Location of Repository

Reviewing phosphorus and nitrogen targets for the SSSI meres

By Stephen C. Maberly and Laurence Carvalho


1. The meres in the North-West Midlands of England are a regional cluster of lakes of high conservation importance, yet impacted by high nutrient concentrations from surrounding land-use and population settlements. Despite their long-history of being in a eutrophic state, the most rapid enrichment and consequent changes to aquatic flora have occurred over the last 50-100 years. In many lowland temperate lakes, these changes have largely resulted from phosphorus enrichment and national legislation has set target concentrations for this element to prevent or reduce nutrient enrichment. However other nutrients, such as nitrogen, can also limit or co-limit productivity and is probably of more widespread importance than generally recognised, but currently no target concentrations exist for this nutrient.\ud 2. The purpose of this review, therefore, was to:\ud • review existing phosphorus target concentrations for 26 meres with SSSI status\ud • assess the applicability of nitrogen target concentrations for these meres\ud • recommend P and N target concentrations for the 26 SSSI meres\ud • recommend further research to address data gaps and improve understanding and management of the meres.\ud 3. The review considered key published and unpublished literature and recent work in setting P and chlorophyll standards for the Water Framework Directive. It also analysed data on current concentrations of nitrogen & phosphorus provided by the Environment Agency and macrophyte species provided by Natural England.\ud 4. There is not a single way to assess nutrient-limitation so eight approaches were used: i) N:P ratios, ii) comparison of Chla: total phosphorus (TP) versus European average response, iii) comparison of Chla: total nitrogen (TN) versus European average response, iv) Chla:TP ratios, v) Chla:TN ratios, vi) seasonal minima of phosphate and nitrate, vii) modelling using a stoichiometric model ‘Metabolake and viii) direct bioassays.\ud 5. Although nitrogen-limitation is becoming recognised as being more widespread then hitherto thought, in the case of the meres, the frequently high concentrations of TP caused by input from glacial deposits, low rates of TP loss caused by low flushing rates or lack of a distinct outflow coupled with potential loss of nitrogen to the atmosphere via denitrification will tend to favour nitrogen-limitation and this was reflected in the analysis.\ud 6. The analyses indicated that at the 22 sites where data were available, 7 sites were mainly phosphorus-limited (32%), 8 sites were mainly nitrogen-limited (36%) and 7 sites were either mainly co-limited or not limited by nutrients (32%). Direct bioassay data showed that nutrient-limitation can vary over a year.\ud 7. Nutrient targets are needed to manage water quality or achieve conservation objectives. They are not something essential of themselves.\ud 8. Macrophyte diversity, a possible conservation objective, does not appear to relate to nutrient availability in the meres, possibly because most of the meres have relatively high nutrient concentrations and possibly because the macrophyte populations have adapted to these concentrations over many decades. Total number of species vs nitrate in the meres is not markedly different from the UK data-set used to establish the relationship between macrophyte diversity and nitrogen so this is unlikely to be the cause of the lack of a relationship.\ud 9. In terms of setting targets, many of the meres have high alkalinities so high-baseline targets for TP would be expected. Site-specific Water Framework Directive TP targets for the meres at the High/ Good boundary range from 14 µg L-1 for Oak Mere (low alkalinity) to 46 µg L-1 for Quoisley Little Mere. The Good/ Moderate boundary targets range from 21 to 57 µg L-1. These concentrations are generally below the inferred TP concentrations from around 1850 which range between 31 and 50 µg L-1 at the five sites where cores have been analysed. Consequently, even Good/ Moderate Water Framework Directive targets may not be achievable. \ud 10. The presence of nitrogen limitation or co-limitation in the meres means that nitrogen targets are also appropriate at some of the sites. Total nitrogen targets were derived from European datasets relating concentrations of chlorophyll a to total nitrogen for different types of lakes. In turn, the chlorophyll a target was derived from Water Framework Directive standards for different lake types. The Good/ Moderate total nitrogen target varied between 0.4 mg to 1.4 mg L-1. \ud 11. Where possible, mere-specific targets are preferable to general targets, but these require better data to implement. A range of different future projects addressing data gaps and uncertainty over the functioning of the meres are suggested. In particular, the role of trophic-interactions, for example grazing of phytoplankton by zooplankton and control of zooplankton by fish, in moderating water quality needs to be assessed and managed.\u

Topics: Ecology and Environment
Publisher: Centre for Ecology and Hydrology
Year: 2010
OAI identifier: oai:nora.nerc.ac.uk:10565

Suggested articles


To submit an update or takedown request for this paper, please submit an Update/Correction/Removal Request.