Major Phytoplankton Species under Elevated CO2 and Climate Change

The anthropogenic CO2 perturbation and the associated climate change have a huge effect on the ocean surface temperatures and on precipitation, which lead to a stronger stratification of the surface ocean. These changes influence phytoplankton communities directly. Phytoplankton are responsible for a high percentage of carbon fixation by photosynthesis. About 30% of this organic carbon is exported into the deep ocean, which makes phytoplankton an important sink for CO2. With climate change the phytoplankton net primary production (NPP) can be altered. Increased stratification can, on the one hand, lead to a reduced nutrient availability as the upward transport of nutrients from the deep ocean to the surface is reduced. Thereby, primary production is reduced. On the other hand, stratification leads to better light conditions, which results in higher NPP production. How NPP will change in the future remains unclear as different studies project different results. In order to understand how NPP will change in the future one needs to understand how the phytoplankton species composition in the ocean will alter. To get a better comprehension, I have a closer look at two important phytoplankton functional groups, diatoms (silicifying phytoplankton) and coccolithophores (calcifying phytoplankton), which are major drivers of the biogeochemical cycling in the ocean. I focus on how the distribution of these two phytoplankton groups change with increasing atmospheric CO2 concentrations and how this affects the NPP and in turn the climate. It has been shown that under increased oceanic CO2 concentrations coccolithophores have a competitive advantage in comparison to other phytoplankton functional groups. Besides, smaller sized phytoplankton will be in general more likely produced under nutrient limited conditions. Generally, diatoms will have a disadvantage and coccolithophores an advantage under these conditions. There are different feedbacks that diatoms and coccolithophores have on the climate. Diatoms for example contribute a lot to the export production of carbon (about 40%). As they are projected to be reduced under climate change, a considerable part of the export production will be reduced, too. This implies a positive feedback to the atmospheric CO2 concentration. In contrast, due to increased dissolved CO2 in the ocean, the pH decreases, which leads to a reduction of calcification by coccolithophores. Thus, this leads to a negative feedback on atmospheric CO2, as the production of CO2 during calcification is reduced. However, there are many other feedback loops (positive and negative), which makes it difficult to predict in what direction the marine biosphere will finally influence the CO2 concentration in the atmosphere and the climate. A lot of research needs to be done in the future in order to get a clearer idea how the climate influences phytoplankton structure and in turn, how the different phytoplankton groups influence the climate.