Management of artificial wetlands for waterfowl - a novel technique to increase the availability of invertebrate food items.

Abstract

Barley straw (Hordeum sp.) was added to parts of a lake complex formed by gravel digging. This treatment, in March 1978 was an experimental input of allochthonous material which normally forms the major nutrient and energy supply to natural wetlands, and which is lacking in wetgravel workings with no inflow streams. The aim of the treatment was to significantly increase the standing crop of those macroinvertebrates which are an important food source for many species of duck in summer. Nine similar, interconnected lagoons were chosen, 6 were treated with straw and 3 were used as untreated control sites. The invertebrate fauna of the sediments was sampled using diver's hand cores. Insect emergence was monitored by floating sticky traps and the colonising fauna of the straw and other detritus was studied by use of Ekman Grabs and straw filled nylon mesh litter-bags. The existing fauna at all sites was similar. It was dominated by Tubificid worms, chironomid larvae were sub-dominant and there were very small numbers of other insects and molluscs. Initially, the existing chironomid larval populations were adversely affected. Following addition of the straw the biomass and number of organisms was reduced at all sites, including the controls, but with a greater part of the reduction at the treated sites being due to a fall in chironomid numbers. Emergence of chironomids in 1978 was also lower from the treated sites and total chironomid emergence showed a decrease with an increase in amount of straw added. Further examination however, showed that while the initial fauna, particularly the chironomids of the underlying sediment decreased, there was an increase in numbers and diversity of aquatic invertebrates within the straw itself, with leaf 'conditioners' predominant in fresh straw and 'decomposters' including chironomids in older straw. Straw filled litter-bags showed a dramatic increase in the fauna of the straw in 2 freshly treated lagoons and chironomids emergence began sooner in 1979 from treated sites than from control sites. Despite the initial adverse effects on the Chironomidae, which are an important duckling food, the technique promises to be a valuable treatment for artificial wetlands, providing nutrient and energy supply to stimulate and increase the production of invertebrate food for waterfowl. My thanks are due to the Amey Roadstone Corporation Limited who finance the research project of which this study is a part.