Encyclopedia of Environmental Science and Engineering, Volume I and II

396 EUTROPHICATION

varies with the organism and with the type of nitrogen. For

ammonia nitrogen the optimum range varies from 0.3 to 5.3

ppm and for nitrate nitrogen the optimum range falls between

0.3 and 0.9 ppm. Below these values the growth rate decreases

as the concentration of nitrogen decreases.

Apparently the use of the various forms of nitrogen by

algae is not constant throughout the year. Tests conducted

at Sanctuary Lake in Pennsylvania (1965) indicate that the

order of preference for the three forms of nitrogen—ammonia-

nitrogen, nitrate-nitrogen, and nitrite-nitrogen—are defi ned

by three seasonal periods, which are:

Spring (1) Ammonia nitrogen

(2) Nitrate nitrogen

(3) Nitrite nitrogen

Midsummer (1) Ammonia nitrogen

(2) Nitrite nitrogen

(3) Nitrate nitrogen

Fall (1) Ammonia nitrogen

(2) Nitrate nitrogen

(3) Nitrite nitrogen

The amount of nitrogen in the aquatic environment is

important to algae because it determines the amount of chlo-

rophyll that may be formed. Too much nitrogen, however,

inhibits the formation of chlorophyll and limits growth.

Laboratory studies on algae conducted by Gerloff indicate

that of all the nutrients required by algae, only nitrogen, phos-

phorus and iron may be considered as limiting elements, and

of these three, nitrogen exerts the maximum limiting infl u-

ence. Approximately 5 mg of nitrogen and 0.08 mg of phos-

phorus were necessary for each 100 mg of algae produced.

The corresponding nitrogen/phosphorus ratio is 60 to 1.

Hutchinson cites phosphorus as being the more impor-

tant element since it is more likely to be defi cient. When

phosphorus enters a body of water, only about 10% is in the

soluble form readily available for algal consumption. During

midsummer total phosphate may increase greatly during the

formation of algal blooms, while soluble phosphate is unde-

tectable due to rapid absorption by the growing algae. Very

often during warm weather these blooms are stimulated by

the decomposition and release of soluble phosphates from

the bottom sediments, deposited by the expired blooms of

previous seasons. Thus when phosphates are added to a

lake, only a portion of the phosphates are used in produc-

ing blooms. The blooms thrive and consume phosphates for

only a short time, and a signifi cant amount fi nds its way to

the bottom sediments where it will be unavailable to further

growth of aquatic vegetation.

Prescott examined a number of algae and concluded

that most blue-green algae are highly proteinaceous.

Aphanizomenon fl os-aquae, for example, was shown to con-

tain 62.8% protein. Green algae were found to be less pro-

teinaceous. Spirogyra and cladophora, for example, contain

23.8 and 23.6% respectively. Thus it can be concluded that

the nitrogen requirement (for the elaboration of proteins)

depends on the class of algae, and that blue-green algae

would require more nitrogen than green algae.

Provasoli examined 154 algal species to determine the

requirements for organic micronutrients. He found that

although 56 species required no vitamins, 90 species were

unable to live without vitamins such as B 12 , thiamin and

biotin, either alone or in various combinations. He concluded

that these vitamins are derived from soil runoff, bottom muds,

fungi and bacterial production (B 12 ), and from a natural resid-

ual in the water.

Ketchum and Pirson conducted a series of examina-

tions on the inorganic micronutrient requirements of algae

and concluded that a number of elements are necessary for

growth. No numerical values were assigned to the require-

ment levels. Those elements shown to be essential were C,

H, O, P, H, S, Mg, Ca, Co, Fe, K and Mo. Those elements

which may be essential (subject to further study) were Cu,

An, B, Si, Va, Na, Sr, and Rb.

In summation, absolute values and nutrient thresholds

cannot be set at this time because too little is known regard-

ing the requirements of individual species. It might be stated

in general terms, however, that nitrogen and phosphorus are

two essential nutrient elements related to the production of

blooms, and that if they are present in the neighborhood

of 0.2 ppm and 0.05 ppm, respectively, algal growths will

increase signifi cantly.

NUTRITIONAL THRESHOLDS FOR THE GROWTH

OF AQUATIC PLANTS

Studies conducted by Harper and Daniel indicate that sub-

merged aquative plants contain 12% dry matter of which

1.8% are nitrogen compounds and 0.18% are phosphorus

compounds. Hoagland indicates that when the nitrate content

of water is high, nitrates may be stored in aquatic plants to

be reduced to the usable ammonia nitrogen form as required.

Subsequent investigations show that ammonia nitrogen can

be substituted for nitrate nitrogen and used directly. Light

apparently is not a necessary factor in the reduction of the

nitrogen.

Muller conducted a number of experiments on both algae

and submerged aquatic plants, and concludes that exces-

sive growths of plants and algae can be avoided in enriched

waters if the concentration of nitrate nitrogen is kept below

0.3 ppm, and if the concentration of total nitrogen remains

below 0.6 ppm.

OXYGEN BALANCE

Recently, attention has been given to the effect of the intense

growths of algae on the oxygen balance of natural water-

ways. It has been established that the dissolved oxygen

concentrations may exhibit wide variation throughout the

course of the day. This variation is attributed to the ability of

algae to produce oxygen during the daylight hours, whereas

they require oxygen for their metabolic processes during the

hours of darkness.

C005_012_r03.indd 396C005_012_r03.indd 396 11/18/2005 10:26:02 AM11/18/2005 10:26:02 AM