Project Description
Our main objective in this proposed research is to investigate the relative importance and potential interactive effects of iron, light and CO2 levels in structuring algal assemblages and growth rates in the Ross Sea. We hypothesize that the interaction of these three variables largely determines the bottom-up control on these two dominant Southern Ocean phytoplankton taxa. Grazing and other loss processes will also be important variables in determining the relative dominance of these two taxa; however, the study proposed here will primarily focus on the bottom-up control mechanisms. It is important to understand such environmentally-driven taxonomic shifts in primary production, since they are expected to impact the fixation and export of carbon and nutrients, and the production of DMS, thus potentially providing both positive and negative feedbacks on climate.

Within the context of this proposal, we consider a range of ambient iron, light and pCO2 levels that span those typically observed in the Ross Sea during the growing season. That is, dissolved iron ranging from ~0.1 nM (“low iron”) to >1 nM (“high iron”) (Fitzwater et al. 2000; Sedwick et al. 2000); mean irradiance (resulting from vertical mixing/self shading) ranging from <10% Io (“low light”) to >40% (“high light”) (Arrigo et al., 1998, 1999), which may be adjusted based on our field observations (see section 6.3.3); and pCO2 ranging (Sweeney et al. 2001) from ~150 ppm (“low CO2”) to the probable higher levels of pCO2 — 750 ppm as a conservative estimate — that are likely to be attained later this century due to anthropogenic perturbation of the global carbon cycle (IPCC, 2001).

From the information currently available from both field observations and experiments, we have formulated the following specific hypotheses regarding the interactive role of iron, light and CO2 in regulating algal composition in the Ross Sea. Principally, we propose that diatoms bloom in the southern Ross Sea only under optimum conditions of high iron, light and pCO2; colonial Phaeocystis dominate under conditions of high iron with either (or both) low light or low pCO2; and solitary Phaeocystis are predominant under conditions of low iron with either (or both) low light or low pCO2. Two cruises are planned to investigate the interactions between the primary productivity of the Ross Sea and pCO2, iron and other trace elements.

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