A number of biological changes may occur as a result of eutrophication. Some of these are direct (e.g. stimulation of algal growth in water bodies), while others are indirect (e.g. changes in fish community composition due to reduced oxygen concentrations). This section summarizes some of the typical changes observed in aquatic, marine and terrestrial ecosystems following eutrophication.
Some typical changes observed in lakes following artificial eutrophication are summarized in Table 2.1. Similar characteristic changes are observed in other freshwater systems.
Table 2.1 Typical changes observed in lakes experiencing ‘artificial’ eutrophication.
• Turbidity increases, reducing the amount of light reaching submerged plants. |
• Rate of sedimentation increases, shortening the lifespan of open water bodies such as lakes. |
• Primary productivity usually becomes much higher than in unpolluted water and may be manifest as extensive algal or bacterial blooms. |
• Dissolved oxygen in water decreases, as organisms decomposing the increased biomass consume oxygen. |
• Diversity of primary producers tends to decrease and the dominant species change. Initially the number of species of green algae increases, causing temporary increase in diversity of primary producers. However, as eutrophication proceeds, blue-green bacteria become dominant, displacing many algal species. Similarly some macrophytes (e.g. bulrushes) respond well initially, but due to increased turbidity and anoxia (reduced oxygen) they decline in diversity as eutrophication proceeds. |
• Fish populations are adversely affected by reduced oxygen availability, and the fish community becomes dominated by surface-dwelling coarse fish, such as pike (Esox lucius, see Figure 2.1) and perch (Perca fluviatilis). |
• Zooplankton (e.g. Daphnia spp.), which eat phytoplankton, are disadvantaged due to the loss of submerged macrophytes, which provide their cover, thereby exposing them to predation. |
• Increased abundance of competitive macrophytes (e.g. bulrushes) may impede water flow, increasing rates of silt deposition. |
• Drinking water quality may decline. Water may be difficult to treat for human consumption, for example due to blockage of filtering systems. Water may have unacceptable taste or odour due to the secretion of organic compounds by microbes. |
• Water may cause human health problems, due to toxins secreted by the abundant microbes, causing symptoms that range from skin irritations to pneumonia. |
In oligotrophic systems, even quite small increases in nutrient load can have relatively large impacts on plant and animal communities.
Figure 2.1 The pike (Esox lucius), a species of fish that can tolerate eutrophic water.
Plant species differ in their ability to compete as nutrient availability increases. Some floating and submerged macrophyte species are restricted to nutrient-poor waters, while others are typical of nutrient-rich sites (see Table 2.2). Figure 2.2 shows turbid water in a polluted drainage ditch associated with localized growth of algae. There are no aquatic plants present.
Table 2.2 Species restricted to nutrient-poor or typical of nutrient-rich standing water.
Trophic state | Associated macrophytespecies |
---|---|
oligotrophic | alternate water-milfoil (Myriophyllum alternifolium) bog pondweed (Potamogeton polygonifolius) |
oligo-mesotrophic | bladderwort (Utricularia vulgaris) |
eutrophic | hairlike pondweed (Potamogeton trichoides) |
tending towards hypertrophic | spiked water-milfoil (Myriophyllum spicatum) fennel-leaved pondweed (Potamogeton pectinatus) |
Figure 2.2 A polluted drainage ditch with luxuriant growth of algae, forming a carpet over its surface.
In rivers, the presence of plant species such as the yellow water-lily (Nuphar lutea) and the arrowhead (Sagittaria sagittifolia, Figure 2.3) are likely to indicate eutrophic conditions. In some rivers, the fennel-leaved pondweed (Potamogeton pectinatus) is tolerant of both sewage and industrial pollution.
Figure 2.3 Arrowhead (Sagittaria sagittifolia), a macrophyte species that is tolerant of eutrophic conditions.
Whereas some species can occur in waters with quite a wide range of nutrient levels, some are relatively obligate to specific trophic bands and are unable to survive if nutrient levels alter significantly from those to which they are adapted. In 1989, Michael Jeffries derived ranges of tolerance for a number of macrophyte species by studying literature on their occurrence and distribution in relation to different aspects of water quality. He also reviewed results of scientific studies reported in the literature to determine what concentrations of nitrate, ammonia, phosphorus, suspended solids and biological oxygen demand (BOD) appeared to be associated with severe or total loss of macrophyte species due to eutrophication (see Table 2.3). Research has suggested that changes to certain macrophyte communities can occur at soluble reactive phosphorus concentrations as low as 20 μg 1(1(0.02 mg l(1). Soluble reactive phosphorus (SRP) is the term commonly used to describe phosphorus that is readily available for uptake by organisms. It is used in contrast to measures of total phosphorus, which include forms of the element that are bound to sediment particles or locked up in large organic molecules. These forms are unavailable for immediate uptake, but they may become available over time.
Water samples from two lowland rivers, A and B, are found to contain the following concentrations of plant nutrients.
Nutrient | Concentration/mg l-1 | |
---|---|---|
River A | River B | |
nitrate | 2.2 | 12.1 |
ammonia | 0.07 | 0.6 |
SRP | 0.18 | 0.13 |
By reference to Table 2.3, what conclusions can you draw about the probable diversity of aquatic macrophytes in each of the rivers?
Table 2.3 Typical concentrations (in mg l−1) of selected water quality parameters associated with ‘natural’, degraded and severe loss of plant species.
Condition | SRP/mg P l−1 | Nitrate/mg N l−1 | Ammonia/mg N l−1 | Suspended solids | BOD |
---|---|---|---|---|---|
‘natural’ | <0.1 | <3.0 | <0.2 | <30 | <2.0 |
degraded (partial loss of species found under ‘natural’ conditions) | 0.1-0.2 | 3.0-10 | 0.2-5.0 | 30-100 | 2.0-6.0 |
severe loss of species | >0.2 | >10 | >5.0 | >100 | >6.0 |
BOD, biological oxygen demand; SRP, soluble reactive phosphorus.
One of the symptoms of extreme eutrophication in shallow waters is often a substantial or complete loss of submerged plant communities and their replacement by dense phytoplankton communities (algal blooms). This results not only in the loss of characteristic plant species (macrophytes) but also in reduced habitat structure within the water body. Submerged plants provide refuges for invertebrate species against predation by fish. Some of these invertebrate species are phytoplankton-grazers and play an important part in balancing relative proportions of macrophytes and phytoplankton. Submerged macrophytes also stabilize sediments and the banks of slow-flowing rivers or lakes. Bodies of water used for recreation (boating for example) become more vulnerable to bank destabilization and erosion in the absence of well-developed plant communities, making artificial bank stabilization necessary (Figure 2.4). Submerged plants also have a role in the oxygenation of lower water layers and in the maintenance of aquatic pH.
Figure 2.4 River bank stabilization
Name three species of submerged macrophytes that are tolerant of eutrophic water.
The enrichment of water bodies by eutrophication may be followed by population explosions or ‘blooms’ of planktonic organisms.
Bursts of primary production in an aquatic ecosystem in response to an increased nutrient supply are commonly referred to as ‘algal blooms’. Can you explain why this term is taxonomically incorrect?
‘Algal blooms’ are a well-publicized problem associated with increased nutrient levels in surface waters. The higher the concentration of nutrients, the greater the primary production that can be supported. Opportunistic species like some algae are able to respond quickly, showing rapid increases in biomass. Decomposition of these algae by aerobic bacteria depletes oxygen levels, often very quickly. This can deprive fish and other aquatic organisms of their oxygen supply and cause high levels of mortality, resulting in systems with low diversity. The odours associated with algal decay taint the water and may make drinking water unpalatable. Species of cyanobacteria that flourish in nutrient-rich waters can produce powerful toxins that are a health hazard to animals. Such problems are well documented for a number of famous lakes. The Zurichsee in Switzerland has been subject to seasonal blooms of the cyanobacterium Oscillatoria rubescens due to increased sewage discharge from new building developments on its shores. For lakes in Wisconsin, USA, ‘nuisance’ blooms of algae or bacteria occur whenever concentrations of phosphate and nitrate rise.
Increased productivity tends to increase rates of deoxygenation in the surface layer of lakes. Although phytoplankton release oxygen to the water as a byproduct of photosynthesis during the day, water has a limited ability to store oxygen and much of it bubbles off as oxygen gas. At night, the phytoplankton themselves, the zooplankton and the decomposer organisms living on dead organic matter are all respiring and consuming oxygen. The store of dissolved oxygen thus becomes depleted and diffusion of atmospheric oxygen into the water is very slow if the water is not moving.
What s the relative rate of oxygen diffusion in water compared with its rate in air?
Still waters with high productivity are therefore likely to become anoxic.
Figures 2.5 and 2.6 give an example of the change in aquatic invertebrate species following eutrophication. In unpolluted water, mayfly larvae may be found. In polluted water, these species cannot survive due to reduced oxygen availability and are likely to be replaced by species, such as the bloodworm, which can tolerate lower oxygen concentrations.
Figure 2.5 (a) Mayfly larva, found in unpolluted water. (b) Adult mayfly.
Figure 2.6 Pollution-tolerant species, the bloodworm.
Many species of coarse fish, such as roach (Rutilus rutilus, a cyprinid fish, Figure 2.7), can also tolerate low oxygen concentrations in the water, sometimes gulping air, and yields of fish may indeed increase due to the high net primary production (NPP) of the system. However these species are generally less desirable for commercial fishing than others such as salmon (Salmo salar, a salmonid fish, Figure 2.8), which depend on cool, well-oxygenated surface water. Populations of such species usually decline in waters that become eutrophic (Figure 2.9); they may be unable to live in a deoxygenated lake at all, resulting in fish kills (Figure 2.10). They may also be unable to migrate through deoxygenated waters to reach spawning grounds, resulting in longer-term population depressions.
Figure 2.7 Roach, a species of coarse fish (a cyprinid) able to benefit from eutrophic conditions.
Figure 2.8 Salmon (Salmo salar), a salmonid fish intolerant of eutrophication.
Figure 2.9 Changes occurring in northern temperate lakes due to eutrophication.
Figure 2.10 Fish kill caused by deoxygenation of the water.
Lake Victoria is one of the world’s largest lakes and used to support diverse communities of species endemic to the lake (i.e. species that are found only there), but it now suffers from frequent fish kills caused by episodes of deoxygenation. In the 1960s deoxygenation was limited to certain areas of the lake, but it is now widespread. It is usually associated with at least a tenfold increase in the algal biomass and a fivefold increase in primary productivity.
When eutrophication reaches a stage where dense algal growth outcompetes marginal aquatic plants, even relatively tolerant fish species suffer from the consequent loss of vegetation structure, especially young fish (Figure 2.11). Spawning is reduced for fish species that attach their eggs to aquatic plants or their detritus, and fish that feed on large plant-eating invertebrates, such as snails and insect nymphs, suffer a reduced food supply.
Figure 2.11 Newly-hatched salmon are particularly demanding of oxygen-rich water.
In a southeast Asian village where cyprinid fish from the local pond are an important source of protein, eutrophication of the water by domestic sewage is seen as advantageous. Why?
Why do you think nitrogen is becoming increasingly available to terrestrial ecosystems in many parts of the world)
Some suggested global scenarios for the year 2100 identify nitrogen deposition (together with land use and climate change) as one of the most significant ‘drivers’ of biodiversity change in terrestrial ecosystems.
Atmospheric deposition of nitrogen, together with the deposition of phosphorus-rich sediments by floods, can alter competitive relationships between plant species within a terrestrial community. This can cause significant changes in community composition, as species differ in their relative responses to elevated nutrient levels. As is the case with aquatic vegetation, terrestrial species that are able to respond to extra nitrogen and phosphorus with elevated rates of photosynthesis will achieve higher rates of biomass production, and are likely to become increasingly dominant in the vegetation. Atmospheric deposition of nutrients can reduce, or even eliminate, populations of species that have become adapted to low nutrient conditions and are unable to respond to increased nutrient availability. Some vegetation communities of conservation interest are directly threatened by atmospheric pollution.
In Britain, rare bryophytes are found associated with snowbeds (Figure 2.12). Most of the late-lying snowbeds in Britain are in the Central Highlands of Scotland, which are also areas of very high deposition of nitrogenous air pollutants. Snow is a very efficient scavenger of atmospheric pollution and melting snowbeds release their pollution load at high concentrations in episodes known as ‘acid flushes’. The flush of nitrogen is received by the underlying vegetation when it has been exposed following snowmelt. Concentrations of nutrients in the meltwater of Scottish snowbeds have already been shown to damage underlying bryophytes, including a rare species called Kiaeria starkei. Recovery from damage is slow, and sometimes plants show no signs of recovery even four weeks after exposure to polluted meltwater. Given the very short growing season, this persistent damage can greatly reduce the viability and survival of the plants. Tissue nitrogen concentrations in Kiaeria starkei have been shown to be up to 50% greater than that recorded in other upland bryophytes. This example emphasizes the potential threat of atmospheric pollution to snowbed species, and suggests that some mountain plant communities may receive much higher pollution loadings than was previously realized.
Figure 2.12 Typical habitat for Kiaeria starkei is a late-lying snowbed, such as this in the Scottish Cairngorms, where the meltwater in spring will carry a flush of atmospheric pollutants, including nitrogen compounds.
The deposition of atmospheric nitrogen can be enhanced at high altitude sites as a consequence of cloud droplet deposition on hills. Sampling of upland plant species at sites in northern Britain has shown marked increases in nitrogen concentration in leaves with increasing nitrogen deposition, which is, in turn, correlated with increasing altitude. The productivity of the species was also found to increase in line with the amount of nitrogen deposited. Plant species can therefore respond directly to elevated levels of nitrogen. In the longer term, the relative dominance of species is likely to alter depending on their ability to convert elevated levels of deposited nitrogen into biomass.
What will be the effect on species diversity of increasing biomass?
Atmospheric pollution can also affect plant-insect interactions. Unusual episodes of damage to heather moorland in Scotland have been caused by the winter moth (Operophtera brumata) in recent years. It has been suggested that this may be due to the effects of increased nitrogen supply on heather plants, including increased shoot growth and a decrease in the carbon : nitrogen ratio in plant tissues. Winter moth larvae have been shown to grow faster on nitrogen-treated heather plants, so it is possible that increased atmospheric deposition of nitrogen may have a role in winter moth outbreaks and the associated degradation of heather moorland in upland Britain.
Although uplands are more susceptible to atmospheric deposition of nitrogen, the effects can be seen in lowland areas too. Nitrogen deposition and the consequent eutrophication of ecosystems is now regarded as one of the most important causes of decline in plant species in the Netherlands. Figure 2.13 shows how the number of grassland species of conservation interest in south Holland declines as the nitrogen load increases. The maximum percentage of species (approximately 95%) is possible at a nitrogen load of about 6 kg N ha−1 yr−1. At loads higher than 10 kg N ha−1 yr−1 the number of species declines due to eutrophication effects, and below 5 kg ha−1 yr−1nitrogen may be too limiting for a few species.
Figure 2.13 Relationship between potential number of protected grassland species in grassland and nitrogen load in South Holland.
A significant proportion of important nature conservation sites in Britain are subject to nitrogen and/or sulfur deposition rates that may disturb their biological communities. Lowland heath ecosystems, for example, have a high profile for conservation action in Britain. They typically have low soil nutrient levels and a vegetation characterized by heather (Calluna vulgaris). Under elevated atmospheric deposition of nitrogen, they tend to be invaded by taller species, including birch (Betula spp., Figure 2.14), bracken (Pteridium aquilinum) and the exotic invader, rhododendron (Rhododendron ponticum).
Figure 2.14 Invasion of lowland heath by birch trees.
A large number of SSSIs in the UK, designated as such on account of their terrestrial plant communities, are considered to have been damaged by eutrophication. This has been identified as a factor in the decline of some important UK habitats, including some identified for priority action under the UK’s Biodiversity Action Plan (BAP). Wet woodlands, for example, occur on poorly drained soils, usually with alder, birch and willow as the predominant tree species, but sometimes oak, ash or pine occur in slightly drier locations. These woodlands are found on floodplains, usually as a successional habitat on fens, mires and bogs, along streams and in peaty hollows. They provide an important habitat for a variety of species, including the otter (Lutra lutra, Figure 2.15), some very rare beetles and craneflies. They also provide damp microclimates, which are particularly suitable for bryophytes, and have some unusual habitat features not commonly found elsewhere, such as log jams in streams which support a rare fly, Lipsothrix nigristigma. Wet woodlands occur on a range of soil types, including relatively nutrient-rich mineral soils as well as acid, nutrient-poor ones. Nevertheless, many have been adversely affected by eutrophication, resulting in altered ground flora composition and changes in the composition of invertebrate communities.
Figure 2.15 Otter, Lutra lutra.
Nutrient enrichment can also affect habitats found in drier sites. Eutrophication caused by runoff from adjacent agricultural land has been identified as a cause of altered ground flora composition in upland mixed ash woods for example. These woods are notable for bright displays of flowers such as bluebell (Hyacinthoides non-scripta), primrose (Primula vulgaris) and wild garlic (Allium ursinum, Figure 2.16a). They also support some very rare woodland flowers which are largely restricted to upland ash woods, such as dark red helleborine (Epipactis atrorubens, Figure 2.16b) and Jacob’s ladder (Polemonium caeruleum).
Figure 2.16 (a) Wild garlic (Allium ursinum) and (b) dark-red helleborine (Epipactis atrorubens) are species of the woodland floor that may be displaced if nutrient availability in the soil increases.
Other terrestrial UK BAP habitats that may be adversely affected by nutrient enrichment from agricultural fertilizers or atmospheric deposition include lowland wood pasture, lowland calcareous grassland, upland hay meadows and lowland meadows; again the result can be altered plant species composition.
Coastal marshes and wetlands in many parts of the world have been affected by invasion of ‘weed’ or ‘alien’ species. Eutrophication can accelerate invasion of aggressive, competitive species at the expense of slower growing native species. In the USA, many coastal marshes have been invaded by the common reed (Phragmites australis, Figure 2.17). Phragmites is a fierce competitor and can outcompete and entirely displace native marsh plant communities, causing local extinction of plants and the insects and birds that feed on them. Phragmites can spread by underground rhizomes and can rapidly colonize large areas. However, it is the target of conservation effort in some areas, including Britain, because the reedbeds it produces provide an ideal habitat for rare bird species such as the bittern (Botaurus stellarus). But its spread is not always beneficial for nature conservation, as it often results in the drying of marsh soils, making them less suitable for typical wetland species and more suitable for terrestrial species. This is because Phragmites is very productive and can cause ground levels to rise due to deposition of litter and the entrapment of sediment. Thus eutrophication can also play an indirect part in the loss of wetland habitats.
Figure 2.17 An extensive stand of the common reed (Phragmites australis), which is found around the globe in nutrient-rich, shallow freshwater. It is expanding its extent in many areas in response to eutrophication of previously nutrient-poor ecosystems.
In the marine environment, nutrient enrichment is suspected when surface phytoplankton blooms are seen to occur more frequently and for longer periods. Some species of phytoplankton release toxic compounds and can cause mass mortality of other marine life in the vicinity of the bloom. Changes in the relative abundance of phytoplankton species may also occur, with knock-on effects throughout the food web, as many zooplankton grazers have distinct feeding preferences. In sheltered estuarine areas, high nutrient levels appear to favour the growth of green macroalgae (‘seaweeds’) belonging to such genera as Enteromorpha and Ulva (Figure 2.18).
Figure 2.18 The macroalga Ulva taeniata, which can grow to several metres in length, given a sufficient supply of nutrients.
Nutrient runoff from the land is a major source of nutrients in estuarine habitats. Shallow-water estuaries are some of the most nutrient-rich ecosystems on Earth, due to coastal development and the effects of urbanization on nutrient runoff. Figure 2.19 shows some typical nitrogen pathways. Nitrogen loadings in rainfall are typically assimilated by plants or denitrified, but septic tanks tend to add nitrogen below the reach of plant roots, and if situated near the coast or rivers can lead to high concentrations entering coastal water. Freshwater plumes from estuaries can extend hundreds of kilometres offshore (Figure 2.20) and the nutrients within them have a marked effect on patterns of primary productivity. Localized effects of eutrophication can be dramatic. For example, increased nitrogen supplies lead to the replacement of seagrass beds (e.g. Zostera marina) by free-floating rafts of ephemeral seaweeds such as Ulva and Cladophera, whose detritus may cover the bottom in a dense layer up to 50 cm thick.
Figure 2.19 Nitrogen pathways in developed coastal estuaries.
Figure 2.20 Phosphorus concentration with increasing distance from New York Harbour
Estuarine waters enriched by nitrogen from fertilizers and sewage have been responsible for the decline of a number of estuarine invertebrate species, often by causing oxygen depletion of bottom water. Intertidal oyster beds have declined considerably as a result of both over-harvesting and reduced water quality. Harvesting tends to remove oysters selectively from shallow-water habitats, reducing the height of oyster beds and making remaining oysters more vulnerable to the damaging effects of eutrophication. In estuaries, elevated rates of microbial respiration deplete oxygen, and periods of anoxia occur more frequently, especially in summer when water temperatures are high and there is slow water circulation. Oysters in deeper water are more likely to be exposed to anoxic conditions, being further removed from atmospheric oxygen inputs, and to die as a result.
Seagrass distributions are very sensitive to variation in light. Seagrasses, like any other plant, cannot survive in the long term if their rate of photosynthesis is so limited by light that it cannot match their rate of respiration. Light transmission is a function of water column turbidity (cloudiness), which in turn is a combination of the abundance of planktonic organisms and the concentration of suspended sediment. Seagrass distributions are therefore strongly affected by eutrophication and effects on water clarity. In Chesapeake Bay, eastern USA, seagrass beds historically occurred at depths of over 10 m. Today they are restricted to a depth of less than 1 m. Runoff from organic fertilizers has increased plankton production in the water column, limiting light transmission. In addition, large areas of oyster bed have been lost (also partly as a result of eutrophication) with an associated reduction in natural filtration of bay water. Oysters, like many shellfish, clean the water while filtering out microbes, which they then consume. Seagrass beds now cover less than 10% of the area they covered a century ago (Figure 2.21). Similar patterns have occurred elsewhere.
Figure 2.21 Decline of seagrass beds (green) in the estuary of Waquoit Bay, Cape Cod, over the past century.
Seagrass beds play an important role in reducing the turbidity of coastal waters by reducing the quantity of sediment suspended in the water. Seagrasses slow down currents near the bottom, which increases the deposition of small sediment particles and decreases their erosion and resuspension. Seagrass roots also play an important part in stabilizing sediments and limiting disturbance caused by burrowing deposit-feeders.
Many species depend on seagrass beds for food or nursery grounds. Seagrass increases the structural complexity of habitat near the sea floor, and provides a greater surface area for epiphytic organisms. Seagrass leaves support rich communities of organisms on their surface, including microalgae, stationary invertebrates (such as sponges and barnacles) and grazers (such as limpets and whelks). The plants also provide refuges from predatory fishes and crabs. Predation on seagrass-associated prey such as the grass shrimp is much higher outside seagrass beds than within them, where the shrimps can hide and predator foraging is inhibited by grass cover. Without seagrasses, soft-substrate communities on the sea bed are simpler, less heterogeneous and less diverse. By reducing the health of seagrasses, eutrophication contributes directly to biotic impoverishment (Figure 2.22).
Figure 2.22 Changes in sea floor communities in shallow coastal waters following eutrophication. (a) The structural diversity afforded by the plants and the availability of oxygen in the sediment promote a diverse community of animals. (b) The loss of structural diversity and oxygen from the sea-bed causes the animal community to be replaced by one of bacterial decomposers.
Seagrass (Zostera marina) is a key species for maintaining the biodiversity of estuaries. Why is its abundance reduced following eutrophication of estuarine waters? Can you identify a mechanism in which the decline of seagrass promotes its further decline?
Marsh plant primary production is generally nitrogen limited, so saltmarsh vegetation responds readily to the artificial eutrophication that is now so common in nearshore waters. Eutrophication causes marked changes in plant communities in saltmarshes, just as it does in freshwater aquatic and terrestrial systems. Biomass production increases markedly as levels of eutrophication increase. Increases in the nitrogen content of plants cause dramatic changes in populations of marsh plant consumers: insect herbivores tend to increase (Figure 2.23) and so do numbers of carnivorous insects. Thus, increasing the nitrogen supply to saltmarshes has a dramatic bottom-up effect on marsh food webs. Eutrophication can also alter the outcome of competition among marsh plants, by changing the factor limiting growth. At low levels of nitrogen, plants that exploit below-ground resources most effectively, such as the saltmarsh rush (Juncus gerardii) are competitively dominant, but at higher nutrient levels dominance switches to plants that are good above-ground competitors, such as the common cord grass (Spartina anglica, Figure 2.24). In other words, as nitrogen availability increases, competition for light becomes relatively more important.
Figure 2.23 Effects of nutrient enrichment on herbivorous insect abundance (measured as dry mass) in saltmarshes.
Figure 2.24 The cord grass (Spartina anglica), which has spread rapidly around the coasts of Britain in the past 100 years, aided by the increased nutrient supply to saltmarshes in addition to being widely planted to help stabilize bare mud.