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CHAPTER 1. ENVIRONMENTAL ASPECTS OF EUTROPHICATION

1.4. Causes of Eutrophication

1.4.3. Results of Assessments of Eutrophication

When the data from mid-latitude freshwater lakes are combined with information from lakes worldwide, it is evident that phosphorus limitation is not universal in lakes and reservoirs. Regional differences in land use and geochemistry can lead to ample supply of phosphorus. Within individual lakes, seasonal variations in phosphorus and nitrogen supply are not uncommon. Moreover, it is important to recognize that short-term variations in the relative magnitude of various routes of nutrient supply with different nitrogen to phosphorus ratios can change the importance of nitrogen or phosphorus as limiting nutrients. Further, as lakes become eutrophic, there is a tendency for nitrogen limitation to become more important than in less nutrient-rich waters.

Based on the assumption that phosphorus is the primary limiting nutrient, regression equations relating phosphorus, usually mean annual total phosphorus, to algal biomass, usually expressed as mean annual chlorophyll a, have been developed with the extensive data available from freshwater lakes in north temperate and subarctic regions. Similar relations have been found to apply in other areas, such as south African and South American impoundment. However, the confidence interval about the chlorophyll-phosphorus regressions can span an order of magnitude. Apparently, limitation by nitrogen or light, differences in nitrogen to phosphorus ratios and trophic structure among lakes can cause substantial variability. Hence, relations between nutrients and chlorophyll derived from regional data should be only one of several types of evidence used in management decisions about the cause of eutrophication.

Results from experimental enrichments used to assay for nitrogen or phosphorus limitation are available from numerous freshwater lakes in the north temperate region. Overall, combined nitrogen plus phosphorus enrichments enhance algal growth more frequently and more substantially than additions of nitrogen or phosphorus alone. Apparently, both nitrogen and phosphorus are often in sufficiently short supply such that enrichment with one without the other produces only a brief period of enhanced growth before depletion of the not enriched nutrient limit growth. Although nitrogen can be supplied from the atmosphere by fixation of gaseous nitrogen, ecological constraints on gaseous nitrogen fixation may prevent adequate rates of nitrogen supply via gaseous nitrogen fixation.

The role of nutrients in the eutrophication of temperate lakes in the south hemisphere has been evaluated in a variety of lakes and reservoirs in New Zealand, Australia and south Africa. In general, the lakes of New Zealand have low total nitrogen to total phosphorus ratios and have about half the nitrogen concentrations of comparable lakes in North America and Europe. Lakes and reservoirs located in the semi-arid regions of south Africa and Australia tend to be phosphorus limited to a greater extent than lakes and reservoirs located in the semi-arid southwest United States. Impoundment with low nutrient concentrations were usually phosphorus limited, and, as phosphorus loading increased, nitrogen limitation became more pronounced.

Systematic evaluation of the role of nutrient limitation in tropical lakes is not possible because too few of the wide variety of tropical lakes have been examined. East African lakes have received relatively more attention than other lakes. Nitrogen limitation may be widespread because of the low nitrate concentrations and moderate to high phosphate concentrations common in eastern African lakes. However, nitrogen to phosphorus ratios and uptake rates of radioactive phosphorus provide strong evidence for phosphorus limitation in some Kenyan lakes. In South American tropical floodplain lakes, seasonal and regional differences in the relative importance of nitrogen or phosphorus limitation occur. Concentrations of both total nitrogen and total phosphorus in South American reservoirs correlate with chlorophyll. Physiological assays and enrichment experiments carried out in Lake Titicaca, Boliva and Peru provide good evidence for adequate phosphorus supply, while increased algal growth after nitrogen additions occurred in Lake Valencia, Venezuela.

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