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This paper is an extension of an earlier paper on a Trophic Index TRIX, a linear combination of the log of 4 state variables: ChA, aD%O, minN & TP (Vollenweider et al. 1998), to characterizing succinctly the trophic levels of coastal marine areas, and to discuss questions that since have arisen in applying and developing the index further. This entails in depth review of some procedural aspects such as data processing including transformation of rough data; the statistical distribution of TRIX; comparisons over different spatial and temporal scales; discrimination limits between two contiguous means of TRIX; interrelationships between TRIX and its components, and between TRIX and other water related variables. Further to statistical properties of TRIX, attention is given to integrative features and other implicit functional relationships pertinent to the description of trophic characteristics of coastal systems, among which to mention the term "efficiency". This theoretical framework has been applied to data from two trophically different responding coastal systems, i.e. the Emilia-Romagna coastal area of the Adriatic Sea and the Tuscany coastal area of the Tyrrhenian Sea to illustrate some of the arising interpretative problems in using TRIX. Finally, analyses of the two system are juxtaposed in an alternatively slightly modified reference diagram in which the trophic location and the extent of variability are shown in relation to the N/P ratio and related efficiency.
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