For nutrient determination samples were collected in 100 ml polyethylene bottles and kept continuously under deep freeze (-20°C) until their analysis in the laboratory. PO4 concentrations were measured on a Perkin Elmer Lambda 2S UV/VIS Spectrometer according the protocol of Murphy and Riley (1962).NO2 concentrations were measured by a Bran and Luebbe autoanalyser according to the protocol of Bendschneider et al. (1952).NO3 concentrations measured by a Bran and Luebbe autoanalyser according to the protocol of Stickland and Parsons (1968).Si(OH)4 concentrations were measured by a Bran and Luebbe autoanalyser according to the protocol of Mullin and Riley (1955).