imageNow we see that we may be returning back to a sustained period of strong seasonality, so maybe we weren't that far off with our original interpretations. But why the changing pattern? What is causing this flip-flop in the seasonality of the export processes? At the present time we simply do not quite understand what's driving the carbon flux at our study site despite 13 years of approximately monthly sampling. This reinforces my earlier comments about undersanpling and a lack of comprehensive understanding of even the most basic carbon cycle processes. If one examines carbon export versus primary production, which we considered to be fairly well understood at the start of our study, we don't see the "expected" trends. In fact, there's very little relationship between the contemporaneous primary production and the contemporaneous carbon flux at either the HOT site or at our sister program near Bermuda.

imageSo, again, these examples underscore some of the complexities and the uncertainties of the biological carbon pump. The same thing is true for nitrogen but not for phosphorus. In fact, when we examine the N:P ratios and compare them to the expected Redfield Ratio of image16N:1P, we see that the mean export N:P at Station ALOHA is much greater than the Redfield Ratio, which means that we're exporting more N relative to P. Furthermore, the system was pretty stable for the first several years, and now we've entered a period of large oscillation of N:P -- it looks like a model gone awry, but these are real data from the Earth's largest biome.

imageI would like to make one final comment before closing; it has to do with the perceived, well-publicized oligotrophic nature of the North Pacific subtropical gyre and the role of aperiodic shifts in community structure and carbon export. The ocean around Hawaii has always been considered to be a low nutrient, low biomass (chlorophyll) and low export ecosystem; apparently this is not always true. This satellite image shows the Hawaiian Island chain in red (false color) and a large open ocean bloom of phytoplankton (green) in the middle of the otherwise vast blue (low biomass) sea.


How can such a low nutrient system sustain such a large bloom? One possibility, as Dr. Chisholm has just shown, is to provide a pulse of a growth limiting nutrient; in this case bioavailable Fe. The primary natural mechanism for Fe deposition in these open ocean regions is as dust, transported across the North Pacific from deserts in China.

When deposited into the open North Pacific this dust can stimulate the local growth of large diatom cells, leading to near surface accumulations. Some of these blooms can produce aggregates (the event-driven carbon pump) and others can enhance carbon export by endosymbiotic N2-fixing cyanobacteria (the N2-primed prokaryote carbon pump). The diatom populations are displaced toward the sea surface where the Fe is first made available, which is why we can detect these events via color-sensing satellites. We hypothesize that these diatom blooms are a result of Fe deposition, and that even in these low- nutrient environments such aperiodic processes can significantly impact the biological carbon pump. If this can happen in the oligotrophic North Pacific gyre, it can probably happen everywhere on Earth.

imageThe importance of these open ocean diatom blooms for carbon export processes has recently been confirmed by collecting the exported materials in time-series sediment traps moored near the seafloor. The time-series record of particulate matter exported from the euphotic zone shows a dramatic depositional event approximately 1 month after the bloom lasting for a period of about 2 weeks. The exported materials escape remineralization and are deposited into the deep sea. With these returned sample materials in hand we have begun to use microscopes, mass spectrometers and other instruments to find out exactly who the organisms are, and why they bloomed then disappeared from the euphotic zone. Most importantly, we hope to also determine whether they were fueled by a Fe-deposition event and whether they thrived off N2-fixing endosymbionts. These are works "in progress."

imageSo, in conclusion, we have a lot to think about at this symposium, and I would just like to end with a little bit of good advice from Louis Agassiz, who said, "Study nature, not books." The information that we seek and the processes that we endeavor to understand about the ocean's carbon cycle are not yet in any books. They are, however, in nature. We must go to the field, perturb and systematically manipulate the habitat to fully understand it, and we should also "strive to interpret what really exists."

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