316 Organic waste recycling: technology and management
industrial and agricultural wastewaters. In fact, the low palatability of aquatic
weeds to livestock has been attributed to a high mineral content (see section
7.6).
7.3.4 Miscellaneous
Some aquatic plants have carotene and xanthophyll pigment concentrations that
equal or exceed those of terrestrial forages such as alfalfa. These pigments are
important ingredients in poultry rations.
Pesticides have been found in aquatic plant samples collected when the
waterway has been recently treated with herbicides or insecticides. Also, traces
of cyanide, oxalate, and nitrate have been found. However, no evidence of
toxicity to mice, sheep or cattle has yet been found in water hyacinth or hydrilla
samples.
7.4 Productivity and problems caused by aquatic weeds
Aquatic weeds, especially rooted, emergent species and floating species are
some of the most productive freshwater ecosystems. Typical values for the net
production of different types of aquatic vegetation from fertile sites (recorded in
terms of unit weight per unit area of the earth's surface per unit time) are as
follows (Westlake 1963): lake phytoplankton 1-8, submerged weeds 3-18, and
emergent weeds 27-77 and floating weeds 35-90 tons of dry organic matter/(ha-
yr). At that time, the highest net productivity recorded was for sugar cane, 85
tons dry matter / (ha-yr).
The productivity of submerged weeds is usually low because the water
reflects and absorbs some of the incident light, coloured substances in the water
absorb light, and the diffusion of carbon dioxide in solution is slow compared to
its diffusion in air. The presence of phytoplankton in the water column also
reduces the light available for submerged plants, and, in eutrophic waters may
be dense enough to cause the elimination of aquatic weeds. On the other hand,
emergent weeds are particularly productive since they make the best use of all
three possible states, with their roots in sediments beneath water and with the
photosynthetic parts of the plant in the air. The surrounding mud around the
roots may be a good source of soluble nutrients, which can diffuse to the roots
via the pore water in the sediments. Light and carbon dioxide are more readily
available in the air than in water. Thus, they make the best of both aquatic and
terrestrial environments.