be considered highly significant). As can be seen, d' is positively correlated to TSI, although the relation is not very sharp, because of the similarity of TSI for Lake Trasimeno and Lake Mendota. The index d' shows a good negative linear correlation with the lake's mean depth: the intercept value given by the linear regressions (30.9 X 10"4 K"1) could approach the higher values for d' at the limits of existence of an aquatic ecosystem, which is reached at a rate of 0.1 X 10"4K"1m"1.
The indices d and d' could be considered measures of the ability of the ecosystems to dissipate the incoming solar energy into the system; the positive correlation between these indices and the trophic state of the lakes indicates that they could account for the influence of the biological productivity on the whole entropy production of the system. As high nutrient concentrations increase the whole biological production as well as the energy flow through an ecosystem, an increase in d and d' values with eutrophication is expected because of the irreversibility of the biological processes.
Furthermore, the efficiency of the energy transfer between the trophic levels in eutrophic systems was found to be lower than in oligotrophic systems (Jonasson and Lindegaard, 1988). In ecological terms, this should mean that a higher nutrient availability in more eutrophic systems induces the achievement of a biological community possessing a better ability to dissipate energy, following a development strategy based on the maximization of the productivity, rather than optimization of the energy exploitation.
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