G3g

Sediment C (mgl-1) 35GgCm-2 5GGgCm-2

Sediment C (mgl-1) 35GgCm-2 5GGgCm-2

From an ecological management point of view the Zostera dominated community is preferred because the oxygen concentration is higher, the water is clearer, and no crash due to anaerobic conditions takes place.

Starting from the hypothesis that the ecosystem structure having the highest eco-exergy among the possible ones would prevail, two models (one for an Enteromorpha dominant community and another one for a Z. noltii dominant community) were developed to compare eco-exergy for several conditions, using STELLA. The growth was described as a function of internal concentrations of nutrients, temperature, light, and salinity (Duarte, 1995).

If the hypothesis is correct, comparing the models for the two types of ecosystems, the highest exergy under eutrophied and medium to high-salinity conditions should be found for ecosystem (1), while the highest exergy should be found for ecosystem (2) under low-nutrient and low-salinity conditions.

The models show that if the freshwater with high concentration of nutrients (particularly nitrogen) is discharged during the last part of the year Enteromorpha will be dominant. The eco-exergy calculations show that the exergy is approximately the same for the two models, which may be interpreted as the initial value, may be crucial for the final results.

The results of the five simulations suggest that the ecological management of the freshwater discharge is a key factor for the prevailing of the two communities (Enteromorpha, Z. noltii). From a management viewpoint at least two possibilities can be considered: artificial control of the freshwater discharges through the use of sluices, increasing the discharge during the first part of the year; or reduction of the nutrient input from freshwater (and if possible also from tide water). The joint use of these two alternatives should give the Zostera dominated community better conditions.

Eco-exergy, specific eco-exergy, and diversity

The spatial and temporal variation of eco-exergy, specific eco-exergy, species richness, and heterogeneity were analyzed to examine in what extent these ecological indicators would capture changes in benthic communities along the gradient of eutrophication.

The benthic communities in the Mondego Estuary (Portuguese western coast) were monitored during a yearly cycle. Samples of macrophytes, macroalgae, and associated macrofauna were taken fortnightly at three different sites, during low water, along an estuarine gradient of eutrophication in the south arm of the estuary, from the non-eutrophicated zone, where the Z. noltii community is present, up to the heavily eutrophi-cated zone, in the inner areas of the estuary, where Enteromorpha spp. blooms have been observed. An overview of the major taxonomic groups contributing to the exergy in this system is provided in Table 9.14.

With regard to eco-exergy, values were consistently higher in the Z. noltii community than in the eutrophicated areas. Also, eco-exergy values were higher in the most heavily eutrophi-cated area when compared with the intermediate eutrophicated area, especially during spring and early summer. This was related to the intensity of the Enteromorpha bloom, which gave rise to much higher values for total biomass in the most eutrophicated area.

Table 9.14 Major contributors to the exergy in the Mondego Estuary benthic communities along the gradient of eutrophication

Contributors Non- Intermediate Eutrophicated Eutrophicated eutrophicated eutrophicated area before the area after the

Table 9.14 Major contributors to the exergy in the Mondego Estuary benthic communities along the gradient of eutrophication

Contributors Non- Intermediate Eutrophicated Eutrophicated eutrophicated eutrophicated area before the area after the

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