Zooplankton community along full salinity gradient of river-ocean continuum showed four planktonic copepod communities and three microzooplankton (tintinnid) communities in the Pearl River–ocean continuum. The community between fresh and ocean communities is the transitional group. However, whether transitional group consistently exist in other river-ocean salinity gradient is an open question. We hypothesize that spatial scale of the salinity gradient play a role in determining the existence of transitional group because of the role that small habitat resulted in species extinction. In the present research, we investigated meso-zooplankton and micro-zooplankton communities along salinity 0-30 gradient along the Jiulong River Estuary (JRE) continuum in four seasons: spring (24, April, 2025), summer (8, August, 2025), autumn (17, November, 2025), and winter (7, January, 2026). Our findings showed that there were fresh and oceanic communities in both tintinnid and meso-zooplankton communities separated by a salinity boundary of 10–15 without transitional group in all four seasons. Both the fresh community and oceanic community showed simple seasonal cycle: one community peaked in summer and diminished in winter. Our research proved that there is no transitional group in small rivers with salinity range span in a short distance. The small spatial extension of the mesohaline water limited the existence of the transitional group.
Key words: Zooplankton; Transitional group; Salinity gradient, River-ocean continuum; Jiulongjiang River