Contrasting marine phytoplankton responses to meltwater inputs from Arctic and Antarctic glaciers revealed by bioassay experiments

Along the coastal periphery of the Greenland and Antarctic Ice Sheets, global warming has increased freshwater discharge and associated fluxes of terrigenous material from the cryosphere into the ocean. Shifts in the availability of light and bio-accessible nitrogen, phosphorous, silica, iron, manganese, and cobalt can influence seasonal patterns of marine primary production. Yet the spatial and temporal scales of changes to these drivers from ocean-cryosphere interaction remain unclear. Using bioassay experiments, we tested the response of coastal phytoplankton from Antarctica and Greenland to freshening and changes in micronutrient availability. In both polar regions, additions of freshwater >2% by volume resulted in significant short-term negative responses by the primary producers, yet their responses to changing micronutrient stoichiometry diverged. In the Western and Northern Antarctic Peninsula, 7 of 8 bioassay experiments, which incubated the ambient microbial community, suggested that phytoplankton experienced conditions replete with macro- and micronutrients and growth was light-limited during austral summer. Conversely, at one Antarctic incubation site upstream of local micronutrient sources, results indicated a novel cobalt and iron co-limitation with serial manganese limitation of phytoplankton growth. In contrast, bioassay results from West Greenland evidenced a more variable situation: phytoplankton responses suggested a combination of light limitation, nitrogen limitation with and without serial silica limitation, and phosphorous limitation. Phosphorous limitation is not thought to be common in polar marine waters yet increasingly may be plausible in Arctic fjords subject to increased freshening and stratification. Future responses of primary producers to increasing freshwater discharge around the Antarctic Peninsula and West Greenland may therefore diverge. Around Greenland, runoff may drive inshore communities increasingly towards nitrogen and possibly phosphorous limitation, while alleviating serial silica limitation for siliceous microalgae. Conversely, around the Antarctic Peninsula increasing runoff may alleviate light limitation via stratification and result in more efficient macronutrient drawdown.