Community Structure and Temporal and Spatial Distribution of Zooplankton in Zhelin Bay, China(2000～2001)

Zhelin Bay is one of the most important bays for marine culture in Guangdong Province of China. Due to the increasing human population and mariculture in the last 10 years, the ecological environment has changed dramatically. Although Phaeocystis blooms of 1997 and 1999 had brought the serious economic loss and ecological damage to this area, few studies were followed. To understand why Phaeocystis blooms frequently hit this area and how the increased marine culture affects this ecosystem, we have investigated the temporal and spatial distribution of phytoplankton, zooplankton, nutrients, and other biochemical parameter since May 2000. This paper describes an annual (7/2000～7/2001) variation of the community structure and temporal and spatial distribution of zooplankton in Zhelin Bay. Water samples were collected from eight stations weekly (March to November) or biweekly (December to February). Sampling was scheduled to cover the period around high tide (±1.5h). In each station, water samples for zooplankton analysis was collected by double vertical hauls of a JP-2 net (200μm) from the bottom to the surface. Two batch samples were collected each time. One batch was fixed immediately with formalin (5% in the samples) in situ and then transferred to laboratory for species identification and biomass analysis. Thereinto, copepod, and cladocera were identified to species, others such as hyperiidea, euphausiacea, mysidacea, polychaeta, chaetognotha, tunicata, medusa were identified to genus. The other batch was used for dry weight measurement. Samples were filtrated through GF without fixation, rinsed with 6.5% isoosmotic ammonium formate, desiccated with the vacuum pump, and dried in the oven at 60℃ for 24 h. Then they were cooled to indoor temperature in the desiccator, and measured with OHAUS analytical balance. Our investigation found 57 copepods, three cladocera, and others such as hyperiidea, euphausiacea, mysidacea, polychaeta, chaetognotha, tunicata, medusa, and planktonic shrimps. In terms of individual biomass, copepod accounted for 66.3%, planktonic larvae accounted for 13.1%, cladocera accounted for 10.7%, and the rest accounted for 9.9%. The dominant species in the Zhelin Bay are Paracalanus crassirostris, Oithona brevicornis, Penilia avirostris, Calocalanus pavoninus and Euterpe acutifrons. Based on their ecological and distributional characteristics, copepod and cladocera can be classified into four ecotypes: estuarine species, nearshore and warm-water species, nearshore and warm-temperature species, and offsea extensive species. Based on the data of copepod and cladocera only, species abundance increased from 26 species in the inner bay (Station 4) to 47 at the outer bay (Station 7). Temporally, the highest abundance was found in August (38 species) and the lowest was in January (17 species). In all identified copepod and cladocera, there were 45 species with body length smaller than 1.3 mm, which accounted for 75.0% of the total species abundance. Particularly, the body lengths of most dominant species were less than 0.6 mm. Therefore, micro-zooplankton dominated the zooplankton community in Zhelin Bay. The annual average dry weight increased from the inner bay (47.3 mg/m 3 at Station 4)to the outer bay (211.8 mg/m 3 at Station 7) . Monthly average dry weight ranged from 19.4 mg/m 3 in January to 264.1 mg/m 3 in May with the annual mean of 115.7 mg/m 3. It also exhibited two peaks during our investigation period: the biomass was low in January (19.4 mg/m 3), but increased from February with water temperature and phytoplankton abundance, and it reached the first peak in May (264.1 mg/m 3), which was associated with the biomass of Noctiluca scientillans, then it decreased until reaching the second peak in September (224.4 mg/m 3). Biomass expressed as individual number increased from 2.4×10 3ind./m 3 in the inner bay (Station 4) to 13.4×10 3ind./m 3 at the outer bay (Station 7). The large-scale filter-feeding oyster culture and marine fish culture were suspected to be the rea