about 50% of the controls, two harvests will result in about 75% reduction, 

 and three harvests almost totally eliminate the plants for that year. The 

 researchers recommended two harvests, one in June and the other in July, 

 for that climate. None of the treatments had an appreciable effect on the 

 subsequent year's growth. So far little is known of the effects of har- 

 vesting of higher aquatic plants on the phytoplankton. 



A demonstration in the State of Florida provides another example of 

 weed harvesting. The St. John's River is the largest river entirely within 

 the state, approximately 480 km long. Problem weeds such as water hya- 

 cinths occur in areas where water use for navigation is extensive. The 

 weeds also retard irrigation and drainage and reduce game fish and water 

 fowl. The decomposition of detritus from these plants also depletes the 

 oxygen from the lower waters. Control of water hyacinths on the St. John's 

 River has been carried out with chemicals for a long time. Now researchers 

 are experimenting with harvesting techniques. In Florida alone, more than 

 40,000 ha of water are covered with water hyacinths, despite extensive and 

 continuous programs of control by various governmental agencies. 



Dredging --A procedure that can be thought of as an extension or modifi- 

 cation of harvesting is dredging of the sediments of a eutrophic lake. Per- 

 haps in many lakes the sediments are an important source of nutrients that 

 may be cycled to the overlying waters, especially at certain times of the 

 year. In theory, dredging would remove this nutrient source, but there are 

 several problems, not the least of which is disposal of dredged material. 

 These problems are being addressed in an extensive research program 

 recently undertaken by the U.S. Army Corps of Engineers. 



Dilution or F1ushing --Another method of eutrophication control is flush- 

 ing or dilution. Use of this method is limited by the availability of 

 fresh water. Two nutrient dilution procedures have been attempted: (1) 

 pumping water out of the lake, thus permitting increased inflow of nutrient- 

 poor groundwater, and (2) routing additional quantities of nutrient-poor 

 surface waters into the lake. The first has been used at Snake Lake, Wis- 

 consin. The second has been tried in several places. One of the most 

 successful experiments was done at Green Lake, Washington, where, after 5 

 years of flushing (plus some initial dredging), the blue-green algal 

 standing crop was suppressed, with elimination of Aphanizomenon . Flushing 

 has also been tried on a small scale at Moses Lake, Washington. 



Aeration --It is also possible to immobilize nutrients in eutrophic 

 lakes through aeration of hypolimnetic waters where large reservoirs of 

 phosphorus may accumulate. Aeration methods generally fall into two 

 groups: those that destratify the lake and thus affect all depths, and 

 those that aerate only the bottom waters and do not destratify the lake, 

 i.e., hypolimnetic aeration. When destratif ication is accomplished, the 

 lake becomes isothermal with oxygen present at all depths and other chemi- 

 cal conditions fairly uniform. Hypolimnetic aeration has certain 

 advantages over destratif ication. Nutrients are not upwelled into the 

 surface waters where they may promote algal growth. Further, hypolim- 



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