A Chlorophyll Bloom in the Equatorial Pacific

The open ocean is considered a barren desert, however little oases of life are known to propagate anywhere. Papa Mau recently encountered an increase in chlorophyll-A concentration on his leg between Hawaii and the first TAO mooring [6/20/2012 15:20:04 (UTC); 9.8106 lat., -167.6827 long.]. Benjamin, who was trailing Papa Mau by about 180 miles at the time encountered the bloom 6 days later [6/26/2012 15:40:05 (UTC); 9.7325 lat., -167.7568 long.]. An increase in chlorophyll-A is indicative of a greater density of phytoplankton, microscopic ocean plants that form the base of the oceanic food chain. Distance traveled since the initial response is over 300 miles, this bloom is extensive. The computer generated image you can see is a screen grab of the fluorometer response displayed in Google Earth.

Embedded is a graph of the raw data. Notice the per diem fluctuation of phytoplankton density as they alter their height in the water column (at night moving deeper to respire and at day shallower to better access light needed for photosynthesis). The fact that Benjamin is starting to see an overlapping cycle as he enters the bloom adds weight to the discovery.

The discovery of this bloom confirms studies that describe a relatively recent increase in phytoplankton blooms in the Pacific Ocean (Venrick et al. 1987). “Since 1968 a significant increase in total chlorophyll a in the water column during the summer in the central North Pacific Ocean has been observed. A concomitant increase in winter winds and a decrease in sea surface temperature suggest that long-period fluctuations in atmospheric characteristics have changed the carrying capacity of the central Pacific epipelagic ecosystem.”

In other words, our changing climate is affecting the activity of the fundamental base of the ocean food chain. This will have widespread consequences for the entire ocean ecosystem in the coming decades.

Typically, phytoplankton blooms occur when an unusual abundance of a growth-limiting nutrient (such as iron, phosphorous or, more typically, nitrogen) becomes available near the ocean, allowing for a greater density of organisms to coexist than normal. Changes to ocean convection and deep-water nutrient upwelling could be providing the extra nutrients to support such a phytoplankton bloom, along with temperatures more favorable to growth. Scientists have sometimes poured large doses of these nutrients into the middle of the ocean and witnessed a consequent phytoplankton bloom (Martin et al. 1994).

Embedded above are graphs of water temperature and salinity inside, and before coming across, the bloom. Water temperatures inside are significantly higher than outside. The concomitant increase in salinity is likely unrelated to the bloom itself; increased water temperature quickens evaporation, leaving behind higher salinity concentrations. The precise correlation of the different data sets is in itself fascinating to see.

An interesting feature of this data is the distinct boundary between ocean conditions within and outside of the bloom. Notice how Papa Mau’s temperature and salinity measurements rapidly spike when entering the bloom, and how 6 days later Benjamin records exactly the same phenomenon (particularly salinity). It is possible that the increased phytoplankton biomass and its respiration/productivity create a warmer patch of water. The two related processes of blooming and water temperature together form a positive feedback system in which the warmer water temperature creates a thermocline that traps that water and phytoplankton near the surface (warm water is less dense), thereby causing the water to heat further, enhancing the thermocline, and so on and so forth. This kind of feedback loop plays a large part in enabling mid-ocean blooms to propagate so successfully.

Data like that gathered by these two gliders is crucial to better understanding the life cycles and processes of the ocean in order to better inform conservation efforts, sustainable fishing, as well as a host of other applications.

Here’s to other, exciting, future discoveries!

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References

Venrick, E. L., McGowan J. A., Cayan, D. R., and Hayward T. L. 1982 Climate and Chlorophyll a: Long-Term Trends in the Central North Pacific Ocean. Science Vol. 238, no. 4823, pp. 70-72.

Martin J. H. et al.  1994 Testing the Iron Hypothesis in Ecosystems of the Equatorial Pacific Ocean. Nature Vol. 371, pp. 123-129.