There is a marked contrast between the theory and the practice of conservation in terrestrial environments on the one hand and in most aquatic environments, including those of the coastal marine zone, on the other. It is effectively axiomatic in the terrestrial sphere that conservation requires active management. It is not sufficient simply to safeguard an area from external threats, although that could also be important. Without active intervention, a given terrestrial system is likely to change into another, almost certainly of lesser conservation interest. The habitat must be manipulated so as to maintain it in a form suitable for the species and the communities of interest, and the populations of certain species-most especially various potentially superior competitors and often significant consumers-must be controlled in order to favour others. Burning, grazing and many other tools are routinely used to prevent natural changes. To a degree, the same is also true of freshwater habitats: control of water tables, of water quality, of fringing stands of vegetation and of submerged aquatics are all well-known and much practised measures.
Marine and brackish-water systems are treated differently, however. Through much of the past, these have been conserved only insofar as they lay within the boundaries of conservation areas declared-and managed-for other purposes, whether these have been botanical or ornithological. More recently, however, they have become the focus of conservation interest in their own right with the declaration of Marine Nature Reserves, or Special Areas of Conservation as a consequence of the 'Habitats Directive' (Council Directi6e 92/43/EEC; Anon., 1992), etc. In these, in contrast to terrestrial conservation practice, management measures have largely been restricted to preventing human activities that might impinge adversely on them, with no attempt being made to manipulate the contained populations.
In part this could be a consequence of the recency of their designation, but it also reflects divergent philosophical approaches. In essence, it is necessary to interfere in terrestrial systems because most of them are anthropogenic in origin and are naturally subject to successional changes. In contrast, the sea is supposed to be a natural system and is thought to be able to maintain itself in the desired state provided that human interference can be removed or at least controlled. It is essential to prevent natural succession from occurring on land but the aquatic coastal system is already in its climax state and it is this status quo that must be maintained, not a degraded version of it. This may (or may not) be true of much of the coastal sea, but is it also the case in coastal brackish waters including lagoons?
Here the argument is rendered more complex by another system of divergent philosophical approaches. Some habitats are perceived as being under physical control whilst others are structured by biotic