Final Project Report
Our primary goal in this research project was to elucidate the photobiological
responses of phytoplankton to the addition of iron in the Southern Ocean. To this
end, we fabricated one new fast repetition rate fluorometer (FRRF), refurbished an
existing instrument, and repaired a commercial instrument. At the invitation of
Professor Victor Smetacek from the Alfred Wegener Institute, we deployed two fast
repetition rate fluorometers and one senor researcher on the German research vessel,
Polarstern, to participate in a European fertilization experiment Southern Ocean in
the winter of 2000-2001.
In the winter of 2000-2001 we participated on both research vessels in the SoFEX
iron addition experiments. We deployed 4 instruments with 4 researchers. One
researcher was a professional photographer who, in addition to standing watch on the
FRRF, documented the experiment with both still and moving pictures. A movie made by
him was used in subsequent outreach projects for K-12 students.
On each ship we mapped the horizontal and vertical profiles of the effect of iron on
quantum yields, effective absorption cross sections, and photosynthetic efficiency in
real-time.
From post cruise data processing, we developed quantitative estimates of the effect of
iron fertilization on the change in primary production, the change in chlorophyll
synthesized, and the change community response to the iron addition.
Our instruments clearly showed that iron fertilization enhanced the photosynthetic
efficiency of phytoplankton in the near surface waters of
the Atlantic sector within 10 days, but that near the base of the euphotic zone, iron
and macronutrients appeared to be sufficient to lead to high quantum efficiencies.
Our experimental data revealed that iron limitation clearly forces a physiological
constriction on the diatom community in that sector of the Southern Ocean. The data
indicate that the response time to iron addition was at least an order of magnitude
longer than that observed in the Equatorial Pacific in the IronEx experiments; we
attributed the difference to temperature. The results of this work, published in
Limnology and Oceanography, remain the only published results from the EISENEX
experiment to date.
The FRRF signals were the first to reveal the effect of iron on stimulating primary
production, and the maps we generated at sea showed the spatial and temporal evolution
of, what would become, blooms of phytoplankton in the two fertilized patches.
Our core results were summarized in the SOFEX paper published in Science
(Coale et al. 2004). An additional paper is being prepared for Deep-Sea Res.
(Gorbunov et al, in press).
Finally, Sasha Tozzi, a graduate student who participated in the SOFEX experiment,
developed a thesis from lab and field work (Tozzi 2001), and a publication (Tozzi et
al, 2004).
Publications
Gervais, F., Riebesell, U., Gorbunov, M.Y. (2002). Changes in primary productivity and
chlorophyll a in response to iron fertilization in the Southern Polar Frontal Zone.
Limnology and Oceanography 47, 1324-1335.
Coale, K.H. et al. (2004) Southern Ocean iron enrichment experiment (SOFeX): Iron,
silicon, and light interactions in Antarctic waters. Science 304: 408-414.
Tozzi, S, S. Schofield, and P. G. Falkowski. 2004. Historical climate change and ocean
turbulence as selective agents for two key phytoplankton functional groups. Global
Change Biol. (in review).
Competition and succession of key Marine phytoplankton functional
groups in a variable environment
by Sasha Tozzi
Dissertation Director:
Paul G. Falkowski
The goal of this thesis is to understand how the physiological ecology of phytoplankton
impacts the competitive success between biogeochemically-significant phytoplankton
taxa. More specifically, the effect of a variable nutrient regime on resource competition
and successions between diatoms and coccolithophores was studied using a prognostic
numerical model. Numerical simulations assessing the impact of nutrient vacuole present
in diatoms but not in coccolithophorids were conducted and results were verified with
laboratory experiments. The vacuoles advantage diatoms with pulses up to 24 hours or
few divisions. Diatoms result then to prevail in a high turbulent mixing environment
with high nutrient concentrations and coccolithophores in a more stable and depleted
water column.