Response of a phytoplankton community to nutrient addition under different CO2 and pH conditions

Effects of nutrient enrichment on 400-L coastal phytoplankton community cultures were examined under 3 pCO(2) levels [ambient (400), 800 and 1200 mu atm]. Three days after addition of the nutrients, rapid increases in phytoplankton pigments and cell numbers were noticed. Relative growth rates of diatoms and dinoflagellates were higher than prasinophytes and haptophytes during early stages of the culture experiment, and only limited effects of increased CO2 were observed on nutrient consumption, biomass and cell numbers. Ocean acidification showed significant effects on phytoplankton composition during the post-blooming period with negligible dissolved nutrients; up to 70 % of total cells were picoplankton in the 1200-mu atm condition as compared to 20 % in the ambient condition. An increase in chlorophyll b and a flow cytometry analysis of the cultured strains strongly suggested Micromonas-like (Prasinophyceae) picoplankton dominated in the acidified conditions. It is likely that the effects of ocean acidification are significant in low-nutrient conditions such as during the post-blooming period.