Proposal Abstract:
Oceanographers recognize the importance of phytoplankton photosynthesis in affecting atmospheric pCO2 and global climate change. Little attention has been given, however, to the opposite question: How will rising anthropogenic pCO2 levels and associated climate changes impact algal community structure and biogeochemistry? Climate-related shifts in phytoplankton assemblages may have profound implications for oceanic feedbacks on the atmosphere, and for human use of marine resources. This is because particular algal groups are largely responsible for crucial processes like vertical carbon export, biogenic calcification, production of climatically active gases like dimethylsulfide (DMS), and for supporting food webs that lead to economically valuable higher trophic levels. Increasing pCO2 and temperature, as well as changes in light and nutrient supplies from a shoaling mixed layer and a stronger thermocline, can drive major switches in algal diversity. These dominance changes include dramatic shifts between siliceous, calcareous, and non-mineralizing phytoplankton, between groups that are major or minor contributors to the biological pump, and between taxa that produce negligible or copious amounts of DMS. The PIs are investigating the potential impacts of pCO2 and climate change on algal community structure and biogeochemistry in the Bering Sea, an area that already exhibits signs of rapid climate regime shifts. Two independent experimental methods are being used: Semi-continuous incubations, to simulate non-equilibrium conditions due to episodic events such as storms, and a unique shipboard chemostat system to simulate quasi-steady state stratified conditions. Experimental treatments include: (1) Ambient pCO2 and temperature conditions, and (2) Elevated pCO2 (750 ppm) and temperature (+ 3o C). Each treatment is subdivided into two mixed layer depth simulations: (1) Ambient nutrient and light conditions and (2) A higher light/lower nutrient treatment to simulate a future decreased MLD. Levels of pCO2 and physical variables chosen were based on current model predictions for the surface ocean by the year 2100. Phytoplankton diversity and carbon and nutrient cycling are being closely followed in these greenhouse ocean simulations, with particular attention to shifts between biogeochemically critical groups like diatoms, coccolithophorids, and Phaeocystis.