The productivity of some salt marshes is similar to that of seaweeds. Spartina altemiflora has been grown at net annual yields of about 33 dry t/ha-year, including underground material, on optimum sites (Westlake, 1963). Other emergent communities in brackish water, including mangrove swamps, appear to have annual organic productivities of up to 35 t/ha-year (Westlake, 1963), but insufficient information is available to judge their value in biomass energy systems. Freshwater swamps are believed to be highly productive and offer opportunities for energy production. Both the reed Arundo donax, and bulrush Scirpus lacustris appear to produce 57 to 59 t/ha-year yields (Westlake, 1963); if these can be sustained, they should be suitable candidates for biomass energy usage. Cattail (Typha spp.) is a wetland biomass that has been proposed as an energy resource (Pratt and Andrews, 1980). It grows naturally in monocul­tures, is highly productive, has few insect pests, and can be grown on marginal lands. Managed stands are reported to yield 25 to 30 dry t/ha-year of cattail in the northern climates of the United States (Minnesota).

A strong aquatic biomass candidate for energy applications is the water hyacinth (Eichhomia crassipes) (Klass, 1974). This biomass species is highly productive, as might be expected because it grows in warm climates and has submerged roots and aerial leaves like reedswamp plants. It has been estimated that water hyacinth could be produced at rates up to about 150 t/ha-year if the plants were grown in a good climate, the young plants always predominated, and the water surface was always completely covered (Westlake, 1963). Some evidence has been obtained to support this growth rate (McGarry, 1971; Yount and Grossman, 1970). If such yields can be maintained on a steady-state basis, water hyacinth could possibly turn out to be a prime aquatic biomass candidate as a nonfossil carbon source for synfuels manufacture as well as other potential applications such as the manufacture of paper. Water hyacinth currently has no competitive uses and is considered to be an undesirable species on inland waterways. Many attempts have been made to rid navigable streams in Florida of water hyacinth without success; the plant is a very hardy, disease-resistant species (Del Fosse, 1977).