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.

The zooplankton of 18 reservoirs of South China was investigated in 2000. 61 Rotifera species, 23 Cladoceras and 14 Copepodas were identified. The most frequent Rotifera genera were Keratella, Brachionus, Trichocerca, Diurella, Ascomorpha, Polyarthra, Ploesoma, Asplanchna, Pompholyx and Conochilus. Bosmina longirostris, Bosminopsis deitersi, Diaphanosoma birgei, D. brachyurum and Moina micrura were typical of Cladocera in the reservoirs. Phyllodiaptomus tunguidus, Neodiaptomus schmackeri and Mesocyclops leuckarti were the most frequent Copepoda and M. leuckarti dominated Copepoda in most reservoirs. High zooplankton species richness with low abundance was characteristic of the throughflowing reservoir, whereas low species richness with low abundance was found in the reservoir with the longest retention time. Relative high abundance and medium species diversity were the distinction of intermediate retention time reservoirs.

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.

In this study, a 1D model of reservoir hydrodynamics DYRESM has been applied to Sau Reservoir, a river valley reservoir in the North-Eastern Spain. Simulation is undertaken for 3 years (1995-1997). Meteorological input data measured at the dam are only available from May of 1997. In this case the simulation results fit measured temperatures very well. In the remaining periods, some meteorological data (radiation, wind and rainfall) were obtained from two nearby stations. Simulated temperature distribution in 1996 is close to the observed one. In 1995, however, the simulated result is far from the observed data. Inflows, outflow and local meteorological events such as storms and gusts of wind seem to be responsible for the differences. By changing some parameters, the effects of flow, light extinction coefficient and outlet elevation on thermal stratification are investigated. Simulations demonstrate that the inflow with high temperature is the main factor controlling the thermal structure in Sau Reservoir and demonstrate that the effect of residence time on thermal stratification is manifested mainly by the changes in the depth of thermocline.

Based on a model of light lindted growth, Huisman and Weissing found that in a well mixed water column with constant light supply (energy reaching the water surface), equilibrium growth and competition of phytoplankton for light can be characterised by a critical light intensity at the base of the colunm (I*out). The present study attempts to give a further insight into this model. We first analyse the dependence of the critical light intensity on four parameters: initial slope of the photosynthesis-inntensity (p-I) curve, maxinaal photosynthetic rate, the light-saturated parameter Ik and specific carbon loss rate. Increases in the first two parameters tend to reduce the critical light intensity and increases in the last two tend to increase the critical light intensity. Then we analyse the performance of the model under variable light supply with a time-scale of 1 day (24hr). Within this time-scale, the critical light intensity changes with time. However, the equilibrium growth and the outcome of competition for light can be adequately characterised by critical light extinction defined as the upper lindt of total light extinction due to both biomass and non-living matter in the water column. Under constant light supply, a critical light intensity uniquely corresponds to a critical light extinction. Therefore, critical light extinction can be utilised to predict the equilibrimn growth and the outcome of competition under both constant and variable light supply. By changing the maximal light supply at noon, seasonal succession of species composition of conmmnities is investigated. The possible effect of two typical photoresponses, photoadaptation and photoinhibition, on growth and competition are discussed.