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The PSII herbicide equivalent concentration (PSII-HEq) method measures the toxicity of photo-system II herbicides in a mixture. These herbicides impact on plant photosynthesis (food production). The method assumes that the herbicides act together in the receiving environment (e.g. cumulatively) resulting in more environmental harm. To calculate PSII-HEq each herbicide is compared to a reference PSII herbicide: diuron. As a reference chemical, diuron is given a value of 1. If another pesticide is more potent than diuron it is given a value of more than 1 and if another pesticide is less potent than diuron it is given a value of less than 1. Toxicity is then calculated from a whole water sample by multiplying the value assigned to each herbicide by its respective concentration. These values are added together to produce the final toxicity score. This method is widely used and simple to calculate, however it is limited by the fact that it can only account for chemicals that impact aquatic organisms in the same manner (e.g. chemicals that impact the photosynthetic process at the same point such as PSII herbicides).

It is expected that reporting pesticides in the inshore marine zones will move to using the ms-PAF method by the release of the 2019 report card. This will allow for direct comparison of scores between basins, estuaries and inshore marine zones.

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