THE CAPACITY OF SEAGRASSES TO SURVIVE INCREASED TURBIDITY AND SILTATION - THE SIGNIFICANCE OF GROWTH FORM AND LIGHT USE

Seagrasses are submerged or intertidal angiosperms that form extensive meadows in shallow coastal waters. Tropical as well as temperate seag rass beds are subject to man's interference. Most human activities aff ect seagrasses either through reductions in light availability or chan ges in sediment dynamics, the latter often caused by hydrodynamic chan ges. This paper offers practical ranges within which seagrasses can be expected to occur for these two main environmental factors: light ava ilability and sediment deposition. With a few exceptions, comparativel y little variation was found among species in light requirements for p hotosynthesis. Predicted compensation depths calculated from photosynt hesis data correlated well with independently estimated maximum coloni zation depths. In contrast, considerable differences exist in architec tural characteristics. Large shoot size or the capacity to elongate ve rtical 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 y r(-1) can probably be coped with, depending on the species. Observed h orizontal 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 t o track the migration of sand waves and, thus, horizontally escape sed imentation.