Abstract: The effect of pollution in an aquatic mass is monitored. A living sessile organism (2) is placed in said aquatic mass and a series of first measurements of a first parameter of said organism (2) is carried out during a first time period. Each of said first measurements is recorded with an associated measurement of a second parameter to give a series of first data points (82c). A first envelope (80c) is determined and recorded with respect to said first parameter for said first data points (82c). A series of second measurements of said first parameter of said organism (2) is carried out during a second time period. Each of said second measurements is recorded with an associated measurement of a second parameter to give a series of second data points (86c). A second envelope (84c) is determined and recorded with respect to said first parameter for said second data points (86c). Said first and second envelopes (80c, 84c) are compared to determine the presence of pollution during said first or second time period.