Submerged aquatic macrophytes have their photosyn thetic tissue entirely submerged (Figures 2e and 16). Various experiments have proven that minerals can be
taken up directly by shoot tissues of submerged plants; however, there also is no question regarding the uptake capability of nutrients by the roots of these plants. Most studies confirmed that although shoot uptake occurs, the major source for nutrients is the sediment.
Submerged plants are commonly covered by periphy tic algae, especially under conditions of higher nutrient concentrations in water. These algae usually take up large amounts ofnutrients which may add to nutrient removal from the water. However, if macrophytes are not harvested, nutrients return back to the water column after algal die off. The development of epiphytic com munities on the leaves ofvascular plants may reduce net production through several mechanisms other than photosynthetically available radiation (PAR) attenuation, including the reduction ofdiffusive transport ofinorganic carbon, nitrogen, and phosphorus. Reduction of photo synthesis ofsubmerged plants was observed to correspond with increasing epiphytic material associated with nutri ent enrichment. It has been reported that biomass of submersed macrophytes decreased significantly with increasing nutrient concentrations in the water. They concluded that this reduction was because of increased periphyton and phytoplankton growth due to nutrient additions which resulted in increased light attenuation.
Depletion of dissolved inorganic carbon in water and increase of dissolved oxygen content caused by photosynthetic activity create optimal conditions for volatilization ofammonia, chemical precipitation ofphos phorus, and for the mineralization of organic matter. Large amounts of nutrients from water and released from senescent tissues are also assimilated by epiphytic algae which are often present. Much ofthe detrital matter produced in these systems will be accumulated and retained in the sediments.
Submerged plants, however, only grow well in oxygen ated water and therefore cannot be used in wastewater with a high content of readily biodegradable organic matter because the microbial decomposition ofthe organic matter will create anoxic conditions. In addition, the turbidity of the water must not be too high to prevent light transmis sion to the plants to support their photosynthetic activity. As a result ofliterature survey on submerged macrophytes it seems that the use oftreatment systems with submerged plants is limited to waters with low nutrient concentrations, low turbidity, and low concentrations of organics. The prime potential use ofsubmerged macrophyte based was tewater treatment systems is therefore for 'polishing' secondarily treated wastewaters, although good treatment ofprimary domestic effluent has been obtained in an Elodea nuttallii based system in New Hamphshire. Submerged vegetation very often develops voluntarily in CWs with emergent vegetation. For example, submerged plants Potamogeton pectinatus (sago pondweed) and Eleocharis dulcis (spikerush) grow extensively in a wetland system treating livestock wastes at Pontotoc, Mississippi. Other submerged macrophyte Vallisneria americana (wild celery) together with P. pectinatus grow in a large treatment wetland at Minot, North Dakota. Submerged plants based CWs are also successfully used to treat agricultural runoffin order to prevent eutrophication of the Florida Everglades. The major species occurring in these wetlands are Ceratophyllum demersum (coontail) and Najas guadalupensis (southern water nymph).
The use of submerged macrophytes for the waste water treatment is still in the experimental stage, with species like Egeria densa (Brazilian elodea), Elodea cana densis (common waterweed), E. nuttallii (Nuttall waterweed), C. demersum, Hydrilla verticillata (hydrilla), Cabomba caroliniana (fanwort), Myriophyllum heterophyllum (variable leaf water milfoil), and Potamogeton spp. (pondweeds) being the most promising. This category may also include systems with macrophytic algae from the order Characeae, that is, Chara and Nitella, which are used, for example, for treatment of mine waste waters in Canada.
See also: Constructed Wetlands, Subsurface Flow.
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