| Summary: | 博士 === 國立臺灣大學 === 海洋研究所 === 86 === Recently, water pollution problems in coastal areas are becoming more serious with increasing discharge of domestic and industrial wastewater into the sea. Masan Bay is known of its severe eutrophication, exemplified by frequent incidences of red tide anoxic condition. The exact causes of eutrophication are not well known so far, but it has been suggested that the chlorophyll-a concentration levels of phytoplankton can be an indicator of eutrophication.
With the above background, this study was carried out with the purpose of evaluating relative importance of environmental factors and zooplankton grazing on phytoplankton biomass-chlorophyll-a based on observations of long-term (monthly) changes (at Kuisan station) from January to November, and on short-term (daily) changes (at Masan Inner Harbor Station) before and after the summer rainy seasons in June and July of l996. The factors affecting the concentration of chlorophyll-a were evaluated statistically by using the best subsets correlation and principal component analyses.
In the relatively short survey period, physico-chemical parameters such as temperature, silicate, dissolved oxygen and phytoplankton and zooplankton composition changed dramatically.
An euglenoid, Eutreptiella marina, dominated in June, and then Prorocentrum micans, a dinoflagellate, in July. For zooplankton, Noctilaca scintillans, harpacticoid copepods and cladocerans dominated in June, and then cyclopoid copepods and rotifers in July.
Grazing rate of copepods was higher in June, when the euglenoid alga, Eutreptiella marina was dominant. During dinoflagellate bloom in July, however, copepods grazing was not a significant factor in affecting chlorophyll-a concentration.
From best subsets correlation coefficients, daily change of chlorophyll-a concentration was mainly correlated with grazing rate and dissolved oxygen in June, and silicate, nitrite, phosphate, nitrate concentrations and dissolved oxygen in July. Other factors were not significant in short-term of chlorophyll-a concentration. Based upon principal component analysis, the increased chlorophyll-a concentration was resulted from the increase of nitrite concentration, diatom and dinoflagellate cell densities and zooplankton grazing in June and July.
Also, the resluts of long-term survey indicated that change of phytoplankton biomass was found to be significantly related to temperature, salinity, dissolved oxygen, nitrate and silicate concentrations as well as compositions of phytoplankton and zooplankton composition. Meanwhile, chlorophyll-a concentration was positively affected by nitrite concentration and dinoflagellate density.
Based upon monthly occurrence, the dinoflagellates Heterocapsa triquetra, and Gonyaulax sp. were dominant during January, followed by a diatom Pseudonitzchia pungens in February, March and April. In June, an euglenoid Eutreptiella marina was predominant in phytoplankton assemblages, accounting for 85.3% of the total phytoplankotn standing crop. However, its dominance was replaced by Prorocentrum triestinum which accounted for 86.8% of the total crop in July. Thereafter, the diatom Skeletonema costalum was predominant and had a contribution of 96% of the total crop. in the last two months (October and November), Cylindrotheca closterium and Gymnodinium sanquineum were present as dominant species, but with much smaller amounts, i.e. < 23.8 % of the total crop.
Monthly abundance of zooplankton also varied greatly, with a mean of 5-86 inds. / l. Copepods were dominant throughout January and February, whereas copepod nauplii and copepodites were dominant in March. In April, copepods were few; on the other hand, polychaete larvae became dominant in zooplankton assemblages. Individual numbers of copepods in summer season (June to August) were the lowest as compared to those in other seasons. However, copepods together with barnacle larvae were dominant in September and October, followed by copepodites, copepods and polychaete larvae in November.
Best subsets correlation coefficients showed that chlorophyll-a concentration was mainly correlated with concentrations of phosphate, nitrite and nitrate. Meanwhile, by using principal component analysis, high chlorophyll-a concentration was significantly influenced by nitrite concentration and dinoflagellate cell density. During the long-term survey (January to November), grazing rates by calanoid copepods were considerably low while dinoflagellates, diatoms and euglenoids were dominant in phytoplankton assemblages. Selective grazing on the above algal groups was not observed. It suggested that mesozooplankton grazing pressure was not a major factor in controlling phytoplankton populations in the study area.
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