Threats to UK Freshwater Habitats

 

UK freshwater habitats and their associated species are threatened by a range of factors. The table below provides a summary of the major threats – details are given below. These are based on information in the 3rd UK Report on Implementation of the Habitats Directive and the UK Biodiversity Habitat Action Plans.

 

 

Eutrophic standing waters

Mesotrophic lakes

Oligotrophic and dystrophic lakes

Aquifer-fed naturally fluctuating water bodies

 Ponds

 Rivers

Pollution

*

*

*

*

*

*

Abstraction and flow regime

*

*

*

*

*

*

Invasive alien species

*

*

*

 

*

*

Recreation

*

*

*

 

 

 

Fisheries management

*

*

 

*

*

 

Climate change

*

 

 

*

 

*

Grazing/control of scrub & trees

 

 

 

*

*

 

Morphological alterations

 

 

 

 

*

*

 

Pollution

Freshwater threatsA well-known problem affecting the freshwater environment is pollution. This can threaten the quality of all categories of freshwater habitats. Pollution can come from:

  • point sources, for example pipes discharging effluents from industrial sites, wastewater treatment plants or mines, sewage tanks, and fish farms;
  • non-point sources, i.e. widespread diffuse pollution, for example land-use activities such as farming, forestry and urban areas; this type of pollution generally exceeds that from point sources.

Individual pollutants include pesticides, poisonous metals and other chemicals, and spillages of slurry or milk. Agricultural fertilisers, including animal manures, can cause eutrophication (nutrient enrichment), which can result in excessive algae and weed growth.

Increased levels of eroded silt can have polluting effects. Silt erosion can be produced by ploughing of grassland, drainage associated with upland afforestation, and peat-cutting on moorland catchments. Eroded silt increases sediment and nutrient loads. This can inhibit the growth of rooted aquatic plants, promote algal growth, and smother the bottom of rivers and lakes, damaging fish spawning beds.

Freshwater threatsWater acidification is also a factor in catchments where the underlying soil/bedrock offers limited buffering capacity. Although there has been a general decline in the acidifying effects of sulphurous air pollution, nitrogen oxide deposition remains an issue as a cause of acidification and eutrophication.

 

Abstraction and flow regulation

Water is abstracted from freshwater habitats to supply drinking water, for agricultural irrigation, and for industrial usage. It can be taken directly from a water body or indirectly via the ground water or from underground aquifers. In some places, dams have been constructed to allow for water abstraction. These modify river flows and can causing environmental problems downstream. Abstraction and flow regulation can exacerbate pollution by concentrating pollutant levels and limiting subsequent flushing.

Although some seasonal variation in water availability is normal, abstraction can significantly reduce water levels. In extreme cases, it can cause a lake, pond or river to dry up. This can have devastating impacts on wildlife, although in some aquatic habitats (turloughs and temporary mere and ponds) it is a natural phenomenon in summertime.

 

Freshwater threatsInvasive alien species

Alien species are non-native plants or animals. Some of these have become invasive in native freshwater ecosystems. As a result, they have displaced resident wildlife and, in some cases, had serious economic impacts. 

Examples include Australian swamp stonecrop (Crassula helmsii) and parrot’s feather (Myriophyllum spicatum). Both have rapidly colonised and changed the ecology of small standing water habitats. Lowland oligotrophic lakes are particularly vulnerable to invasions by Crassula. This now occurs in >10,000 sites in England and is spreading in Scotland and Wales. At present, there is no effective means of controlling it.

The invasive signal crayfish (Pacifastacus leniusculus) is native to North America. It has spread across 25 European countries, including the UK, having been introduced in 1976. Not only has it eaten large amounts of aquatic vegetation, but eliminated many populations of native crayfish by spreading crayfish plague.

Complete eradication of invasive species is costly and difficult. Preventing them establishing in the first place is by far the most cost-effective approach.

 

Freshwater threatsMorphological alterations

Freshwater life has evolved to occupy a range of micro-habitats. This includes the edges and deeper water in lakes and ponds. On rivers this includes the bankside vegetation, pools and riffles, and areas of exposed sand and shingle. Changes to the physical structure of a freshwater body, notably river canalisation or dredging of lakes and ponds, can alter the availability of such habitats and result in substantial damage. Weirs, dams and barrages can alter water and sediment movements, and may impede passage of migratory fish such as salmon.

 

Recreation

There are a number of recreational activities that can damage freshwater habitats. Boats can damage vegetation directly or through wave erosion. This can also increase turbidity by stirring up sediment, contributing to enrichment and encouraging the growth of algae.

Freshwater threatsRecreational and sporting use of lakes can create disturbance affecting bird and mammal populations. Marginal vegetation can suffer from trampling. The construction of marinas and other leisure facilities may destroy valuable habitat and can lead to increased pollution. Angling and associated fisheries management (see below) is a widespread problem for many lowland lakes and ponds.

 

Fisheries management

Introduction of fish and manipulation of existing stocks can damage freshwater wildlife interests in various ways. It can reduce resident fish and many other animal and plant populations. By altering the structure of the food web, it can increase predation of small invertebrates that graze algae. Fish threaten amphibian populations by eating their tadpoles.

Heavy stocking of bottom-feeding fish, especially carp, results in increased disturbance of sediments. This increases turbidity and mobilises nutrients, which encourages algal blooms and cause other fundamental changes.

 

CFreshwater threatslimate change

Climate change is likely to have significant impacts on freshwater habitats. Changes in the amount, timing and distribution of rainfall and run-off are bound to affect the character and ecology of lakes, ponds and rivers. Higher intensity rainfall could increase sewer overflow rates, lead to more severe flooding and increased erosion. Summer droughts would reduce river flows and increase abstraction demands. A rise in water temperature would have wide-ranging effects, causing changes to food-web structure and a general increase in fish and algal and other phytoplankton growth.

 

Grazing/control of scrub & trees

An appropriate level of grazing is important to maintain the open character of turloughs and temporary meres and ponds. For many lowland ponds, some periodic cutting of marginal scrub and trees is required to control shading; this should normally be undertaken piecemeal basis.