Aquatic invertebrates

Overall vulnerability

Feature assessed:

• Aquatic beetles and invertebrates

Special qualities:

• Internationally important and locally distinctive wildlife and habitats

Feature description:

Aquatic invertebrates are a broad category, covering all non-vertebrate animals living in PDNP streams and ponds. This assessment focusses mostly on freshwater insects, insects whose larvae have an aquatic stage, and native crayfish. Caddisflies, stoneflies, and mayflies are common freshwater species, their larvae sharing habitat with dragonfly nymphs and water beetles. The native white-clawed crayfish can be found in some PDNP streams and still water bodies. Other invertebrates present in the PDNP include freshwater bivalves such as the swan mussel, water snails, and freshwater shrimp. Some crustaceans can be found living in the subterranean waters of PDNP caves and mines.

Freshwater invertebrates can be found in most water bodies in some form. Species with high dispersal potential, such as those with a flying adult stage, being able to make use of even isolated and ephemeral water bodies. Rivers and other watercourses contain the most diverse assemblage of invertebrates, being a large and well-connected habitat. These areas are where crayfish and larger invertebrates will be found. Dewponds are also important invertebrate habitat, being some of the only areas of standing water in the White Peak.

Freshwater invertebrates play a vital part in the ecosystem, an essential link in the food chain of freshwater as well as terrestrial systems. Fish and other aquatic animals depend upon freshwater invertebrates for their food. The flying adult stage of some freshwater invertebrates is also an essential part of the diet of many birds, such as the pied flycatcher. Freshwater invertebrates are also an invaluable indicator of water quality, a healthy community indicating absence of pollution or nutrient enrichment.

How vulnerable are aquatic invertebrates?

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

Aquatic invertebrate populations in the PDNP likely mirror the national decline, with many species threatened by pollution and invasive species. As aquatic organisms, these invertebrates depend on good water quality, and so they are most sensitive to climate change effects that reduce water quality such as increased siltation and pollution. Changes in annual water cycles, such as altered flow rates and drying out of some habitat, are also likely to have a significant effect. Aquatic invertebrates have a moderate adaptive capacity due to their high diversity and dispersal, and may benefit from economic and institutional efforts to improve water quality.

Current condition:

Freshwater invertebrates are threatened globally, with 25% of species under threat of extinction, though sparse data for the PDNP makes it difficult to assess whether this is the case locally. Rivers and streams in the PDNP are heavily impacted by their proximity to human population centres and agricultural land, mostly in the form of pollution and sedimentation. The main bodies of standing water in the White Peak are dewponds. Despite being sheltered from some human impacts, they are still at risk of loss due to abandonment or poor management. As many as 50% of dewponds have been lost since the 1970s, and more may be lost if not maintained.

Invasive species are a persistent problem, especially for crayfish. The signal crayfish has displaced native white-clawed populations and spread crayfish plague. As a result, white-clawed crayfish have been lost from most areas of the PDNP. Only a few small populations are still present in some streams and still water bodies. Introductions to so-called ‘ark sites’ are underway. Other invasive crustaceans such as the zebra mussel, killer shrimp, and demon shrimp have also been found in PDNP waters. Invasive Himalayan balsam is a persistent problem, shading areas near riverbanks and increasing siltation of the water via bank erosion.

Pollution of PDNP waters is pervasive, and has a greater immediate effect on invertebrates than larger freshwater species. Rivers such as the Derwent have their headwaters in the Dark Peak, where heavily contaminated and acidic peat is washed into the watercourse. Heavy metals in the peat are soluble in acidic water and so can be carried downstream through the PDNP. As these heavy metals can be toxic even at low concentrations, their effect is felt far from the source.

Runoff from agricultural land can carry with it a variety of harmful chemicals, which are washed into streams to the detriment of invertebrate populations. Pesticides such as neonicotinoids are designed to kill insect crop pests, and so have a pronounced effect on the insect part of the aquatic assemblage. Sheep dip has played a similar role on pastureland, with the now banned cypermethrin able to cause damage many kilometres downstream. Herbicide use has been harmful, increasing nitrogen availability and encouraging algal blooms, making watercourses unsuitable for many freshwater invertebrates. Fertiliser runoff, including nutrients from slurry, has had a similar effect in the form of nitrate pollution but can also cause phosphates to enter the watercourse. Phosphates increase the growth of diatoms and make river sediments and water quality unsuitable for many invertebrates.

Pollution near population centres, especially downstream of wastewater treatment facilities, is a particular problem. Pharmaceuticals can have detrimental effects on invertebrates, with some chemicals commonly found in household products with anti-microbial properties affecting smaller invertebrate species and bioaccumulating in larger ones. Microplastics, found in the same areas, may also cause similar problems with bioaccumulation. Phosphates can also be released from industrial sites and wastewater treatment sites, having the same effects as fertiliser runoff on water quality and river sediment. Accidental releases of industrial chemicals such as washing materials have the greatest negative effects, eliminating invertebrates from some areas temporarily.

Silt can enter the watercourse from various sources, including runoff from agricultural land and roads. As many PDNP roads follow the course of rivers, this may be a significant effect. High siltation can damage invertebrates in numerous ways including burial, abrasion, and clogging of gills. This runoff can also contain de-icing chemicals during winter that can reduce diversity downstream for some distance.

Modifications such as weirs and dams that change river course and flows can change invertebrate communities and prevent the spread of invertebrates upstream. Impoundment of water behind these structures also creates poor habitat for invertebrates, and build-up of sediment limits benthic riverbed habitat. Bridges can also be detrimental to some invertebrates, slowing the spread of adult mayflies upriver. Conversely, they may also provide good habitat for caddisflies. Artificial light can draw adult mayflies away from watercourses, preventing them from completing their life cycle. Roads can also have this effect to a lesser extent by reflecting horizontally polarised light.

What are the potential impacts of climate change?

Overall potential impact rating

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Direct impacts of climate change

Increases in annual average temperatures may have a significant effect on freshwater ecosystems. Higher water temperatures will at best change the species composition and at worst cause the loss of many invertebrate species. Spring macroinvertebrate numbers could be reduced by about 20% for every 1⁰C rise, and a 3⁰C rise could result in a reduction of over 40%. Some species may be replaced by invertebrates better adapted to the new conditions, meaning that ecosystem function may persist but community composition is altered. As climatic ranges contract upwards in altitude and latitude, the PDNP may prove an important refuge for those species lost in more lowland environments. Data Certainty: High

Changes in annual flow patterns could affect freshwater invertebrate populations in numerous ways. Increased winter rainfall would cause a higher flow rate in most rivers. Invertebrate communities are sensitive to flow rate, so this could be detrimental to some species and cause a change in the community composition. Conversely, reduced summer rainfall and increased drought would have the opposite effect: slower flows and reduced habitat would favour different species to those benefitting from faster winter flow. Variable systems, such as the Lathkill and other ‘vanishing rivers’ would be affected most by these changes. Drought may also dry out ponds, especially dewponds with low natural water input. These ponds may become ephemeral, and lose some of their invertebrate community. Species with high dispersal ability and those able to survive seasonal drying out may persist. Reduced water levels in ponds also results in lower dissolved oxygen content, which can have a significant effect on invertebrate communities. Data Certainty: Low

Sedimentation or erosion

Drier ground conditions in summer followed by severe summer storms and increased winter rainfall is likely to lead to greater erosion and runoff. Runoff from bare ground in pastoral and arable land, as well as from roads via drains would increase siltation and sedimentation in water bodies. Siltation damages freshwater invertebrates and reduces habitat, especially shallower benthic habitat, by settling on river and pond beds. The speed at which this material enters the water system will also likely increase, reducing the potential to remove it by natural or artificial means. Data Certainty: High

An increase in summer wildfires would compound this effect. Fires open up bare ground and introduce silt in the form of ash from burnt plant matter, potentially increasing siltation and sedimentation during the summer months, especially in the Dark and South-West Peak. Data Certainty: Low

Nutrient changes or environmental contamination

Increased atmospheric carbon dioxide and nitrogen deposition may lead to a higher growth rate in riverbank plants. This may have effects on freshwater invertebrate populations due to increased shading of some sections of the riverbank. Shading of the water leads to a different invertebrate community, meaning some rivers would have a changed species assemblage. However, the extent to which this would be a problem is unclear, as some shading of the riverbank is good for macroinvertebrate communities, and shading may offset some of the warming effects of climate change. Increased plant growth may also lead to increased nutrient input from fallen leaves. However, this effect would likely be negligible compared to the existing issues of nutrient input from agricultural land and wastewater. Eutrophication could also increase due to contamination, which can decrease freshwater invertebrate populations greatly, especially mayflies. Data Certainty: High

Reduced summer flows and lower water levels in ponds may amplify existing issues in the water bodies. A smaller body of water, moving more slowly through the system, would have higher concentrations of both nutrients and pollutants. The negative effects of these inputs would be heightened, leading to reduced invertebrate abundance and changes in community composition as tolerant species outcompete sensitive species. Eutrophication may also increase in the warmer, nutrient loaded waters. Conversely, higher winter flows could cause more flushing of the river systems, leading to lower nutrient availability during the winter months. Data Certainty: Low

Increased runoff due to winter rainfall may also increase nutrient loading of water. This could occur through increased fertiliser entering the watercourse, and eroded soil deposited in rivers and streams. This again would alter invertebrate communities, and enhance other effects such as eutrophication. Data Certainty: Low

Invasive or other species interactions

Increased annual average temperatures may lead to invasive species moving into the PDNP and increasing in prevalence. Those invasive species already present that are from warmer climates will have an even greater competitive advantage. Species that are not yet invasive may become so due to conditions that are more favourable. Data Certainty: Low Increased flooding may connect water bodies more regularly and over greater distances. This would provide greater opportunities for invasive species present in one water body to colonise new areas. This increased spread would allow invasive species to replace native species over a greater area of the PDNP. Data Certainty: Low

Human behaviour change

Hotter, drier summers may lead to increased abstraction of water to supply nearby towns and cities. This could be in the form of more demand on existing installations or creation of new installations to keep up with demand. This would lower flow rates in some areas and provide physical barriers to dispersal for some invertebrate species, changing the community composition and fragmenting habitat. Some rivers may also be culverted, diverted, or concreted for flood prevention. This would reduce the habitat available to most species and further reduce connectivity. Models suggest that caddisfly populations have a variable response to climate change mitigation measures, with some worse off. Data Certainty: Low

Other indirect climate change impacts

Increased average water temperatures may also have effects on oxygen and carbon availability. Warmer water holds less dissolved oxygen and organic carbon, meaning that conditions are likely to change for freshwater invertebrates as temperatures rise. Some invertebrates require high oxygen levels so these species could be lost, but microbial activity is also heavily dependent on oxygen and carbon levels. As a result, many small invertebrates could have reduced food availability as the bottom of the food chain is reduced. This change in the microbial community would have knock on effects on all trophic levels, leading to reduced abundance and change in community composition of freshwater invertebrates and the species that depend on them. Data Certainty: Low

What is the adaptive capacity of aquatic invertebrates?

Overall adaptive capacity rating

Freshwater invertebrate populations can recover from high levels of devastation, mostly through their high dispersal ability. Studies from abroad have shown that populations can recover from complete loss in around a year. Pond invertebrates adapted to small and ephemeral habitat will be much faster to recover than river species such as white-clawed crayfish, especially as these have already been lost from many rivers. Data Certainty: High The diverse assemblage of freshwater invertebrates found in the PDNP will bolster their recovery ability. Data Certainty: Low

Freshwater invertebrate habitats have generally high connectivity. Although freshwater systems could be seen as fragmented, especially when it comes to ponds and other small water bodies, the invertebrates themselves often have excellent dispersal. Some species, such as the white-clawed crayfish, will only be able to travel short distances between water bodies. Others, such as some insects with flying adult stages will be able to colonise any suitable habitat. Artificial structures such as dams, weirs, and bridges create some barriers to dispersal, but can be removed or modified. Data Certainty: Low

Some funding is available for habitat works, meaning water quality can be improved at a catchment level. The current complexity of environmental land management schemes may mean that the incentives for landowners to take part are limited. Catchment partnerships may be a good resource to influence whole system management. The withdrawal from the EU Common Agricultural Policy may provide an opportunity to improve payment for public goods, and therefore ecosystem services. Data Certainty: Moderate

Some institutional support is available for freshwater invertebrates in the form of projects such as ‘Crayfish in Crisis’ in the South-West Peak. Strong legislation in the form of the European Union Water Framework Directive and strict regulatory processes monitor water quality and can enforce better practice, although this only applies to main rivers. Institutional guidance on management will be vital to success in conserving these habitats. Data Certainty: Moderate

Water management will be important for invertebrate populations in the future. Flow-slowing measures will generally benefit invertebrates, but artificial flood prevention measures can fragment habitat. Not enough is known about conservation of freshwater invertebrates to be fully effective in response to future changes, but it is likely that whole catchment practices will be needed to be effective. Data Certainty: Low

Key adaptation recommendations for aquatic invertebrates:

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.

• Contamination of watercourses must be taken seriously, and efforts made to reduce the entry of fertilisers, slurry, pesticides, pharmaceuticals, road runoff, and wastewater pollutants.
• Changes or modifications made to river channels (for example for renewable energy sources) must be made with aquatic invertebrates in mind.
• Floating dead wood and woody debris is invaluable invertebrate habitat. Allow natural processes to occur in river systems, and allow for a less ‘tidy’ looking environment.
• Conduct surveys of invertebrate communities across the PDNP. Dewponds may be of particular interest.
• Remove impoundments and allow natural meandering where feasible and retro-fit artificial berms or meanders to improve natural sediment transport.
• Buffer strips to trap sediment before it reaches a watercourse should be established wherever possible.
• Combatting the spread of invasive non-native species will be key to the recovery of many invertebrate populations, especially the white-clawed crayfish.
• Continue dewpond restoration work.
• Encourage further uptake of environmental land management schemes by farmers within the PDNP.

Improve current condition to increase resilience: Increase structural diversity to improve resilience at a landscape scale

The current condition of a feature is an important factor alongside its sensitivity and exposure, in determining its vulnerability to climate change. These recommendations focus on increasing the structural diversity of the area or habitat in which the feature is found. This can help to offset the effects of climate change on the feature, as well as to allow it to be in a better position to recover from future climate changes.

• Establishment of more native trees around riverbanks will assist not only with flood management and water quality but also shade the river and cool the water, although too much shade may be detrimental to aquatic plant life. Increased tree and scrub cover in upland catchments would also be beneficial.
• A greater diversity of habitat surrounding rivers will be beneficial to the invertebrate populations. Variability in shading, as well as wetness of the surrounding habitat is desirable.