Increasing Transparent Exopolymer Particle (TEP) formation during diatom blooms as a result of elevated temperature and pCO2 have been suggested to result in enhanced aggregation and carbon flux, therewith potentially increasing the sequestration of carbon by the ocean. We present experimental results on TEP and aggregate formation by Thalassiosira weissflogii (diatom) in the presence or absence of bacteria under two temperature and three pCO2 scenarios. During the aggregation phase of the experiment TEP formation was elevated at the higher temperature (20°C vs. 15°C), as predicted. However, in contrast to expectations based on the established relationship between TEP and aggregation, aggregation rates and sinking velocity of aggregates were depressed in warmer treatments, especially under ocean acidification conditions. If our experimental findings can be extrapolated to natural conditions, they would imply a reduction in carbon flux and potentially reduced carbon sequestration after diatom blooms in the future ocean.

A laboratory experiment was designed to investigate the effects of ocean acidification (three levels) in the presence or absence of the clay mineral illite (five concentrations) on the coagulation of organic particles. The formation of aggregates . 1 mm from marine detritus and phytoplankton, and their characteristics and sinking velocity, were monitored during the 48 h experiment. Aggregation of particulate organic carbon (POC) was independent of both partial pressure of carbon dioxide (P CO2 ) and illite addition, implying that the fraction of POC available for export is not affected by either mineral supply or ocean acidification conditions up to ambient + 52.7 Pa P CO2 (+ 520 ppm). This was true even though the illite appreciably influenced aggregate size, number, and characteristics, including the percentage of transparent exopolymer particles (TEP) incorporated in aggregates. Carbonate chemistry, in the presence of illite, did affect particle formation by clay, carbon, and TEP at the micrometer scale (allocation between dissolved and particulate pools). Our experiment did not resolve processes on this scale well and it remains to be seen if such shifts in the size spectrum of organic carbon and minerals are relevant for the biological pump. High illite content led to small aggregates with a low average sinking velocity. In the absence of biological changes to particle production or loss, coagulation of POC, which is central to the biological pump, is not influenced by ocean acidification or dust input, but sinking velocity and, hence, flux attenuation of POC are likely to be affected by changes in dust input.