Abstract
Seagrasses are submerged or intertidal angiosperms that form extensive meadows in shallow coastal waters. Tropical as well as temperate seagrass beds are subject to man's interference. Most human activities affect seagrasses either through reductions in light availability or changes in sediment dynamics, the latter often caused by hydrodynamic changes. This paper offers practical ranges within which seagrasses can be expected to occur for these two main environmental factors: light availability and sediment deposition. With a few exceptions, comparatively little variation was found among species in light requirements for photosynthesis. Predicted compensation depths calculated from photosynthesis data correlated well with independently estimated maximum colonization depths. In contrast, considerable differences exist in architectural characteristics. Large shoot size or the capacity to elongate vertical stems enabled several species to raise their leaf canopy closer to the water surface and thus suffer less in turbid water. The latter also allows a response to siltation: sedimentation rates of 2-13 cm yr(-1) can probably be coped with, depending on the species. Observed horizontal rhizome elongation rates differed considerably among species: colonizing seagrasses expand horizontally at rates between 1 and 10 m yr(-1). The higher horizontal growth rates are probably sufficient to track the migration of sand waves and, thus, horizontally escape sedimentation. [KEYWORDS: Dutch wadden sea; zostera-marina l; cymodocea-nodosa; eelgrass; patterns; biomass; productivity; communities; beds; variability]