BIODIVERSITY PACK HABITAT GUIDE LAKES

Lakes, broadly defined as standing waterbodies of over 2 hectares in surface area, exhibit great variety in terms of size, depth, productivity and characteristic species. They are typically classified across a gradient of productivity; from nutrient poor ‘oligotrophic’ lakes common across the uplands, to the naturally more nutrient rich ‘eutrophic’ lakes typical of the lowlands.

Freshwater and terrestrial habitats enhance our many freshwater habitats is excavation and reservoir construction. are formed by the flow and retention of intrinsically linked; with works to rivers and Man-made water bodies can have similar water in the landscape. Their nature is lakes able to benefit wetland habitats, and physical, chemical and biological properties determined by landform and hydrological vice versa. Delivery on a catchment scale to natural lakes, so their origin does not pathways, the characteristics of the water can take account of these synergies, and preclude them being of conservation value. supply, and climatological and biological can look to secure opportunities to achieve influences which generate a mosaic of biodiversity benefits across the full range of Those that are naturally functioning (in rivers, lakes, wet , reedbeds, habitats present within a catchment. terms of water chemistry and quality, and other habitats of various degrees of hydrological regime, wetness and types of hydrochemistry. morphology WFD AND and biological The UK Biodiversity Action Plan (published B2020 assemblages) in 1994), described the biological resources Man-made water provide the best and of the UK which were identified as SYNERGIES: bodies can have similar most sustainable being the most threatened and required VERY physical, chemical and expressions of conservation action – our priority species biological properties freshwater habitats and habitats. Detailed plans set out actions ACHIEVABLE and the biodiversity to protect and restore our threatened to natural lakes, so they support. Some wildlife, and work continues today, as a Targets for Good their origin does not artificial systems key part of the delivery within Biodiversity Ecological Status under cannot operate 2020 and the Water Framework Directive the Water Framework preclude them being in this way due to (WFD). Across catchments, action to Directive place a focus of conservation value their very nature, on the biology (aquatic requiring constant plants, and intervention to © Ben Hall/2020VISION invertebrates), physico- maintain their chemistry (nutrients, dissolved oxygen, biodiversity interest. Examples include: acidity), hydrology and morphology of our lakes. Status improvements under WFD can • Lakes generated by impounding make a contribution to all targets under rivers; these prevent natural river Outcome 1 of Biodiversity 2020 which calls habitat function, act as silt traps and for Priority Habitats to be maintained in will inevitably become in-filled and favourable condition, degraded ecosystems eutrophic; to be restored, and areas of importance for • Reservoirs with artificial hydrological biodiversity and ecosystem services to be regimes or highly artificial margins; safeguarded. these features restrict hydrological connectivity with adjacent wetland LAKES IN A habitats. CATCHMENT CONTEXT These systems remain valuable for the species they support, and are particularly important where no good-quality natural GLACIAL LANDSCAPES CONTAIN MANY Lakes form naturally via a range of LAKES FORMED AFTER THE LAST ICE AGE hydrological and geological process, such as standing water habitats remain. However, if glacial deepening, whilst others have been naturally functioning lakes can be restored, created by human activity such as mineral these will provide a more sustainable

1 BIODIVERSITY PACK: LAKES habitat to support this biodiversity in a Lake substrate varies naturally with lake type TROUT MAY FEED IN LAKES, MOVING OUT way that also allows natural functioning of and size, with large water bodies having INTO RIVERS TO SPAWN other adjacent habitats, supporting the full a greater fetch and therefore exposure. range of biodiversity at the landscape scale. Sheltered shores and deeper water are dominated by fine silts and muds, whilst NATURAL ECOSYSTEM coarser particles dominate exposed shores. Seasonally exposed muddy banks are FUNCTION IN LAKES important for plants such the rare cut-grass, and a number of priority invertebrates, Natural water quality is the most whilst clean gravels that do not dry out important requirement for a lake to provide spawning grounds for fish. support a natural biological community. This includes nutrients, acidity, oxygen, Lake water levels fluctuate naturally; some lack of other pollutants and colour. Under dry out completely, such as the aquifer- natural conditions, the fed fluctuating nutrient status of the meres in Breckland, water should reflect and others hardly the geology, soils and Natural water quality fluctuate at all. Bare vegetation of the is the most important substrate (rock, catchment, giving rise pebbles, sand or silt) to a range of lake types requirement for a lake exposed by water from nutrient poor to to support a natural level drawdown rich. There will also be provides habitat for a strong correlation biological community insect larvae and between alkalinity their associated © Alexander Mustard/2020VISION and nutrient status, as predators. both originate from the Water levels also surrounding geology; readily weathered determine the hydrological connectivity rocks will lead to both higher nutrient of lakes; when levels are high they may AT A GLANCE GUIDE concentrations and higher alkalinity. This is be naturally connected, and as they fall reflected in work undertaken for the Water may become isolated (either permanently LAKES Framework Directive which classifies lakes or temporarily), shaping their biological according to alkalinity and depth. assemblages. Lakes may be longitudinally Phosphorous and nitrogen connected by their inflows and outflows, or limit the productivity of a lake, In clear water lakes phosphorous and/ laterally by connections between a lake and determining abundance, type and or nitrogen limit the productivity of the its surrounding . system, determining the abundance, diversity of AQUATIC PLANTS type and diversity of aquatic plants. Connectivity results in the expression of a natural hydrosere; this transition from Light availability also limits productivity; particularly for lakes in peaty catchments fully aquatic (downslope) to terrestrial Lake water-levels fluctuate where organic matter makes the water communities (upslope) is critically naturally. Rocks, sand and silt brown, limiting light penetration and dependent on natural water levels and exposed by ‘draw-down’ provide resultant biomass. shorelines. The extent of the hydrosere habitat for INSECT LARVAE will depend on the morphology of the lake Acidity varies with geology, with and the surrounding topography and will photosynthetic activity (CO2 uptake encompass a range of water depths, light INVERTEBRATES, reduces acidity) and as a consequence climates, wave exposure and sediment and of hydrological events (via dilution or types. As well as being important habitats, , FISH concentration). It influences all chemical natural hydroseres also play a role in BIRDS feed, breed and take and biological processes in lakes, so can the maintenance of water quality and refuge among emergent vegetation impact considerably on water quality and dissipation of wave energy. found on the fine silts and muds of lake ecology. Plants dominate the littoral zone; sheltered shores Dissolved oxygen, essential for respiration, submerged stoneworts and pondweeds is another important aspect of water dominate the deeper areas, and are joined BACTERIA take advantage of quality. Water naturally holds less oxygen in the shallows by emergent species, organic matter that settles on the at higher temperatures, salinity and whilst periphyton (attached algae) may altitude, and concentrations will also dominate wave-exposed rocky shores. lake bed vary between lakes depending on depth, The littoral zone also provides habitat for season, productivity and exposure. invertebrates, zooplankton, fish and birds, The cold and dangerous open Organic material that settles on a lake bed which feed, spawn/nest and take refuge water zone is dominated by decomposes, consuming oxygen, and in amongst the vegetation, roots and deep lakes particularly this oxygen may not deadwood. Fringing vegetation aids the PLANKTON and ROACH be replenished as the lower waters become emergence of dragonfly and damselfly cut off from the atmosphere by the water species, and is essential for the terrestrial above, either as surface waters become part of the lifecycle of many organisms. In Migratory fish such as EEL and warm and less dense in summer, or when many smaller, very shallow lakes the whole LAMPREY require connections they freeze in winter. Low oxygen levels water body is essentially one large littoral zone to flowing waters to enable them impact fish like shelly and vendace. and there is little if any truly ‘open water’. to complete their life cycle

2 BIODIVERSITY PACK: LAKES © Zsuzsanna Bird

abundant, as are freshwater shrimps, molluscs and chironomids. Moderate nutrient concentrations, clear water, suitable oxygen levels, and a mix of substrates and emergent vegetation provide habitat for this diverse range of fauna.

Unimpacted nutrient-rich eutrophic lakes support a range of pondweeds and floating and submerged vegetation such as frogbit and greater bladderwort. Rich vegetation supports good numbers of water beetles, water bugs and molluscs, and a cyprinid- dominated fish assemblage, many of which require submerged vegetation for spawning.

A rare habitat type, Marl lakes such as the well-known Malharm Tarn occur in limestone catchments where calcium carbonate is precipitated out of the water FLOATING-LEAVED SPECIES LIKE WHITE as marl. Low nutrient levels in the water WATER LILY DOMINATE THE VEGETATION OF DYSTROPHIC LAKES, WHERE FEW limit floating algae, but dense charophyte SUBMERGED SPECIES CAN THRIVE beds (freshwater green algae that are structurally like plants) prevail in these lakes. Marl lakes often support white- clawed crayfish and abundant molluscs The open water zone of lakes is dominated water bodies may contain Arctic charr, eel, due to the high alkalinity, while the fish by phyto- and zoo-plankton, and the lake three-spined sticklebacks and trout. assemblage is largely dictated by the bed by bottom-dwelling animals and nature of the shoreline. bacteria able to take advantage of settling Most frequently found on hard, acid rocks organic matter. Many macro-invertebrates of the uplands, oligotrophic lakes are The Brackish lakes of coastal regions range such as water beetles avoid the deep open characterised by zones of rosette forming from oligotrophic to eutrophic depending water as it is both cold and dangerous, shoreweed, water lobelia and quillwort. on the geology of the catchment. They sticking to shallow warm waters and the Their waters are characteristically clear can support marine macro-algae such as protection of emergent vegetation. Fish and support plants at depths greater than serrated wrack, egg wrack and gutweed move freely between all the zones to 6 m, due to the naturally nutrient-poor as well as tasselweeds, some charophytes find food, shelter and suitable areas for conditions which limit algal productivity. and fennel pondweed. Brackish waters can spawning, although some such as bream Oligotrophic lakes support a predominantly support interesting assemblages of macro- feed mainly in the sediments, whilst others salmonid fish assemblage including trout invertebrates, particularly molluscs, water like roach feed in mid-water. Lakes can and, in the Lake District, potentially the beetles, water bugs and flies, including be less hospitable to amphibians (whose rarer Arctic charr, shelly and vendace. brackish-water specialists like the priority larvae are predated by fish), but in good These species require wave-washed coarse species lagoon sand shrimp. quality standing waters with sufficient substrates and relatively high oxygen structural marginal complexity and concentrations in the hypolimnion (cooler PRESSURES ON OUR extensive shallows to provide refuges, bottom waters). Characteristic insects these species can coexist. include mayflies, stoneflies, beetles and LAKES flies, and terrestrial ground beetles that Some species require lakes to be prey upon emerging aquatic insects. Lakes are generally not completely lost as hydrologically connected to flowing waters a result of anthropogenic impacts since to enable them to complete their life cycle. In some heathland areas of lowland their size often prevents this, although Eels, lampreys and trout may migrate into England like the New there are there are exceptions such as the draining lakes taking advantage of habitat and oligotrophic waters on sandy soils; these of Whittlesea . However, lakes can available food, whilst trout and Arctic charr rare lakes display elements of dystrophic be seriously damaged by pollution (point may move out into streams to spawn. and oligotrophic lakes and often support a source and diffuse, from air and water), Other species carry out their entire lifecycle rich community more usually physical habitat modification (mostly of the in one lake: this can lead to genetically associated with upland lakes. shoreline and littoral zone), hydrological unique populations, such as with vendace. modification (including abstraction and Mesotrophic lakes are the most botanically impoundment), non-native species and Dystrophic lakes, typically found in areas of diverse, supporting the rosette forming fisheries management. Currently only a peatland habitat, often have little submerged species common in oligotrophic waters as single lake in England, Burnmoor Tarn in vegetation because of their naturally peat- well as a range of taller growing species the Lake District, is at WFD ‘High’ status. stained water, although they can support such as pondweeds, and sometimes rare Sphagnum and floating-leaved species such plants like floating water-plantain and Nutrient enrichment leads to increases as white water-lily. Molluscs are often absent least water-lily. Similarly, fish assemblages in phytoplankton and reductions in water due to the naturally acidic water chemistry, are a mixture including both salmonids clarity and plant growth. Dead organic whilst dragonflies, beetles, water bugs and and cyprinids, and again within the Lake matter then accumulates, and is broken chironomids are abundant, accompanied District may include the rarer whitefish down by bacteria which use up oxygen, by copepods and cladocerans, some such as vendace. Mayflies and caddisflies reducing its availability to other lake specific to such waters. Larger dystrophic associated with aquatic vegetation are species; this oxygen depletion can lead to

3 BIODIVERSITY PACK: LAKES fish death. Termed ‘eutrophication’, the create impermeable alter communities and process generates turbid, algal-dominated barriers affecting add nutrients to a water lakes and has led to the loss of many both long Climate change is body, whilst fishing plants and animals from previously-diverse migrations, such as platforms and access water bodies such as the Norfolk Broads. those undertaken already altering routes can damage Community changes and species loss can by eels, and short environmental waterside vegetation. occur even at less-severely affected sites, migrations, such as conditions in UK lakes However, the presence and not just when water bodies become those sometimes of anglers can deter completely dominated by algae. undertaken by Arctic dumping, vandalism charr, to reproduce and illegal stocking, and On base-poor upland geology, there is a in tributaries. anglers can report water quality issues. legacy from air pollution where historically deposited sulphur is still leached from soils Shoreline reinforcement interrupts the Climate change is already altering and pH is recovering only very slowly. Where natural continuity of the substratum and environmental conditions in UK lakes, and water chemistry has begun to recover, moisture gradient, truncating or eliminating predicted extreme rainfall events and drier, biological recovery has been slight and not the hydrosere and directing wave energy warmer summers will continue to alter always predictable, with acid-tolerant species to other areas, causing erosion. Use of regimes of temperature, sediment and water being replaced by species not recorded the shoreline for activities such as boat quality to a considerable degree. Different before acidification, perhaps due to nitrogen moorings can impact plants directly lake types will be differentially sensitive, enrichment and/or climate change. (breaking, dislodging) and indirectly particularly to changes in rainfall, based on (shading and disturbing sediments). their depth and hydrological connectivity. Shoreline habitats can be impacted by Overall, the impacts of human activities in artificial water level fluctuations, shore Invasive and non-native species can catchments greatly reduce the ability of still reinforcement, , and also increased damage lake habitats, and predate or waters to cope with climate change. wave action and direct disturbance due to compete with other species. For example, increased use. Because of the value of these the feeding behaviour of the widely- The distribution and abundance of individual littoral zones, for biodiversity and other introduced common carp destroys species, including priority species, are ecosystem services, shoreline modification macrophyte beds and re-suspends affected by these anthropogenic impacts. can be considered among the most severe sediments, releasing nutrients to the water Whilst species characteristic of unimpacted threats to the integrity of lake ecosystems. column and contributing to eutrophication. conditions will generally suffer, some species Water bodies which are hydrologically will benefit. The change in flora associated Abstraction from water bodies or their connected are more likely to have invasive with nutrient enrichment is an example catchments, or drainage of surrounding species due to the ease of dispersal, as are of this. Moderate nutrient enrichment land, can lower water levels, decreasing a waters popular with visitors, indicating the can allow a naturally oligotrophic lake to lake’s area and reducing the extent of the role of humans as vectors of dispersal. support a greater number of plant species, hydrosere. Impoundments also reduce resulting in conflicting management water level fluctuations resulting in the Fisheries management can either promote perspectives if these new communities are loss of typical plant communities of the or put pressure on characteristic biological deemed to be of value. natural drawdown zone. Dams and weirs communities. Fish stocking and bait use can

KEY PRESSURES ON LAKES

ABSTRACTION: ENRICHMENT: HABITAT LOSS: Abstraction or Nutrient enrichment The modification of drainage from lakes can lead to increases water levels, shoreline or their catchments in phytoplankton and reinforcement, lowers water levels, dead organic matter, siltation and increased decreasing lake size which reduces water use all severely disturb and reducing the clarity, plant growth the littoral zone – a extent of the hydrosere and oxygen supply biodiversity hotspot

CLIMATE CHANGE: MANAGEMENT: INVASIVE SPECIES: Predicted extreme Fisheries management Non-native and rainfall events, may put pressure on invasive species can and drier, warmer characteristic biological impact characteristic summers will alter communities, add biological communities temperature regimes, nutrients to the through direct sediment and water water and damage competition or the quality in lakes waterside vegetation alteration of habitats

4 BIODIVERSITY PACK: LAKES KEY MANAGEMENT MESSAGES

© Mark Hamblin2020VISION • TAKING THE • RATIONALISING CHANGES LONG VIEW IN SPECIES DISTRIBUTION Whilst physical restoration may be AND ABUNDANCE needed to trigger recovery of lakes, The current distribution of many rare taking a longer-term approach allows (and more common) lake species is natural recovery to play the fullest limited as a result of previous habitat role possible. For instance, waiting loss or degradation. Plans for species for lakes to ‘flush’ out (or lock up conservation and ecosystem restoration in deep sediments) accumulated should therefore take into account the phosphorus after many years of (positive and negative) implications for pollution may be a better option species of the restoration of natural than expensive dredging. A long- processes, and of climate change. term vision encourages management Suitable habitat needs to be maintained decisions which are more sustainable, or created to prevent local or regional particularly if the seemingly extinctions and to aid species recovery. ‘immovable’ socioeconomic constraints Direct management, including of today may be resolved in the longer reintroduction, can also be considered term. to assist in the transition to restored RESERVOIRS AND GRAVEL PITS OFTEN LACK THE NATURAL MARGINS THAT environmental conditions. PROVIDE FEEDING OPPORTUNITIES FOR • SPECIES WADING BIRDS MANAGEMENT • SUCCESSION The artificial manipulation of fish Tackling increased sediment and assemblages (removal of planktivores nutrient loads is inherently more or carp, addition of piscivores) and sustainable than repeatedly • RESTORATION OF the introduction of macrophytes undertaking expensive removal of NATURAL PROCESSES have been widely applied to lake sediment to halt succession. Where restoration. Such biomanipulation natural succession/infilling produces Measures that seek to restore natural techniques are most likely to be new habitats of conservation value, processes in a lake and its catchment – successful after external nutrient it may be most appropriate to allow natural water quality, geomorphological reduction, and should be undertaken this to happen, particularly in areas and hydrological regimes – make with the long-term aim of restoring where careful restoration after mineral significant contributions towards lake native and appropriate mixed fish and extraction can allow the creation of restoration. Practitioners should seek plant communities. new early-successional water bodies. to understand the system as it would operate under natural processes and plan from that foundation, factoring © Amy Lewis in implications for related adjoining habitats. • LARGE-SCALE PERSPECTIVE The condition of a lake is heavily influenced by what is happening in the catchment and in the atmosphere above it. Restoring natural water quality, sediment and hydrological regimes is key – it is not only about addressing direct impacts within the lake itself. • TAKING ACTION IN THE RIGHT ORDER

Expensive interventions undertaken within the water body, such as sediment removal or biomanipulation, LAKESIDE FENCING MAY ALLOW will not deliver the greatest possible NON-NATIVE SPECIES TO TAKE HOLD IN biodiversity benefits unless pollutant UNGRAZED AREAS inputs are reduced first.

5 BIODIVERSITY PACK: LAKES © Linda Pitkin/2020VISION ARTIFICIAL STRUCTURES CAN HINDER THE MOVEMENTS OF MIGRATORY FISH SPECIES SUCH AS ARCTIC CHARR

• IN-WATER • WATERSIDE wetland biodiversity. Specialist advice can be valuable; for example, work STRUCTURES VEGETATION being undertaken by the Freshwater Weirs and dams have a range of Semi-natural waterside vegetation is Habitats Trust to identify Important‘ physical effects on lake habitats, block part of a fully functioning hydrosere, Freshwater Areas’ could inform local the free movement of some species, providing habitat for characteristic fauna, delivery. and particularly affect animals that stabilising the shoreline and reducing naturally use rivers and streams for part nutrient and sediment loads. Tree of their life cycle. To eliminate these roots and woody debris are particularly impacts completely, structure removal important for many invertebrates, and should be the aim wherever possible. tree cover provides shade, mitigating Modification to minimise impacts is against rising air temperatures. the next best option, preferably using a Shoreline fencing should be avoided by-pass channel, permeable to relevant where livestock grazing intensity can be priority species as a minimum. reduced to suitably low levels that avoid REFERENCES AND heavy damage to in-water and adjoining FURTHER READING • SHORELINE habitats. Alternatively, set-back fencing STRUCTURES should provide a sufficiently wide zone • A narrative for conserving to allow the development/maintenance freshwater and wetland habitats Dealing with these is a key step in of the hydrosere, with access for periodic in England restoring a naturally functioning water grazing or manual management. body with lateral connectivity to • Climate Change Adaptation the wider environment. Removal, or • UNDERSTANDING THE Manual – Standing Open Water modification to minimise all impacts of LOCATION OF EXISTING these structures, should be considered, FRESHWATER BIODIVERSITY e.g. soft engineering options. CONTACT US To maximise the benefits of restoration • SEASONALITY work, and eliminate damage to priority or endangered species, it is  catchmentbasedapproach.org Seasonally exposed sediments important to obtain a clear picture of support an array of characteristic flora the distribution of local freshwater @CaBAforum and fauna but can be destroyed by biodiversity, (indeed, this knowledge drainage, infilling or deepening. Natural is legally necessary for some species). @ [email protected] water-level fluctuations are essential Practitioners should take account of for their persistence. standing water, running water and

Edited by The Wildlife Trusts on behalf of the CaBA Biodiversity Group 2018. Design: lonelycottage.co.uk. Header image P1: © Charlotte Hall.