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4.1 Measuring and monitoring eutrophication

During the 1990s there was increased demand in the UK for effective methods of monitoring eutrophication. There was also considerable interest in the development of monitoring systems based on biotic indices. Several ‘quality indices’ based on a variety of organisms were explored. For monitoring tools to have practical application, they must satisfy certain requirements:

  • sampling must be quick and easy;

  • monitoring must be based on a finite number of easily identified groups; and

  • indices for evaluation must be straightforward to calculate.

Within-year variability in nutrient concentrations can be high, particularly for enriched waters. A high sampling frequency may therefore be required to provide representative annual mean data. In nutrient-enriched lakes, annual means are more likely to provide appropriate estimates of phosphorus than winter-spring means, due to the importance of internal cycling of nutrients in summer. This is an important consideration when designing sampling strategies for use in predictive models of trophic status.

The large group of algal species collectively known as diatoms has been used as indicators of eutrophication in European rivers. Individual species of diatom vary in their tolerance of nutrient enrichment, some species being able to increase their growth rates as nutrients become more available, whilst others are outcompeted and disappear. As diatoms derive their nutrients directly from the water column, and have generation times measured in days rather than months or years, the species composition of the diatom community should be a good indicator for assessing eutrophication. Convincing correlations have been demonstrated between aqueous nutrient concentrations and diatom community composition, but there are a number of other physical and chemical factors that also affect diatom distribution, such as water pH, salinity and temperature, which also need to be taken into account.

The UK Environment Agency has assessed the extent of eutrophication on the basis of concentrations of key nutrients (primarily nitrogen and phosphorus) in water, and the occurrence of obvious biological responses, such as algal blooms. There is an intention to rely more heavily in future on biological assessment schemes. One such system is based on surveys of the aquatic plant populations in rivers. Known as the mean trophic rank (MTR) approach, this uses a scoring system based on species and their recorded abundances at river sites. Each species is allocated a score (its species trophic rank, STR) dependent on its tolerance to eutrophication (Table 4.1); then, for a given site, the mean score for all species present is calculated. Tolerant species have a low score, so a low MTR tends to indicate a nutrient-rich river. In Britain, rivers in the north and west tend to have the highest MTR scores, whereas rivers in the south and east of England have the lowest. These scores reflect the influence of numerous factors, such as differences in river flow, patterns of agricultural intensification and variations in population density.

Table 4.1 Sensitivity of aquatic plants to nutrient enrichment, as indicated by species trophic rank (STR).
Species STR Species STR Species STR
Algae Angiosperms Angiosperms
Batrachospermum spp. 6 (a) Broadleaved species (b) Grassleaved species
Hildenbrandia rivularis 6 Apium inundatum 9 Acorus calamus 2
Lemanea fluviatilis 7 A. nodiflorum 4 Alisma plantago-aquatica 3
Vaucheria spp. 1 Berula erecta 5 A. lanceolatum 3
Cladophora spp. 1 Callitriche hamulata 9 Butomus umbellatus 5
Enteromorpha spp. 1 C. obtusangula 5 Carex acuta 5
Hydrodictyum reticulatum 3 Ceratophyllum demersum 2 C. acutiformis 3
Stigeoclonium tenue 1 Hippurus vulgaris 4 C. riparia 4
Littorella uniflora 8 C. rostrata 7
Liverworts Lotus pedunculatus 8 C. vesicaria 6
Chiloscyphus polyanthos 8 Menyanthes trifoliata 9 Catabrosa aquatica 5
Jungermannia atrovirens 8 Montia fontana 8 Eleocharis palustris 6
Marsupella emarginata 10 Myriophyllum alterniflorum 8 Eleogiton fluitans 10
Nardia compressa 10 M. spicatum 3 Elodea canadensis 5
Pellia endiviifolia 6 Myriophyllum spp. Footnotes   * 6 E. nuttallii 3
P. epiphylla 7 Nuphar lutea 3 Glyceria maxima 3
Scapania undulata 9 Nymphaea alba 6 Groenlandia densa 3
Nymphoides peltata 2 Hydrocharis morsus-ranae 6
Mosses Oenanthe crocata 7 Iris pseudacorus 5
Amblystegium fluviatilis 5 O. fluviatilis 5 Juncus bulbosus 10
A. riparium 1 Polygonum amphibium 4 Lemna gibba 2
Blindia acuta 10 Potentilla erecta 9 L. minor 4
Brachythecium plumosum 9 Ranunculus aquatilis 5 L. minuta/miniscula 3
B. rivulare 8 R. circinatus 4 L. trisulca 4
B. rutabulum 3 R. flammula 7 Phragmites australis 4
Bryum pseudotriquetrum 9 R. fluitans 7 Potamogeton alpinus 7
Calliergon cuspidatum 8 R. omiophyllus 8 P. berchtoldii 4
Cinclidotus fontinaloides 5 R. peltatus 4 P. crispus 3
Dichodontium flavescens 9 R. penicillatus pseudofluitans 5 P. friesii 3
D. pellucidum 9 R. penicillatus penicillatus 6 P. gramineus 7
Dicranella palustris 10 R. penicillatus vertumnus 5 P. lucens 3
Fontinalis antipyretica 5 R. trichophyllus 6 P. natans 5
F. squamosa 8 R. hederaceus 6 P. obtusifolia 5
Hygrohypnum luridum 9 R. sceleratus 2 P. pectinatus 1
H. ochraceum 9 Ranunculus spp. Footnotes   * 6 P. perfoliatus 4
Hyocomium armoricum 10 Rorippa amphibia 3 P. polygonifolius 10
Philonotis fontana 9 R. nasturtium-aquaticum 5 P. praelongus 6
Polytrichum commune 10 Rumex hydrolapathum 3 P. pusillus 4
Racomitrium aciculare 10 Veronica anagallis-aquatica 4 P. trichoides 2
Rhynchostegium riparioides 5 V. catenata 5 Sagittaria sagittifolia 3
Sphagnum spp. 10 V. scutellata 1 Schoenoplectus lacustris 3
Thamnobryum alopecurum 7 Viola palustris 9 Scirpus maritimus 3
Sparganium emersum 3
Fern-allies S. erectum 3
Azolla filiculoides 3 Spirodela polyrhiza 2
Equisetum fluviatile 5 Typha latifolia 2
E. palustre 5 T. angustifolia 2
Zannichellia palustris 2


Response to eutrophication: STR 1-3 most tolerant; STR 4-5 moderately tolerant; STR 6-7 moderately sensitive; STR 8-10 most sensitive. Back to main text


* Average values for the genus are used when individual species cannot be identified. Back to main text