The development of a classification system for inland aquatic ecosystems in South Africa

Abstract

A classification system is described that was developed for inland aquatic ecosystems in South Africa, including wetlands. The six-tiered classification system is based on a top-down, hierarchical classification of aquatic ecosystems, following the functionally-oriented hydrogeomorphic (HGM) approach to classification but incorporating structural attributes at the lower levels of the hierarchy. At Level 1, a distinction is made between inland, estuarine and shallow marine systems using the degree of connectivity to the open ocean as the key discriminator. Inland systems are characterised by the complete absence of marine exchange and/or tidal influence. At Level 2, inland systems are grouped according to the most appropriate spatial framework for the particular application. At Level 3, four primary Landscape Units are distinguished (Valley floor, Slope, Plain, Bench) on the basis of the topographic position within which a particular inland aquatic ecosystem is situated, in recognition of the influence that the landscape setting has over hydrological and hydrodynamic processes acting within an aquatic ecosystem. Level 4 identifies HGM Units, defined primarily according to landform, hydrological characteristics and hydrodynamics. The following primary HGM Units (or HGM Types), which represent the main units of analysis for the classification system, are distinguished at Level 4A: (1) River; (2) Floodplain Wetland; (3) Channelled Valley-Bottom Wetland; (4) Unchannelled Valley-Bottom Wetland; (5) Depression; (6) Seep; (7) Wetland Flat. Secondary discriminators are applied at Level 5 to classify the hydrological regime of an HGM Unit, and Descriptors at Level 6 to categorise a range of biophysical attributes. The HGM Unit at Level 4 and the Hydrological Regime at Level 5 together constitute a Functional Unit, which represents the focal point of the classification system. The utility of the classification system is ultimately dependent on the level to which ecosystem units are classified, which is in turn constrained by the type and extent of information available.