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Progress in Researches on Dynamical Processes of Phytoplankton Ecology in Maximum Turbidity Zone of Estuary

The physical processes effects on plankton, and the interactions between physical processes and biological processes are the main research field of GLOBEC. Estuary is an important area of human activities, complex dynamical processes and frail ecology. The importance of research on estuary has been gradually recognized since the research plan of LOICZ was put forward by IGBP. Estuarine ecosystem is similar to marine ecosystem in many aspects. However, because the biological condition and transport processes are different, the structure of biological population and spatial variation are also different, a special ecosystem is formed in the estuary. The special horizontal transport caused by the interaction between the dynamics of river and ocean, front, stratification, turbulence mixing are the important dynamical processes. The functions of these processes in estuarine ecosystem have not been understood well yet. The dynamical processes of phytoplankton ecology in maximum turbidity zone of estuary are the distinguishing features of estuarine ecosystem. In recent decades, progress in research on this field has been achieved. The maximum turbidity zone is one of the special dynamical phenomena in tidal estuary. It is the cloudy waters in which the concentration of suspended particular matter (SPM)is higher than that in the upper river and down sea area, and it often moves upper and down regularly in a certain range. Maximum turbidity zone can been found in every tidal estuary in every climate condition in the world, specially it develops very well in part mixing estuary and full mixing estuary. The initial stage research of maximum turbidity zone has been focused on deposition dynamics in estuary, then on the characteristic and regular pattern of geochemistry. In recent years, the research has been focused on its biogeochemistry, ecology and environment. The results indicated that the maximum turbidity zone has the function of enriching heavy metals and other pollutants. The pollutants can be absorbed by phytoplankton, and then affect the whole estuarine ecosystem by food chain. In other way, the maximum turbidity zone is the area for fish feeding and spawning, and fish population is also abundant in this area. Therefore, it is important to study phytoplankton ecology in maximum turbidity zone. A few research results indicated that the dynamical processes of turbulence mixing in maximum turbidity zone have obvious influence on phytoplankton ecology. Generally speaking, estuary is nutrient\|rich but turbid, so that algal productivity and population growth rate are limited primarily by light availability. Therefore, assume it to be true that turbid estuaries are unproductive ecosystems and that primary production is delayed relative to the clearer marine waters. However, very high rate of production have also been found in estuarine turbidity zone. For example, algal bloom occurred in the maximum turbidity zone of Great Ouse estuary, England. Due to relative thinness of the euphotic zone, mixing depth was assumed to be the main factor influencing phytoplankton production in turbid systems. Production processes in these environments depends on sequential exposure to the light due to the turbulent mixing that carries algal cells up and down the water column. In the case of the Great Ouse estuary the absence of stratification demonstrated that, the whole water column was well mixed, the mixing depth being therefore equal to the water depth. Consequently, phytoplankton development is possible if the critical depth is more than the water depth. The ‘entrapment effects' in the maximum turbidity zone of estuary is mainly caused by gravitational circulation. Concerning photosynthesis only occurring in light time and large amount of nutrient being necessary during algal bloom, the long retention time of nutrient caused by the ‘entrapment effects' is the important factor of algal bloom in the maximum turbidity zone. In another way, the ‘entrapment effects' can also bring phytoplankton staying in maximum turbidity zone for a l