intermediate and strongly eutrophic areas, primary production is largely the result of these macroalgal blooms. Production appears as a strong pulse during this specific time, but remains at very low levels during the rest of the year. This limited temporal interval of primary production results in a significantly lower figure for the cumulative annual primary production and TST in these areas as compared with the corresponding measures in the Zostera beds. Comparing the AMI values of the flow structure for the three areas, it is possible to discern a very small decrease in the measure among the three zones, suggesting that, as regards trophic structure, these areas are indeed different.
The three zones appear nevertheless much more distinct by eye than is illustrated by the AMI values. In the light of these results, the network definition of eutrophication appears to be inappropriate for the Mondego estuarine ecosystem. It would be more accurate to describe the enrichment processes occurring in this ecosystem as "pulse eutrophication." This process could be characterized as a disturbance to system ascendency in the form of an intermittent supply of excess nutrients that, when coupled with a combination of physical factors (e.g., salinity, precipitation, temperature, etc.), causes both a decrease in system activity and a drop in the mutual information of the flow structure. Even though a significant rise in the TST occurs during the period of the algal bloom and at that time there is a strong increase of the system ascendency, the annual picture nevertheless suggests that the other components of the intermediate and strongly eutrophic communities were unable to accommodate the pulse in production. The overall result was a decrease in the annual value of the TST and, as a consequence, of the annual ascendency as well.
Regarding the results of the trophic analysis, the Zostera community has one more trophic level than those counted in the strongly eutrophic chain, implying that this community possesses a more complex web with additional top consumers. At the same time, the Zostera community exhibits lower transfer efficiency at the first trophic level, probably because the production of Z. noltii meadows usually cannot be eaten directly, but needs to be decomposed first (Lillebo et al., 1999). Concerning the analysis of cycled materials, the overall percentage of cycled matter increases as the degree of eutrophica-tion rises. This is indicated by the Finn Cycling Index (FCI) that reveals the proportion of TST that is devoted to the recycling of carbon (Finn, 1976).
Eugene Odum (1969) suggested that mature ecosystems recycle a greater percentage of their constituent material and energy than do pioneer or disturbed communities. Hence, according to Odum, the progressive increase in the FCI would suggest the maturation of the ecosystem.
It has been observed, however, that disturbed systems also often exhibit greater degrees of recycling. The speculation is that such an increase in cycling in disturbed systems is the homeostatic response that maintains in circulation resources, which before the perturbation had been stored as biomass in the higher organisms (Ulanowicz and Wulff, 1991). This latter scenario seems consistent with the present results.
When the whole-system properties of the three areas were compared, the measures associated with the system considered to lie between the two extremes in nutrient loading, did not plot intermediate to the other two.
From this viewpoint, the intermediate eutrophic area appears to be the most disturbed of the three areas, since it has the lowest ascendency, AMI, TST, and development capacity values and the highest figures for redundancy and FCI.
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