Oxic sediment

Anoxic sediment po3-


Figure 15.19 Phosphorus interaction with sulfur and iron in lakes. (Based on Home and Goldman, 1994.)

In the fall turnover the phosphate is mixed back into surface layers, where it is available to plants and algae. With the transport of oxygen back to the depths, the iron and sulfur are oxidized again, and the cycle repeats. The process can be summarized by the following reaction, in which oxic conditions push the reaction to the right and anoxic conditions force it to the left.

4 Fe(II)S + 4 PO43~ + 4H+ + 9O2 , 4 Fe(III)PO4 + 4 SO42~ + 2H2O

The phosphate concentration falls in the epilimnion throughout the summer stratification as phytoplankton take it up, then die and settle toward the bottom. Phytoplankton are highly efficient at taking up phosphate, even at very low environmental concentrations. The Monod half-rate coefficient for phosphate uptake is about 1 to 3 mg/L PO4-P. Thus, the uptake mechanism is rarely saturated unless pollution increases the concentration greatly. Differences in ability to compete for phosphorus may contribute to the succession of algal species during the summer stratification. Algae also exhibit luxury consumption of phosphorus, in which they store as much as 20 times the amount needed for cell division in cellular bodies called polyphosphate granules. Asterionella can store 100 times its immediate needs. By such hoarding they can continue their growth after the water column has been depleted of phosphorus.

Phosphorus is recycled rapidly in the water column. Zooplankton eat the phytoplank-ton, then excrete phosphorus at the rate of 10% of their body store daily. Half of this is phosphate and is readily reused. The other half is organic phosphate. Phytoplankton have the unusual capability to excrete the extracellular enzyme alkaline phosphatase. This acts in the water, outside the phytoplankton cell, to break the bond between the phosphate and the rest of the organic molecule. This represents a risky investment for the phyto-plankton, since other species may benefit from the cost they bear for excreting the enzymes. When phosphorus is plentiful, the enzyme production slows considerably.

Diatoms need large amounts of silica (SiO2) for their cell walls. About half of the dry weight of diatoms is silica. An absence of silica can result in replacement by cyanobacter.


Figure 15.20 Changes in algal species composition with degrees of eutrophication. (From Connell and Miller, 1984.)


Figure 15.20 Changes in algal species composition with degrees of eutrophication. (From Connell and Miller, 1984.)

Rivers typically have about 13 mg SiO2 per liter, derived from weathering of feldspars. Lakes range from 0.5 to 60 mg/L.

Iron, besides its role in the phosphorus cycle of lakes, is often a limiting nutrient. In the eplimnion much of the iron is chelated with organics. Some algae can only use inorganic iron, others only the chelated form, and others use both. Cyanobacter produce powerful iron-chelating organics, which limit their competitors' access to iron.

Eutrophication produces changes in the community species structure. As nutrient levels increase, the dominant algal species shift from diatoms to green algae to cyanobac-ter (Figure 15.20). In the more extreme forms of cultural eutrophication the phytoplankton growth exceeds the ability of zooplankton to control them. More bacteria and detritus feeders are found. Respiration by the plants at night or by bacteria degrading dead plants can deplete oxygen, affecting higher organisms including fish. Species diversity and richness may increase in moderate eutrophy, then decrease in the extreme form, although abundance continues to increase. At such high abundance these blooms may produce phytotoxins affecting higher forms of life, such as fish or cattle that drink the water.

Cultural eutrophication can be controlled by eliminating nutrient loading, reducing nutrients in the lake, or by addressing the symptoms, whether they be low dissolved oxygen or plant accumulation. Nutrients can be eliminated by using advanced wastewater treatment or by diverting the wastewater to another watershed. In the case of Lake Tahoe, a highly oligotrophic lake on the border between California and Nevada, the wastewater is both treated and pumped over the mountains surrounding the lake. This has eliminated enough phosphorus so that the lake has changed from being nitrogen limited to being phosphorus limited. However, significant nitrogen loading still results from atmospheric deposition of nitrogen oxides fixed by combustion in automobile engines and wood-burning stoves.

Dredging of lakes is used to remove phosphorus and nitrogen stored in sediments. Phosphorus can be precipitated from the water column by adding alum. This also removes suspended solids including bacteria, increasing the clarity of the water. Swimmers in small recreational lakes can tell if this treatment has recently been applied, because the alum flocs are easily resuspended by waving the hand over the bottom.

Algae can be killed or inhibited by application of 500 mg Cu/L using copper sulfate. Phytoplankton will be killed, although any taste and odor or toxic compounds may remain. Cyanobacter are especially sensitive to copper. Fish and zooplankton are relatively insensitive, but algae can develop resistance. In the case of macrophytes, physical removal both eliminates the nuisance and takes nutrients out of the system.

The problem may be low dissolved oxygen in the hypolimnion. In this case, mechanical mixing can be used to destratify the lake, mixing the oxygen-rich epilimnion and the deficient hypolimnion. Sometimes it is desired not to destroy the stratification: for example, to support cold-water fish such as trout. In this case a variety of hypolimnetic aeration techniques can be applied in which hypolimnion water usually is pumped to the surface, aerated, then returned below the thermocline. Oxygen can also be dissolved directly into the water of the hypolimnion.

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