Light availability, water availability, temperature and the supply of plant nutrients are the four most important factors determining NPP. Altered availability of nutrients affects the rate of primary production in all ecosystems, which in turn changes the biomass and the species composition of communities.
Which two elements most often limit NPP?
Compounds containing these elements are therefore the causal agents of eutrophication in both aquatic and terrestrial systems. Let us consider them in turn.
Phosphorus has a number of indispensable biochemical roles and is an essential element for growth in all organisms, being a component of nucleic acids such as DNA, which hold the code for life. However, phosphorus is a scarce element in the Earth’s crust and natural mobilization of phosphorus from rocks is slow. Its compounds are relatively insoluble, there is no reservoir of gaseous phosphorus compounds available in the atmosphere (as there is for carbon and nitrogen), and phosphorus is also readily and rapidly transformed into insoluble forms that are unavailable to plants. This tends to make phosphorus generally unavailable for plant growth. In natural systems, phosphorus is more likely to be the growth-limiting nutrient than is nitrogen, which has a relatively rapid global cycle and whose compounds tend to be highly soluble.
Human activities, notably the mining of phosphate-rich rocks and their chemical transformation into fertilizer, have increased rates of mobilization of phosphorus enormously. A total of 12 × 1012 g P yr−1 are mined from rock deposits. This is six times the estimated rate at which phosphorus is locked up in the ocean sediments from which the rocks are formed. The global phosphorus cycle is therefore being unbalanced by human activities, with soils and water bodies becoming increasingly phosphorus-rich. Eutrophication produces changes in the concentrations of phosphorus in all compartments of the phosphorus cycle.
The mechanisms of eutrophication caused by phosphorus vary for terrestrial and aquatic systems. In soils, some phosphorus comes out of solution to form insoluble iron and aluminium compounds, which are then immobilized until the soil itself is moved by erosion. Eroded soil entering watercourses may release its phosphorus, especially under anoxic conditions.
What changes occur to iron(III) compounds (Fe3+) as a result of bacterial respiration in anoxic environments, and how is their solubility affected?
Once in rivers, retention times for phosphorus may be short, as it is carried downstream either in soluble form or as suspended sediment. Algal blooms are therefore less likely to occur in moving waters than in still systems. In the latter, there is more time for the phosphorus in enriched sediments to be released in an ‘available’ form, increasing the concentration of soluble reactive phosphorus (SRP), and thus affecting primary production.
Phosphorus is generally acknowledged to be the nutrient most likely to limit phytoplankton biomass, and therefore also the one most likely to cause phytoplankton blooms if levels increase. However, there do appear to be some systems that are ‘naturally eutrophic’, with high phosphorus loadings. In these systems, nitrogen concentrations may then become limiting and play a dominant role in determining phytoplankton biomass.
Nearly 80% of the atmosphere is nitrogen. Despite the huge supply potentially available, nitrogen gas is directly available as a nutrient to only a few organisms.
Why cannot the majority of organisms utilize gaseous nitrogen?
Nitrogen gas is very unreactive and only a limited number of bacterial species have evolved an enzyme capable of cleaving the molecule.
Once ‘fixed’ by these bacteria into an organic form, the nitrogen enters the active part of the nitrogen cycle. As the bacteria or the tissues of their mutualistic hosts die, the nitrogen is released in an available form such as nitrate or ammonium ions - a result of the decay process. Alternatively, the high temperatures generated during electrical storms can ‘fix’ atmospheric nitrogen as nitric oxide (NO). Further oxidation to nitric acid within the atmosphere, and scavenging by rainfall, provides an additional natural source of nitrate to terrestrial ecosystems. Nitrates and ammonium compounds are very soluble and are hence readily transported into waterways.
Nitrogen is only likely to become the main growth-limiting nutrient in aquatic systems where rocks are particularly phosphate-rich or where artificial phosphate enrichment has occurred. However, nitrogen is more likely to be the limiting nutrient in terrestrial ecosystems, where soils can typically retain phosphorus while nitrogen is leached away.
In addition to the natural sources of nutrients referred to above, nitrogen and phosphorus enter the environment from a number of anthropogenic sources. These are considered below.
Pollution of the atmosphere has increased rates of nitrogen deposition considerably. Nitrogen has long been recognized as the most commonly limiting nutrient for terrestrial plant production throughout the world, but air pollution has now created a modern, chemical, climate that often results in excess supplies of nitrogen due to atmospheric deposition.
The main anthropogenic source of this enhanced nitrogen deposition is the NOx (mainly as NO) released during the combustion of fossil fuels — principally in vehicles and power plants. Like that generated within the atmosphere, this fixed nitrogen returns to the ground as nitrate dissolved in rainwater.
Patterns and rates of deposition vary regionally, and between urban and rural areas. Concentrations and fluxes of nitrogen oxides tend to decline with distance from cities: deposition of inorganic nitrogen has been found to be twice as high in urban recording sites in New York City than in suburban or rural sites. Some natural ecosystems, particularly those near industrialized areas, now receive atmospheric nitrogen inputs that are an order of magnitude greater than those for pre-industrial times. Figure 3.2 shows intensive industrial land use adjacent to the River Tees and its estuary in Teesside, UK. The estuary is still important for wildlife, including seals and a variety of birds, but its quality has declined markedly due to atmospheric and water pollution. In the UK, atmospheric deposition can add up to 150 kg N ha−1 yr−1. For comparison, the amount thought to trigger changes in the composition of species-rich grassland is 20-30 kg Nha−1 yr−1, and a typical dose farmers apply as inorganic fertiliser to an intensively managed grassland is 100 kg N ha−1 yr−1.
Figure 3.1 Industrial emissions. These introduce nitrogen compounds into the atmosphere.
Figure 3.2 Large-scale industrial development adjacent to the River Tees.
Domestic detergents are a major source of phosphorus in sewage effluents. Phosphates are used as a ‘builder’ in washing powders to enhance the efficiency of surfactants by removing calcium and magnesium to make the water ‘softer’. In 1992, the UK used 845 600 tonnes of detergent of various types, all of which have different effects on the environment. Estimates of the relative contribution of domestic detergents to phosphorus build-up in Britain’s watercourses vary from 20-60%. The UK’s Royal Commission on Environmental Pollution (RCEP) reviewed the impacts of phosphate-based detergents on water quality in 1992, focusing on the effects on freshwater. The RCEP concluded that eutrophication was widespread over large parts of the country, and recommended a considerable investment in stripping phosphates from sewage as well as efforts to reduce phosphate use in soft-water areas. The main problem is that many of the ingredients of detergents are not removed by conventional sewage treatment and degrade only slowly.
Why did the RCEP recommend that phosphate use be reduced particularly in ‘soft’ water areas?
Other compounds added to detergents may also contribute to eutrophication. Silicates, for example, particularly if used as a partial replacement for phosphates in detergents, can lead to increased growth of diatoms. These algae require silicates to build their ‘skeleton’ and their growth can be limited by silicate availability. When silicates are readily available, diatoms characteristically have ‘spring blooms’ of rapid growth, and can smother the surfaces of submerged macrophytes, depriving them of light. A loss of submerged macrophytes is a problem because it results in the loss of habitat for organisms feeding on phytoplankton, and therefore the risk of blooms by other species is enhanced.
Runoff from intensively farmed land often contains high concentrations of inorganic fertilizer. Nutrients applied to farmland may spread to the wider environment by:
drainage water percolating through the soil, leaching soluble plant nutrients;
washing of excreta, applied to the land as fertilizer, into watercourses; and
the erosion of surface soils or the movement of fine soil particles into subsoil drainage systems.
Some water bodies have been monitored for long periods, and the impact of agricultural runoff can be demonstrated clearly. In the 50 years between 1904 and 1954, for example, in Loch Leven, Scotland, there were major changes in the species composition of the community of photosynthetic organisms. The species composition of the green alga community changed and the numbers of cyanobacteria rose considerably. Increasingly since then, large blooms of filamentous cyanobacteria have been produced in the loch. These changes have been linked with trends in the use of agricultural fertilizers and other agrochemicals.
In Europe, large quantities of slurry from intensively reared and housed livestock are spread on the fields (Figure 3.3). Animal excreta are very rich in both nitrogen and phosphorus and therefore their application to land can contribute to problems from polluted runoff. Land use policies have concentrated livestock production into purpose-built units, increasing the pollution risks associated with handling the resultant slurry or manures.
Figure 3.3 Muck spreading.
European agricultural policies that subsidize agriculture on the basis of productivity have also encouraged the use of fertilizers. Use of fertilizers has undergone a massive increase since 1950. In the USA, by 1975, total use of inorganic fertilizer had reached a level equivalent to about 40 kg per person per year. A recent European Environment Agency report estimated that the groundwater beneath more than 85% of Europe’s farmland exceeds guideline levels for nitrogen concentration (25 mg l−1), with agricultural fertilizers being the main source of the problem. Pollution of surface waters also occurs on a large scale. A survey by the UK’s Environment Agency in 1994 found that over 50% of the 314 water bodies surveyed in England and Wales had algal blooms caused by fertilizer runoff (Figure 3.4).
Figure 3.4 Algal blooms caused by fertilizer runoff.
Patterns of fertilizer use do differ considerably between countries. In those with poorly developed economies, the costs of artificial fertilizers may be prohibitive. In hotter climates, irrigation may be used, resulting in higher nutrient runoff than for equivalent crops that are not irrigated. The high solubility of nitrate means that agriculture is a major contributor to nitrogen loadings in freshwater. Agriculture accounts for 71% of the mass flow of nitrogen in the River Great Ouse in the Midlands, UK, compared with only 6% for phosphorus.
What are the main sources of phosphorus and nitrogen that enrich rivers in a developed country
Studies evaluating the effects of nutrient loading on receiving water bodies must take account of the range of land uses found within a catchment.
As shown in Table 3.1, phosphate exports increase considerably as forests are converted to agricultural land and as agricultural land is urbanized. Agricultural runoff is known to be a potential source of nutrients for eutrophication, but the degree of mechanization may also be important. In catchments where agriculture is heavily mechanized, higher levels of sedimentation are likely. Most sediments arise as a result of soil erosion, which is promoted by tilling the land intensively. This destroys the soil’s natural structure as well as removing vegetation which helps to stabilize soil.
To cite just one example, high sediment input in the latter half of the 20th century has caused shrinkage of the area of open water in the Mogan Lake system near Ankara, Turkey. Undoubtedly, mechanization and intensification of agriculture have played their part, but so too has the drainage of adjacent wetlands. The drained wetlands no longer trapped sediments, and themselves became vulnerable to erosion. This further increased sediment loadings in the lake. Levels of phosphorus have also risen. Draining the wetlands exposed the organic matter in their soils to oxidation, ‘mobilizing’ the phosphorus that had accumulated there over many years. This was then carried into the lake in drainage water.
Table 3.1 Quantities of nitrogen and phosphorus (g m−2 yr−1) derived from various types of land use in the USA and from the atmosphere.
Land use | Total phosphorus | Total nitrogen | |
---|---|---|---|
Losses from land to water courses | urban | 0.1 | 0.5 |
rural/agriculture | 0.05 | 0.5 | |
forest | 0.01 | 1.3 | |
Additions to land | atmospheric sources: | ||
rainfall | 0.02 | 0.8 | |
dry deposition | 0.08 | 1.6 |
Sediments have a variable but complex role in nutrient cycling in most aquatic systems, and are a potential ‘internal’ source of pollutants. Release of phosphorus from lake sediment is a complex function of physical, biological and chemical processes and is not easy to predict for different systems. Nitrogen is not stored and released from sediments in the same way, so its turnover time within aquatic systems is quite rapid. Nitrogen concentrations tend to fall off relatively quickly following a reduction in external nitrogen loading, whilst this is not true for phosphorus because the sediments can hold such a large reservoir of this nutrient that input and output rates may become decoupled.
In some shallow coastal areas, tidal mixing is the dominant nutrient regeneration process, as the sediments are regularly disturbed and redistributed by changing water currents, making nutrient exchange with the water much more rapid.
Why is a lake in a catchment dominated by arable agriculture much more prone to eutrophication than one in a forested catchment?
Direct effects of eutrophication occur when growth of organisms (usually the primary producers) is released from nutrient limitation. The resulting increased NPP becomes available for consumers, either as living biomass for herbivores or as detritus for detritivores. Associated indirect effects occur as eutrophication alters the food supply for other consumers. Changes in the amount, relative abundance, size or nutritional content of the food supply influence competitive relationships between consumers, and hence the relative success and survival of different species. Nutrient-induced changes in plant community composition and productivity can therefore result in associated changes in the competitive balance between herbivores, detritivores and predators. Consumers may also be affected by changes in environmental conditions caused indirectly by eutrophication, for example reduced oxygen concentrations caused by bacterial decay of biomass.
In freshwater aquatic systems, a major effect of eutrophication is the loss of the submerged macrophyte community (Section 2.1.1). Macrophytes are thought to disappear because they lose their energy supply in the form of sunlight penetrating the water. Following eutrophication, the sunlight is intercepted by the increased biomass of phytoplankton exploiting the high availability of nutrients. In principle, the submerged macrophytes could also benefit from increased nutrient availability, but they have no opportunity to do so because they are shaded by the free-floating microscopic organisms. Research in the Norfolk Broads has supported the view that the rapid replacement of diverse macrophyte communities by algal communities is attributable to light attenuation, caused by raised turbidity, but has also suggested that there may be more complex mechanisms operating, which must be understood if practical measures are to be undertaken to tackle eutrophication problems. There is evidence to suggest that either a plant-dominated state or an algal-dominated state can exist under high-nutrient conditions (Figure 3.5). Once either state becomes established, a number of mechanisms come into play which buffer the ecosystem against externally applied change. For example, a well-established submerged plant community may secrete substances that inhibit algal growth, and may provide refuges for animals that graze large quantities of algae. On the other hand, once an algal community becomes well established, especially early in the year, it can shade out the new growth of any aquatic plants on the bottom and compete with them for carbon dioxide in the water.
Figure 3.5 Probability plot of two stable states in shallow freshwater ecosystems. Over a broad range of phosphorus concentrations in the eutrophic-hypertrophic range, either state may potentially occur. However, once established, that state promotes processes that result in it becoming stabilized, and switches between the two states are only rarely observed.
Research in the Norfolk Broads into possible trigger factors for switches from communities dominated by submerged macrophytes to those dominated by algae suggests that pesticides could play a role. Some herbivores are thought to be susceptible to pesticide leaching from surrounding arable land. Pesticide residues in sediments were found at concentrations high enough to cause at least sub-lethal effects, which could reduce the herbivore population for long enough to reduce algal consumption. This could help to explain the observation that most of the Norfolk Broads that have lost their plants are directly connected with main rivers draining intensive arable catchments, whereas those that have retained plant dominance are in catchments where livestock grazing predominates.
Clear relationships can be seen between human population density and total phosphorus and nitrate concentrations in watercourses (Figure 3.6). In 1968 the anthropogenic contribution amounted to some 10.8 g N per capita per day and 2.18 g P per capita per day. Outputs have continued to rise since then. Worldwide, human activities have intensified releases of phosphorus considerably. Increased soil erosion, agricultural runoff, recycling of crop residues and manures, discharges of domestic and industrial wastes and, above all, applications of inorganic fertilizers, are the major causes of this increase. Global food production is now highly dependent on the continuing use of supplementary phosphates, which account for 50-60% of total phosphorus supply.
Figure 3.6 (a) The relationship between human population density and the concentration of phosphorus (P) in rivers across Europe. (b) The relationship between the percentage of catchment in agriculture and the concentration of nitrogen (N) in the same European rivers.
Studies of nutrient runoff have shown a mixture of inputs into most river and lake catchments: both point source (such as sewage treatment works) and diffuse source (such as agriculture). Point sources are usually most important in the supply of phosphorus, whereas nitrogen is more likely to be derived from diffuse sources.
Using the data presented in Figure 1.13 and Table 2.3, comment on whether the remediation activities on the broads neighbouring the River Ant were likely to have resulted in a recovery of plant species diversity by 2000. Assume that 80% of the total phosphorus in the water is in the form of SRP.