Although coastal waters make up only about 10% of the surface area of the ocean, they harbor most of its life. Measuring biological activity in these regions can reveal their impact on fisheries, low-oxygen dead zones, and the global carbon cycle, but coastal zones remain understudied. Now new research by Teeter et al. suggests how to improve the accuracy of a method that uses oxygen and argon measurements to quickly estimate marine biological activity.
Traditionally, measurement of coastal biological activity has required time-consuming experiments with water samples in bottles. A faster method estimates net community production: the balance between oxygen consumption and oxygen production by all organisms in the ecosystem, including microbes, plants, and fish. This approach uses shipboard measurements of the ratio of dissolved oxygen to argon in ocean water, which reflects the amount of oxygen produced by marine organisms that is released into the atmosphere. In coastal zones, however, upwelling delivers cold, low-oxygen water to the surface, altering oxygen-argon ratios and resulting in underestimation of net community production.
The researchers quantified these inaccuracies across a stretch of the California Current that flows southward off the west coast of Canada. They used a computational framework called the Regional Ocean Modeling System to calculate net community production in the region over the course of a few weeks and compared these values to those estimated from oxygen-argon ratios.
The analysis showed that oxygen-argon ratios can accurately capture net community production for a large coastal region when averaged over several weeks. However, the researchers confirmed that upwelling results in underestimation of net community production on smaller time and spatial scales, and they quantified the degree of this inaccuracy under different upwelling scenarios.
The scientists also investigated several factors associated with upwelling to see whether any of them could be used to identify when net community production measurements are likely to be inaccurate. They considered sea surface salinity, partial pressure of carbon dioxide, and chlorophyll levels but found that sea surface temperature was the best indicator of inaccuracies.
These findings suggest that sea surface temperature measurements could aid the use of the oxygen-argon method by pinpointing net community production estimates that may be inaccurate. Future research could provide additional insights by considering other settings, such as the windy California-Oregon coast, and using more complex modeling strategies. (Global Biogeochemical Cycles, https://doi.org/10.1029/2017GB005874, 2018)
—Sarah Stanley, Freelance Writer