Modelling the spatial and temporal variability of diatoms in the River Murray

We have developed a simple mechanistic model, which simulates the temporal and spatial variability of concentrations of the diatom Aulacoseira granula ta in the River Murray, NSW Australia, over large distances under different flow conditions. The model incorporates a simple growth function determine d by the availabilities of silica and light, and by temperature. It also in cludes losses due to sinking and downstream advection. The model is tested against weekly measurements for the period 1981-1990, at three sites separa ted by 1100 km. The seasonal cycle of Aulacoseira concentrations along the river is well represented by the model. During times of low flow, Aulacosei ra concentrations increase through substantial growth in the upstream part of the river. Further downstream, Aulacoseira concentrations diminish where losses exceed growth, which is reduced due to silica limitation. During ti mes of high Bow, the concentrations of Aulacoseira remain fairly uniform al ong the river as the diatoms have insufficient time to grow before being ad vected downstream.