the river. The upper Poulter, above Welbeck Lake, is rated at 'B' on the GQA scale, despite the fact that the flows are augmented by the outflow from Langwith Sewage Treatment Works, which contributes around 1.2 Ml/d (megalitres per day) and by water from Langwith fish farm, which can be up to 0.9 Ml/d, but which is seasonal in nature. Water quality remains good through the length of the river. Figures for the final section before it joins the River Idle show that the levels of ammonia and dissolved oxygen are those which would be expected in a natural ecosystem. Between 1991 and 2006, the BOD has been reduced by more than 50 percent, but there is still room for improvement. Levels of phosphates and nitrates, which are affected by agricultural activity, are quite high, but some improvements have occurred. Water quality is also measured by two biological indexes. These are NTAXA, which relates to the total number of invertebrate species found in samples of the water, and ASPT, (the Average Score Per Taxon), calculated from the numbers of specific invertebrates which are graded according to their sensitivity to organic pollution. The assessment for the years 2004 to 2006 rated the final section as grade A (very good) on the first index, and grade B (good) on the second. This indicates that the water is not far short from unpolluted. The Environment Agency has since changed the system for assessing water quality in the river systems in England. Each is given an overall ecological status, which may be one of five levels: high, good, moderate, poor and bad. There are several components that are used to determine this, including biological status, which looks at the quantity and varieties of invertebrates, angiosperms and fish. Chemical status, which compares the concentrations of various chemicals against known safe concentrations, is rated good or fail. The water quality of the Poulter was as follows in 2019. The quality of the Millwood Brook has improved, as it was rated bad for ecological status in 2009. Like many rivers in the UK, the chemical status changed from good to fail in 2019, due to the presence of polybrominated diphenyl ethers (PBDE) and mercury compounds, neither of which had previously been included in the assessment. Flora and fauna The river's upper reaches are diverse in terms of their physical nature and their ecological importance. They include an