The planktonic copepod assemblages off the Danshuei River estuary, at the boundary of the East China Sea and the northeastern tip of the Taiwan Strait, were studied as a function of conductivity-temperature-depth (CTD) data obtained from the area from Oct. 1998 to July 2004. From seasonal copepod samples, 120 copepod species were identified off the Danshuei estuary during a 6-yr study period. In a methodical comparison, the rank-abundance (RA) was compared to quantitative estimates and diversity indices of copepods (i.e., the occurrence rate, mean abundance, Shannon-Wiener diversity index, and evenness and species richness index). The present study shows for the first time long-term patterns of planktonic copepod communities, thus demonstrating the usefulness of the RA tool in characterizing plankton bioindicator species to monitor environmental changes. A general model of species successions with season is exemplified by 1 coastal species that dominated the copepod communities during warm-water situations: Temora turbinata. Calanus sinicus was an indicator species belonging to relatively cold-water species transported from the Yellow Sea and East China Sea during the northeastern monsoon in winter.