Award: OPP-1643652

Phytoplankton in Antarctic coastal waters grow more rapidly relative to waters farther offshore. This growth is limited by the availability of light for photosynthesis and the supply of dissolved iron. Dissolved iron is a trace nutrient which means it has low concentrations. The supply of dissolved iron to these coastal waters can be increased by melting sea ice, resuspension of bottom sediments, inputs by onshelf intrusions of an oceanic water mass, Circumpolar Deep Water, and from melting of the ice sheets that surround the Antarctic continent. For the latter source, melting of the bottom of the floating margins of the Antarctic ice sheet, the ice shelves, produces melt water that is less dense and rises along the base of the ice shelf. This upward moving water causes deeper waters with high dissolved iron concentrations to rise towards the surface. In this project, simulations provided by a computer model that coupled the ocean circulation with sea ice and the ice shelves showed that this overturning circulation driven by the melting ice shelves is an important source of dissolved iron to the well-lit surface waters in many locations around Antarctica. This implies that phytoplankton growth may be vulnerable to changes in ice shelf melt. The heat that melts the bottom of the ice shelf comes from the adjacent ocean waters that circulate under the ice shelf. Additional simulations with the ocean-sea ice-ice shelf model that used changes in different atmospheric conditions that are projected for 2100 showed an increase in the ice shelf melt, which produced an increase in the overturning circulation driven by the ice shelf melt. The result is an increase in the total dissolved iron supplied to the surface waters of the Antarctic continental shelf. The atmospheric changes also reduced the summer sea ice cover, making more light available to the ocean surface. The implication is that atmospheric changes projected for 2100 may result in an increase in phytoplankton growth and productivity around Antarctica. A second computer model that includes the growth dynamics of two primary phytoplankton taxa, Phaeocystis antarctica and diatoms, in response to light and dissolved iron availability provided evaluations of the importance of different sources of dissolved iron in supporting phytoplankton blooms in the Ross Sea. The simulations showed that blooms of P. antarctica are triggered by the beginning of light availability and the simultaneous availability of dissolved iron from the previous winter and melting sea ice. As surface dissolved iron is depleted, diatoms begin to grow in the spring and rapidly out compete P. antarctica and dominate the later stages of a bloom. Over a growing season, about four times more dissolved iron is associated with diatoms than with the P. antarctica biomass. As the phytoplankton bloom progresses it is supported by different sources of dissolved iron, resulting in different vulnerabilities to climate-induced modifications to environmental conditions. The simulations generated by this project provide a resource for other types of studies, such as those focused on Southern Ocean ecosystems, marine policy, and projects that are being developed as part of the UN Decade of Ocean Science and Sustainable Development. In particular, the circulation simulations are informing discussions of location and implementation of marine protected areas in the Southern Ocean. These applications represent value-added impact from this project. Last Modified: 01/11/2022 Submitted by: Eileen E Hofmann