Analytical Study On Multi-environment Factors That Influencing The Phytoplankton Growth In The Changjiang Estuary And Its Adjacent Area

In the vicinity of the Changjiang Estuary and Hangzhou Bay exists a sea area between 27°N and 32°N where the phytoplankton biomass is higher than its adjacent area. The water depth of this area is of 30-50m and the salinity is of 26-30. Consequently, this leads to the formation of the famous Zhoushan fishery ground, however, this also leads to the frequently occurrence of harmful algal blooms (HABs) and hypoxia area in the bottom water there. In addition to its own ecological characters, the environmental factors such as seawater temperature, salinity, nutrients, solar radiation, trace elements and so on jointly influence the growth of phytoplankton. For the Changjiang Estuary and its adjacent area, it has been widely accepted that the phytoplankton growth is basically controlled by three key factors: nutrients, temperature and the light penetration owing to the concentration of turbidity. However, the combined effect of these environment factors has not been studied through sufficient in situ observations derived from multi-surveys, especially, to which extent of the influence of respective factors on the growth of phytoplankton is seldom reported. To answer this question, based on the observations of all the cruises from 2002 to 2007 the contribution of nutrients, seawater temperature, turbidity, salinity, petroleum carbons, Cu, Pb, Zn and Cd was analyzed in this dissertation using the method of multivariate statistical analysis and simulation of effective factors. This work definitely provides a valuable foundation for revealing the occurrence mechanism of HABs in Changjiang estuary and its adjacent area. The main work and key results are listed bellow:1. The horizontal pattern of phytoplankton biomass since 1980s was analyzed and summarized.The concentration of chlorophyll a (Chla) increased from coast to certain distance then decreased in further offshore direction. Higher Chla concentrations were observed neither in the inshore waters with higher nutrient concentrations, nor in the offshore waters with higher transparences. It occurred in the sea area with the saliniy of about 29.8psu±1.3psu. The Chla distribution there was normally in patchiness and generally has several higher-concentration centers. The numbers, the value and the location of these centers varied seasonally and annually. For the seasonal scale, the monthly average Chla concentration tended to show a"double-cycled", higher in spring and summer whereas lower in autumn and winte. In addition, the spatial variation in spring and summer was larger than that of other seasons and the higher Chla concentration water mass withdrew to the estuary in winter. Nowdays, the Chla concentration was generally in a higher level; especially some certain areas are at their historically high level.2. Based on the data retrieved from 5 cruises around Changjiang Estuary and its adjacent area during 2002-2003, the main environmental factors that controlling the phytoplankton growth were fixed by means of principal component analysis.The first two principal components were retained for further interpretation among the five components extracted. The first principal component (PC1) accounted for 30.2% of the variance and clearly identified as the influence of terrigenous input of Changjiang River et al., which changed the nutrients concentration and water transparence. The second principal component (PC2) explained 17.4% of the variance and identified as the impact of temperature and petroleum hydrocarbons. From the eigenvector matrix and explained variance, the nutrients, light and temperature status of seawater were the main environmental factors that controlling the phytoplankton growth in the study area, while other polluted factors such as heavy metals had relatively minor effect.3. Based on the data of 14 field cruises during 2002-2007, it further confirmed that the most important factors that affected the phytoplankton growth are nutrients, light and temperature.Two principle components were extracted. The PC1 represented the impact of land runoffs such as Changjiang River and explained 50.9% of the variance. The component matrix showed that the source of nutrients and turbidity was homogenous and both were negatively correlated with salinity. The PC2 represented the impact of seawater temperature and explained 19.7% of the variance. To summarize, the growth of phytoplankton was controlled basically by terrigenous input, which changed the nutrient concentration and light penetration of the Changjiang Estuary and its adjacent area. the seawater temperature played a second role in this respect.4. When the relationship between the Chla and the environmental factors changed from the nonlinear to linear, the relative importance of these factors was evaluated by means of multiple linear regressions.The quantitative relationship between Chla and the environmental factors was established and the path coefficient (Pi) was calculated. The path analysis showed that phosphorus (PO4-P) had the biggest direct effect on the Chla. Based on the importance of their relationship with phytoplankton growth, the order of the five main growth parameters was concluded as: PO4-P, silicate (SiO3-Si), turbidity, dissolved inorganic nitrogen (DIN) and seawater temperature (ST).5. The horizontal pattern of Chla in the Changjiang Estuary and its adjacent area was simulated using the growth–effect-factor theory. Moreover, the controlling factors of the spatial and temporal variation of the Chla concentration were analyzed.Without nutrients input, the similarity index (SI) between the simulated distributions and the measured distributions of the Chla was (0.65±0.13). The Pearson correlation coefficients was (0.56±0.21) with P<0.01. This result showed that the calculation of the growth–effect-factor and the parameters chosen was reasonable. Based the analysis of the multiple effect factors, it showed that the contribution of the nutrients and the irradiation to the phytoplankton growth was similar, 45% and 46% respectively. The contribution of the ST was minimum as 9%. Further study showed that the nutrients and seawater temperature controlled the seasonal variation of the Chla, while the irradiation controlled the spatial variation of the Chla.