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Figure 9.14 Variation of exergy and specific exergy in comparison with heterogeneity (A) and species richness (B) along the gradient of eutrophication gradient. For each situation, respectively, non-eutrophicated (ZC), intermediate eutrophicated (INT), and eutrophicated (EUT), we indicate the average values and the standard deviation, taking into account the entire yearly data set. The spatial variation of exergy and specific exergy was significantly correlated (r = 0.59; p < 0.05). The spatial variation of heterogeneity was not significantly correlated with exergy or specific exergy (r =- 0.48 and r = 0.38, respectively; p < 0.05).

strongly eutrophic area were chosen. Estuarine food webs were constructed at the three sites and these quantified food webs were examined using network analysis. Taken together with Table 9.15 these provide the measures that were used to characterize the trophic status of the three estuarine ecosystems. Although the three habitats are clearly distinct in physical appearance, network analyses revealed both differences and similarities among their trophic structures that had not been apparent at first glance.

It was possible to observe (Table 9.15) that the Zostera dominated community had the highest TST, followed (unexpectedly) by the strongly eutrophic system and finally by the intermediate eutrophic area. The development capacity was highest in the Zostera beds and lowest in the intermediately eutrophic area.

The index differed significantly among the three areas. Due to the logarithmic nature of this index, small differences can represent appreciable disparities in structure. The average mutual information (AMI) was slightly higher in the non-eutrophic area, followed closely by the eutrophic area and was lowest in the intermediate eutrophic area. Concerning ascendency, it increased in order from the intermediate eutrophic to the heavily eutrophic zone to the Zostera meadows, while redundancy increases in the opposite direction.

The long-term study in the Mondego Estuary indicated that years of low precipitation tended to be associated with reductions in turnover rates and increases in water column stability, temperature, salinity, and light penetration (Martins et al., 2001). These changes in habitat conditions encouraged blooms of macroalgae that gradually replaced the resident macrophytes (Marques et al., 1997; Martins et al., 2001). In the

Table 9.15 Network analysis ecosystems indices for the three areas

Information indices

Non-

Intermediate

Strongly

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