WHAT STORY IS TOLD BY OCEANIC TRACER CONCENTRATIONS?

Reference
Gruber, N, K. Keller, and R. M. Key. What story is told by oceanic tracer concentrations?
Science, 290, 455-456, 2000.

Text

In their report "Temporal trends in deep ocean Redfield ratios", Pahlow and
Riebesell suggest that the marine biota has changed in the last few decades
in response to human activities. These findings challenge the steady-state
paradigm of ocean biogeochemistry and might have important implications
for the global carbon cycle. However, the signals that Pahlow and Riebesell
analyzed are subject to numerous methodological uncertainties [discussed
elsewhere (1), and their interpretation of the signals hinges critically on the
exclusion of alternative explanations. Here we propose alternative explanations
for the reported trends consistent with existing data and knowledge.

For North Atlantic deep waters, Pahlow and Riebesell report an increase in
nitrate to phosphate (N:P) ratios and suggest increased nitrogen deposition
as a cause. However, this mechanism would decrease the ratio of apparent
oxygen utilization (AOU) to nitrate, because this mechanism should lead to
an increase in nitrate without changing oxygen. This effect is not seen in
Pahlow and Riebesell's analysis. A small change in preformed nutrients
[resulting in a cumulative decrease in deep-water phosphate of about 0.02
micromoles per kilogram (umol/kg) over 20 years and a cumulative increase in
deep-water nitrate of 0.14 umol/kg over the same period (2) could also explain
the reported N:P trends. Such a change in preformed nutrients, caused for example
by a slight shift in the deep-water source regions rather than in biology, would be
undetectable with the presently available data because of large seasonal variability
and inadequate sampling.

For North Pacific deep waters, Pahlow and Riebesell report a small increase
in AOU and suggest enhanced export production as a cause. However,
increasing AOU could also be caused by decreasing water ventilation
rates--a change predicted as a response to global warming (3) and perhaps
manifested in recent chlorofluorocarbonbudgets (4). Testing this alternative
ventilation mechanism poses a formidable (and so far open) challenge.

There are many observations that demonstrate that the marine biota is
variable, especially on local scales (5). However, the conclusions by Pahlow and
Riebesell that the marine biota has changed ov er large spatial and temporal scales
are unsubstantiated because alternat ive hypotheses cannot be firmly excluded.

Nicolas Gruber
Institute of Geophysics and Planetary Physics and Department of Atmospheric Sciences,
University of California, Los Angeles,
Los Angeles,
CA 90095, USA

Klaus Keller
Robert M. Key

Department of Geosciences,
Princeton University,
Guyot Hall,
Princeton, NJ 08544, USA

References and Notes

1. L. I Gordon et al., Eos Trans. Am. Geophys. Union 80 (suppl.), OS45 (1999 );
   C. W. Mordy et al., Eos Trans. Am. Geophys. Union 80 (suppl.), OS43 (1999).
2. Computed by using the rates of changes reported by Pahlow and Riebesell and
   their equation 3, solved for the change in nutrient concentration (n_t1 - n_t0) and
   adopting a value of 120 umol/kg for AOU_t0.
3. S. Manabe et al., J. Climate 4, 785 (1991); J. L. Sarmiento et al., Nature 393, 245 (1998).
4. W. S. Broecker et al., Science 286, 132 (1999).
5. D. M. Karl et al., Nature 373, 230 (1995).

Response

Gruber, Keller, and Key suggest alternative explanations for the trends in
deep-ocean Redfield ratios obtained in our analysis. They suggest that a
small decrease in preformed nutrients could explain the trends observed in
the North Atlantic and would be undetectable with presently available data.
As we outlined in our report, however, the decrease in preformed phosphate
concentrations necessary to generate the observed increase in the N:P ratio
would have to be at least 12% or about 0.2 micromolar over 20 years. A
change in preformed phosphate concentrations of this magnitude, should it
exist, could be easily detected in the data available for the time period
covered in our analysis.

Gruber et al. say that increased aeolian nitrogen (N) deposition
"would decrease the ratio of apparent oxygen utilization (AOU) to
nitrate" in the North Atlantic. This would be true only if primary
production in the North Atlantic was not limited by nitrogen. Although a
transition from nitrogen to phosphorus limitation has been suggested for so
me areas, nitrogen is considered to be the dominant limiting nutrient in
the North Atlantic (1). A recent study by Wu et al. (2) reports phosphate depletion
in parts of the western North Atlantic. The authors conclude, however, that the
concentration of available nitrogen is decreased to limiting levels in these regions
because of the formation of refractory dissolved organic nitrogen. Although
increasing aeolian nitrogen deposition may have in fact reduced the AOU:N ratio
locally in phosphorus-limited regions, the effect would not be detected by our
large-scale analysis.

For the North Pacific, Gruber et al. suggest that decreasing
deep-water ventilation rates as a result of global warming may explain the
observed increase in AOU. North Pacific deep waters originate almost
entirely from the South Pacific(3). If decreasing deep-water ventilation had
caused the trends indicated for the North Pacific, similar trends should also
have occurred in the South Pacific 2E. Because this is not observed, changes
in deep-water ventilation are not likely to be the main cause for the changes in
AOU and oxidative ratios observed in the North Pacific.

In essence, although the nonbiologically mediated processes suggested by
Gruber et al. may have contributed to the observed temporal trends
in deep-ocean Redfield ratios, they cannot account for their magnitude 2E
The changes in marine biota that we proposed in our report thus remai n the
most plausible explanation for the observed trends.

Markus Pahlow*

Ulf Riebesell
Alfred Wegener Institute for Polar and Marine Research, <BR>
Post Office B ox 120161, <BR>
D-27515 Bremerhaven, Germany

*Present address: Bedford Institute
of Oceanography,
Dartmouth, Nova Scotia,
B2Y 4 A2, Canada

References

1. K. A. Fanning, Nature, 339, 469 (1989).
   
2. J. Wu, W. Sunda, E. A. Boyle, D. M 2E Karl, Science, 289, 759 (2000).
3. A. M. Macdonald, Prog. Oceanogr., 41, 281 (1998).