Elsevier

Water Research

Volume 35, Issue 17, December 2001, Pages 4053-4062
Water Research

Application of the kohonen neural network in coastal water management: methodological development for the assessment and prediction of water quality

https://doi.org/10.1016/S0043-1354(01)00151-8Get rights and content

Abstract

Kohonen neural network (KNN) was applied to nutrient data (ammonia, nitrite, nitrate and phosphate) taken from coastal waters in a Spanish tourist area. The activation maps obtained were not sufficient to evaluate and predict the trophic status of coastal waters. To achieve this aim, a new methodology is proposed which uses as its starting point the activation maps obtained from KNN. Firstly, to evaluate the trophic status of the coastal waters, it consists of the development of a quadrat system which enables a better classification than the traditional classification based simply on standardized data. The new classification allows clear differentiation of water quality within the mesotrophic band. Secondly, and in order to use the activation maps as predictive tools, the trophic classification, obtained from activation maps, was transposed onto new activation maps. To do this, the activation maps of the sampling points which defined each trophic group were superimposed. To avoid unnecessary complexity and to facilitate the process, this superimposition was undertaken only where the frequency exceeded 0.05. In this way, four frequency maps related to the trophic status of coastal waters (potentially eutrophic, high mesotrophic, low mesotrophic and oligotrophic) were obtained. There was no loss of relevant information in the new maps thus obtained. These frequency maps served as the basis for the successful prediction of the trophic status of random samples of coastal waters. This methodology, based on KNN, is proposed as a tool to aid the decision-making in coastal water quality management.

Introduction

The quality of the coastal environment is very important for the sustenance of both natural ecosystems and human activities (Karydis and Coccossis, 1990). Since the 1970s, the relationship between environmental quality and human activities has attracted the interest of both scientists and policy makers (OECD, 1978). In the Mediterranean coastal areas, the massive nutrient inflow to the sea, as a result of urbanization, has not only had a negative impact on coastal ecosystems, but also on tourism and other economic activities (Clark, 1996; Moriki and Karydis, 1994). In this context, coastal water quality managers have a keen interest in the development and application of analytical and operational tools which can facilitate the decision-making process. Statistical evaluations (Ignatiades et al., 1985; Austen and Warwick, 1989) based on univariate methods have low interpreting value for decision makers. Multivariate procedures have proved more suitable, given the complicated nature of pollution-induced ecological disturbances (Warwick, 1988), and they have been successfully applied in environmental quality assessment and management (Ignatiades et al., 1992; Karydis, 1996). Recently, artificial neural networks (ANN) have become the focus of interest across many scientific disciplines including ecology (Walley and Fontama, 1998; Foody, 1999; Brosse et al., 1999; Barciela et al., 1999; Lek and Guegan, 2000), analytical chemistry (McAlernon et al., 1999; Li et al., 1999), food analysis (Dolmatova et al., 1998) and water quality (Trautmann and Denceux, 1994; Mappa et al., 1996; Schleiter et al., 1999). Until now, the range of applications of ANN developed over recent years has not included its application in coastal water quality management.

ANN are computer techniques that attempt to simulate the functionality and decision-making processes of the human brain (Eberhart and Dobbins, 1990). They comprise a set of simple, interconnected units which work in parallel to categorize input data into output classes. Such networks may classify data more accurately than conventional statistical classifiers (Dolmatova et al., 1997; Kocjancic and Zupan, 1997). Different types of networks have been developed (Beale and Jackson, 1990; Haykin, 1994; Bishop, 1995). They all have in common the ability to learn from data. They may be broadly classified into networks which learn in a supervised or unsupervised way.

In supervised learning the network is given many different examples of a particular problem, including the desired solutions. Thus, the network is able to compare its outputs with the desired outputs to determine the extent of its errors. A training algorithm then modifies the internal parameters of the network so that the next time the same examples are presented to the network its predictions would be less erroneous. This process is repeated many times until the performance of the network is maximized. In unsupervised learning, the desired outputs are not given and the network learns to classify the examples by recognizing different patterns. Kohonen self organizing maps are a type of neural network designed for unsupervised pattern recognition tasks (Kohonen (1982), Kohonen (1989)). The goal of Kohonen neural network (KNN) is to map the spatial relationship between clusters of data points in hyperdimensional space. Once trained, they may be used to identify unknown data patterns.

In this paper, KNN has been applied to nutrient data obtained from the coastal waters of Almerı́a (Spain). The objective of this study is the development of a methodology, based on the output from the KNN, that makes it possible to assess and predict coastal water quality. To be of use in the management of coastal water quality, such a methodology had to be able to distinguish the mesotrophic conditions. KNN is evaluated as a possible tool in the decision-making process in coastal water quality management.

Section snippets

Sampling design

The coast of Almerı́a is one of the most important tourist areas in southern Spain. As in other Mediterranean countries, sewage discharges are the main contributory factors to nutrient enrichment of coastal waters (Aguilera, 1997). Taking this fundamental characteristic into account, sampling stations were established along the coast (Fig. 1): nine sampling stations (AD1, GV1, UR1, RM1, AG1, CC1, CB1, MJ1 and GR1) were located at the main sewage outfalls; nine sampling stations (AD2, GV2, UR2,

Interpretation of the activation maps

All data obtained for each sampling point were used on the Kohonen network. The mean nutrient concentrations of raw data are given in Table 1. Fig. 3 shows, for every sampling station, an activation map of 6×6 neurons. The convergence was mostly reached in 50,000 iterations. The activation maps obtained represent coastal water quality. Spatial placement of the activated neurons is indicative of different water qualities. For example, the activation maps for the two stations SB and CG indicate

Methodology of classification

Variables that enable the information in the activation maps to be extracted need to be based on the system of quadrats described in the methods section. Accordingly, the variables selected were: (1) quadrat containing the greatest activation, (2) quadrat containing the neuron with the highest degree of activation, (3) quadrat containing the centroid of the activation map. Table 2 shows the values obtained for these variables. Each variable comprises two values and each observation is defined

Conclusions

KNN can be an effective tool for the evaluation and prediction of the trophic status of coastal waters.

The information contained in the activation maps was successfully extracted using the quadrat system. This system preserves the salient features of the topographic map (Kohonen, 1990). The information drawn from the activation maps enables a classification of the trophic status of the coastal waters into four classes (potentially eutrophic, highly mesotrophic, slightly mesotrophic and

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