Climate change impacts on southern Ross Sea phytoplankton composition, productivity, and export
The Ross Sea, a highly productive region of the Southern Ocean, is expected to experience warming during the next century along with reduced summer sea ice concentrations and shallower mixed layers. This study investigates how these climatic changes may alter phytoplankton assemblage composition, primary productivity, and export. Glider measurements are used to force a one-dimensional biogeochemical model, which includes diatoms and both solitary and colonial forms of Phaeocystis antarctica. Model performance is evaluated with glider observations, and experiments are conducted using projections of physical drivers for mid-21st and late-21st century. These scenarios reveal a 5% increase in primary productivity by midcentury and 14% by late-century and a proportional increase in carbon export, which remains approximately 18% of primary production. In addition, scenario results indicate diatom biomass increases while P. antarctica biomass decreases in the first half of the 21st century. In the second half of the century, diatom biomass remains relatively constant and P. antarctica biomass increases. Additional scenarios examining the independent contributions of expected future changes (temperature, mixed layer depth, irradiance, and surface iron inputs from melting ice) demonstrate that earlier availability of low light due to reduction of sea ice early in the growing season is the primary driver of productivity increases over the next century; shallower mixed layer depths additionally contribute to changes of assemblage composition and export. This study further demonstrates how glider data can be effectively used to facilitate model development and simulation, and inform interpretation of biogeochemical observations in the context of climate change. Plain Language Summary Understanding how the global ocean responds to climate change requires knowing the natural behavior of individual regions and anticipating how future changes will affect each region differently. It is especially important to determine these behaviors for regions changing in unique ways and for regions relatively undisturbed by human influences. One such region is the Ross Sea, which has some of the most productive marine plants and animals around Antarctica. Significant changes in the Ross Sea environment are likely over the next century, but it is not known how these changes will impact the marine food web. In this study, computer simulations give us an idea of how warmer temperatures combined with other changes related to melting sea ice may impact the base of the Ross Sea food web over the next century. The simulations show changes in algae species, increases in the amount of plant matter produced, and increases in the amount of plant matter that sinks from the well-lit ocean surface to deeper waters. The details of what cause these changes in the simulations give us new ways of thinking about change in the Ross Sea and point us toward parts of the system warranting further study.