Historical Comparison On Evolution Of Phytoplankton Community And Its Relations With Environmental Factors In Yangtze Estuary And Its Adjacent Sea

This paper mainly studied the changes of phytoplankton communities and its relations with environmental factors in Yangtze estuary and its adjacent Sea, by collecting the historical data from the survey of 50 years ago in Yangtze estuary, and recent data of three cruises in Yangtze estuary and its adjacent sea (32°00'～29°30'N, west of 123°E) in July and November, 2005 and May, 2006, carried by both the State Key Laboratory of Estuarine and Coastal Research (East China Normal University) and Zhoushan Marine Ecological Environmental Monitoring Station (Zhejiang Province). Based on above large scale surveys and historical data, the evolution characters and trend of phytoplankton communities in Yangtze Estuary and its adjacent sea were analyzed, the relationship between phytoplankton species and environmental factors were checked by using Canonical Correspondence Analysis (CCA) ( CANOCO4.5), and the reasons of phytoplankton community evolution were analyzed also based on climate change and human activities.The main conclusion as follows:In comparing with 50 years survey data, species composition, cell abundance and dominant species of phytoplankton in Yangtze Estuary have taken an obvious change. Although diatom still was dominant species in phytoplankton species composition of this area, its quantity proportion showed an obvious decline, and the percentage of dinoflagellate was increasing with the number of dominant species decreased. Skeletonema costatum was still dominated in Yangtze Estuary, but the percentage of the dominant species of diatom descended a lot. Many original dominant species of diatom appeared as undominated, such as Chaetoceros sp., Pseudonitzschia pungens, Planktoniella sol, Thalassionema nitzschioides, etc. The amounts of dinoflagellate in dominant species were increasing with some new dominant species appeared, such as Prorocentrum dentatum, Noctiluca scientillans, Ceratium fusus, and so on. The quantity of phytoplankton cell abundance revealed an increase tendency, and community diversity was falling in evidence. The ascendancy of Skeletonema costatum and unbalance distribution of increasing dinoflagellate resulted in a marked downward trend of diversity.CCA analysis showed that nutrients and light conditions played the most important role in affecting distribution of phytoplankton, and water temperature and salinity were also crucial. It also displayed good relationship between phytoplankton species and environmental variables especially reflected the relation between distribution of dominant species and environmental factors. Skeletonema costatum distributed widely and adapted to extension environmental conditions, so it was able to be the only long term dominant species in Yangtze Estuary.An apparent succession of phytoplankton communities in Yangtze Estuary has happened under the influences of both climate change and human activities. Global warming caused rising of sea water temperature in Yangtze Estuary. It was unfavorable to the growth of diatom which was sensitive to water temperature, but favorable to reproduction of dinoflagellate. The increasing precipitation enhanced discharge of Yangtze River, and it was help for advancing the input flux of nutrient matters. At the same time, human activities aggravated the status of eutrophication in Yangtze Estuary. Increasing nutrients caused blooming growth of algal, and resulted in red tide frequently. Besides, the construction of hydraulic engineering in upstream lessoned sediment discharge form Yangtze River and reduced transport flux of dissolved silicon into estuary, so that the development of diatom community was limited as well. The phytoplankton community of Yangtze Estuary revealed an obvious succession trend from diatom to dinoflagellate.