Patterns of zooplankton distribution along marine, estuarine and riverine portion of Danshuei ecosystem, northern Taiwan
Résumé
The Danshuei River embouchure is the largest estuarine ecosystem in northern Taiwan. The interacting effects of wastewater discharge from Taipei City and Taipei County into the river and seawater intrusion are the main determinants of the community composition, distribution, and abundance of zooplankton in this estuarine ecosystem. In the present study, we analyzed the abundance, distribution, and diversity of zooplankton, synoptically collected at 25 (19 marine and 6 estuarine) stations in this ecosystem. In total, 28 zooplankton groups were identified, including the top 3 most abundant groups: Calanoid copepods, larval stages, and Noctilucales. Among copepods, 46 species were identified from the estuarine stretch, whereas during the same period, 86 species of copepods were recorded in waters offshore of the estuary mouth, which is the boundary of the East China Sea and northeastern tip of the Taiwan Strait. Mesozooplankton abundances and distributions were highly variable among stations, indicating a highly fluctuating ecosystem. Zooplankton abundances were not correlated with chlorophyll a levels. Copepod diversity but not abundance showed a significant positive correlation with salinity (r = 0.764). The recorded salinity levels and zooplankton species compositions suggest that seawater intrusion up to station 3 of our study acts as a major driving force for zooplankton distribution and diversity in the Danshuei Estuary. Hierarchical classification revealed higher interannual variability in the occurrence of copepods species; at the 1st hierarchical level, samples collected in Oct. 2001 at stations 2, 3, and 4 were completely separated from other samples. Among these groups, the upstream station 4 was exclusively represented by the euryhaline estuarine calanoid copepod, Pseudodiaptomus annandalei, at a density of 1631 individuals (ind.)/m3, station 3 was represented by Pse. annandalei and Apocyclops borneoensis with respective densities of 552.5 and 1.8 ind./m3, and the seaward station 2 recorded Pse. annandalei, Paracalanus aculeatus, and A. borneoensis at respective densities of 364.9, 6.9, and 1.2 ind./m3. At the 2nd hierarchical level, samples collected in Apr. 2002 were segregated from the others and exclusively contained coastal marine and euryhaline species. The 3rd hierarchical level included species occurring with no significant variations and classified species into 3 groups: (i) species with relative abundances of 1.4%-2.9% occurring during the post-spring period but without significant temporal variations, (ii) the freshwater species of cyclopoid copepod, Mesocyclops pehpeiensis, and (iii) copepod species represented by local coastal, marine, and brackish-water species characteristic of warm tropical and subtropical waters. The Euclidean distance measurement separated the copepod community into 5 groups. The most indicative euryhaline species at upstream stations, Pse. annandalei was separated at the 1st level withoutclustering with any other species. The 2nd most abundant species, Mesocyclops pehpeiensis, grouped with other coastal species at the 2nd level of grouping; however it was separated at the 3rd level of grouping without associating with any other coastal or marine species.