems, in approach pioneered by Boaz Luz (Hebrew University). In the absence of photosynthesis, the Δ17O of dissolved O2 in seawater will be similar to that of air. Photosynthesis adds O2 that is normally fractionated. Hence as photosynthetic O2 is added, the Δ17O anomaly decreases. Δ17O constrains the fraction of dissolved O2 in a water sample that is produced by photosynthesis in situ. Taking this value for the mixed layer along with an estimate for the rate of air-sea gas exchange, one can calculate the rate at which photosynthetic O2 is being transferred to the atmosphere. At steady state, this rate is the gross photosynthesis rate.
O2 supersaturation in the mixed layer gives a measure of net O2 production (photosynthesis minus respiration). A complication is that in the absence of net production O2 would still commonly be supersaturated in seawater because of the physical processes of warming and bubble entrainment. Warming of the mixed layer induces supersaturation by lowering the solubility. Breaking waves produce bubbles which induce supersaturation by partly dissolving in seawater. As Jenkins, Emerson, Quay and their colleagues have shown, Ar is an inert gas with similar solubility properties to O2. Biological O2 supersaturation equals O2 supersaturation in excess of Ar supersaturation. Knowing biological O2 supersaturation and an estimate of the rate of gas exchange, one can calculate net O2 production.
We collect seawater samples from the mixed layer into preevacuated flasks. In the laboratory, we measure the triple isotope composition of O2 in these samples, along with the O2/Ar ratio. We then use these results, together with an estimate of the gas exchange rate, to calculate rates of net production, gross production, and O2 consumption. This method is far more efficient than any other experimental technique for determining these properties, allowing us to make measurements at unprecedental scale and resolution.
A major extension of this work is the development in our lab, by Jan Kaiser, of a membrane inlet mass spectrometer for continuous measurements of the dissolved O2/Ar ratio. This instrument continuously pumps surface seawater through a Teflon AF tube located within a vacuum chamber. Gases diffuse through the Teflon tube, into the vacuum chamber, and into a quadropole mass spectrometer, which measures the O2/Ar ratio. From these measurements and estimates of the gas exchange rate, we can make continuous calculations of the air-sea biological O2 flux, and net production, along cruise tracks.
We are focusing our work on two regions: the equatorial Pacific and the Southern Ocean. The equatorial Pacific is of interest in ocean biogeochemistry as it spans oligotrophic to eutrophic waters, allowing the study of a wide range of planktonic ecosystems. The equatorial Pacific is relatively well described with models of ocean circulation and biogeochemistry, allowing one to quantitatively examine the mass balances of carbon and oxyen from the perspective of the oceans as well as the atmosphere. Our work in the equatorial Pacific, carried out by Melissa Hendricks (Ph. D., University of California), has involved a 3-D study in which constrains the variations in the triple isotope composition of O2 between 8° N and 8° S latitude for the basin east of 165° E. The results show that the triple isotope composition of O2 reflects the 3-D patterns of photosynthesis, respiration, advection, and diffusion in the study region. A particularly intriguiging finding is that, in highly undersaturated waters 10's of meters below the euphotic zone, on the order of half the dissolved O2 is of photosynthetic origin. It probably originates from photosynthesis in the lower part of the euphotic zone closer to the water mass source regions. The results are being evaluated quantitatively in work of John Dunne (Geophysical Fluid Dynamics Laboratory), using a 3-D ocean biogeochemistry general circulation model.
Our second study area is the Southern Ocean. This region is of particular interest because of the major role the basin plays in setting the partial pressure of CO2 in air, and the availability of nutrients to the other oceans of the world. Our work here to date has involved only studies of samples collected underway by collaborators aboard ships of opportunity. The work was initiated by Melissa Hendricks, and is primarily the effort of Matt Reuer (Ph. D., M. I. T.), currently at Colorado College; it is also a collaboration with Professor Paul Falkowski (Rutgers University). Matt and Melissa have analyzed samples from over 20 crossings of the Southern Ocean. The key result is that net and gross production increae to the north. This result is somewhat unexpected since colder temperatures, higher silica concentrations, and iron supplied by upwelled waters are all thought to be limiting, and all favor higher net production at the southern end of the transect
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Net and gross production in the Southern Ocean: Net and gross production on three transects across the Southern Ocean, south of Australia and New Zealand (Hendricks et al., 2004; Rever et al., 2005). |
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A summary of our results for the distribution of air-sea O2 fluxes in
the Southern Ocean. Each circle represents the location and time of
a measurement of O2/Ar in the oceanic mixed layer. We calculated air-
sea biological O2 fluxes from the biological O2 supersaturation (O2
supersaturation in excess of Ar supersaturation) and a
parameterization of gas exchange in terms of windspeed. Filled
circles correspond to positive fluxes from ocean to atmosphere; open
circles correspond to fluxes from atmosphere to ocean. The size of
the circle is proportional to the magnitude of the flux; the largest
diameter corresponds to a flux of 170 mmol m-2 day-1. Colors
record seasons: orange is fall, blue is summer, and green is spring.
Positive fluxes predominate during summer. They are sustained by net
community production and estimate the rate of that term. Negative
fluxes reflect net heterotrophy or (more likely) ventilation during
spring and fall. This figure summarizes work of Melissa Hendricks,
Jan Kaiser, Nicolas Cassar, Bruce Barnett, and outside collaborators,
and was assembled by Nicolas Cassar.
Click on figure to expand. |