Seaweeds and seagrasses thrive on many sheltered to fully exposed coasts, occupying rocky shores, coral reefs, and many other coastal habitats. The contribution of seagrass meadows and seaweed-based ecosystems to the global ocean is disproportionate to their small area. Seagrasses, being rooted angiosperms, commonly flourish as meadows in soft sediments at and below the intertidal zone. Estimates of standing crop and primary productivity of seagrasses and seaweeds are as variable as the methods that have been devised to measure them. Rates of seagrass production vary seasonally, even in the tropics. Biomass and production estimates of kelps and other seaweeds are as numerous as those for seagrasses and equally variable, doubtless for the same reasons. Seagrasses and seaweeds differ in some aspects of photosynthetic potential, such as their ability to harvest light. The growth and productivity of seagrasses and seaweeds are determined by the way in which individual plants balance their carbon requirements.