Studies On Community Structure Of Zooplankton And Its Relationships With Environmental Factors In Zhelin Bay Based On SOM And CCA

Zhelin Bay, covering 68-70 km², is one of the most important bays for large-scale marine culture in South China. Since 1980s, as a good place for typhoon-sheltering, it has seen its mariculture develop rapidly. Hence, together with traditional nutrient from the waste discharge and agricultural activities, the increasingly intensive aquacultural activities accelerated the eutrophication during the past decade, thus in turn impeded aquacutural production, e.g., the large-scale of harmful algal blooms (Phaeocystis) occurred at the end of 1997. The degenerative and eutrophic environment has made caged fish culture much tougher, bringing huge economic loss, but this problem was paid little attention to and few studies were done about it. From May 2000, our group conducted an ecological investigation around Zhelin Bay, where we monthly sampled the phytoplankton, zooplankton, nutrients, water temperature, salinity, and other environmental variables. In this study, zooplankton communities and their relationships to environmental factors were researched at 9 stations in Zhelin Bay from May 2000 to April 2004.The aim of this work was to (1) study the zooplankton composition, the yearly variation and the spatial differances of species number, abundance, biomass, diversity and evenness throughout the 4-year period, (2) identify the zooplankton community structure and the dominant species and indicator species for each community, and (3) investigate the relationship between zooplankton community and environmental parameters. Communities, however, are difficult to analyze since the data consist of diverse taxa in a non-linear way. Therefore, a self-organizing map (SOM) was utilized to classify the zooplankton communities, and their relationships to environmental factors were analyzed through canonical correspondence analysis (CCA). An average from the water surface and bottom was taken for all environmental factors in the analysis. Since some environmental factors were absent during the course from May 2000 to April 2001, datasets used for SOM and CCA analyses were those from May 2001 to April 2004. Correspondingly, the 4-year study period was divided as follows: the first study period (May 2000-April 2001), the second study period (May 2001-April 2002), the third study period (May 2002-April 2003), and the fourth study period (May 2003-April 2004). Results as following:A total of 79 taxa, including 65 species of copepods, 4 species of cladocerans were found throughout the study period, among which calanoid Paracalanus crassirostris, cyclopoid Oithona brevicornis and cladoceran Penilia avirostris were the most abundant species, contributing 28.05%, 20.81% and 5.07% of the total zooplankton abundance, respectively. Besides, in terms of species number and density, copepods were the most abundant group (69.76%).Based on results of Kruskal-Wallis ANOVA, samples from the outer bay and stations at the edges of the large-scale caged-fish farm had significantly greater abundance, biomass, species number and diversity than those from the inner bay (p<0.05) and the station in the center of the largest caged-fish farm (p<0.05) with the lowest values observed in S2, which is frequently perturbed by human activities. Samples from eastern boundaries of the bay had significantly greater abundance and species number than those from western boundaries (p<0.001), while biomass and diversity had no significant differences between the eastern and western parts of the bay (p>0.05).As for the yearly variation, the Kruskal-Wallis ANOVA results revealed that the abundance, biomass, species number and diversity had significant differences among the investigated years (p<0.01). Moreover, the abundance, biomass, speceis richness and diversity all showed a decreasing tendency through the investigation, which might be attributed to the increasing of mean temperature and the abnormal weather in southern China.Six clusters with dominant species compositions and indicator species were distinguished by SOM method, i.e. cluster I (Neritic warm water community), cluster II (Eurytopic community), cluster III (Neritic mesotrophic community), cluster IV (Neritic eutrophic community), cluster V (Estuarine low-saline community) and cluster VI (Pelagic high-saline community). The pelagic high-saline community, which occurred in the outer bay with relative good water quality, was the dominant community in the study area. The dominant species for this community were Paracalanus crassirostris, Oithona brevicornis, pelagic larvae, Oithona similis, Penilia avirostris, Calocalanus pavoninus, Evadne tergestina, tunicata, Paracalanus parvus and Euterpina acutifrons. However, the neritic eutrophic community, which occurred in eutrophic waters, was the poorest community, and dominated by cirriped larvae and Clytemnestra scutellata. Furthermore, seven, one and seven indicator species were identified for cluster I, V and VI, respectively, while no indicator species were observed in cluster II-IV. The differences among clusters could be mainly ascribed to the changes in environmental factors; especially the intensive aquaculture structures suspended on waters and in waters have a great impact on the distribution patterns of organisms in the bay. Differences of environmental variables for each cluster derived from the SOM were analyzed by ANOVA. The ANOVA on environmental variables in the 6 clusters gave significant differences (p<0.05) for temperature (Temp), salinity (Sal), Si, DIP, NH₄, NO₂, NO₃ and phytoplankton (Phy), showed highly significant differences among clusters (p<0.01), suggesting the existence of relationships between these variables and zooplankton communities. Moreover, each variable in cluster IV (aquaculture areas situated at the inner and western parts of the bay) showed larger variation than that in other clusters, which might be attributed to the influence of anthropogenic activities in this area, such as discharge from domestic and mariculture waste from aquaculture effluents.The CCA analysis not only confirmed the above findings, but also showed that, with descending importance, water temperature, nutrients (DIN and Si), salinity and phytoplankton were the main determinants of spatial and temporal distributions of zooplankton in Zhelin Bay. Moreover, the CCA analysis revealed that the eurytopic species, with the deterioration of eutrophication, has become the dominant species in Zhelin Bay, while species that are vulnerable to organic pollution decreased or disappeared completely. These results also reflected a shift in zooplankton community structure due to environmental deterioration, which implies that the zooplankton community in Zhelin Bay could be directly or indirectly affected by global warming, local climate fluctuations, seasonal cycles of phytoplankton, river discharge which alters salinity distribution and residence time of plankton, and other environmental variables that affected by the pollution originated from land, mariculture and other human activities.