The development from kinetic coefficients of a predictive model for the growth of Eichhornia crassipes in the field. IV. Application of the model to the Vernon Hooper Dam — a eutrophied South African impoundment

Abstract

A model developed for  Eichhornia crassipes (Mart .) Solms was used to identify the limiting nutrient in the Vernon Hooper Dam and to predict population sizes, yields, growth rates and frequencies and amounts of harvest under varying conditions of nutrient loading and climate. Predicted data were used to evaluate the effectiveness of harvesting measures currently being employed for controlling both nutrient inputs and the population size in this impoundment. Predictions o f the population size, before harvesting was initiated, generally compared favourably with th a t estimated visually. Predictions of the quantities of P that could be removed daily by a 20 ha population indicate that such a population in the impoundment could reduce P concentrations in the epilimnion during summer stratification to levels limiting for algae. This may explain the observed reduction in algal concentrations since the introduction of harvesting. Estimates of the amounts and frequencies of harvest required to contain the predicted potential yields of a 20 ha population indicate that 100 metric tonnes of fresh water hyacinths harvested daily from the impoundment, under present conditions of reduced nutrient loading, are adequate during winter, but not during summer.