The levels of nutrients present determine the trophic state of a water body, where trophic means ‘feeding’.
Give another example of the adjective trophic being used in a scientific context.
The adjective eutrophe (literally ‘well fed’) was first used by the German botanist Weber in 1907, to describe the initially high nutrient conditions that occur in some types of ecosystem at the start of secondary succession. Scientists studying lakes at the beginning of the 20th century identified stages in plant community succession that appeared to be directly related to trophic state or nutrient status. They described a series of stages:
‘oligotrophic — mesotrophic — eutrophic — hypertrophic’
where oligotrophic meant ‘low in nutrients’, mesotrophic ‘with intermediate nutrient concentration’, eutrophic ‘high in nutrients’ and hypertrophic ‘very high in nutrients’. At the time, these definitions were derived from comparative estimates between water bodies with different nutrient status, judged according to their phytoplankton communities. Phytoplankton is a collective term for the free-floating photosynthetic organisms within the water column. It encompasses both algae (from the kingdom Protoctista) and photosynthetic members of the kingdom Bacteria. Thus an oligotrophic lake would have clear water with little phytoplankton, whereas a eutrophic lake would be more turbid and green from dense phytoplankton growth, and a mesotrophic lake would be intermediate between the two. Table 1.1 summarizes some of the general characteristics of oligotrophic and eutrophic lakes. A further definition, dystrophic, describes ‘brown-water lakes’, which have heavily stained water due to large amounts of organic matter usually leached from peat soils. The presence of these organic compounds can reduce the availability of nutrients to organisms, making the water body even less productive than an oligotrophic one.
Table 1.1 Some general characteristics of oligotrophic and eutrophic lakes.
Characteristic | Oligotrophic | Eutrophic |
---|---|---|
primary production | low | high |
diversity of primary producers | high species diversity,with low population densities | low species diversity,with high population densities |
light penetration into water column | high | low |
toxic blooms | rare | frequent |
plant nutrient availability | low | high |
animal production | low | high |
oxygen status of surfacewater | high | low |
fish | salmonid fish (e.g.trout, char)often dominant | coarse fish (e.g. perch, roach, carp) often dominant |
Why is light penetration poor in eutrophic lakes?
More recently, trophic bands have been defined in relation to levels of nutrients measured by chemical analysis. Table 1.2 shows trophic bands as defined in relation to concentrations of total phosphorus.
Table 1.2 Trophic bands for standing waters. (Phosphorus concentrations tend to be higher in running waters that carry suspended sediment.)
Trophic band | Total phosphorus/mg l-1 |
---|---|
dystrophic | <0.005 |
oligotrophic | 0.005-0.01 |
mesotrophic | 0.01-0.03 |
eutrophic | 0.03-0.1 |
hypertrophic | >0.1 |
The trophic state of water bodies and rivers varies depending on a number of factors, including position in the landscape and management of surrounding land. In general, upland areas are more likely to have nutrient-poor (oligotrophic) water, characterized by relatively fast-flowing rivers (Figure 1.1) and lakes that have clear water with limited higher plant communities.
Figure 1.1 Fast-flowing upland stream with clear water and few plants.
By contrast, lowland waters in more fertile river catchments tend to be nutrient-rich (eutrophic), and lakes in lowland areas are more likely to be turbid with lush fringing vegetation. Lowland rivers have slower flow and are likely to be more nutrient rich as a result of soluble compounds having been washed into them. They are likely to have fringing vegetation and some floating and submerged aquatic plants (Figures 1.2 and 1.3). In aquatic systems, the term macrophyte is used to describe any large plant (macro, large; phyte, plant). The term is used to distinguish angiosperms (whether emergent, floating or submerged) from small algae such as diatoms (which are strictly not plants at all, but are often lumped together with plants when considering the productivity of ecosystems).
Figure 1.2 Lowland river, rich in aquatic plant species.
Figure 1.3 Rich community of macrophytes. The tall plants growing out of the water are described as emergent.
What is the process by which nutrient elements are lost from the soil profile by the action of excess rainfall draining through it, which may eventually deliver them to a surface water body?
The term ‘eutrophication’ came into common usage from the 1940s onwards, when it was realized that, over a period of years, plant nutrients derived from industrial activity and agriculture had caused changes in water quality and the biological character of water bodies. In England and Wales, eutrophication has been a particular concern since the late 1980s, when public awareness of the problem was heightened by widespread toxic blue-green bacterial blooms (commonly, but incorrectly, referred to as algal blooms) in standing and slow-flowing freshwaters. Figure 1.4 shows blue-green bacteria (cyanobacteria) growing at the margins of a lake. Cyanobacteria are not typical bacteria, not only because some of them are photosynthetic, but also because some of them can be multicellular, forming long chains of cells. Nonetheless, cyanobacteria clearly belong to the kingdom Bacteria because of their internal cellular structure.
Figure 1.4 A cyanobacterial bloom.
Why are cyanobacteria so productive in eutrophic water bodies (Figure 1.4) compared with oligotrophic ones?
A wide range of ecosystems has been studied in terms of their species diversity and the availability of resources. Each produces an individual relationship between these two variables, but a common pattern emerges from most of them, especially when plant diversity is being considered. This pattern has been named the humped-back relationship and suggests diversity is greatest at intermediate levels of productivity in many systems (Figure 1.5).
Figure 1.5 The species richness of samples of vegetation from South Africa shows a classic humped-back relationship with ecosystem productivity as inferred from amount of biomass per unit area.
How does species diversity differ from species richness?
An explanation for this relationship is that at very low resource availability, and hence ecosystem productivity, only a limited number of species are suitably adapted to survive. As the limiting resource becomes more readily available, then more species are able to grow. However, once resources are readily available, then the more competitive species within a community are able to dominate it and exclude less vigorous species.
In most ecosystems it is the availability of mineral nutrients (especially nitrogen and phosphorus) that limits productivity. In eutrophic environments these nutrients are readily available by definition, so species diversity can be expected to be lower than in a more mesotrophic situation. It is for this reason that eutrophication is regarded as a threat to biodiversity. Eutrophication of the environment by human-mediated processes can have far reaching effects, because the nutrients released are often quite mobile. Together with habitat destruction, it probably represents one of the greatest threats to the sustainability of biodiversity over most of the Earth.
Eutrophication of habitat can occur without human interference. Nutrient enrichment may affect habitats of any initial trophic state, causing distinctive changes to plant and animal communities. The process of primary succession is normally associated with a gradual eutrophication of a site as nutrients are acquired and stored by vegetation both as living tissue and organic matter in the soil.
There is a long-standing theory that most water bodies go through a gradual process of nutrient enrichment as they age: a process referred to as natural eutrophication. All lakes, ponds and reservoirs have a limited lifespan, varying from a few years for shallow water bodies to millions of years for deep crater lakes created by movements of the Earth’s crust. They fill in gradually with sediment and eventually became shallow enough for plants rooted in the bed sediment to dominate, at which point they develop into a closed swamp or fen and are eventually colonized by terrestrial vegetation (Figures 1.6 and 1.7).
Figure 1.6 Cross-section through a water body that is gradually becoming filled with silt deposits and organic matter as a result of vegetation growth.
Figure 1.7 A floodplain water body becoming colonized with emergent macrophytes, which may eventually cause it to disappear through an accumulation of silt and organic matter.
Nutrient enrichment occurs through addition of sediment, rainfall and the decay of resident animals and plants and their excreta. Starting from an oligotrophic state with low productivity, a typical temperate lake increases in productivity fairly quickly as nutrients accumulate, before reaching a steady state of eutrophy which might last for a very long time (perhaps thousands of years). However, it is possible for the nutrient status of a water body to fluctuate over time and for trophic state to alter accordingly. Study of sediments in an ancient lake in Japan, Lake Biwa (believed to be around four million years old) suggests that it has passed through two oligotrophic phases in the last half million years, interspersed with two mesotrophic phases and one eutrophic phase. Evidence such as this has led to the suggestion that the nutrient status of lakes reflects contemporary nutrient supply, and can increase or decrease in response to this. The processes by which nutrients are washed downstream or locked away in sediments help to ensure that reversal of natural eutrophication can occur.
Rivers vary in trophic state between source and sea, and generally become increasingly eutrophic as they approach sea-level.
While eutrophication does occur independently of human activity, increasingly it is caused, or amplified, by human inputs. Human activities are causing pollution of water bodies and soils to occur to an unprecedented degree, resulting in an array of symptomatic changes in water quality and in species and communities of associated organisms. In 1848 W. Gardiner produced a flora of Forfarshire, in which he described the plants growing in Balgavies Loch. He talked of ‘potamogetons [pondweeds] flourishing at a great depth amid the transparent waters, animated by numerous members of the insect and finny races’. These ‘present a delightful spectacle, and the long stems of the white and yellow water lilies may be traced from their floating flowers to the root’. By 1980, the same loch had very low transparency and dense growths of planktonic algae throughout the summer. The submerged plants grew no deeper than 2 m, and in the 1970s included just three species of Potamogeton, where previously there were 17.
For any ecosystem, whether aquatic or terrestrial, nutrient status plays a major part in determining the range of organisms likely to occur. Characteristic assemblages of plant and associated animal species are found in water with different trophic states. Table 1.3 shows some of the aquatic macrophyte species associated with different concentrations of phosphorus in Britain.
Table 1.3 Concentrations of phosphorus (in rivers) with which plant species are correlated.
Phosphorus present as soluble reactive phosphorus (SRP)*/mg P l-1 | Plant species (see Figure 1.8 for illustrations) |
---|---|
<0.1 | bog pondweed, Potamogeton polygonifolius river water-crowfoot,Ranunculus fluitans |
0.1-0.4 | fennel-leaved pondweed, Potamogeton pectinatus |
0.4-1.0 | yellow water-lily,Nuphar lutea arrowhead,Sagittarias agittifolia |
>1.0 | spiked water-milfoil, Myriophyllum spicatum |
* This term is explained in Section 2.1.
What impression would you gain from an observation that a population of river water-crowfoot in a particular stretch of river had been largely replaced by fennel-leaved pondweed over a three-year period?
Figure 1.8 Three aquatic macrophyte species which differ in their tolerance to eutrophication: (a) river water-crowfoot (Ranunculus fluitans) is intolerant, (b) yellow water-lily (Nuphar lutea) is intermediate and (c) spiked water-milfoil (Myriophyllum spicatum) is tolerant.
Figure 1.9 illustrates the relationship between levels of total phosphorus in standing water and the nutrient status of lakes. Above a level of 0.1 mg phosphorus per litre, biodiversity often declines. Using the trophic bands defined in Table 1.2, this is the concentration at which lakes are considered to become hypertrophic. This is way below the standard of 50 mg l−1 set as the acceptable limit for phosphorus in drinking water. Nutrient loadings this high are generally caused by human activities. Extremely high levels of eutrophication are often associated with other forms of pollution, such as the release of toxic heavy metals, resulting in ecosystems that may no longer support life (Figure 1.10).
Figure 1.9 Relationship between levels (in mg l−1) of total phosphorus in standing water and the nutrient status of lakes (STW, sewage treatment works).
Figure 1.10 Polluted river in an industrial area.
For lakes with no written historical records, the diatom record of sediments can be used to study earlier periods of natural change in water quality, and to provide a baseline against which to evaluate trends in artificial or human-induced eutrophication. Diatoms are microscopic photosynthetic organisms (algae of the kingdom Protoctista), which live either free-floating in lakes or attached to the surface of rocks and aquatic vegetation. It is well established that some species of diatom can tolerate oligotrophic conditions whereas others flourish only in more eutrophic waters. When they die, their tiny (< 1 mm) bony capsules, which can be identified to species level, sink to the bed and may be preserved for thousands of years. A historical record of which species have lived within a water body can therefore be constructed from an analysis of a core sample taken from its underlying sediment.
Studies of diatom remains have demonstrated that current levels of eutrophication far exceed those found historically. In the English Lake District, productivity and sediment input increased in some lakes when vegetation was cleared by Neolithic humans around 5000 years ago, and again when widespread deforestation occurred 2000 years ago. However the greatest increases in productivity, sediment levels and levels of carbon, nitrogen and phosphorus, have occurred since 1930. Figure 1.11 shows the general pattern of changes in productivity in Cumbrian lakes through history as the type and intensity of human activities has changed.
Figure 1.11 Relationship between historical human activities and productivity of lakes in Cumbria, UK. (BP means ‘years before present’.)
In the Norfolk Broads, the waters of the River Ant had a diverse macrophyte flora during the 19th century. The submerged species known as water soldier (Stratiotes aloides, Figure 1.12) was common, but by 1968 the only macrophytes remaining were those with permanently floating leaves, such as water-lilies. During that period, throughout the Broads, there was a general trend away from clear-water habitats, typified by, for example, the diminutive angiosperm known as the holly-leaved naiad (Najas marina), towards habitats containing more productive species, such as pondweeds (Potamogeton spp.) andhornworts (Ceratophyllum spp.). In some cases, they eventually became eutrophic habitats with turbid water, typified by free-floating green algae and cyanobacteria, with very few macrophytes at all. For example, hornwort (Ceratophyllum demersum) was almost choking Alderfen Broad in 1963, but had almost disappeared by 1968 to be replaced eventually by algal blooms in the 1990s.
Figure 1.12 A free-floating macrophyte, water soldier (Stratiotes aloides), spends most of its life submerged, but rises to the surface to flower.
Sediment cores from the River Ant and neighbouring broads suggest that observed changes in plant community composition were linked to rising levels of total phosphorus: mean levels in the area rose dramatically between 1900 and 1975 (Figure 1.13), but have since fallen as a result of actions taken to remove phosphorus from the system.
Figure 1.13 Annual peak concentration of total phosphorus in the water bodies linked to the River Ant, Norfolk Broads, England, where a programme of remedial action to address eutrophication has been implemented over the past 25 years.
Using the trophic bands in Table 1.2, describe the change in the River Ant broads between 1800 and 1975.
Eutrophication has damaged a large number of sites of special scientific interest (SSSIs) designated in the UK under the Wildlife and Countryside Act of 1981: English Nature has identified a total of 90 lake SSSIs and 12 river SSSIs that have been adversely affected. Artificial eutrophication in rivers is even more widespread than in lakes and reservoirs. Human activities worldwide have caused the nitrogen and phosphorus content of many rivers to double and, in some countries, local increases of up to 50 times have been recorded.
Eutrophication has also become a problem for terrestrial wildlife. Deposition of atmospheric nitrogen and the use of nitrogen-rich and phosphorus-rich fertilizers in agriculture has resulted in nutrient enrichment of soils and has caused associated alteration of terrestrial plant and animal communities.
Some of the effects of large-scale eutrophication have adverse consequences for people, and efforts to manage or reduce eutrophication in different countries now cost substantial sums of money. Removing nitrates from water supplies in England and Wales cost £20 million in 1995. Higher frequency of algal blooms increases the costs of filtration for domestic water supply and may cause detectable tastes and odours due to the secretion of organic compounds. If the bloom is large, these compounds can accumulate to concentrations that are toxic to mammals and sometimes fish. Furthermore, the high productivity of the blooms means that although oxygen is released by photosynthesis during the day, the effect of billions of cells respiring overnight can deplete the water of oxygen, resulting in fish dying through suffocation even if they tolerate the toxins.
Are fish most at risk from suffocation in warm or cool water?
Another problem caused to the water industry by algal blooms is the production of large quantities of fine organic detritus, which, when collected within waterworks’ filters, may support clogging communities of aquatic organisms such as nematode worms, sponges and various insects. These may subsequently find their way into water distribution pipes and on occasion appear in tap water!