Mapping Strategies in Chemical Oceanography - American Chemical

1 New Problems for Chemical Oceanographers E D W A R D J.

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Downloaded by MCGILL UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: August 1, 1985 | doi: 10.1021/ba-1985-0209.ch001

Office of Naval Research, Arlington, VA 22217

The development of new methodologies in chemical oceanography—techniques such as underway sampling, fiber-optic technology, laser-induced fluorescence, flow-injection analysis, and other new autoanalyzer methods—promises to initiate a revolution in the scope of problems that can be addressed by marine chemists. The new problems are to be found in chemical variations occurring on short spatial scales and short time scales in upwelling and other frontal regions where chemical patchiness and biological patchiness are strongly interactive. D I T T M A R P U B L I S H E D H I S R E S U L T S F R O M T H E F I R S T 2 years of the H . M . S . Challenger expedition almost 100 years ago (I). His results demonstrated that, to a very good first approximation, seawater could be considered a two-component system (water and sea salt). Consequently, 99.5% (by weight) of all the chemical components of seawater became relatively uninteresting to water-column chemists. However, 80 of the elements of the periodic table could be found i n the remaining 0 . 5 % . Consequently, marine chemists have tended to concentrate on the minor and trace components of seawater because the interesting variations occur i n them; that is, they reveal information about the processes and mechanisms that influence the chemistry of seawater. Chemical oceanographers, as all scientists do when their field of study matures, naturally progress from the survey mode to process-oriented investigations, from correlations of observations to hypotheses of mechanisms, and from the general understanding of the chemistry of seawater to highly specialized and focused interests. W I L L I A M

Evolution

of Chemical

Oceanography

Chemical oceanography has evolved from the study of the major elements to the investigation of minor components—the nutrients, silica, and dissolved oxygen—then on to radiotracers, trace metals, organics, and bio0065-2393/85/0209-0001 $06.00/0 © 1985 American Chemical Society

In Mapping Strategies in Chemical Oceanography; Zirino, A.; Advances in Chemistry; American Chemical Society: Washington, DC, 1985.


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genie gases (Figure 1). Simultaneously, analytical methods and techniques have also evolved. Initially our basic tool was based on the hydrographic wire and the Nansen bottle, a technology rooted in the 19th century. Despite limited vertical and horizontal resolution, the data obtained still represented the most valuable base for discussion of the general circulation of the oceans. In the past 20 years the Nansen-bottle technology has been replaced w i t h the conductivity, temperature, and depth (CTD)-rosette system. Vertical resolution of samples is now within centimeters, temperature measurements are within the accuracy of the best reversing thermometers, and data are transmitted directly to computer, which not only eliminates tedium but also safeguards against transcription and calculation errors. The advent of the CTD-rosette has, however, some disadvantages. Nansen bottles are purely mechanical, and, therefore, can be deployed, maintained, and repaired by technicians w i t h little specialized training. C T D systems are sophisticated electronic instruments that require personnel trained especially i n their maintenance. Moreover, they are linked to complex computers and software that also need highly skilled and trained operators. Therefore, data acquisition has not only become much more expensive, it is now limited, at least i n the modern sense, to large institutions. The net result of these changes is that much more coordination and planning are

Figure 1. The evolution of chemical oceanography, 1884-1984. Oceanic fronts are characterized by areas of enhanced biological activity—the major process by which the chemistry of the upper ocean varies on a short time scale.



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required of scientists who wish to justify and mount major field efforts i n oceanography. These trends w i l l continue and the specialization and sophistication of data gathering w i l l intensify. A major driving force in this direction is the increasing interest i n problems characterized by shorter time and space scales. For many years the equipment and techniques available to physical oceanographers made it almost impossible to study any phenomenon other than the general circulation i n the ocean. These very large scales of motion, the general circulation, are set by the large-scale w i n d stress pattern, the large-scale solar heating and cooling, and the broad bathymetry of the ocean basins. However, as advanced techniques and equipment allowed measurement on finer grids and shorter time scales, the mesoscale fluctuations were found to be much more energetic than the general circulation. The great bulk of the energy supplied to the ocean does not drive the general circulation, but rather drives the finer scale fluctuations in the ocean, with horizontal space scales of tens to several hundreds of kilometers, and time scales not of decades or years, but of many days to several months. Possible New

Tools

W e foresee a parallel development i n chemical oceanography. In the past 7 years, considerable attention has been paid to the chemistry of Gulf Stream rings, features w i t h spatial scales of tens of kilometers and lifetimes of months. T o study still smaller phenomena—ocean fronts that may have spatial scales of tens of meters and evolution periods of days—the old hydrographic cast method of observation is inadequate. One possible direction for the future is the deployment of underway towed pumping systems. Some investigators have already made significant progress toward this goal. Advances i n plankton sampling have been made by Alex W . H e r m a n . He deploys his Batfish vehicle to measure chlorophyll a fluorescence, conductivity, temperature, and depth with a C T D device, and zooplankton numbers with a conductivity sensor. Eugene Traganza has been surface sampling with an on-deck pump i n his studies of the California current upwelling system. I n this region, where strong chemical variability extends all the way to the surface, significant work can be done with surface sampling. I n most cases, however, sampling over a range of depths within the photic zone w i l l be necessary. D a v i d R. Schink and his colleagues have employed a modified version of a commercially available towed pump to study biogenic dissolved gases i n the ocean. Richard J . Mousseau and his 1

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Bedford Institute of Oceanography. Naval Postgraduate School. Texas A & M University. G u l f Science and Technology Company.


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coworkers have had a similar hydrocarbon "sniffer" operational i n petroleum exploration for several years. The National Oceanic and Atmospheric Administration has put some effort into developing a six-port pumping system for the study of dispersion of wastes in the sea.


Other Needed

Instrumentation

Underway water sampling is only part of the solution. Underway chemical analyses are essential to the investigation of small-scale, rapidly evolving features. Continuous, in-situ specific electrodes for all the chemical parameters of interest would be ideal, but currently we have only the p H electrode and the dissolved oxygen sensor, and very few ideas on how to proceed with in-situ sensors for other chemical species at trace concentrations. The next best thing is to mate underway water sampling with continuous or very rapidly repetitive, on-board chemical analyses. Considerable progress is being made i n this area also. The autoanalyzer represented a substantial advance in the ability to make repetitive on-board chemical measurements. The development of flow-injection analysis promises to enlarge enormously the scope and speed of such methods. The flow-injection methods were pioneered in clinical chemistry, and are being developed for oceanography by Kenneth S. Johnson and Robert L . Petty. D a n a Kester and Richard W . Zuehlke are also developing new autoanalyzer techniques for the rapid analysis of certain trace metals i n seawater. One of the most technologically exciting prospects for the future involves the use of fiber optics to transmit a spectroscopic signal from an in-situ sensor to the ship's deck. Just as with underway water sampling, these analytical systems are becoming complex, expensive, and require highly trained technical specialists to maintain and operate them. This effort and expense can only be justified if significant oceanographic problems are being tackled. Understanding the biology and chemistry of frontal processes is such a significant problem. 5

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The Importance

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Fronts

Ocean fronts are regions where vertical advection and the exchange of momentum and other properties are intense. In addition, two-sided convergences are very effective i n collecting and concentrating suspended particulate matter. For these reasons, i n part, fronts are regions of enhanced chemical variability. Moreover, an understanding of the role of biological fronts is needed i n both diagnostic and prognostic biological productivity models. A number of intensely interesting questions can be posed about the 5 6

University of C a l i f o r n i a , Santa Barbara. University of Rhode Island Graduate School of Oceanography.


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biology and chemistry of fronts: D o distributions of chemical properties at ocean fronts follow general patterns; what mechanisms—physical, biological, or chemical—are important for these patterns in chemical properties; can predictive chemical models be developed based on physical distributions and remote sensing imagery; what diffusional processes occur across ocean fronts and how can understanding of these processes be used for ocean prediction purposes? M a n y of the contributors to this volume are i n the forefront i n answering these questions.


Recent Progress in Upper Ocean

Chemistry

Traganza and his students have studied the eddies of the California upwelling (2-3). His system allows continuous chlorophyll a and temperature measurements. Nutrients are processed by autoanalyzer every 2 m i n (~500-m spacing at 9 knots); adenosine triphosphate (ATP) and guanosine triphosphate (GTP) are processed every 10 m i n (~2.5-km spacing). These densely spaced data show the expected close correlation of temperature and nutrient chemistry; the cold water is high i n nutrients, the w a r m water is low. Most interesting, however, is the location of biomass concentrations as determined by A T P and chlorophyll. Traganza's data show strongly that the blooms are occurring not i n the cold, nutrient-rich water, but rather on the thermochemical gradients. This occurrence is particularly true for the equatorial side of cyclonic cold water spirals. This observation was supported by the agreement between in-situ data and color imagery from the Nimbus-7 coastal zone color scanner. Because of the location of the phytoplankton blooms i n the sharp nutrient gradients, and because these features persist, Traganza has suggested that the gradients may act as "natural chemostats" i n which the specific growth rate is equal to the dilution rate, and the total biomass is proportional to the concentration of the nutrient supply. His study provides evidence that, i n certain situations, the biological patchiness may be set initially by the chemical patchiness that is a consequence of the physical oceanography. The reverse situation may be true with respect to the distribution of dissolved molecular hydrogen in the ocean. Schink's observations (4) of diel variation of hydrogen concentrations i n the controlled ecosystem population experiment containers and in the Marine Ecosystem Research Laboratory tanks have been confirmed i n the open ocean by Herr et al. (5). H y drogen is probably produced either i n the guts of zooplankton or fish, or by cyanobacteria. Dissolved hydrogen typically decreases with depth below the photic zone; apparently, the gas is consumed by microflora in the deep ocean. Such consumption occurs in freshwater (6) and anaerobic (7) systems. The studies i n this book (by Zuehlke and Kester, Herman, Schink, Traganza, and others) indicate that biological activity is the major process changing the chemistry of the upper ocean on a short time scale. Unravel-


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ing the biological-chemical relationships w i l l provide some of the most challenging problems for chemical oceanographers i n the years ahead.


Literature

Cited

1. D i t t m a r , W . i n "Report on the Scientific Results of the Exploring Voyage of H . M . S . Challenger. Physics and Chemistry"; Murray, J., Ed., H.M. Stationery Office: L o n d o n , 1884; v o l . 1, p p . 1-251. 2. Traganza, E. D.; Nestor, D. A.; M c D o n a l d , A. K . J. Geophys. Res. 1980, 85, 4101-6. 3. Traganza, E . D.; C o n r a d , J. C.; Breaker, L. C . i n "Coastal U p w e l l i n g , " Richards, F . A., Ed.; A m e r i c a n Geophysical U n i o n : Washington, D.C., 1981; p p . 228-41. 4. Bullister, J. L.; Guinasso, N. L., Jr.; Schink, D. R. J. Geophys. Res. 1982, 87, 2022-34. 5. Herr, F . L.; Frank, E.; Leone, G. M.; Kennicutt, M. C . Deep-Sea Res. 1984, 31, 13-20. 6. C o n r a d , R . ; Aragno, N.; Seiler, W . Appl. Environ. Microbiol. 1983, 45, 50210. 7. Sorensen, J . ; Christensen, D.; Jorgensen, D . D . Appl. Environ. Microbiol. 1981, 42, 5-11. RECEIVED for review July 15, 1983. ACCEPTED January 19, 1984.


Mapping Strategies in Chemical Oceanography - American Chemical

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