The supply of inorganic carbon affects, directly or indirectly, to some degree, most processes within aquatic ecosystems. Variety of habitat requires that studies cover a broad range of conditions. Experiments, especially field studies, should focus on basic mechanisms so as to facilitate extrapolation to other conditions. Water covers three-fourths of the world's surface. Although productivity per unit area in the submersed environment is relatively low, the total global yield is large. In contrast, emergent aquatic plants inhabit a smaller area, but are among the most productive. Aquatic plant systems have an enormous impact on human society. Aquatic plants are found in a variety of habitats and are usually grouped according to their size and modes of attachment to substrata. By far the largest number of aquatic plant species consist of microscopic algae of freely-floating habit or attached to both living and non-living substrata.