USE OF THE TROPHIC DIATOM INDEX TO MONITOR EUTROPHICATION IN RIVERS

The performance of the trophic diatom index (TDI), a new index of trop hic status in rivers, was evaluated at sites above and below major sew age works where nutrient removal is being considered. Some modificatio ns to the index are proposed as a result. These include changing the s cale from I (low nutrients) to 5 (high nutrients) to 0-100, the remova l of taxa that are predominately planktonic from the index, and some m inor adjustments to taxa sensitivities and indicator values. More deta iled guidelines on the choice of sample site are also proposed. In par ticular, it is important that sites in the ''recovery zone'' downstrea m of sewage works are chosen so that the influence of the gross effect s of organic pollution on the index is minimised. A ''look-up'' chart to aid data interpretation is also described, The revised TDI is now r ecommended to regulatory organisations as a tool for assessing eutroph ication in rivers. (C) 1998 Elsevier Science Ltd. All rights reserved.