Mass loss is a reasonable measure for the degradation of CDA materials because it is wellestablished that these materials biodegrade to CO2 in the coastal ocean . Additionally, because SA/V is a control of biodegradation (Supplementary Discussion 1) if any mass loss were attributed to physical disintegration (fragmentation), this would only increase the fragments' mass loss rate (Equation ). Thus, for mass loss measurements in our continuous flow-through seawater mesocosm, 𝑘 𝑑 was considered the surface erosion rate due to biodegradation processes. Additionally, samples in our system experienced negligible mechanical deformation and abrasion; the low flow rates were unable to deflect hanging samples (average flow velocity: ~4 mm/s), indicating a very low shear rate, and the use of seawater filtered to particulate less than 200 µm in combination with low flow rates presumably minimizes any abrasive removal of material. Previous experiments have determined that no mass loss occurred in sterilized controls .