This study provides a description of mesozooplankton (holo-and meroplankton) abundance, biomass and diversity patterns inside and outside a tropical estuary (Nha Phu Estuary, Khanh Hoa, Viet Nam). In total 185 zooplankton species have been recorded during the study period (2009)(2010), copepods contribute with the largest share of species (more than 100), Tunicata with 20, Cnidaria with 17 and Chaetognatha with 9 species. At the most species rich site the number of zooplankton species varies between 55 and 123. The number of species and the annual variation in numbers declines towards the head of the estuary (14-37 species). In contrast, the highest numbers of individuals occur in the inner part of NPE. Calanoids that are the most abundant group of the copepods occur in densities up to 28.2 ind. L −1 (Aug. 9). At 'Outer NPE' and 'Outside NPE' the maximum density of calanoids is 5.8 and 10.7 ind. L −1 , respectively. The declining diversity of zooplankton towards the head of the estuary is also supported by various indices (Shannon's index, Margalef's index). A cluster analysis on similarity of species supports a clustering of the inner NPE sites vs the other sites. There is a general separation between the dominant copepod species in the inner (Bestiola sp., Acartia pacifica, Pseudodiaptomus incisus) and outer (Paracalanus gracilis, Acrocalanus gibber, Subeucalanus subcrassus, Oithona rigida, Corycaeus andrewsi, Oithona plumifera) part of the estuary though a few species are common in both areas (Paracalanus crassirostris and Euterpina acutifrons). The zooplankton community at the inner NPE is subjected to more variable hydrographic conditions (salinity in particular) than the communities at the other sites where more stable conditions prevail. A short residence time in the inner part of the estuary due to the tide is supposed to impede a strong horizontal structuring of the zooplankton community.
Zooplankton samples were collected at 16 stations in the coastal waters of Son Tra peninsula, Da Nang city in dry season (7/2016). At each station, zooplankton samples were collected by using Juday net (mouth diameter: 37 cm, mesh-zise: 200 µm) from 1 m above bottom to surface. 112 species belonging to 11 zooplankton groups were identified. Copepod was a dominant group with 67 species, followed by Tunicate (13) and Gastropod (7). The Northeast and Northwest areas of Son Tra peninsula have a higher biodiversity than the South and Southeast based on number of species, species richness value. An average density of zooplankton was 7.768 ins.m-3 and was decided by copepod density (3.759 inds.m-3 on average). Station 4 had the highest number of individual and station 18 had the lowest zooplankton density. Oikopleura fusiformis was dominant in most of stations in the north area (22.16%) and in the northwest (15.97%), while copepod species Paracalanus crassirostris was dominant in the northwest (16.51%) and in the south (24.57%) of Son Tra based on SIMPER analysis (90%).
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