The Comparison Of The Zooplanktonic Communities In The North Channel And North Branch Of The Changjiang Estuary

Zooplankton plays an important role in aquatic food webs, both as a resource for consumers on higher trophic levels and as a conduit for packaging the organic material in the biological pump. Changjiang (Yangtze River) Estuary is an important fishery spawning, nursery, and migration ground. The Changjiang Estuary is characterized by multi-order bifurcations, and the salinity is different between branches or channels, especially between the North Channel (NC) and North Branch (NB). Most of the previous studies on zooplankton have only included one branches of water; therefore, these studies would not represent the whole Changjiang Estuary. To further the study of zooplankton in the Changjiang Estuary, the zooplanktonic communities between the NC and NB is compared.Seasonal zooplankton samples from stations in the Changjiang Estuary were collected from May, 2004 to February, 2005. The dominant species are determined by the dominance and then the contribution of each dominant species to total abundance of zooplankton is determinedβ(standard regress index). Moreover, the relationship between the salinity and abundance was studied by stepwise linear regression. During the whole year, the salinity was positively correlated with abundance, while the temperature, negatively. Linear regression analysis also showed a high positive correlation with salinity for total abundance in August and November, while in February and May, no obvious relations. The most abundant communities were composed of neritic and brackish-water species. The NC (salinity <5), was greatly diluted by freshwater while the NB was brackish water with salinity range from 12-28. Consequently, a decline in abundance of zooplankton was evident along the estuarine haloclines from the maximum in area of high salinity to minimum in the limnetic zone. Total zooplankton abundance and biomass were lower in NC than the NB in all seasons. In short, the salinity influences the abundance of each species of zooplankton, finally determines the total abundance of the zooplankton. Further more, a winter peak in the abundance existed, which might be caused by the flourishing of Sinocalanus sinensis, a widely distributed species in the Changjiang Estuary.Then, the feature of zooplanktonic communities and the difference of dominant species of the NC and NB were analyzed. Species diversity indices, K-dominance curves, or other community level indices and analysis of similarities (ANOSIM) were applied to discriminate the communities. Moreover, the effect of ecological factors, especially salinity and temperature on the dominant species and communities in these waters is discussed. The results indicate that, the NC is composed of tidal water and oligohaline water; while the water of NB is featured by mesohaline or polyhaline. Hence, the zooplanktonic communities in these two waters are significantly different. In spring, the abundance and diversity of community in NB were larger than those in NC, the dominant species in NC was only Sinocalanus sinicus, while in NB there were several dominate species, among which Labidocera euchaeta was most important of all. In summer, the diversity indices were peak, the species richness and total abundance of zooplankton in NC is less than those in NB. The NC was made up of oliohaline communities, and NB was composed of brackish species, e.g. Schmackeria poplesia, Tortanus vermiculus, and etc. Thus, the communities in these two waters were significantly different. In autumn, similar to spring, the abundance and diversity of community in NB were larger than that in NC. As usual, in NC S. sinicus was the most importance species, while in NB, zooplankton was composed of neritic community which consists of Paracalanus parvus, P. aculeatus, L. euchaeta, Calanus sinicus, and etc. As a result, there were evident and significant differences between the zooplanktonic communities in NC and NB. In winter, due to low temperature, salinity might be not the limit factor for S. sinicus. This was supported by the fact that the burgeoning of S. sinicus, which is widely distributed, but its occupation of the more saline waters depends upon a seasonal reduction of temperature. In short, salinity and its combined effect with temperature are key factors, which determine the characters of the zooplanktonic communities. Temperature mainly influences the seasonal alteration of the dominant species, thus the communities, while the salinity, influenced by the interaction between runoff and ocean mass, determines the local zooplankton communities in the NC and NB.