Photoinhibition is a central problem for the understanding of plasticity in photosynthesis vs. irradiance response. It effectively reduces the photosynthetic rate. In this contribution, we present a mechanistic model of algal photoinhibition induced by photodamage to photosystem-II. Photosystem-IIs (PSIIs) are assumed to exist in three states: open, closed and inhibited. Photosynthesis is closely associated with the transitions between the three states. The present model is defned by four parameters: effective cross section of PSII, number of PSIIs, turnover time of electron transfer chains and the ratio of rate constant of damage to that of repair of D1 proteins in PSIIs. It gives a photosynthetic response curve of phytoplankton to irradiance (PI-curve). Without photoinhibition, the PI-curve is in hyperbola with the first three parameters. The PI-curve with photoinhibition can be simplified to the same form as the hyperbola by replacing either the number of PSIIs with the number of functional PSIIs or the turnover time of electron transfer chains with the average turnover time.

We explore control mechanisms underlying the vertical migration of zooplankton in the water column under the predator-avoidance hypothesis. Two groups of assumptions in which the organisms are assumed to migrate vertically in order to minimize realized or effective predation pressure (type-I) and to minimize changes in realized or e!ective predation pressure (type-II), respectively, are investigated. Realized predation pressure is defined as the product of light intensity and relative predation abundance and the part of realized predation pressure that really affects organisms is termed as effective predation pressure. Although both types of assumptions can lead to the migration of zooplankton to avoid the mortality from predators, only the mechanisms based on type-II assumptions permit zooplankton to undergo a normal diel vertical migration (morning descent and evening ascent). The assumption of minimizing changes in realized predation pressure is based on consideration of DVM induction only by light intensity and predators. The assumption of minimizing changes in effective predation pressure takes into account, apart from light and predators also the effects of food and temperature. The latter assumption results in the same expression of migration velocity as the former one when both food and temperature are constant over water depth. A significant characteristic of the two type-II assumptions is that the relative change in light intensity plays a primary role in determining the migration velocity. The photoresponse is modified by other environmental variables: predation pressure, food and temperature. Both light and predation pressure are necessary for organisms to undertake DVM. We analyse the effect of each single variable. The modification of the phototaxis of migratory organisms depends on the vertical distribution of these variables.

nity or group, which characterizes the mean rate of energy consumption in a community. Models 1 and 2 generate peaked distributions of biomass spectral density whereas Model 3 generates a flat distribution. In Model 4, the distributions of biomass spectral density and of abundance spectral density depend on the Lagrangian multipler (λ2). When λ2 tends to zero or equals zero, the distributions of biomass spectral density and of abundance spectral density correspond to those from Model 3. When λ2 has a large negative value, the biomass spectrum is similar to the empirical flat biomass spectrum organized in logarithmic size intervals. When λ2>0, the biomass spectral density increases with body mass and the distribution of abundance spectral density is an unimodal curve.

In consequence of interactions between compartments, the matter or energy residence time in an econetwork is in nature distinct from that in a compartment. Based on the analysis of econetwork structure, a strategy is developed to calculate the matter or energy residence time in a general econetwork and the effects of self-, direct-and indirect interaction on econetwork residence time. Two typical examples are used to illustrate the strategy, the results show that total residence time equals the ratio of total standing stock to total system outflow or total system inflow instead of the ratio of total standing stock to total system throughput.

Institute of Hydrobiology, Jinan University, Guangzhou 510632, People's Republic of China Three measures-total residence time, total ecosystem throughput, and ratio of total standing stock to total system throughput-are investigated on the basis of an analysis of two types of structure matrices (or transitive closure matrices). The quantitative expression of total residence time given by Han (Han, B. P., 1997. Total residence time of ecosystem at steady state. Ecol. Modelling 95, 301-310) is proved to be equal to the ratio of total standing stock to total system outflow, that is, total residence time does not depend on ecosystem structure. Whereas total system throughflow is explicitly dependent on ecosystem structure, the ratio of total standing stock to total system throughflow not only depends on system structure but also on system state. Therefore, total system throughflow and the ratio of total standing stock to total system throughflow may measure ecosystem maturity or complexity. The ratio of total system throughflow to total system inflow or total system outflow is defined as the flow multiplying index instead of average path length, for the throughflow which equals the difference between total system throughput and total system inflow results from the interaction between compartments, i. e. reutilization of total system inflow. By dividing two types of structure matrices (transitive closure matrices) into cycling matrices and noncycling matrices, which correspond to first and subsequent passage flows, respectively, the contribution of cycling paths and noncycling paths to the three measures can be understood.