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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).

Batrachospermum spp.6(a) Broadleaved species(b) Grassleaved species
Hildenbrandia rivularis6Apium inundatum9Acorus calamus2
Lemanea fluviatilis7A. nodiflorum4Alisma plantago-aquatica3
Vaucheria spp.1Berula erecta5A. lanceolatum3
Cladophora spp.1Callitriche hamulata9Butomus umbellatus5
Enteromorpha spp.1C. obtusangula5Carex acuta5
Hydrodictyum reticulatum3Ceratophyllum demersum2C. acutiformis3
Stigeoclonium tenue1Hippurus vulgaris4C. riparia4
Littorella uniflora8C. rostrata7
LiverwortsLotus pedunculatus8C. vesicaria6
Chiloscyphus polyanthos8Menyanthes trifoliata9Catabrosa aquatica5
Jungermannia atrovirens8Montia fontana8Eleocharis palustris6
Marsupella emarginata10Myriophyllum alterniflorum8Eleogiton fluitans10
Nardia compressa10M. spicatum3Elodea canadensis5
Pellia endiviifolia6Myriophyllum spp.*6E. nuttallii3
P. epiphylla7Nuphar lutea3Glyceria maxima3
Scapania undulata9Nymphaea alba6Groenlandia densa3
Nymphoides peltata2Hydrocharis morsus-ranae6
MossesOenanthe crocata7Iris pseudacorus5
Amblystegium fluviatilis5O. fluviatilis5Juncus bulbosus10
A. riparium1Polygonum amphibium4Lemna gibba2
Blindia acuta10Potentilla erecta9L. minor4
Brachythecium plumosum9Ranunculus aquatilis5L. minuta/miniscula3
B. rivulare8R. circinatus4L. trisulca4
B. rutabulum3R. flammula7Phragmites australis4
Bryum pseudotriquetrum9R. fluitans7Potamogeton alpinus7
Calliergon cuspidatum8R. omiophyllus8P. berchtoldii4
Cinclidotus fontinaloides5R. peltatus4P. crispus3
Dichodontium flavescens9R. penicillatus pseudofluitans5P. friesii3
D. pellucidum9R. penicillatus penicillatus6P. gramineus7
Dicranella palustris10R. penicillatus vertumnus5P. lucens3
Fontinalis antipyretica5R. trichophyllus6P. natans5
F. squamosa8R. hederaceus6P. obtusifolia5
Hygrohypnum luridum9R. sceleratus2P. pectinatus1
H. ochraceum9Ranunculus spp.*6P. perfoliatus4
Hyocomium armoricum10Rorippa amphibia3P. polygonifolius10
Philonotis fontana9R. nasturtium-aquaticum5P. praelongus6
Polytrichum commune10Rumex hydrolapathum3P. pusillus4
Racomitrium aciculare10Veronica anagallis-aquatica4P. trichoides2
Rhynchostegium riparioides5V. catenata5Sagittaria sagittifolia3
Sphagnum spp.10V. scutellata1Schoenoplectus lacustris3
Thamnobryum alopecurum7Viola palustris9Scirpus maritimus3
Sparganium emersum3
Fern-alliesS. erectum3
Azolla filiculoides3Spirodela polyrhiza2
Equisetum fluviatile5Typha latifolia2
E. palustre5T. angustifolia2
Zannichellia palustris2
Response to eutrophication: STR 1-3 most tolerant; STR 4-5 moderately tolerant; STR 6-7 moderately sensitive; STR 8-10 most sensitive.
* Average values for the genus are used when individual species cannot be identified.

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