[Chap. XLVII THE ALGAE 609 



and phvcoeivthrin and phycocyanin occur in the reds and blue-greens. 



Economic aspects of the algae. To most people the algae of ponds and 

 lakes, as well as those of the ocean deposited along shore after storms, 

 are nuisances. Algae may accumulate in sufficient quantities to become 

 an annoyance to iDathers. Occasionally stock die after using water heav- 

 ily populated with certain blue-green algae. Bad odors and tastes of 

 drinking water are often attributed to the presence of decaying algae in 

 reservoirs. Tea and other subtropical plants are severely injured by 

 algae which grow either on the surface of the leaves, interfering with 

 photosynthesis, or within the leaves as parasites. Small fishes often be- 

 come so entangled in algal mats that they die either there or subse- 

 quently from injuries received in extricating themselves. Algae in fish 

 hatchery ponds may so deplete the oxygen content of the water at 

 night as to cause the death of the young fish through suffocation. 



Algae are rather easily destroyed in small bodies of water by means 

 of certain chemical compounds, particularly arsenites and copper sul- 

 fate. The latter is so effective that one part of it in a million parts of 

 water is sufficient to kill most algae. Copper sulfate in such low con- 

 centrations is not harmful to fishes, nor is the water unfit for human 

 consumption. Although there are on the market numerous commercial 

 products containing copper sulfate with adequate instructions for their 

 use, a few crystals thrown into small pools will usuallv suffice. For 

 larger ponds a quantity of the chemical may be placed in a sack, tied 

 to the rear of a boat, and allowed gradual! \' to dissolve in the water 

 as the boat is moved over the surface. 



Among other practical measures used for eradicating algae from 

 fish ponds are ( 1 ) the removal of the accumulating organic matter bv 

 draining the pond from the bottom instead of the surface, and ( 2 ) the 

 maintenance of high turbidity. Young cra\'fish are often added to the 

 ponds because they increase the turbiditv of the water, preventing light 

 from penetrating; the algae are unable to survive. Reservoirs mav be 

 kept free of algae by stirring up the muddy bottom with large propellers 

 on boats. 



Suppose we now examine the credit side of the ledger. As has been 

 noted before, the algae are the ultimate sources of food and enersv of 

 all strictly aquatic animals. The food chains of animals are numerous 

 and varied (Fig. 275). Some fishes feed directly on the algae, while 

 others secure the energy bound up in the plants by eating animals that 

 feed on algae. Some blue-greens, a few greens, and manv species of 



