Sea-grass communities: structure, distribution and classification
den Hartog, Cornelis
Aims: In the marine biological literature sea-grass beds are generally regarded as being more or less similarly structured, and typically indicated as the sea-grass ecosystem. This assumption regarding their structure is discussed and rejected, as regarded on a worldwide scale sea-grass beds show considerable variation in many qualities to be elucidated in this paper. Study area: Sea-grass beds of the world. Methods: A combination of the formation approach and the phytosociological approach is applied, using genera (instead of species) and some structural vegetation characteristics as variables. The study of sea-grass beds with the two mentioned approaches is elucidated, and the history of their application for the classification is outlined. Results: Six well-defined classes of sea-grass communities are recognised on a global scale (top-down). The classification of the sea-grass communities is presented in the form of an identification key. The descriptions are based on floristic composition, physical structure (stratification, rooting system), relation to the substrate (soft substrate or rock), and degree of permanence (from annual presence to millennia). Conclusions: The assumption that sea-grass communities may be considered as more or less similarly structured ecosystems is an unjustified simplification, as the world's sea-grass beds show, apart from differences in the species composition, considerable variations in their structure, persistence and performance. They have been accepted as a 'formation' in its own right. Seagrass communities are well distinguished from all other plant communities, and show only occasionally some overlap with communities of brackish and continental salt waters. Descriptions of sea-grass communities are generally based on the dominant angiosperm component, and thus present in fact only taxo- or merocoenoses. Consequently, they may show considerable regional variations, and even within the same area, if the algal flora, the fauna and environmental parameters, such as exposition to wave action, salinity, and substrate are being considered. The importance of the proposed classification is that comparisons of sea-grass communities can be made at the right level, and that generalisations should be considered in a more critical manner.