We're going to talk a little bit more about the biology of the plants and other contributors to the productivity of seagrass meadows. Now there are different kinds of seagrasses, it's not all the same. And here I've got two pictures, one from the Mediterranean, not tropical, but definitely showing these very large seagrasses in the Mediterranean that dominate, called Positonia. And the second image I have is of this large strappy-like species from Singapore, again, and Harlus, that we saw the photo of the flowers earlier on. So these are two species that are a large sort of form, but we also have very small forms. Now these species tend to be very common in the Indo-Pacific region, where they can cover vast areas. And they're actually really quite difficult to see sometimes, and they often appear as a green fuzz on the surface because much of the biomass is actually, or much of the material of the plant is actually under the sediment surface. Now this is some of the favoured food of jugongs, which are the large grazes of these systems. Now seagrass, even though they're very different in size, all have a very common plan, and that is that they are modular plants, a little bit like the lawn that you might see in front of a building or in front of your house. They have modules that are made up of leaves, a rhizome, which is actually a word that means underground stem, and roots that are rooted into the sediment. There's some other elements into this picture, and you can see that they actually grow from the tip, so they grow outward, and they can fill a whole meadow by growing along through this underground stem. Now those rammets, which are the modules, can be dispersed if the plant is disturbed. So in this way, these plants have seeds for dispersal, where they can start new meadows, that are dispersed and can start new meadows, but also they can start new meadows from rammets or these modules, and that's a very important part of seagrass biology. Now these different kinds of seagrasses have different ecologies, really, or different suites of traits that lead to differences in the way they respond to disturbances, for example. So the small species, which include Halofala, Rupia, sort of in the middle, but Zostra, and also Haladouli, they're small, thin leaves, they have very small rhizomes, they're short-lived, and they grow very rapidly, and they're easily grazed. Well, in contrast, the large species, like Enhalus and Thalassia that's extremely common in the Caribbean, have large rhizomes, robust, thick leaves, and they actually grow relatively slowly, and don't respond quite as quickly to disturbances. Now, in addition to these differences in size and growth rates, they all have some special adaptations that allow them to live within the coastal zone, underwater, rooted in sometimes sediments that have very low oxygen concentrations. They have gas spaces within them, they're sort of spongy, and that allows the transfer of oxygen to their root systems so that they can grow and take up nutrients. They have very specialised reproduction, they have pollen like other plants in the terrestrial environment, but this pollen works underwater. They have associations with bacteria in the sediment, that fix nitrogen gas from the atmosphere into forms that the plants can use, and that means that they can increase nitrogen for other organisms in seagrass beds. They also can concentrate carbon dioxide from the water, and that can enhance their photosynthetic rates. So this is a very clever adaptation to life underwater. But seagrass are not the only contributors to primary production, or the only contributors to growth and biomass, and to the food web. There are other contributors, and these include algae, so macroalgae seaweeds, but also very small algal species that inhabit the leaves of the seagrassers. There's also microscopic algae that live on the sediment surface, and these can be highly productive and have important roles in supporting some of the animal species that live and use seagrass beds.