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.

Figure 4.7 Weed harvesting methods.

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

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.