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UBC Theses and Dissertations

Preliminary exploration of estuarine ecosystem structure at low trophic levels with a controlled microcosm Wu, Yong


In a laboratory microcosm, two kinds of salinity gradients were created to simulate the process occurring in estuarine circulation with mixing sea water and fresh water. The behaviour of the freshwater and seawater ecosystem components in relation to the salinity gradients were investigated. Seven parameters were chosen as indicators of the different trophic levels in the ecosystem or of the environmental conditions. The interaction among the different trophic levels was reflected in the development pattern of phytoplankton, nutrients, bacteria, and nanozooflagellates. The interaction among the ecosystem and environmental conditions were reflected in the difference between the different experiments with different salinity gradients and different ecosystem origin. In stage I freshwater phytoplankton were tested on two different salinity gradients. The data showed that the freshwater biota could not pass through the salinity gradient. Most of them died or were inhibited during the mixing process. The autotrophic component in fresh water could no longer function as a autotrophic component but served as an organic substrate contributor. The inhibition of phytoplankton growth by the salinity gradient provided a condition in which the bacteria could concurrently develop in the ecosystem. In a homogenous condition with controlled flasks, the development of the autotrophic component was separated from heterotrophic bacteria over time. In stage II, a seawater ecosystem had a different responce from that of fresh water. With an increasing salinity, the growth of phytoplankton could be limited at low salinities. This resulted in a delay of the maximum phytoplankton biomass and provided the first period of time for bacterial development before the bloom of autotrophic components. Thus bacteria formed a peak before phytoplankton developed. Seawater phytoplankton could, on the other hand, actively and quickly respond in their growth, on the salinity gradient. Thus the seawater autotrophic component may play the major role in primary production in a phytoplankton based estuarine ecosystem. The behavior of autotrophic components in both systems can have a strong effect on the rest of the components in the system; their changes could cause a great change in the whole system structure. Different developmental patterns of phytoplankton and bacteria in different experiments were explained with a conceptual diagram which summarizes the idea of energy states of an ecosystem and the function of phytoplankton and bacteria in ecosystem dynamics. Temporal development patterns of the ecosystem components in our experiments may be extrapolated into spatial distributions if a body of water is moving seaward in an estuary. Thus a conceptual model is presented to explain the spatial distribution of many biologically important components which have often been reported in many field investigations of estuaries.

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