Reservoirs

Overall vulnerability

Feature(s) assessed:

• Reservoirs

Special qualities:

• Internationally important and locally distinctive wildlife and habitats

Feature description:

Reservoirs not only store drinking water for millions of people, but are home to various plant and animal species on both a permanent and seasonal basis. Aquatic plants, fish, insects, and birds all thrive within this habitat. Within the PDNP, there are 46 reservoirs covering more than 1100 hectares. 90% (42) of these reservoirs are found in the Dark Peak. These reservoirs are locally important for birds such as breeding common sandpiper and black-headed gulls, as well as rare mosses such as the dwarf bladder-moss which is found at a few sites.

How vulnerable are reservoirs?

Reservoirs in the PDNP have been rated ‘high’ on our vulnerability scale. This score is due to high sensitivity and exposure to climate change variables, coupled with a moderate current condition, and a moderate adaptive capacity.

Land management practices, human behaviour, and invasive species currently affect the water quality in reservoirs and the species that live in and around them. Reservoir water levels, flow rates and quality may be compromised by climate stressors influencing sites at the catchment level. Habitat for wildlife in and around the reservoirs is likely to change. Reservoirs naturally fluctuate with weather events giving them good ability to recover. However a decline in reservoir water quality continues to pose the biggest threat to people and wildlife that rely on it.

Current condition:

Information on the condition of reservoirs as a wildlife habitat is sparse. Of the few reservoirs that have been assessed against the Water Framework Directive (WFD) within the PDNP, the water quality is mostly moderate, suggesting improvements can still be made.

Catchment run-off containing microbes and pesticides affects the colour and nutrient load in the reservoirs. Colour and dissolved carbon are a concern in the Derwent Reservoir along with pesticides in the River Derwent. Severn Trent is working with Moors for the Future Partnership to improve water quality through moorland restoration work and with farmers to reduce environmental contamination risks. Pesticides, bacteria and parasites such as cryptosporidium are a threat to people and also to other animals that use these reservoirs. Minimising sediment load and pollution risks in and around upland reservoirs is an ongoing process that is being carried out by water companies and land owners. This will ultimately benefit both people and wildlife.

Reservoirs are affected by land management practices right across the catchment. Nutrient and microbial enrichment is determined by fertiliser and slurry application, while erosion levels are influenced by stock numbers and machinery use. Intensive farming practices are particularly harmful to reservoir water quality as both ground and surface water makes its way downstream to reservoirs. Wildlife found in and around reservoirs is affected by water quality changes as well as disturbance. Recreational use of reservoirs for fishing, hiking, and cycling can interfere with the activity of some species. For example, common sandpipers at Ladybower Reservoir are vulnerable to disturbance in the breeding season. One study has shown that frequent disturbance can affect choice of nesting sites and the size of the breeding population, however breeding success rates at this reservoir were unaffected. Bird populations at smaller reservoirs may be more vulnerable as they have a smaller area of habitat available to which they can retreat.

Invasive and nuisance species are another threat to reservoir habitats. Plants are particularly problematic. New Zealand pigmyweed is present at Ladybower Reservoir, threatening native plant and animal life. Other reservoirs have large stands of rhododendron growing round the edges which can lead to acidification of the water and lower wildlife diversity. Water transfers from one waterbody to another are a potential risk for spreading invasive species. Risk assessments by water companies are in place to check both rivers and reservoirs before these transfers happen in order to mitigate this risk.

What are the potential impacts of climate change?

Overall potential impact rating

Nutrient changes or environmental contamination

Hotter, drier summers and wetter winters will affect the flow rates of water going into reservoirs. In summer the catchment is likely to stay drier for a longer period with low flow rates and decreased flushing. This may lead to higher concentrations of contaminants, increased risk of eutrophication, higher biological oxygen demand, and a greater frequency of algal blooms. In winter, erosion and run-off are likely to increase. Acid and heavy metal run-off from the uplands may further compromise water quality and increase eutrophication risk. The combined effect could be a change to the habitat and the species that are able to survive in it. A loss of plants could affect aquatic life, particularly fish that rely on them for food and water oxygenation. Data Certainty: Moderate

Intense flow in storm events could also affect reservoir water quality. Run-off from farmland and erosion of peat may increase nutrient load and heavy metal concentrations in reservoir water. While these effects will vary from catchment to catchment, reservoir condition under the WFD could deteriorate. Ecological status is based on both chemical and biological assessments, with the final result based on whichever has the poorer condition. Data Certainty: Moderate

Atmospheric pollution may affect water chemistry as well as growth rates of plants both in and alongside reservoirs. Increased carbon dioxide could cause the acidification of water bodies. Accelerated plant growth could lead to increased eutrophication that becomes harder to reverse. Reservoir habitats are likely to be altered, with varying impacts on plants and animals that depend on them. Data Certainty: Moderate

Sedimentation or erosion

Hotter drier summers and periods of drought are likely to create warmer, shallower reservoirs with increased erosion in the surrounding catchments. Lower rainfall and increased evaporation may lead to changes in the thermal stratification of the water and lower dissolved oxygen levels. Phosphorous may also be released from the sediment back into the water due to deoxygenated conditions, increasing the risk of eutrophication. All of these reservoir habitat changes affect the wildlife that lives in and around them. Aquatic species, such as fish and invertebrates, may be affected by the reduced aquatic habitat and lower oxygen levels. Birds may benefit as it becomes easier to catch aquatic prey or they may be negatively affected as their food sources decline. Data Certainty: Moderate

Wetter winters with increased rainfall and erosion are also likely to affect reservoir water levels. Storage demands may become higher as larger volumes of water fall over shorter periods. Increased silt build up could reduce the total reservoir storage available for these peak rainfall times. Plants in the drawdown zones, such as mudwort and shoreweed, may be affected. Flood risks are likely to increase, threatening neighbouring and downstream areas. Data Certainty: Moderate

Human behaviour change

Reservoir water levels during drier summers are likely to be significantly lowered by demand for water. Water levels will already be low during drier or drought conditions, and will be further decreased by increased demand for water from the nearby towns and cities. PDNP reservoirs are already struggling to meet demand in dry years, and this pressure will likely only increase with climate change. Data Certainty: Low

Recreation near reservoirs is likely to increase as temperatures get hotter. Areas near water and shade could be in greater demand by a range of visitors such as anglers, sailors, and hikers trying to escape the heat. Increased visitor numbers around reservoirs will mean increased pressure on the surrounding infrastructure, and more frequent disturbance of local wildlife. Birds, such as common sandpipers, are particularly vulnerable in the breeding season. Repeated disturbance could affect breeding outcomes at some reservoirs. Data Certainty: Low

Land use decisions affect reservoirs and their neighbouring habitats. With increased floods and droughts forecast, land may become better suited to other uses. New crops may be introduced bringing with them new pesticides and changes to ploughing regimes. Water quality of reservoirs could change as a result. More harmful pesticides may deteriorate aquatic habitat condition and have an adverse effect on wildlife. Conversely lower toxicity pesticides or lower application rates may help improve water quality downstream. Intense ploughing regimes pose an increased risk of erosion and reservoir siltation. In other areas land may be abandoned, allowing scrub to develop. Increased vegetation cover may help wildlife habitat and diversity improve both on the edges of reservoirs and in the reservoirs themselves. Data Certainty: Low

Invasive or other species interactions

Warmer temperatures year round may aid the spread of invasive or nuisance species. Plants could benefit from a longer growing season and increased winter survival rates. Native aquatic and riparian plant species may be lost as more aggressive plants move in to take their place. New Zealand pigmyweed is already present at some reservoirs, and could expand its range which would threaten the ecological condition of reservoir habitats. Data Certainty: Low

Increased disturbance during storm events may create opportunities for invasive plants to colonise. Erosion of stream banks and gaps created in woodland adjacent to reservoirs could become vulnerable to invasion from species such as rhododendron. Vegetation changes surrounding the reservoir have a follow-on effect to the water quality of the reservoir itself. Some species, particularly those that cover a large area, may decrease the ecological condition of the reservoir. Data Certainty: Low

Other indirect climate change impacts

Hotter, drier summers are predicted to lead to an increased risk of wildfire. Many reservoirs in the PDNP are surrounded by plantation woodlands which are at risk. Loss of woodland adjacent to reservoirs would mean the loss of a key habitat utilised by birds and insects that thrive on the water’s edge. Debris from wildfires in land surrounding reservoirs is also likely to lead to an increase in sedimentation and changes to water quality. The combined effect of changes on land and in the reservoir itself will see a change in the available habitat and the species that can thrive there. Data Certainty: Low

What is the adaptive capacity of reservoirs?

Overall adaptive capacity rating

Reservoirs in the PDNP are highly variable in shape and size. The limited storage capacity means that in periods of heavy rain and flooding, water reaches a maximum depth and storage options are restricted. Reservoirs are however by their nature more resilient than river extraction as they have water storage in dry periods. Due to the high density of existing reservoirs and topography of the area, there is limited space for the creation of new ones. Data Certainty: Moderate

Wildlife living in and around reservoirs has varying levels of dispersal ability. Fish and rarer plants are the most limited as they are often restricted to specific sites. Some reservoirs such as Ladybower are stocked with fish every year so they are less at risk from a climate stressor damaging the population. Birds are less restricted as they can move to other sites, however breeding sites may be limited for some species, such as the black-headed gull which currently only breeds at Longdendale. Loss of habitat may lead to a change in the bird species present. Woodland damaged by wildfire or storm events can be re-established, but the time it takes to grow back into a mature forest may affect species that utilise this habitat. Plants that grow on the edges of reservoirs in the drawdown zones should be able to move to new drawdown zones as long as the substrate remains similar. Data Certainty: High

Reservoirs have a natural ability to recover from drought and flooding events provided regular checks are carried out on infrastructure. Water quality in reservoirs can potentially recover from change more readily than wildlife - impacts on which may be severe and long-term. Data Certainty: High Reservoir habitat quality is determined by management of the reservoir itself and management of the land surrounding it. Legislation is in place for the continual improvement and monitoring of water quality in the UK, under the WFD. The Environment Agency oversees this monitoring and identifies local issues that are contributing to poor water status. Water companies have their own monitoring and management plans in place for their reservoirs. Drought trigger zones are in place to help manage water levels at different sites. Adaptability in water use behaviour during dry periods can also help prevent water depletion. Data Certainty: High

Financial resources are available for improving water quality at the catchment level. However schemes to improve habitats are often complex and there is limited incentive for private land owners to participate. There may be opportunities to improve existing schemes and compensation for ecosystem services in the future. Water companies also offer funds to support catchment level projects, such as the Boost for Biodiversity Fund available from Severn Trent. Water companies are also able to manage supplies and move water within the PDNP and import water in extreme scenarios. Reservoirs therefore have some protection from dangerously low water levels. Data Certainty: Moderate

Key adaptation recommendations for reservoirs:

Improve current condition to increase resilience

The current condition of a feature is an important factor alongside its sensitivity and exposure, in determining its vulnerability to climate change. These recommendations are aimed at improving the condition of the feature at present, therefore making it better able to withstand future changes to climate.

• Continue reservoir management of water levels with strict usage policies in place for drought periods.
• Continue catchment management to improve vegetation cover to reduce erosion and nutrient run-off entering reservoirs.
• Continue monitoring and control of non-native species in reservoirs and in neighbouring habitats.
• Improve access and recreation infrastructure to minimise habitat damage and wildlife disturbance.
• Increase riparian tree cover at reservoirs lacking in riparian vegetation, shade can help regulate reservoir water temperatures.
• Minimise agricultural inputs, especially slurry, fertilisers and pesticides. Give consideration to good management of waste to improve catchment quality, including effective slurry store management.

Improve current condition to increase resilience: Targeted conservation efforts for important sites and at risk areas

The current condition of a feature is an important factor alongside its sensitivity and exposure, in determining its vulnerability to climate change. These recommendations are conservation measures aimed at those sites that will have the biggest impact for this feature – either because they are particularly important for the feature or because they are most at risk from climate change.

• Strengthen biosecurity measures by raising awareness: install signs at busy recreation sites, particularly where fishing and boating are common.
• Manage disturbance levels during bird breeding season, for example have access restrictions to vulnerable sandpiper territory.