Introduction

Abstract

Freshwater ecosystems represent a major group of habitats around the world. Not only are the habitats themselves important, for a wide variety of reasons, but the medium itself — fresh water — is of fundamental significance to human welfare everywhere. Fresh water is essential to humans for drinking water, transport, irrigation, energy and as a vehicle to eliminate waste material; the biota provide important food resources and utilizable materials. All freshwater bodies are dynamic systems: not only are their organisms affected by the physicochemical conditions (and thus by human activities), but also the plants and animals interact and may influence both the habitat and one another. They have a major influence on the physical and chemical conditions, while inter- and intraspecific relationships among plants and animals may be of critical importance to both water quality and the structure of communities (Maitland, 1990).