4 Managing eutrophication

Introduction

The degree to which eutrophication is considered a problem depends on the place and people concerned. A small lake in South-East Asia, heavily fertilized by village sewage, can provide valuable protein from fish. In other parts of the world, a similar level of nutrients would be regarded as damaging, making water undrinkable and unable to support characteristic wildlife. In Europe, nitrates in drinking water are regarded as a potentially serious threat to health. Eutrophication has also damaged important fisheries and caused significant loss of biodiversity. Worldwide, efforts to reduce the causes and symptoms of eutrophication cost huge sums of money.

There is no single piece of existing legislation dealing comprehensively with the problem of eutrophication in the UK. However, one aim of the European Community’s Urban Wastewater Treatment Directive (EC UWWTD) is to protect the environment from the adverse effects of sewage. This should help to reduce the problem of eutrophication in coastal waters where large discharges contribute significant nutrient loads. In the UK, 62 rivers and canals (totalling 2500 km), 13 lakes and reservoirs and five estuaries have been designated as sensitive areas (eutrophic) under this directive, and there are requirements for reducing nutrient loads from sewage treatment works in these areas (Figure 4.1).

Figure 4.1

Figure 4.1 Map of eutrophic-sensitive waters in the UK, designated by European Directive.

Under the EC UWWTD, areas designated as ‘eutrophic sensitive’ must have phosphorus-stripping equipment installed at sewage treatment works (STWs, Figure 4.2) that serve populations of 10 000 or more. However, the majority of nature conservation sites classified as sensitive are affected by smaller, rural STWs for which such equipment is not yet required. Phosphorus stripping involves the use of chemicals such as aluminium sulfate, which react with dissolved phosphates, causing them to precipitate out of solution.

Another piece of European legislation that has some bearing on problems of nutrient enrichment is the EC Nitrates Directive. This is intended to reduce nitrate loadings to agricultural land, particularly in areas where drinking water supplies have high dissolved nitrate levels. The directive requires member states to monitor nitrate levels in water, set up ‘nitrate vulnerable zones’ (NVZ), and produce and promote a ‘code of good agricultural practice’ throughout the countryside. This should include measures to control the storage, handling and disposal of slurry, for example (Figure 4.3). However, the legislation designed to curb nutrient inputs from agricultural sources is primarily directed towards reducing nitrate levels in drinking water rather than protecting nature conservation sites. The EC Nitrates Directive defines eutrophication only in terms of nitrogen compounds, and therefore does nothing to help protect the majority of aquatic sites where many eutrophication problems are attributable to phosphorus loading.

The Declaration of the Third North Sea Conference in 1990 specified that nutrient inputs entering areas of the marine environment that are, or are likely to become, eutrophic, must be reduced to 50% of their 1985 levels by 1995. The Fifth Conference in 2002 went further, aiming to eliminate eutrophication and create a healthy marine environment by 2010. Fine words.

The UK Environment Agency has developed a eutrophication strategy that promotes a coordinated framework for action, and a partnership approach at both national and local levels. The management of eutrophication requires targets and objectives to be agreed for different water bodies. Analysis of preserved plant and animal remains in sediments can be used to estimate the levels of nutrients that occurred in the past, when the water bodies concerned were less affected by eutrophication. These reference conditions can then be used to determine which waters are most at risk, or have already been damaged by eutrophication, and to prioritize sites for restorative action. The ability to measure and monitor levels of eutrophication has therefore become increasingly important.

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.* 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.* 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.

* Average values for the genus are used when individual species cannot be identified.

4.2 Reducing eutrophication

In Britain, water supply companies have tended to regard eutrophication as a serious problem only when it becomes impossible to treat drinking water supplies in an economic way. Threshold concentrations at which action is taken to reduce nutrient loadings thus depend on economic factors, as well as wildlife conservation objectives.

There are two possible approaches to reducing eutrophication:

  1. Reduce the source of nutrients (e.g. by phosphate stripping at sewage treatment works, reducing fertilizer inputs, introducing buffer strips of vegetation adjacent to water bodies to trap eroding soil particles).

  2. Reduce the availability of nutrients currently in the system (e.g. by removing plant material, removing enriched sediments, chemical treatment of water).

4.3 Reducing the nutrient source

Europe is the continent that has suffered most from eutrophication, and increasing efforts are being made to restore European water bodies damaged by nutrient enrichment. If the ultimate goal is to restore sites where nature conservation interest has been damaged by eutrophication, techniques are required for reducing external loadings of nutrients into ecosystems.

Although algal production requires both nitrogen and phosphorus supplies, it is usually sufficient to reduce only one major nutrient. An analogy can be drawn with motor cars, which require lubricating oil, fuel and coolant to keep them moving and are likely to stop if they run short of any one of these, even if the other two are in plentiful supply. As phosphorus is the limiting nutrient in most freshwater systems, phosphorus has been the focus of particular attention in attempts to reduce inputs. In addition, nitrogen is less easily controlled: its compounds are highly soluble and can enter waterways from many diffuse sources. It can also be ‘fixed’ directly from the atmosphere. Phosphorus, on the other hand, is readily precipitated, usually enters water bodies from relatively few point sources (e.g. large livestock units or waste-water treatment works) and has no atmospheric reserve. However, efforts to reduce phosphorus loadings in some lakes have failed due to ongoing release of phosphorus from sediments. In situations where phosphorus has accumulated naturally (e.g. in areas with phosphate-rich rocks) and nitrogen increases have driven eutrophication, it may be necessary to control nitrogen instead.

4.4.1 Diversion of effluent

In some circumstances it may be possible to divert sewage effluent away from a water body in order to reduce nutrient loads. This was achieved at Lake Washington, near Seattle, USA, which is close to the sea. Lake Washington is surrounded by Seattle and its suburbs, and in 1955 a cyanobacterium, Oscitilloria rubescens, became dominant in the lake. The lake was receiving sewage effluent from about 70 000 people; this input represented about 56% of the total phosphorus load to the lake. The sewerage system was redesigned to divert effluent away from the lake, for discharge instead into the nearby sea inlet of Puget Sound. The transparency of the water in the lake, as measured by the depth at which a white disc could be seen, quickly increased from about 1 to 3 m, and chlorophyll concentrations decreased markedly as a result of reduced bacterial populations.

Diversion of effluent should be considered only if the effluent to be diverted does not constitute a major part of the water supply for the water body. Otherwise, residence times of water and nutrients will be increased and the benefits of diversion may be counteracted.

4.4.2 Phosphate stripping

It has been estimated that up to 45% of total phosphorus loadings to freshwater in the UK comes from sewage treatment works. This input can be reduced significantly (by 90% or more) by carrying out phosphate stripping. The effluent is run into a tank and dosed with a product known as a precipitant, which combines with phosphate in solution to create a solid, which then settles out and can be removed. It is possible to use aluminium salts as a precipitant, but the resulting sludge contains toxic aluminium compounds that preclude its secondary use as an agricultural fertilizer. There are no such problems with iron salts, so Fe(II) ammonium sulfate is frequently chosen as a precipitant. The chemicals required as precipitants constitute the major cost, rather than installations or infrastructure, and the process is very effective: up to 95% of the phosphate can be removed easily, and it is possible to remove more. Despite its effectiveness, however, phosphate stripping is not yet used universally in sewage treatment.

4.4.3 Buffer strips

The interface between aquatic ecosystems and the land is an ecotone that has a profound influence on the movement of water and water-borne contaminants. Vegetation adjacent to streams and water bodies can help to safeguard water quality, particularly in agricultural landscapes. Buffer strips are used to reduce the amounts of nutrients reaching water bodies from runoff or leaching. They usually take the form of vegetated strips of land alongside water bodies: grassland, woodland and wetlands have been shown to be effective in different situations. The vegetation often performs a dual role, by reducing nutrient inputs to aquatic habitat and also providing wildlife habitat. A riparian buffer zone of between 20 and 30 m width can remove up to 100% of incoming nitrate. The plants take up nitrogen directly, provide a source of carbon for denitrifying bacteria and also create oxidized rhizospheres where denitrification can occur. Uptake of nitrogen by vegetation is often seasonal and is usually greater in forested areas with sub-surface water flow than in grassland with predominantly surface flow. The balance between surface flow and sub-surface flow, and the redox conditions that result, are critical in determining rates of nitrate removal in buffer strips (Figure 4.4).

Figure 4.4

Figure 4.4 (a) A field experiment investigating the effectiveness with which a grass buffer strip prevents nutrients applied to the arable field beyond from reaching the stream. In the foreground the V-notch weir is being used to monitor flow, and the small shed houses automatic equipment to sample and assess water quality at regular intervals. (b) Results from such experiments indicate that the majority of the dissolved nitrogen entering the buffer strip is either retained or volatilized, with little reaching the stream even at relatively high loading rates.

The dynamics of nitrogen and phosphorus retention by soil and vegetation can alter during succession. In newly constructed wetlands, nitrogen retention commences as soon as emergent vegetation becomes established and soil organic matter starts to accumulate: usually within the first 1-3 years. Accumulation of organic carbon in the soil sets the stage for denitrification. After approximately 5-10 years, denitrification removes approximately the same amount of nitrogen as accumulates in organic matter (about 5-10 gm−2 yr−1 under conditions of low nitrogen loading). Under higher nitrogen loading, the amount of nitrogen stored in accumulating organic matter may double, and nitrogen removal by denitrification may increase by an order of magnitude or more. Accumulation of organic nitrogen and denitrification can therefore provide for reliable long-term removal of nitrogen regardless of nitrogen loading.

Phosphorus removal, on the other hand, tends to be greater during the first 1-3 years of succession when sediment deposition and sorption (absorption and adsorption) and precipitation of phosphorus are greatest. During the early stages of succession, wetlands may retain from 3 g P m−2 yr−1 under low phosphorus loadings, and as much as 30 g P m−2 yr−1 under high loadings. However, as sedimentation decreases and sorption sites become saturated, further phosphorus retention relies upon either its accumulation as organic phosphate in plants and their litter, or the precipitation with incoming aqueous and particulate cations such as iron, aluminium and calcium.

Nevertheless, in general, retention of phosphorus tends to be largely regulated by geochemical processes (sorption and precipitation) which operate independently of succession, whereas retention of nitrogen is more likely to be controlled by biological processes (e.g. organic matter accumulation, denitrification) that change in relative significance as succession proceeds.

Surface retention of sediment by vegetated buffer strips is a function of slope length and gradient, vegetation density and flow rates. Construction of effective buffer strips therefore requires detailed knowledge of an area’s hydrology and ecology. Overall, restoration of riparian zones in order to improve water quality may have greater economic benefits than allocation of the same land to cultivation of crops.

4.3 Reducing the nutrient source, continued

4.3.4 Wetlands

Wetlands can be used in a similar way to buffer strips as a pollution control mechanism. They often present a relatively cost-effective and practical option for treatment, particularly in environmentally sensitive areas where large waste-water treatment plants are not acceptable. For example, Lake Manzala in Egypt has been suffering from severe pollution problems for several years. This lake is located on the northeastern edge of the Nile Delta, between Damietta and Port Said. Land reclamation projects have reduced the size of the lake from an estimated 1698 km2 to 770 km2. The lake is shallow, with an average depth of around 1.3 m.

Five major surface water drains discharge polluted waters into the lake. These waters contain municipal, industrial and agricultural pollutants, which are causing water quality to deteriorate and fish stocks to decline. Recently, efforts have been made to improve water quality in the most polluted of the five drains. This carries waste water from numerous sources, including sewage effluent from Cairo, waste water from industries, agricultural discharges from farms, and discharges and spills from boat traffic. Several methods for drain water treatment have been proposed, including conventional waste-water treatment plants and other chemical and mechanical methods for aerating the drain water. There are also proposals for construction of a wetland to treat approximately 25 000 m3 per day of drain water and discharge the treated effluent back to the drain.

The treatment process involves passing the drain water through basins and ponds, designed to have specific retention times. The pumped water first passes through sedimentation basins to allow suspended solids to settle out (primary treatment), followed by a number of wetland ponds (secondary treatment). The ponds are cultivated with different types of aquatic plants, such as emergent macrophytes (e.g. Phragmites) with well-developed aerenchyma systems to oxygenate the rhizosphere, allowing the oxidation of ammonium ions to nitrate. Subsequent denitrification removes the nitrogen to the atmosphere.

The waste-water treatment mechanism depends on a wide diversity of highly productive organisms, which produce the biological activity required for treatment. These include decomposers (bacteria and fungi), which break down particulate and dissolved organic material into carbon dioxide and water, and aquatic plants. Some of the latter are able to convey atmospheric oxygen to submerged roots and stems, and some of this oxygen is available to microbial decomposers. Aquatic plants also sequester nitrogen and phosphorus. Species such as common reed (Phragmites australis, Figure 4.5) yield a large quantity of biomass, which has a range of commercial uses in the region. Another highly productive species is the water hyacinth (Eichhornia crassipes, Figure 4.6). This species is regarded as a serious weed on the lake and is regularly harvested to reduce eutrophication. However, it has a potential role in water treatment due to its high productivity and rapid rates of growth. The resultant biomass could possibly be harvested and used for the production of nutrient-rich animal feed, or for composting and the production of fertilizer. Further research is required to develop practical options.

Figure 4.5

Figure 4.5 A small constructed wetland, planted with common reed (Phragmites australis). Such installations are often used to clean water that has undergone primary treatment in a local STW.

The passage of water through emergent plants reduces turbidity because the large surface area of stems and leaves acts as a filter for particulate matter. Transmission of light through the water column is improved, enhancing photosynthesis in attached algae. These contribute further to nutrient reduction in through-flowing water. The mixture of floating plants and emergent macrophytes contributes to removal of suspended solids, improved light penetration, increased photosynthesis and the removal of toxic chemicals and heavy metals.

Estimates for the removal of total suspended solids (TSS), biological oxygen demand (BOD), total phosphorus and total nitrogen by the different wetland components are provided in Table 4.2. These suggest that wetlands, combined with sedimentation and ancillary water treatment systems, could play an important part in reducing nutrient loadings.

Table 4.2 Estimated effluent concentrations and removal efficiencies for the Lake Manzala project.

Parameter Sedimentation pond Wetland treatment system
influent conc./mg l−1 effluent conc./mg l−1 removal efficiency/% influent conc./mg l−1 effluent conc./mg l−1 removal efficiency/%
TSS 160 32 80 32 6.4 80
BOD 40 24 40 24 18.0 25
total P 5 4 20 4 3.2 20
total N 12 12 0 12 8.4 30
organic N 4 4 0 4 3.8 5
ammonium N 5 5 0 5 3.9 22

4.3.5 Domestic campaigns

An important aspect of efforts to reduce nutrient inputs to water bodies is the modification of domestic behaviour. Public campaigns in Australia have encouraged people to:

  • wash vehicles on porous surfaces away from drains or gutters

  • reduce use of fertilizers on lawns and gardens

  • compost garden and food waste

  • use zero- or low-phosphorus detergents

  • wash only full loads in washing machines

  • collect and bury pet faeces.

These campaigns have combined local lobbying with national strategies to tackle pollution from other sources.

4.4 Reducing nutrient availability

Once nutrients are in an ecosystem, it is usually much harder and more expensive to remove them than tackle the eutrophication at source. The main methods available are:

  • precipitation (e.g. treatment with a solution of aluminium or ferrous salt to precipitate phosphates);

  • removal of nutrient-enriched sediments, for example by mud pumping; and

  • removal of biomass (e.g. harvesting of common reed) and using it for thatching or fuel.

In severe cases of eutrophication, efforts have been made to remove nutrient-enriched sediments from lakes. Lake Trummen in Sweden accumulated thick black sulfurous mud after years of receiving sewage effluent. Even when external loadings of phosphorus were reduced to 3 kg P yr−1, there was still an internal load (i.e. that derived from the lake’s own sediment) of 177 kg P yr−1! Drastic action was needed. Eventually nutrient-rich sediment was sucked from the lake and used as fertilizer. The water that was extracted with the sediment was treated with aluminium salts and run back into the lake. This action reduced phosphorus concentrations and improved the clarity and oxygenation of the water. However, removal or sealing of sediments is an expensive measure, and is only a sensible option in severely polluted systems, such as the Norfolk Broads, England.

Removal of fish can allow species of primary consumers, such as the water-flea, Daphnia, to recover and control algae. Once water quality has improved, fish can be re-introduced.

Mechanical removal of plants from aquatic systems is a common method for mitigating the effects of eutrophication (Figure 4.7). Efforts may be focused on removal of existing aquatic ‘weeds’ such as water hyacinth that tend to colonize eutrophic water. Each tonne of wet biomass harvested removes approximately 3 kg N and 0.2 kg P from the system.

Alternatively plants may be introduced deliberately to ‘mop up’ excess nutrients. Although water hyacinth can be used in water treatment, the water that results from treatment solely with floating macrophytes tends to have low dissolved oxygen. Addition of submerged macrophytes, together with floating or emergent macrophytes, usually gives better results. Submerged plants are not always as efficient as floating ones at assimilating nitrogen and phosphorus due to their slower growth, resulting from poor light transmission through water (particularly if it is turbid) and slow rates of CO2 diffusion down through the water column. However, many submerged macrophytes have a high capacity to elevate pH and dissolved oxygen, and this improves conditions for other mechanisms of nutrient removal. At higher pH, for example, soluble phosphates can precipitate with calcium, forming insoluble calcium phosphates, so removing soluble phosphates from water. Various species have been used in this way. One submersed macrophyte, Elodea densa, has been shown to remove nitrogen and phosphorus from nutrient-enriched water, its efficiency varying according to loading rate. Nitrogen removal rates reached 400 mg N m−2 per day during summer, while for phosphorus over 200 mg P m−2 per day were removed.

In terrestrial habitats, removal of standing biomass is an important tool in nature conservation. Reduction in the nutrient status of soils is often a prerequisite for re-establishment of semi-natural vegetation, and the removal of harvested vegetation helps to reduce the levels of nutrients returned to the soil (Figure 4.8). However, if the aim is to lower the nutrient status of a nutrient-enriched soil, this can be a very long-term process (Figure 4.9).

Figure 4.8

Figure 4.8 Mowing and removal of terrestrial vegetation to strip the nutrients contained in the biomass out of the ecosystem.

Figure 4.9

Figure 4.9 Depletion of available phosphorus at Rothamsted long-term experimental husbandry site. Harvesting of arable crops grown without the use of any fertilizer gradually exhausted the reserves of phosphorus in the initially enriched soil.

Question 4.1

A short reach of the River Great Ouse in Bedfordshire was found to contain the following species:

Common name Scientific name
filamentous alga Cladophora spp.
moss Amblystegium riparium
fool’s water-cress Apium nodiflorum
hornwort Ceratophyllum demersum
mare’s-tail Hippuris vulgaris
spikedwater-milfoil Myriophyllum spicatum
yellow water-lily Nuphar lutea
great water dock Rumex hydrolapatham
water speedwell Veronica anagallis-aquatica
sweet flag Acorus calamus
water plantain Alisma plantago-aquatica
lesser pond sedge Carex acutiformis
reed sweet-grass Glyceria maxima
yellow flag iris Iris pseudacorus
greater duckweed Lemna gibba
broadleaved pondweed Potamogeton natans
bulrush Schoenoplectus lacustris

A similar length of the River Eden in Cumbria was found to have the following species:

Common name Scientific name
liverwort Pellia epiphylla
moss Calliergon cuspidatum
river moss Fontanalis antipyretica
water horsetail Equisetum fluviatile
white water-lily Nymphaea alba
lesser spearwort Ranunculus flammula
pinkwater speedwell Veronica catenata
sedge Carex acuta
bottle sedge Carex rostrata
common spike-rush Eleocharis palustris
broadleaved pondweed Potamogeton natans

Using the trophic rank scores in Table 4.1, calculate the mean trophic rank (MTR) for each stretch of river, and comment on whether the watercourse is nutrient-enriched (eutrophic). Assume all the species recorded are of similar abundance and therefore there is no need to weight scores according to relative abundance, as you would do in a real situation.

Question 4.2

List the advantages of preventing eutrophication at source, compared with treating its effects.