流沙湾海草床海域浮游植物的时空分布及其影响因素

Abstract

Phytoplankton is a major component of primary productivity in the ocean ecosystem.Their distribution is affected by physicochemical environmental factors such as light,water temperature,salinity and nutrients and by secondary productivity and marine macroalgae.Phytoplankton community structure,and spatial and temporal changes in phytoplankton richness,combined with correlations to factors influencing community structure can provide understanding of the effects of gradual changes in the environment and of the state of health of an ecosystem. Liusha Bay is famous for the production of seawater cultured pearls and as the southern pearl centre of China.It is also the biggest marine macroalgae resource area in China,with abundant marine macroalgae located to the southwest of Leizhou Peninsula Guangdong(20.36°—20.50°N,109.80°—110.02°E) covering an area of 69 km2.With the development of industry,agriculture and Marine aquaculture in recent years,the bay ecosystem and environment has deteriorated.Cultured pearl production has declined each year and macroalgal abundance has also declined. To examine trends in seasonal variation and the environmental quality of the macroalgal area of Liusha Bay,investigations of phytoplankton community structure and of the factors likely to influence this were carried out from February to November in 2008.Phytoplankton samples were collected using a water sampler and fixed with 5% formalin and 1% Lugol's solution.In a subsequent analysis,species were identified with a microscope after concentration of samples in the laboratory.Temperature and salinity were tested at the time of sample collection and chlorophyll-a content and nutrient composition were determined using spectrophotometry. In total,151 species of phytoplankton were identified,with 57,66,73,and 66 found in winter,spring,summer and autumn,respectively.Within the samples,123 species from 44 genera belonged to Bacillariophyta,accounting for 81.4% of the community,26 species 11 genera belonged to Pyrrophyta,accounting for 17.2% of the community.One species from one genus belonged to Chlorophyta and one species from one genus belonged to Cyanophyta,each accounting for 0.7% of the community respectively.26 dominant species were identified of which the most common were Noctiluca scintillans,Chaetoceros weissflogii,Thalassiosira rotula,and Thalassionema nitzschioides.Dominant species formed catenarian colonies,acerose cells and / or were fairly large individuals(so large that they could not easily be consumed by shellfish).There were no small sized dominant species.11 Bacillariophyta and one Pyrrophyta species occurred in all seasons and 22—43 common species occurred in two seasons.The Jaccard index ranged from 0.211 to 0.448 and implied clear seasonal succession in winter and spring.The average diversity index and evenness were 2.12 and 0.35 respectively,indicating a fairly fragile community composition in this area.The abundance of phytoplankton was rather low throughout the year,due to the feeding pressure of shellfish and competition with macroalgae.Phytoplankton density ranged from 0.24—5.72 ×104 cells/L with maximum density in autumn,followed by summer,spring and winter in a descending order.This finding was notable,since phytoplankton in other sub-tropical areas usually bloom in spring or autumn. The density of phytoplankton significantly positively correlated to silicate and was negatively related to salinity,but no correlation to the other factors was found.The concentration of chlorophyll-a was significantly negatively correlated to water temperature and was negatively correlated to zooplankton abundance.From relative absorption data,nitrogen availability was the main nutritional limiting factor in all four seasons.However,when absolute absorption data were examined both N and P were nutritional limiting factors throughout the year.These findings suggest that the water quality of this area was of a subtropical oligotrophic type.