a Reduction of CDRtheoretical through particulate inorganic carbon (PIC) production by epibiont calcifiers associated with particulate organic carbon (POC) formed by Sargassum. The purple line shows the general relationship, the grey shaded part the range of Sargassum PIC:POC observations, and the cross the Sargassum mean. b Reduction of CDRtheoretical due to nutrient reallocation, which becomes more pronounced the more the seaweed carbon-to-nitrogen (C:N) ratio approaches the phytoplankton C:N ratio. We used a range of phytoplankton C:N ratios22 as indicated by the colour code. The horizontal lines display the range of C:N in Sargassum72 and green/red/brown seaweeds, respectively28 (the height of the lines on the y-axis has no meaning). The solid symbols on the horizontal lines are averages (Sargassum average = 24.8). c Summary of discounts and additions to CDRtheoretical due to calcification and nutrient reallocation following Eq. 1 (upper and lower bounds in square brackets). Percent reductions due to POC formed by plankton (POCplankton, blue); Percent reduction due to PIC associated with seaweeds (PICseaweed, grey); Percent increase due to PIC associated with plankton (PICplankton, black).