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CHAPTER 1. ENVIRONMENTAL ASPECTS OF EUTROPHICATION

1.2. Eutrophication as an Environmental Problem

1.2.2. External Loading to Lakes

Rivers and streams are major routes of transfer of nitrogen and phosphorus to many lakes and reservoirs, and they integrate the various point and non-point sources of nitrogen and phosphorus within their watershed. Processes within flowing water modify the forms of nitrogen and phosphorus and their rates of transport. The mining of phosphate and the industrial fixation of nitrogen and agricultural, industrial and domestic uses of nitrogen and phosphorus have increased during the last few decades. Other activities of modern societies, such as clearing of forests, extensive cultivation and urban waste disposal, have enhanced the transport of nitrogen and phosphorus from terrestrial to aquatic environments. While point and non-point sources of nitrogen and phosphorus contribute to eutrophication, non-point sources often are dominant and present complex management challenges.

Transport of nitrogen and phosphorus in streams and rivers depends on concentration and discharge, and this relation varies among flowing waters and for different forms of nitrogen and phosphorus. Comparative data from around the world with discharge increasing indicate that the concentration of total phosphorus consistently increases. Because a large fraction of the total phosphorus is associated with particles, phosphorus transport occurs disproportionally during high flows. In tropical watersheds with negligible human disturbance, most of the nitrogen is in the dissolved organic and inorganic fractions. Proportions will vary under different land uses with the percentage associated with particles increasing with more disturbances to the watershed.

On an annual basis, streams and rivers usually retain only a small fraction of the dissolved and particulate nutrients that enter from the watershed. Retention is controlled by factors including discharge, current, temperature, solute concentration, light, lithology of sediments and riparian vegetation. Impoundment of rivers by dams reduces downstream transport of suspended sediments and of nitrogen and phosphorus associated with the seston, suspended particulate matter. The proportion of sediment input released through dams varies considerably. Because in many rivers the upper watershed provides the majority of the sediment load, dams can trap most of the sediments leaving the watershed. Furthermore, higher sedimentation rates usually occur in reservoirs than in natural lakes. However, erosion of channels below dams can increase by flow variations and greater scouring by the sediment-depleted waters. Although a number of factors are known to influence sedimentation and retention of phosphorus, the most widely used empirical relations are functions of flushing rates. While the statistical significance of the empirical relations may be high at least for a particular region, they do have considerable variance and must be applied cautiously to individual lakes. For example, north-temperate lakes with anoxic hypolimnion retain less phosphorus than lakes with oxic hypolimnion.

Several approaches can be used to estimate the riverine flux of materials into lakes and reservoirs. The larger the scale of interest, the greater the extrapolation usually required and the more uncertain the estimate. The most direct method is to combine measurements of discharge and concentrations made at the mouths of the rivers that enter the water body of interest. A second approach is to determine the fluvial loss per unit area of land for each ecosystem or land use within a region, and then extrapolate to the whole region based on the area covered by each landscape category. For example, based on data from the U.S.A., as the dominant land use changes from forests or mixed agriculture to pastures, crops and urban uses, the ratio of total nitrogen to total phosphorus yields shifts to lower values. Further, nitrate concentrations in rivers increase as a function of population density worldwide. A tendency toward lower nitrogen to phosphorus ratios favoring nitrogen limitation could be attributed to increased agricultural development and urbanization independent of latitude. Refinements of this approach could consider the spatial organization of the landscape units in relation to the river and temporal aspects of hydrologic conditions. For example, the presence or absence of riparian vegetation is known to strongly influence elemental transport into rivers. Further, average nutrient yields developed in a humid climatic region may not apply reliably to a region with highly seasonal runoff.

As deforested area in the Amazon basin and throughout the tropics has increased, downstream lacustrine waters are influenced by increased inputs of particles and solutes. Nitrogen and phosphorus inputs from cleared areas are of particular concern because of their role in aquatic eutrophication. For example, large increases in solute mobilization from the upper soil horizons to groundwater were observed after slash-and-burning in a partially deforested watershed in the central Amazon. Stream water solute concentrations increased and nutrient ratios were altered subsequent to deforestation. Various logging practices, creation of livestock ranches and forest plantations, development of settlements and cities, and mining activities will cause hydrological and associated hydrochemical changes with varied influences on nitrogen and phosphorus loading.

Atmospheric deposition via rain, snow and aerosols is an increasing important external source of nutrients to lakes and reservoirs. Although initially atmospheric inputs of nitrate were considered a concern because of their contribution to acidification of freshwaters, some oligotrophic waters limited by nitrogen responded to the atmospheric inputs with symptoms of eutrophication. Major sources of nitrogen to the atmosphere, which have increased because of human activities, include burning of fossil fuels and forests, operation of internal combustion engines, and volatilization from feed lots and fertilized fields. Augmented phosphorus deposition may be arising from phosphorus-rich soil particles originating from fertilized and exposed agricultural fields or heavily grazed lands.

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