MAIN V ARIATION TENDENCES IN THE LOTIC ECOSYSTEMS FROM SIERRA NEVADA (GRANADA, S-SPAIN) Physical, chemical and biological characteristics were studied from sixty-one samples taken in seven rivers of Sierra Nevada (S. Spain). Sampling took place in twenty-one stations from November 1980 to June 1981. The results obtained showed a progres~;ve increase of mineral salt content, heterotrophic bacteria! populations, coliform density, soluble phosphate and nitra te concentrations, and Margalef pigment

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... index, according as the altitude decrease, specially down rivers from 800 meters, whereas the oxygen concentration decreased. A principal component analysis transformed the original variables into five new variables which accounted far 82,3°/o of the total variance of the data. 111e interpretation of the five principal components is as follows: 1st, organic contamination; 2nd, water mineralisation; 3rd, phaecal pollution; 4th, water acidity degree; 5th, water velocity.