The present studies were undertaken to assess the geographical distribution of the nutritive richness in the Thau lagoon (Hérault, France). Three descriptors of nutrients in this ecosystem were studied: ammonium, nitrates and phosphates. A systematic sampling strategy was adopted (Frontier, 1983): 63 sampling points described the lagoon; the distance between two contiguous sampling points was 1 km (Fig. 1). The variables were evaluated at each point, during 1 year, at four sampling periods. Analytical methods for nutrient determination were described by Aminot and Chaussepied (1983) and Parsons et al. (1984). The automatic procedure for nitrates and phosphates determination proposed by Tréguer and Lecorre (1975) was used. The following statistical methods were applied: 1. (1) descriptive statistics for each period 2. (2) contiguous grouping linked to: ◦ - hierarchical classification, the similarity between the two points was calculated by the Gower coefficient (Legendre and Legendre, 1984); ◦ - k is a classification method using the later partition as an initial one. Both programs using these computations were part of an R package (Legendre, 1985) 3. (3) discriminant analysis for testing geographical zones obtained by grouping methods (Romeder, 1973). The stepwise discriminant analysis gives the most discriminant period in the different groups. The neighbor discriminant procedure indicates the percent of well-classified sampling points. Statistical characterization of periods for nitrates is shown in Table 1. The two winter periods have high nitrate concentrations. Grouping methods give seven geographical zones (Fig. 2). Statistical characterization of these zones is indicated in Table 2. The most discriminant period for this zonation is February (Table 3), June was not included. February explains 70% of well-classified sampling points (Table 4). Combining February with other periods explains specific groups, i.e. group 4 is confirmed by adding the May period and group 1 by adding the October period. The zonation results from two gradients due one to winter nitrates inflow, and one to summer nitrates consumption. Four sampling points were never well-classified, because of the geographical constraint. Statistical characterization of periods for ammonium ion is given in Table 5. As for nitrates, the two winter periods have high ammonium ion concentrations and summer periods have low ammonium ion concentrations. Grouping methods give six geographical zones (Fig. 3). Statistical characterization of these zones is given in Table 6. The correlation coefficients of the four periods are approximately equal (Table 7), therefore the combination of the four periods give a very good percent of well-classified sampling points (Table 8). October period explains group 2, May period group 4 and February period confirms the percent of groups 1 and 3 given by the June period. For ammonium ion, discriminant analysis shows the contribution of the four periods, each period has a different gradient concentration due to different ecological phenomena (sediment release, inflow, consumption, regeneration). Eleven sampling points are never well classified (Fig. 3), they are on groups borders. Statistical characterization of periods for phosphates is given in Table 9. Grouping methods give five geographical zones (Fig. 4). Statistical characterization of these zones is given in Table 10. The correlation coefficients of the four periods are high (Table 11). Table 12 shows that only group I has always a high percent of well-classified sampling points. The geographical zonation of phosphates seems inadequate. The data collected during 1 year, analysed by classification methods, gave the spatial structure of the lagoon appropriate to any period. Our studies showed different spatial distribution for the three nutrients during 1 year. An attempt was made to link these spatial patterns to environmental changes. In addition the discriminant analysis revealed three kinds of singular sampling points: • - located on the lagoon border • - located between two adjacent groups • - located near waste water treatment plant.
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Jul 1, 1995
A Chapelle 
Open Access