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CHAPTER 6. TECHNOLOGICAL AND MANAGERIAL ASPECTS OF EUTROPHICATION

6.10. Monitoring as a Management and Decision-Making Tool in Water Quality and Eutrophication

6.10.4. Data for Eutrophication Management and Control

There are several different ways of monitoring for the impacts of eutrophication and to determine management options.

Chemical Monitoring

In the majority of lakes and reservoirs studied to date, the limiting nutrient is phosphorus. Therefore, most monitoring and management programmes for eutrophication control have focused on phosphorus management. In most conventional monitoring programmes phosphorus is measured in two forms: one is the total phosphorus (TP) and the other is a measure of the soluble form of phosphorus that is readily bioavailable. Unfortunately, the soluble form is sufficiently unstable that if the analysis is not performed in the field at the time of sampling, any later analysis in the laboratory is usually in error. As a consequence, many national monitoring programmes are reconsidering the value of making laboratory measurements of soluble phosphorus. Fortunately, experience has shown that for most practical purposes, management strategies for phosphorus control can be made using values of total phosphorus. Note that for coastal and estuarine eutrophication it is nitrogen that is usually of concern.

For river environments, phosphorus monitoring for management purposes is to: (a) calculate the loading of phosphorus transport downstream into the receiving water body; (b) determine the relative importance of point versus non-point sources; and (c) determine if remediation measures are producing the desired effects. Phosphorus is, however, closely associated with fine particles such as silt and clay where it is adsorbed by manganese and iron that commonly coat these fine particles. In North America and Europe, the percentage of the annual load of phosphorus that is associated with fine particles has been reported as up to 90% or more. The higher values are associated with rivers that have seasonally high concentrations of suspended solids due to erosion from agricultural lands. In such cases, fixed-interval monitoring programmes tend to greatly underestimate the annual loading of phosphorus - potentially, by up to one order of magnitude. Monitoring for Total-P is generally satisfactory for estimating the gross loading of phosphorus. In some instances it may be useful to know the relative amounts of the various species of sediment-associated phosphorus (see below), however, this is best carried out within a limited survey programme.

Chemical monitoring is more difficult in a lake or reservoir environment. The association of phosphorus with fine-grained sediments (i.e., silts and clays) requires an estimate of the amount of phosphorus, which is transported with the sediment load to the lake, and with the sediment that has been deposited in the bottom of the lake. For sediment deposited in the lake, especially in situations where the bottom of the lake is periodically or seasonally anoxic, solubilization and release of phosphorus from bottom sediments can become a major part of the phosphorus loading to the water column. This is known as the "internal loading" and has been often ignored in many engineering studies for the restoration of lakes.

The phenomenon of phosphorus cycling, from bottom sediments to the water column, uptake by phytoplankton, then deposition back to the bottom sediments as the phytoplankton die, must be broken in order to manage eutrophication. The data required are not included in normal monitoring programmes and require some level of research activity. Fortunately, the data are relatively easy to obtain and the models that predict phosphorus cycling are available. Sediment-associated phosphorus values can be obtained by chemical analysis for several types of phosphorus species.

Bio-Assessment

Eutrophication is associated with abundant growth of biomass. Different levels of eutrophication tend to be associated with different types and quantities of algal species. These are discussed elsewhere in this volume. The most common biological parameter in a monitoring regime for eutrophication is that of chlorophyll-a which is a measure of primary production (algal biomass) in the water column. Measures of chlorophyll-a are included in some trophic classification systems. Another measure of primary production is the concentration of particulate organic carbon (POC).

In the past, there has been considerable work on the use of algal assays as a direct measure of phosphorus that is associated with sediments, i.e., suspended or bottom sediments. Such procedures may be useful in special studies of eutrophic lakes/reservoirs, but should not be considered as part of routine monitoring programmes.

Estimation Techniques

A particular problem in many countries is the lack of necessary and reliable data to estimate the nature and scope of the eutrophication problem. These data are further necessary to develop nutrient loading and to make management decisions on the type of source controls that are likely to make significant improvement in the level of eutrophication. A variety of estimating techniques is available in the literature and may be used in conjunction with water quality monitoring data.

• Point Source Estimation: Databases on effluent characterization from different types of industries can be used to estimate nutrient loading from municipal and industrial sources.
• Non-Point Sources Estimation: Typically, agricultural and urban run-off sources are the principal non-point sources that contribute significant quantities of nutrient to the aquatic environment. Estimation techniques vary from data-intensive modeling procedures that are used in agricultural land management practice, to unit load (i.e., tonnes/km²) of phosphorus, nitrogen, metals, etc., that runs off the land during rainfall or irrigation events. For studies of eutrophication the value of models must be judged against the cost of data collection to run the models.

An alternative that has been useful in the United States has been the calculation of "loading values" which are developed from monitoring (usually) of single-crop areas in small river basins of from a few to several hundred square kilometers in area. Similar data are available in the literature for nutrient run-off from urban areas. Such estimates are, however, crude and may vary over more than an order of magnitude due to the natural variability caused by agronomic, climatic, physical, and spatial factors. Loading values should not be extrapolated beyond the range of conditions included in the case studies from which the loadings are calculated. In studies of eutrophication, monitoring data on phosphorus together with knowledge of river basin characteristics (land use, demography, etc.) can be integrated to provide estimates of loading from non-monitored basins elsewhere in the study region. Other estimation techniques are related to specific activities such as duck-keeping or pig-rearing. Pollutant loading has been calculated on the basis of "x" number of fish (in aquaculture) or ducks etc., per unit of time. Such values are useful, especially for activities occurring directly on, in, or adjacent to, the water. Unfortunately, there appears to be no systematic compilation of these loading factors in the literature.

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