What is a Deep Lake
‘Deep lakes’ range from small bodies of water in sheltered valleys to large tectonic basins like Loch Ness in Scotland. In the temperate zone, the simplest definition of a deep lake is a body of water that is deep enough, or sufficiently sheltered, to remain thermally stratified during the summer.
Why are ‘Deep Lakes’ important.
Many of the environmental problems encountered in deep lakes are similar to those in shallow systems, but there are also specific issues connected with their seasonal division into two layers. Deep lakes are physically more complex than shallow lakes and are often situated in catchments that are topographically diverse. Features that merit special mention include the facts that:
- their physical structure is influenced by the combined effects of the sun, the wind and the rain
- oxygen depletion can occur at depth affecting life there and leading to nutrient release from sediments into the overlying water
- any nutrients that have accumulated in hypolimnion (lower level) during the summer can be drawn up into the epilimnion (upper level) to sustain the growth of phytoplankton
- many are situated in scenically attractive areas and consequently attract large numbers of tourists.
- significant number are reservoirs designed to generate power or supply water to communities.
Threats to ‘Deep Lakes’
Most deep lakes in the UK are located in mountainous areas but their catchments are usually sufficiently diverse to include soil and rock types that buffer the effects of acid deposition. When the impacts of acid deposition were most severe in the 1970s, the larger lakes in the English Lake District suffered little damage. Acidification issues were reported from some upland tarns but these impacts have since been ameliorated. Their rate of recovery is, however, slow and may in future be compromised by changes in climate and land-use.
Many deep lakes in the UK are still subject to enrichment by high levels of N deposition and by the increased inputs of phosphate. Some progress has been made in the reducing the inputs from point sources but controlling the inputs from diffuse sources in the catchment has proved more challenging. Often, the first signs of enrichment are those connected with a change in the composition of the phytoplankton. As lakes become more productive, blooms of cyanobacteria (blue-green algae) become more common and their impact is increased when they float to the surface and accumulate downwind.
- Land use and land management
Changes in land use can have a profound effect on lakes but these effects are often masked or amplified by year-to-year changes in the weather. The management of land in the UK is governed by a complex combination of soil, economic and political factors. In future, the quality of the water in some deep lakes could be adversely affected by the ‘wilding’ policies that are currently gaining ground. Planting trees in the uplands can lead to the drying of peaty soils which, in turn, can lead to the production of more coloured organic compounds which eventually reach the lakes.
- Climate Change
Deep lakes are known to respond in very complex ways to changes in the weather. Just a few hours of strong wind may be sufficient to mix the water column, change the underwater light regime and reverse the ‘normal’ pattern of phytoplankton growth (Reynolds, 1984). There are already signs that some lakes in the English Lake District are stratifying earlier in the year (George & Hurley, 2014) a process that can also disrupt some critical trophic interactions. A general review of the potential effect of climate change on lakes in the UK and Ireland was produced by George et al. (2010). The issues discussed ranged from the effect of dry summers on the leaching of coloured water in the uplands to the droughts that can limit the management options in lowland reservoirs. One area of increasing concern is the impact of ‘extreme’ weather events on the dynamics of lakes. Current climate models suggest that very dry summers and very wet winters will become increasingly common but it is not known when or where these impacts will be most severe. In recent years, the Jet Stream that regulates the weather patterns experienced in the UK and can control lake temperature (Strong & Maberly 2011) has remained in its southerly ‘winter’ position for much of the summer. This explains the recent sequence of wet and windy summers which have also delayed some of the ‘warm summer’ effects predicted by the climate models.
- EU Water Framework Directive (2000/60/E)
The Water Framework Directive was designed to broaden the scope of all water protection legislation and covers ground water as well as surface water. The directive requires all lakes over 50 ha in area to be restored to good ecological status by 2015.
- The Habitats Directive (92/43/EEC)
The Habitats Directive was designed to provide support for a network of protected sites and includes a number of lakes that have been designated on the basis of habitat type or the presence of rare species.
- Bathing Water Directive (2006/7/EC)
Most bathing beaches are located on the coast but a small number are situated on lakes. The revised directive introduced in 2006 was based on an earlier draft and requires Member States to monitor and assess bathing waters using at least two measures of faecal bacteria. Very few lakes in the UK have bathing beaches but there are three on Windermere and more in Scotland.
Principles and practices of ‘Deep Lake’ restoration
Many of the remedial measures proposed for ‘Shallow Lakes’ can also be applied to deep lakes but there are some exceptions. Since most deep lakes are large it is usually impractical to consider measures that involve the removal of sediment or the treatment of the bed with chemicals. A number of authoritative volumes on the principle of lake restoration have been produced over the years and many of the techniques recommended are applicable to deep lakes. A good introduction to the subject can be gained by consulting texts by Klapper (1991) and Cooke et al. (2005).
The main approaches used on deep lakes include reduction of nutrients from point sources by chemical or biological means (a volume published by Gray (2004) provides a useful overview of the available techniques and includes a chapter on natural treatment systems); the reduction of nutrients from diffuse sources (Smith et al. 2015); hypolimnetic water withdrawal to remove anoxic water (Nürnberg 1987); artificial mixing (Jungo et al. 2001) and food-web manipulation (Gulati et al. 1990 and Benndorf et al. 2002).
Reducing the flux of nutrients from point sources
Since the productivity of most deep lakes is limited by the supply of phosphorus, phosphorus stripping plays a key role in most restoration projects. Phosphate can be removed by treating the final effluent by a variety of chemicals but the most widely used are ferric/ferrous chloride or ferric/ferrous sulphate. The chemicals are typically added as the final stage in the process to ensure that most of the organic phosphorus delivered to the works has already been converted to orthophosphate. The sludge thus produced must be removed by settlement or filtration and the dried residues disposed of at a distant site. The amount of sludge produced by tertiary treatment is, typically, two or three times greater than that produced by a conventional systems. This may lead to capacity problems at the works and difficulties with the systems used for handling the dried sludge for disposal. A report produced by the Water Research Centre on behalf of the Environment Agency and English Nature (Mainstone et al. 2000) provides a useful overview of the technique. The report notes that the lack of any immediate ecological benefits should not be interpreted as a lack of progress since all natural systems take some time to recover from a phosphorus overload. This is particularly true for deep lakes where the anoxic hypolimnion provides a long-lived internal loading. The efficiency of the installed tertiary treatment plant may also change from week-to-week is exemplified by work undertaken on lake Windermere (http://www.ceh.ac.uk/windermere-uk-lake-restoration-case-study).
Limiting input of nutrients from diffuse sources
Inputs from diffuse sources, such as agricultural land and stockyards, have proved more difficult to control than those from point sources since they require more intensive monitoring. Two different approached are commonly used to estimate the flux of diffuse nutrients into lakes.
- An intensive programme of field measurements is implemented to measure both the volumes of water entering the lake through its main inflows as well as its chemical composition.
- A hydrological model, driven by historical weather data, is used in conjunction with some generic estimates of the nutrients leached from a range of specified land-use categories. An example of a ‘nutrient budget’ approach to calculating loads is provided by the Loweswater restoration (http://www.ceh.ac.uk/loweswater-uk-lake-restoration-case-study).
In situations where such intensive measurements are not available, simulation modelling provides the only option. The models currently available range from very simple procedures to modular formulations that can be adapted to meet particular need. Examples of the modelling approaches available are the Generalised Watershed Loading Function (GWLF) to model the diffuse as well as the point source loadings to a range of different lakes (Leavesley, 1999) and the MIKE SHE model used by the Danish Hydrological Institute, a much more sophisticated formulation that requires more input variables and more processing power. The modular structure of the model means that it can be used for a wide range of applications and also allows the user to focus on the processes that are of particular interest.
The report produced by Anthony et al. (2008) demonstrates that much more needs to be done to reduce the diffuse inputs into lakes distributed throughout England and Wales. The report includes maps to shows the distribution of different lake types, the annual loss of total phosphorus from agricultural sources and a more detailed analysis of the phosphorus losses in 2004.
Controlling the growth of cyanobacterial blooms
Many of the problems associated with the enrichment of lakes are caused by a change in the dominant species of phytoplankton and their physical behaviour. The most problematic species are the cyanobacteria (‘blue-green’ algae) that float to the surface and accumulate downwind. These blooms are unsightly and can often be toxic so they present a risk to bathers and to animals that drink the water. The factors that regulate the seasonal development of these species are well understood (Reynolds, 1993) but there is still a tendency to underestimate the role played by the weather. It is also important to remember that surface ‘scums’ can appear in unproductive lakes so calls for urgent ‘remedial measures’ can often be misplaced.
Most cyanobacteria only grow well under calm conditions and, since their rate of growth is low, they seldom become a problem until mid-summer. Where the water body concerned is not too large, the growth of such blooms can often be checked by artificial mixing. An example of an inexpensive mixing system that proved effective at limiting the growth of cyanobacteria in Lake Nieuwe, a Dutch reservoir near Amsterdam, has been described by Jungo et al. (2001). This has a surface area of 1.3 km2 and a maximum depth of 30 m and was commonly used as regional recreation area. Summer growths of Microcystis were, however, a persistent problem until a simple mixing system based on a bubble plume was installed in the lake in 1993. The compressor used to drive the system only consumed 85 kW of electricity but, in the following seven years, it reduced the biomass of Microcystis by more than an order of magnitude.
The system used to mix the Queen Elisabeth II reservoir in the Thames Valley was much more costly and was built into the reservoir at the design stage. Here, the pumps used to transfer water from the river into the lake were used to prevent the reservoir from stratifying, reduce the standing crop of phytoplankton and prevent the formation of any surface blooms. Records show that these pumping operations constrained the growth of Microcystis in the reservoir for several decades. In 2010, George et al. (2010) present evidence that they had become less effective in the 1990’s. This was partly due to the number of exceptionally warm summers recorded during the decade which also constrain the amounts of water that can be pumped from the Thames.
The systematic withdrawal of nutrient rich water from the hypolimnion has also proved effective in a limited number of cases. The oxygen content of the water overlying the sediments is thus increased and there is an associated reduction in internal loading from the sediments. This technique is most applicable to lakes that are used as reservoirs since these often have multiple outlet ports in the dam wall. A paper on hypolimnetic withdrawal as an aid to lakes restoration was published by Nürnberg 1987) and Rigosi and Rueda (2012) have recently used multiple discharges to improve the quality of the water abstracted from a Spanish reservoir. They showed that you could also change the qualitative composition of the phytoplankton remaining in the reservoir by selective withdrawal. The factors responsible for this shift were quite complex since they also included the subsequent change in the underwater light regime.
Since cyanobacteria only grow well when the epilimnion is not well mixed, installing some kind of ‘mechanical’ system to mix the water column can check the growth of these undesirable species. Such remedial measures can, of course, only be applied to small lakes or to larger water bodies that have been designed with mixing in mind. A good example of a low cost mixing system is that described Jungo et al. (2001) for Lake Nieuwe Meer in the Netherlands (see Restoration Techniques).
Shapiro (1975) was the first to recommend this biological remedial technique which he termed biomanipulation. The technique is based on the idea that when the numbers of planktivorous fish are reduced, large filter-feeding cladocera become more abundant and there is a consequent improvement in the clarity of the water. Unfortunately, the effectiveness of such manipulation has often been exaggerated since the technique has often been applied in combination with other remedial measures. A book published by Gulati et al. (1990) provides a more balanced view of the technique whilst a review produced by Benndorf et al. (2002) reconciled many of the old arguments.
The programmes used to assess the status of deep lakes should, ideally, be based on those recommended for the implementation of the Water Framework Directive. Some of these recommendations are, however, more difficult to follow for deep lakes; an obvious example being the methods used to sample and map submerged macrophyte. The methods used to assess their fish populations are also challenging and usually have to be combined with hydroacoustic monitoring. There are, however, some simple measurements that provide a very convenient indication of the lake’s trophic status. One of the most powerful indicators of the ‘health’ of a thermally stratified lake are the oxygen concentrations measured at various depths below the thermocline. A temperature / oxygen profile recorded in late summer is particularly informative since it provides a measure of the general productivity of the system.
An issue often discussed but seldom examined systematically, are the numbers and distribution of the samples required to characterise the ecological status of a lake. Samples collected at monthly intervals are often recommended as a good compromise between intensive sampling and occasional surveys but these can still miss some significant events. One study that looked at the effects of the influence of sampling frequency on the detection of change was the one described by George and Hurley (2004). When these time-series were sampled at weekly, bi-weekly and monthly and bi-monthly intervals the results showed that little information was lost by bi-weekly sampling but bi-monthly sampling produced unreliable results.
Another technique that can be used to reduce the cost of synoptic lake surveys is the surveillance of large areas from the air. To date, techniques like airborne remote sensing have not been widely used for lake monitoring but George (2012) has produced a review of potential applications. One, relatively recent development, has been the development of hyperspectral sensors that can identify different functional groups of algae. A paper that explored the potential use of the technique to detect cyanobacterial blooms was published by Hunter et al. (2010) but the cost of deploying such sensors is still a key issue. However, Earth Observation from space, especially as more capable systems are being launched, is opening new opportunities that are currently being explored (www.GloboLakes.ac.uk).
A web site established by the Centre of Ecology & Hydrology (www.ceh.ac.uk/our-science/projects/uk-lake-restoration) includes a number Case Studies on the restoration of lakes. Most of the examples are for sites that are shallow and well mixed but there are a few examples from ‘deep lakes’.
Windermere in the English Lake District
Windermere, the largest lake in the English Lake District and the largest natural lake in England, is divided into two basins by a large island and a region of shallows. The North Basin is less productive than the South Basin since it is surrounded by more mountainous land and is also very much deeper. There are two large sewage treatment plants on the shores of the lake. The one that serves Ambleside is located at the northern end of the lake and produces an effluent of a reasonably high quality. The one serving the busy towns of Windermere and Bowness located at the South Basin has often failed to cope with the summer increase in the number of tourists. In the early 1990s a tertiary treatment plant was installed at this plant and the subsequent recovery of the lake documented in a booklet produced by Pickering (2001).
In recent years, there have been signs that these improvements have not been maintained although the precise cause may be linked to changes in the lake’s food web triggered by climate change, possibly in concert with as well as to the quality of the effluent discharged. Wet summers produce dilute sewage that is more difficult to strip and may produce volumes of sewage that exceed the plant capacity. This problem has recently addressed by building a much bigger storage tanks for the storm water but it is too early to say what effect this will have on the trophic status of the lake. Read more at – http://www.ceh.ac.uk/windermere-uk-lake-restoration-case-study
Loweswater in the English Lake District
Loweswater is a small lake in the north west of the Lake District that is surrounded by relatively rich agricultural land. The catchment covers an area of 8 km2 and is managed by thirteen land-users with oversight from the National Trust. The farms rear sheep and beef and also run self-catering units for tourists. For years, there were signs that the ecological status of the lake was declining since blooms of cyanobacteria were becoming more common.
Waterton et al. 2006 describe how these issues were addressed by the consultation with the end-users. The paper demonstrates the value of a collective approach but also shows that there are conflicts of interest. The paper noted that some schemes planned to improve the environment had unfortunate consequences. At Loweswater, payments made by the Environmentally Sensitive Area (ESA) scheme allowed the farmers to buy more land outside the catchment. In winter, the cattle grazed on this land were returned to the catchment where they increased the phosphorus from the sheds and stock yards. Read more at – http://www.ceh.ac.uk/loweswater-uk-lake-restoration-case-study
Deep Lake Reference List
Anthony, S., Duethmann, D., Turner, T., Carvalho, L. and Spears, B. (2008). Identifying the gap to meet the Water Framework Directive: Lakes Baseline. Final Report. DEFRA. 59 pp.
Benndorf, J. Boing, W., Koop, J. and Neubauer, I. (2002). Top-down control of phytoplankton: the role of time-scale, lake depth and trophic state. Freshwater Biology, 47. 2282-2295.
Cooke, G.D., Welch, E.B., Peterson, S. and Nichols, S.A. (2005). Restoration and Management of Lakes and Reservoirs. CRC Press, Boca Raton, 616 pp.
George, D.G. (2010). The Impact of Climate Change on Lakes. Springer, 507 pp.
George, D.G. (2012). The effect of nutrient enrichment and changes in the weather on the abundance of Daphnia in Esthwaite Water, Cumbria. Freshwater Biology, 57, 360-372.
George, D.G. (2012). Using airborne remote sensing to study the physical dynamics of lakes and the spatial distribution of phytoplankton. Freshwater Reviews, 5, 121-140.
George, D.G. and Hurley, M.A. (2004). The influence of sampling frequency on the detection of long-term change in three lakes in the English Lake District. Aquatic Ecosystem Health and Management, 7, 1-14.
George, G., Jennings, E., and Allott, N. (2010). The Impact of Climate Change on Lakes in Britain and Ireland. In: The Impact of Climate Change on European Lakes (Ed. Glen George), Springer, Aquatic Ecology Series, 359-386.
Gray, N.F. (2004). Biology of Wastewater Treatment. Imperial College Press, London, 1444 pp.
Gulati, R.D., Lammens, E.H.R.R. and Meyer. M.C. and van Donk, E. (1990). Biomanipulation: Tool for Water Management. Proceedings of an International Conference held in Amsterdam, The Netherlands, 8-11 August 1989. Springer, 640 pp.
Hunter, P.D., Tyler, A.N., Carvalho, L., Codd, G.A. and Maberly, S.C. (2010). Hyperspectral remote sensing of cyanobacterial pigments as indicators for cell populations and toxins in eutrophic lakes. Remote Sensing of Environment, 114, 2705-2718.
Jungo, E., Visser, P.M., Stroom, J. and Mur, L.R. (2001). Artificial mixing to reduce growth of the blue-green alga Microcystis in Lake Nieuwe Meer, Amsterdam: an evaluation of 7 years of experience. Water Science and Technology: Water Supply, 1, 17-23.
Klapper, H. (1991). Control of Eutrophication in Inland Waters. Ellis Horwood, Hemel Hemstead, 400 pp.
Leavesley, G.H. (1999). Overview of models for use in the evaluation of the impact of climate change on hydrology. In: Impacts of Climate Change and Climate Variability on Hydrologic Regimes. (Ed. Jan C. van Dam. Cambridge University Press, UK.
Nürnberg, G.K. (1987). Hypolimnetic withdrawal as lake restoration technique. Journal of Environmental Engineering, 113, 1006-1017.
Pickering, A.D. (2001). Windermere: Restoring the health of England’s largest lake. Special Publication No. 11, Freshwater Biological Association, Far Sawrey, Ambleside. 126 pp.
Reynolds. C.S. (1984). The Ecology of Freshwater Phytoplankton. Cambridge University Press, Cambridge, 396 pp.
Reynolds, C.S. (1993). Scales of disturbance and their role in plankton ecology. Hydrobiologia, 249, 157-171
Rigosi, A. and Rueda, F. () Hydraulic control of short-term successional change in the phytoplankton assemblage in stratified reservoirs. Ecological Engineering, 44, 216-226.
Rouen, M., George, D.G., Kelly, J.L., Lee, M.J. and Moreno-Ostos, E. (2005). High-resolution water quality monitoring systems applied to catchment and reservoir monitoring. Freshwater Forum, 23, 2-37.
Shapiro, J., Lamarra, V. and Lynch, M. (1975). Biomanipulation: An ecological approach to lake restoration. In: P.L. Brezonik and J.L. Fox (eds). Proceedings of a Symposium on Water Quality Management through Biological Control, University of Florida, Gainesville, 85-96.
Smith, L., Porter, K. and Hiscock, K. (2015). Catchment and River Basin Management. Routledge.
Strong C. & Maberly S.C. (2011). The influence of atmospheric wave dynamics on the surface temperature of lakes in the English Lake District. Global Change Biology, 17, 2013-2022.
Waterton C., Maberly S.C., Tsouvalis J., Watson N., Winfield I.J., Norton L.R. (2015).
Committing to place: the potential of open collaborations for trusted environmental governance. PLoS Biology 13(3): e1002081.doi:10.1371/journal.pbio.1002081.
Waterton, C., Norton, L. and Morris, J. (2006). Understanding Loweswater: Interdisciplinary research in practice. Journal of Agricultural Economics, 57, 277-293.