An Ecological Study On Zooplankton In A Large-scale Mariculture Zhelin Bay, Eastern Guangdong

Zhelin Bay is one of the most important bays for large-scale mariculture in Guangdong Province, China. Due to the increasing human population and mariculture in the last two decades, the ecological environment has changed dramatically, e.g., the large-scale of harmful algal blooms (Phaeocystis) occurred in 1997 and 1999, respectively. Although degenerative and eutrophic environment has brought serious economic loss and made mariculture becoming more and more difficult, especiatly in caged fish culture, few studies have been performed. To evaluate the environmental condition of Zhelin Bay, from May 2000 our group initiated an ecological investigation around Zhelin Bay. This paper describes the investigated results on zooplanktonic ecology.Samples were monthly collected at least once per month from July of 2000 to December of 2003, but only quarterly in 2004. There were totally 9 sampling stations, including 7 inside the bay and 2 outside the bay. In each station, zooplankton samples were collected by double vertical hauls from the bottom to the surface using a JP36 net (169μm). From two parallel samples, one was immediately fixed with formalin with final concentration of 5%, and another lively took back to the laboratory for biomass analysis, which were determined by dry weight mothed. From fixed samples, copepod and cladocera was identified to species, but others were identified as the groups such as hyperiidea, euphausiacea, mysidacea, polychaeta, chaetognoths, tunicata, and medusa. We also synchronously investigated 13 kinds of environmental factors in the water surface and bottom, including the phytoplankton, water temperature, salinity, nutrients and so on. The diversity and evenness of copepod were calculated by Shannon-Weaver formula and Pielou formula. The relationships between zooplankton, copepods and cladocean and their environmental factors were analyzed by means of the Canonical Correspondence Analysis (CCA) using the CANOCO 4 package. An average from the water surface and bottom was taken for all environmental factors in the analysis. Yearly division was from January to December in a same year, and only 2001 to 2003 were used for CCA.Through this investigation, we identified 67 species (31 genus, 21 families, and 3 orders) of copepods, 3 species of cladoceran, and 12 groups such as hyperid anmphipod, euphausiid, mysid, polychaete, chaetognoth, tunicate, medusa, and planktonic larva and so on. Among them, thedominant species in Zhelin Bay are Paracalanus crassirostris, Oithona brevicornis and Penilia avirostris. The community sturcture of zooplankton was rich in species and abundance (12.0 X 103 indiv./m3), but charactered by obvious miniaturize. Although body length of copepods and cladocrans ranged from less than 0.4 mm to 3.8 mm, the body length less than 0.8 mm in terms of individuals accounted for 93.1% of the total abundance bacause the dominant species such as Paracalanus crassirostris and Oithona brevicornis were all small with body lengths of less than 0.6 mm. In the spatial distribution, the species richness, abundance and biomass of zooplankton were all higher outside the bay than those inside the bay. In the seasonal distribution, the abundance and biomass of zooplankton show a more or less similar pattern with the water temperature, that is, increasing in the warm seasons and low in the cold seasons. However, the speceis were rich in spring and summer and low in late autumnm and winter. But during 2003, the maximum lied in winter, the minmum found in summer. In yearly variation, the speceis richness, abundance and biomass all showed a decreasing tendency through the investigation. During 2003 and 2004, all these three parameters greatly decreased in the stations outside the bay, resulting a much more uniform spatial distribution over the investigated area.The copepods were the most important community, which accounted for 73.08% of the total abundance of zooplankton. Therefore, the spatial-temporal distributions of species, abundance, and diversity of copepods were more or less similar to those showed by total zooplankton. During the investigation, Paracalanus crassirostris and Oithona brevicornis were most dominant speceis in the copepods. The total abundance of these two speceis accounted for 65.1% of copepod abundance and 44.5% of total zooplankton.The most important species in the cladocerans was Penilia avirostris, whose abundance accounted for 90.0% of the cladocerans and 7.50% of total zooplankton. The abundance of cladocerans was higher in the stations outside the bay than those inside the bay, but the estuary of Huanggang River in the inner bay was also a high abundant area. The abundance of cladocerans showed a distinct seasonal variation with a pattern high in the warm seasons and low in the cold seasons. However, The cladocerans outside the bay greatly decreased in 2003 and 2004, which made the spatial distribution of cladoceran abundance become much uniform over the investigated area.The CCA analysis revealed that the environmental factore such as salinity (S), phytoplankton (Phy), nutrients (Si, N, P), and water temperature (T), playing significant roles inthe spatial-temporal distributions of zooplankton in the invetigated area; and salinity, nutrients, turbidity, water temperature, and phytoplankton in the copepods; and phytoplankton, water temperature, salinity, and nutrients in cladocerans. However, only water temperature, phytoplankton, and persistent deterioration of the environmental factors inside the bay caused by the land-sourced pollutant and large-scale mariculture was the most important driving force.