Effects of suspended and accumulated kaolinite particles on the adhesion and the development of Undaria pinnatifida zoospores on the substratum were investigated under the following three types of condition; Case-1: suspended particles together with zoospores, Case-2: zoospores released after accumulation of particles on the substratum, and Case-3 : accumulated particles on zoospores adhered on the substratum.
Case-2 was the most effective inhibition for the adhesion of zoospores, and 50% of effective concentration of particles (EC₅₀) was 29 μg·cm^-2. It was difficult for gametophytes to develop to sporophytes in Case-3. Furthermore, in comparing Undaria zoospores with Porphyra conchospores, the zoospores showed higher tolerance to suspended particles.