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
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.
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."
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."