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CHAPTER 1. ENVIRONMENTAL ASPECTS OF EUTROPHICATION

1.3. Impacts of Eutrophication

1.3.2. Effects of Eutrophication (suite)

Increased Fish Yields

Yields of fish tend to increase as primary productivity increases in lakes, reservoirs and in aquacultural systems (Figure 1.7.). Over a wide range of values, primary productivity-fish yield relationships have a logistic (i.e., sigmoid) form. Hence, greater increases in fish yields are likely for smaller increments in primary productivity in oligotrophic or mesotrophic waters than in eutrophic systems. However, oxygen depletion or elevated pH and ammonia levels under hypereutrophic conditions can lead to lesser increases in fish yields as primary production rises. Assuming that the fish whose yields are improved are edible and marketable, the increase in primary productivity often associated with nutrient enrichment can have a positive result.

Figure 1.7. Relation between photosynthetic rates and fish yields in tropical lakes and reservoirs. FY is the annual commercial fish yields in kg per hectare; PG is the daily average gross photosynthetic rate in g of oxygen per m². For explanation of the letters representing different lakes, please, see Melack (1976).

Nutrient Reuse

Aquaculture of fishes is well established in many parts of the world and can be an effective way to obtain a positive benefit from nutrients that cause eutrophication. The biomass of fish in an aquaculture system can comprise a large portion of the nutrients packaged in a harvestable, marketable form. Although there are few examples of the level of eutrophication being reduced by fish harvest, if markets exist for the fish most common in the eutrophic system, the approach offers a way to reuse nutrients in a manner that can improve water quality.

Phytoplankton and floating aquatic macrophytes can be very effective at nutrient uptake and are capable of reducing dissolved inorganic nutrient concentrations to very low levels. Hence, if the plants are subsequently removed from the water by flocculation or harvesting, they may function in tertiary municipal wastewater treatment or as sources of organic matter for biogas generation or even production of foodstuffs. However, the necessary technology and markets are in need of further development.

Nutrient-acidification Interactions

Lakes that have become acidified by airborne pollutants commonly have low biological productivity, but often contain adequate nitrate, which is received as nitric acid from the atmosphere. If phosphate is added, it is possible to generate sufficient base by enhanced assimilation of nitrate to raise the pH. Each molecule of phosphate added is likely to generate 16 molecules of base as a result of nitrate assimilation. Hence, only modest additions of phosphate are required to raise the pH, and phytoplankton growth is not likely to be excessive, but productivity does increase.

Nutrient-contaminant Interactions

In many lakes and reservoirs toxic contaminants (some metals and organic compounds) have become an increasingly significant problem. Hundreds of industrial chemicals have been found to be toxic to organisms. Persistent and widespread toxic substances include polychlorinated biphenyls (PCBs), DDT and its metabolites, dieldrin, toxaphene, dioxin (2,3,7,8-TCDD), furan (2,3,7,8-TCDF), mirex, hexachlorobenzene (HCB), mercury, alkylated lead, and benzo(a)pyrene. These compounds tend to bioaccumulate in organisms, biomagnify in food webs and persist for long periods in the aquatic environment. They cause acute, subchronic or chronic toxicity, carcinogenicity, mutagenicity and reproductive effects (i.e., teratogenicity, immuno-toxicity and behavioral effects).

There may be interactions between eutrophication and impacts of toxic contaminants. For example, although loads of PCBs to Lakes Erie and Ontario are about the same, fish from oligotrophic Lake Ontario are more contaminated than those from eutrophic Lake Erie. It is possible that eutrophic Lake Erie has a faster sedimentation rate, greater competition for organic chemicals by particles, greater degradation due to higher microbial densities, or simply a greater biomass to dilute organic chemicals than in Lake Ontario (Figure 1.8.). Hence, the negative impacts of eutrophication may be mitigated, at least partially, by lesser impacts from toxic contaminants. Further analysis of nutrient-contaminant interactions is required before net benefits can be assured.

Figure 1.8. PCB levels in smelt (i.e., small, silvery fish of the genus Osmerus inhabiting the Great Lakes) from Lake Ontario and Lake Erie, 1972-1983 (1972-1977 data: edible portion; 1977-1983 data: whole fish) (from Bird and Rapport, 1986).

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