Report No SR95(06)F
HINDCASTING OF IN-LOCH PHOSPHORUS CONCENTRATIONS BASED ON LAND COVER CLASSIFICATION
A new standing water classification scheme was developed for the 1995 quinquennial Scottish Water Quality Survey. The scheme is based upon the extent to which human activity has altered or downgraded water quality and aquatic ecology, and it focuses on three main forcing functions. These are: change in nutrient status, acidification, and toxic substances. The Standing Waters Classification Scheme identifies four classes of water quality, and has established standards for each class within the three main criteria. The classes range from Class 1 (lochs whose water quality and aquatic ecology is not significantly altered by anthropogenic activity), to Class 4 (severely polluted lochs incapable of supporting fisheries due to gross anthropogenic enrichment, acidification, presence of toxins, or deoxygenation).
A changed state temporal framework was adopted in order to produce a quality classification of the nutrient status of standing waters. This compares current condition with that pertaining to a date before the import of phosphorus into catchments caused any significant change. The parameter selected for this analysis is mean total phosphorus concentration (TP), although it is accepted that this is a simple measure of eutrophication. The water quality change associated with nutrient enrichment is determined in the scheme by the ratio of current and hindcasted total phosphorus concentration (TPcurrent / TPoriginal), and this is separated into one of the four classes.
As part of the classification scheme it is necessary to ascribe a range of P loss coefficients to the spatial units of land cover covering the whole of Scotland. lt. is important to balance the detail contained in the smallest functional unit with that of the requirement for the classification of the greatest number of types. Variability within units is a function of catchment specific attributes such as management, slope, soil type, and rainfall, but for classification purposes this must be less than between unit variability. lt. was decided that for the purpose of this research, to utilise the loss coefficients already developed for the PLUS (Phosphorus Land Use and Slope) model developed collaboratively between the Macaulay Land Use Research Institute (MLURI) and the Forth River Purification Board. These loss coefficients were calibrated for all the lochs in the scheme by back calculating the total phosphorus load into the waterbody from the currently best available estimate of total phosphorus concentration in the lochs (TPc). Similarly historical loads of phosphorus were calculated by applying these catchment specific loss coefficient values to an historical land cover dataset, and an historical phosphorus concentration TPo) calculated.
Current TP concentrations in the study lochs sampled to date can be classified into 4 broad trophic categories. Lochs with TP< 10µg/l would be considered oligotrophic (124 sites); those with TP 10-40 µg/l mesotrophic (34 sites); TP 40-100 µg/l eutrophic (9 sites), and those > 100µg/l hypertrophic (3 sites). All trophic states are represented in the Standing Waters Classification Scheme, but over 80% of the lochs would be considered oligotrophic. Results of the hindcasting proceedure identify that of the 170 studied, 18 are Class 2, one is Class three, and one is Class 4. This report identifies and discusses sources of uncertainty within the scheme, and elucidates the important interactions of point versus non-point sources of phosphorus. The scheme also identifies lochs, currently classified as Class 1, where there has been an historical anthropogenic impact and which may change status in future. Recommendations for extending the flexibility of the scheme to produce a data management system, and interfacing this scheme with other databases are presented.
Keywords: Standing waters classification, nutrient hindcasting, land cover classification, phosphorus.
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