I conducted studies examining the effects of iron supply, temperature, and/or pCO2 on algal community structure in three important oceanic regimes, the Peruvian upwelling, Southern Ocean, and Bering Sea. Global climate change is already having large impacts on primary producers in the ocean, including food web structure and nutrient cycles. These studies were conducted using a new shipboard method that I helped to develop, which adapted laboratory continuous culture techniques for bottle incubations at sea using natural algal communities.; In the iron-limited Peru upwelling regime and the Southern Ocean, I examined shifts in phytoplankton community structure and nutrient biogeochemistry following simulated upwelled iron inputs. In the Peru study, the continuous addition of iron at increasing concentrations progressively shifted the community towards diatoms and away from cyanobacteria and nanophytoplankton. In the Southern Ocean, I found that as the rate of upwelling increased, the assemblage shifted towards diatom species that were smaller and faster-growing, as well as non-silicified algal groups. In the Bering Sea, I incubated phytoplankton communities with elevated sea surface temperature and/or pCO2 similar to predicted year 2100 values. In these "greenhouse ocean" simulations, maximum biomass-normalized photosynthetic rates increased 2.6--3.5 times and community composition shifted away from diatoms and towards nanophytoplankton. These effects were driven largely by elevated temperature, with secondary impacts from increased pCO2.; Global climate change is predicted to have large impacts on the ocean, causing alterations in algal dominance, nutrient cycles, and carbon export. Gaining new insights into questions about environmental change will require the development of novel, versatile methods. The shipboard continuous culture system (Ecostat) is a new tool allowing researchers to closely examine fine-scale shifts in the phytoplankton community. This experimental design can simulate some natural oceanic processes more realistically than is the case with existing experimental techniques. The combination of different types of manipulative experiments with observational and modeling approaches will ultimately allow us to both predict the likely directions of future global changes, and help to answer questions about changes that occurred in ocean ecosystems in the past.
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