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Many shallow lakes in delta areas across the globe are characterized by a deteriorating water quality. This knowledge page gives background information on this change and provides insight into Building with Nature interventions to influence sedimentation and ecological processes.
Many different delta lakes can be found scattered across the globe. In spite of large differences, for example climatological zones, many similarities can be found. Of the large lakes (with a surface area greater than 500 km2) in the world, about a third have mean depths less than five meters and nearly half only reach mean depths of ten meters (Herdendorf, 1984). Some lakes have been artificially created, like the Dutch IJsselmeer and Markermeer, others have formed more naturally. Despite differences in location, surface area and depth, there are recurring issues when it comes to the ecological status of lakes.
Water quality is one of the most important and persistent issues. In many of the large delta lakes water quality has changed over the last century, mainly due to elevated nutrient levels. There are two main sources of nutrients in lakes: external (incoming nutrients from outside the system) and internal (nutrients stored in the lake sediment). External nutrient loads have increased over the last century, largely resulting from fertilizer inputs from agriculture and other human activities in the watershed. Besides, there are internal causes for eutrophication of delta lakes. One element in particular is responsible for the deteriorating water quality of lakes; phosphorus. Studies have shown that lake sediments can accumulate large amounts of phosphorus from excessive external loads (CISRERP, 2008). These large buffers of phosphorus can become a major internal nutrient source to the water column. For example, sediments in lake Okeechobee in the United States, produce an internal phosphorus load which is approximately equal to external loads on an annual basis (Jin and Ji, 2004). The potential release of these nutrients is greater under disturbance than under static conditions. However, whether sediments are either a source or a sink for nutrients depends on the local situation. If nutrient loading in the water column is high, lake sediment can act as a buffer and is able to store large amounts of phosphorus, up to 90% of the total amount of phosphorus present in the system. Another important source for internal loading of nutrients are the remains of algae, plants and aquatic wildlife. No matter the source – internal or external – an increased nutrient load can significantly disturb the natural balance of lakes, by or example stimulating algal blooms, which have the potential to disrupt ecosystems, by creating anoxic and shaded conditions.
Clear and turbid lakes
Theory on clear and turbid lakes
In general, two types of shallow lakes can be distinguished. One is characterized by high water transparency, dominance of aquatic vegetation, small amounts of nutrients and a diverse fish community. The other type is characterized by turbid water, with no or small amounts of aquatic vegetation, large amounts of nutrients, large seasonal algal blooms and a fish community which is dominated by large benthivorous fish species like bream. Both states can be stable under the same environmental conditions. Under such conditions, larger disturbances may cause the system to shift from one state to the other: a ‘regime shift’. The theory of alternative stable states explains why ecosystems (in general) can have multiple stable states.
Alternative stable states
The following overview of the alternative stable states theory has been provided by Ingrid van de Leemput, PhD candidate at the aquatic ecology and water quality management group of the Wageningen University. For a more elaborate insight please read the Wikipedia page dedicated to the topic, which provides an impression of the current knowledge.
A system may have alternative attractors, or states, if it has internal positive feedbacks that can amplify the effect of a disturbance. For example, a nutrient pulse in a shallow lake can increase algal biomass temporarily, which results in low water transparency, leading to increased competitive strength of algae over macrophytes, leading to increased levels of sediment disturbance, which lowers the water transparency even more. As a result of this cascade of processes, the system is trapped in a turbid situation even after the nutrient pulse has ceased. Similarly, temporary removal of benthivorous fish species in a turbid lake may trigger a shift to a clear state, through a decreased level of sediment disturbance, higher water transparency, and the growth of aquatic plants. In general, if a system that has multiple stable states is in a certain stable state, it tends to stay there, unless its driving factors exceed a certain threshold and push it to another stable state. Moving from one stable state to another and back may involve ‘hysteresis’, implying that the return to the initial state requires a stronger change of the driving factors than the one that triggered the regime-shift.
One way to explain the theory of alternative stable states is the ball-in-a-cup analogy (adjacent figure; Scheffer et al., 2001). In this representation the valleys in the stability landscape represent the different stable equilibriums; the hilltops unstable equilibriums .The ball represents the current state of an ecosystem. If the ball is situated in a deep valley, the resilience of that particular system is high, because a small perturbation will not be able to push the system out of its equilibrium. In contrast, if the valley is shallow, ecosystem resilience is low, and the system is more easily shifted by a disturbance. Environmental conditions, will alter the shape of the landscape, and thus the resilience of the system. For example, increased nutrient inflow into a shallow lake with clear water will disturb the state of the system. If the nutrient inflow reaches a critical level, the clear water state is not stable anymore, and the system shifts to a turbid stable state. This point is called a ‘tipping point’, and the state shift that occurs at this point is called a ‘critical transition’.
The adjacent figure shows the alternative stable states within an ecosystem. Environmental conditions, for example nutrient load in shallow lakes, can influence the stability landscape in an ecosystem. At low nutrient load only one state, the clear state, is stable, and a shift to a turbid state is not possible. However, when nutrient load increases, a stable turbid state appears, such that a disturbance, like a short-term nutrient pulse, can trigger a system-wide shift to the turbid situation. Then, if nutrient load increases, the resilience of the clear water state decreases (the valley becomes more shallow), and the system becomes more likely to shift. At even higher nutrient load, the clear state becomes unstable and a shift to the turbid state is inevitable. After a shift, the potential of recovery is small (hysteresis).
Alternative stable states in a complex world
So far virtually all theoretical underpinning of this view of ecosystem stability is based on models of homogeneous well-mixed systems. However, the complexity in real ecosystems is much higher: temporal and spatial scales at which processes and feedbacks occur vary, thereby influencing ecosystem resilience and the occurrence of alternative stable states. While experiments enable studying individual processes, and field measurements reveal the behaviour of the system in all its complexity, models may be used to simulate any combination of processes at any scale. In this way, models may provide a better conceptual understanding of the underlying mechanisms involved in complex system dynamics. Using models, it has been shown, for example, that heterogeneity of large shallow lakes decreases the likelihood of a catastrophic shift, and increases the likelihood of a gradual change. In other words, on a large scale, for example on a landscape with many small ponds or a shallow lake with many sub divisions, heterogeneity will ensure that each individual pond or sub division shifts at another driver level, such that the shift on the scale of the entire landscape or shallow lake seems gradual. Connectivity or dispersal of key species plays an important role, as it reduces heterogeneity. In highly connected systems the likelihood of a catastrophic shift is larger, and therefore decreases the potential for repair.
In spatially homogeneous systems with a relatively local connectivity, a front of collapse or recovery can move through space, like a domino-effect. Therefore, in truly homogeneous systems, alternative stable states cannot co-exist in space. If local dispersal rates of key species, or exchange rates of nutrients, vary in space, however, this moving front can come to a halt, leading to stable co-existence. As an example, one may think of a macrophyte field that gradually spreads in time over a shallow lake, but comes to a halt due to high mixing of turbid water in a certain area of the lake.
Spatially explicit models with alternative stable states also show that recovery time of local disturbances may be used as a resilience indicator, and therefore as an early warning signal. If resilience of a system decreases, so if the system comes closer to a tipping point, recovery becomes slower. If this model finding is true, one can monitor recovery of experimental local disturbances (i.e. removed vegetation in a spatial plot with a specific size) to determine whether a system is moving towards a tipping point.
Besides nutrient loads, high turbidity can be disruptive for lake ecosystems. Take for example the shallow lake Markermeer in the Netherlands, created in 1976 by damming the south-western part of the IJsselmeer with the so called Houtribdijk. The initial idea was to reclaim the lake and create new land in the vicinity of Amsterdam. Protests aiming to preserve this ecologically and recreationally important area, combined with a more complex (ground) water dynamics than initially thought, ultimately resulted in cancellation of the plans. The ecological value of the lake, however, started to deteriorate not long after its creation. Especially bird populations dropped significantly. Ecological problems are often multifaceted and difficult to analyse. An important factor in this change of the ecosystem was the complex sediment behaviour, which severely increased turbidity. Nowadays the Markermeer is characterized by unclear waters, locking the system in a turbid state. The diagram below describes the complex and dynamic interactions that drive sediment dynamics in the lake. (adjacent figure; Miguel de Lucas; 2012, under review).
Adjacent figure shows a conceptual image of the complex sediment dynamics of Markermeer. The image represents the sediment layer of lake Markermeer with the water column above. The numbers in the figure refer to the table and text below (Miguel de Luca, in prep).
Wind driven currents
Wind driven waves
Erosion and re-suspension of sediments
Bioturbation in the form of surface palletisation, and accretion of pellets on the bed
Bioturbation in the form of surface disruption
Primary production of algae in the water column.
Flocculation processes, including biotic-abiotic interactions
Settling of flocks and sediments particles
Diffusion of oxygen into the soil facilitated by burrowing of meio-fauna
Oxidation of sediments due to diffusion of oxygen
Reduction of sediments due to bacterial respiration
Availability of large quantities of soft, easily mobilized sediment in combination with strong wind dynamics in the relatively shallow lake are the main causes of the turbidity. Wind is the driver of the mobilization and mixing process. The relatively large surface (about 700 km2) of the lake makes it very susceptible to wind action. These winds induce waves and currents in the water column (resp. 1 and 2 in the figure). Wave and water current power continuously erode and suspend sediments into the water column (3), creating high levels of turbidity.
The sediment is loosened by bioturbation by meiofauna living in or on the lake bed, such as tubifex (4) and ostracoda (5). Algae living in the water column (6) may flocculate with the particles in the lake (7), creating large flocks that gradually become heavier and settle on the lake bed (8). As the layer of sediment is gradually growing thicker, the deposits are slowly consolidated and reduced. However, the time scales of these consolidation processes are still unknown. These more solid deposits are much more difficult to re-suspend than the loose and fluffy sediment. It is possible that, without burrowing meiofauna, the waters of lake Markermeer would be less turbid. However, as the worms create tunnels in the thick and more solid soil layer (10), oxygen can diffuse deep into the soil eventually even reaching anoxic layers. In the presence of oxygen, the sediments will gradually oxidate (11). Not the redox state of the sediment but the bioturbated state determines the bed strength. A loosened bed is much more susceptible to wave and current action (1 and 2). Without alterations to this system, this mechanism will keep the waters of lake Markermeer turbid.
Nutrient dynamics in delta lakes
The paragraphs below provide an overview of the close interaction between nutrient loads and sediment in lake ecosystems.
Nutrient loads on a lake ecosystem (mainly dissolved phosphate: PO4 3- play an important role in the dynamics of aquatic ecosystems. Lakes in the clear water state with an abundance of aquatic macrophytes can withstand high levels of phosphate whilst maintaining clear water. This is due to the fact that phosphate is mainly stored in the sediment and vegetation and therefore not accessible for algal growth. The maximum level of phosphate loading at which the system remains in a clear state is called critical phosphate loading.
When a shallow lake turns to a turbid state, phosphorus will mainly be stored in fish and algae. Both can have an important negative influence on water transparency and can cause a flux of soluble phosphorus from the bed sediment to the water column, through resuspension, excretion and release from the sediment as a result of anoxia. As discussed in the previous paragraphs, the critical phosphorus loading in turbid lakes – i.e. the loading which allows the system to return to a more clear state in which vegetation can grow – is much less than the critical phosphorus loading that in clear lakes.
Internal and external loading
As stated in the introduction, there are two main nutrient sources found in lake ecosystems: internal (from the lake sediment) and external (from incoming waters). However, high external loadings do not always have to result in a high concentration of nutrients in the water column. Sediments can act as a buffer and are able to store large amounts of phosphorus, even up to 90% of the total amount of phosphorus present in the system. In the case of internal loading, the phosphorus stored in the sediment is released to the water column.
Decomposition and nutrients
The organic material accumulated in the bed can be an important source of minerals. Dead and slowly decomposing plants, algae and other organic material in the water column sink to the lake bed. There it forms an organic layer called detritus. As bacteria decompose these organic components, soluble nutrients like nitrogen and phosphorus become available. Through mineralization, organic phosphate and nitrogen are converted into phosphate and nitrate. These nutrients are stored in the sediment and can gradually diffuse to the water column. Both algae and macrophytes are able to use these nutrients, thus closing this part of the nutrient cycle.
Nutrients and macrophytes
In most shallow eutrophic lakes macrophytes reduce the availability of nutrients in the water column as they take up nutrients from the water for their growth. Part of these nutrients is lost from the system as organic matter / plant detritus is buried in the sediment. Macrophytes are especially efficient in reducing the nutrient availability if the nutrient concentration in the water column is relatively high. This for instance is the case in lakes that are recovering from the turbid algae-dominated state (Kufel 2002). Macrophyte beds can reduce nutrient availability further, as these beds can trap suspended particles from the water and prevent their resuspension (Madsen et al. 2001).
In some cases macrophytes can act as a nutrient source. In temperate lakes a large part of the macrophyte biomass dies at the end of the growing season. Decomposition of this plant material can result in a temporary nutrient release. In lakes with low nutrient concentrations in the water column, macrophytes have been found to act as a nutrient source, also during the growing season. Under such oligotrophic conditions macrophytes derive their nutrients primarily from the sediment. Decomposition of old plant parts (leaf turnover) and nutrient leaching from the plant can form a flux of nutrients from the sediment into the water column in these oligotrophic systems (Graneli and Solander 1988).
Macrophytes are able to absorb nutrients from sediment through their roots, creating a small phosphorus difference between the sediment and water column. With lower nutrient levels in the sediment, only a relatively small amount will diffuse into the water column. Also by lowering nutrient availability, macrophytes create less favourable conditions for algae to grow and roots of macrophytes prevent settled algae on the bottom to suspend again and start producing in the water column. However, in changing conditions where clear lakes are becoming more turbid or for example the decay of leaves in autumn, the macrophytes population can decay. The decomposition of large quantities of macrophytes in combination with a limited amount of nutrient absorption to the sediment can result in a flux of nutrients from the sediment to the water column.
Nutrients and algae
Large densities of algae occur only in situations with large quantities of nutrients in the water column. As the algae grow and multiply to ever larger concentrations, phosphorus levels in the water column will drop. At a certain point, as the gradient grows, the balance between nutrients stored in sediments and those in the water column is disrupted. As a result, there will be a flux of phosphorus from the sediment into the water column until the equilibrium across the sediment-water barrier is restored again. This keeps the system in a turbid and highly productive state.
Turbidity and sediment
Turbidity in relation to the sediment
The stable state theory states that aquatic ecosystems can switch between clear and turbid conditions. The example of the turbid waters of lake Markermeer shows that turbidity does not always originate from high nutrient concentrations and associated algae growth. Continuous erosion and resuspension of soft bottom material, driven by wind action on the shallow and wind-exposed lake causes high turbidity and low light transparency of the lake waters (Kelderman et al. 2012). A gradual increase of the suspended sediment concentration / turbidity and the associated increase of light attenuation in the water column causes ecosystem deterioration (Vijverberg et al. 2011). Below several characteristics of a water body are described that influence sediment resuspension, giving insight into the complexity of sediment dynamics in this lake.
Type of sediment
The type of sediment present in a water body is one of the most important factors contributing to turbidity. In the Netherlands, a high diversity of substrates occurs, ranging from of gravel, sand, clay, sediment and peat in a variety of combinations. Dependent on grain size and density, several types of these substrates are especially susceptible to resuspension. The smallest particles, inorganic lutum (with a grain size of less than 2µm) and fine sediment (between 2 and 63 µm) are easily brought into suspension by wave and current action, especially in lake Markermeer with its relatively shallow water and its large wind fetch. Organic peat particles, although of large size, are also suspended easily because of their porous characteristics and low density. Sediments with larger grain sizes like sand (0,063µ - 2mm) do not contribute significantly to lake turbidity. They settle relatively fast after resuspension.
Fish and sediment
The resuspension of sediment particles in shallow lakes is often caused by feeding activities of bottom dwelling – or benthivorous – fish like bream and common carp sieving through the sediment in search of small prey like small crustaceans, worms, insects or snails. As the sediment is loosened by the foraging fish, it becomes more susceptible to resuspension by wind and waves. Benthivorous fish also contribute indirectly to turbidity via the release of soluble phosphate from sediment particles (desorption and excretion). Algae can take these elements from the water column, resulting in an increase in algal densities and subsequently in an increase in turbidity. Besides, with their sieving activities, these fish frequently disturb the bottom and thus prevent macrophytes to settle. These disturbances also keep sediments loose, making them more susceptible to other forms of disturbances.
Waves in relation to the water body
Wave and current action is an important factor in the resuspension of sediment. The actual resuspension depends heavily on wind speed, fetch, depth and type of bottom substrate of the lake. The fetch is the maximum distance (in the direction of the wind) that air can travel over the lake to produce waves. In general it can be stated that, with lower wind velocities and an increasing lake depth, the proportion of the lake where wave resuspension occurs decreases. Still, the actual amount of suspended sediment strongly depends on the type of sediment. Therefore, turbidity of lakes with similar fetch and depth can differ largely due to differences in the sediment composition. In lake Markermeer, for instance, there is a virtually unlimited supply of new fluffy sediment which is highly susceptible to suspension. This, in combination with the shallowness of the lake and its relatively large open water areas causes the high turbidity of the lake waters (Kelderman et al. 2012).
Macrophytes and sediment
Macrophytes in shallow lakes can prevent resupension of sediments and therefore promote water clarity. Several species (e.g. charophytes) form dense mats on the lake bottom making it difficult for waves and currents to suspend sediment and preventing fish to forage in the substrate. Wave action is also limited to the water column above the vegetation mats. In general, beds of macrophytes also act as a net sink for sediments and algae. Strands of macrophytes reduce wind-induced current, thus enabling small particles to settle.
Fluctuating water levels
Fluctuating water levels can also be a factor in increasing the transparency of shallow lakes. During periods of low water levels, iron available for binding phosphorus regenerates through oxidation on the emerged banks. If water levels rise again, the iron keeps on binding phosphorus and phosphorus levels and algal growth decrease, thus enhancing water transparency. Whether and to what extent this occurs and what impact it has on the entire lake ecosystem depends on many factors, such as water level variation, size of the lake and availability of iron.
Restoring ecological states
Measures to restore the ecological state of delta lakes
The ecological status of many of the delta lakes has deteriorated over the last couple of decades, due to anthropogenic and/or natural causes. Several strategies can be chosen to help restoring their ecological quality. Within Building with Nature we look for integrated solutions whereby infrastructure is planned, designed and operated whilst creating new opportunities for nature and at the same time utilising natural forces whenever possible.
As described in the section on alternative stable states, all agents in a system are interconnected and may create a stable system. Even though there are many differences between delta lakes across the globe, many of the issues are of similar origins. In this chapter we describe Building with Nature measures that address two major issues in lake Markermeer: high turbidity and associated ecological degradation. Of course, these problems cannot be considered separately, they are closely linked. Experiences gathered in this lake can be a showcase on how to deal with these problems in other shallow delta lakes. The paragraphs below draw upon ongoing – and often unpublished - research in the frameworks of the studies ‘Autonome Neergaande Trend (ANT, Autonomous Downward Trend) and ‘Natuurlijk(er) Markermeer - IJmeer’ (NMIJ, More Natural Markermeer - IJmeer). The following paragraphs give an overview of (preliminary) research results achieved in these frameworks.
Reducing sediment resuspension in lakes
Removing sediment from a turbid system – or at least preventing it from getting suspended – can initiate a chain of events that breaches the turbidity cycle described above. Sediment reducing measures enhance light transmission at certain places, which favours the establishment of aquatic vegetation that may bring the ecosystem into a stable clear state. Three measures to reduce the sediment lakes are described below: sediment trapping, shelter structures and sediment capping.
As described in a previous section, the availability of sediment in relatively shallow parts of a lake, combined with strong wave and current action, may result in turbid waters. Without any intervention, this sediment would be suspended over and over again. Dredging parts of the lake bed and thereby creating deeper zones where the fluffy material can accumulate may help reducing sediment availability (adjacent figure). Once settled in these sediment traps, where wave and current action is less, the sediment has the opportunity to consolidate and become less erodible. Less sediment availability helps to create more clear waters in – at least parts of – the lake. As described in the section on alternative stable states, a shift towards less turbid waters may push the system back into more favourable ecological state. Less turbid water creates opportunities for – depending the local situation – aquatic vegetation and mussels, which in turn attract several species of waterfowl. The adjacent figure shows a schematic overview of the functioning of sediment traps. Wave action can disturb settled sediment in this lake up to a depth of about 5 m.
Different designs can be considered, such as one large trap in the middle of the lake, a gully close to the shore, a series of pits, or pits in combination with shelter zones. The impact on the sediment content depends strongly on location and design of the pit(s).
Three aspects need to be considered in relation to sediment traps:
- Effectiveness for sediment capture. Focusing on the primary objective of reducing suspended sediment concentrations, a combination of pits with shelter structures may well be the most effective option. Sediment-trapping can be constructed in the ‘supply routes’ of sediment-laden water to a natural or manmade depression in the lake bed.
Ecological aspects. Based on ecological criteria one can draw a suitability map for sediment traps. The adjacent second figure shows such a map for lake Markermeer - IJmeer, based on the suitability of the area for aquatic vegetation, zebra mussels and waterfowl (the meaning of the colours will be explained below). One aspect to be considered is that in pits of more than 8 to 10 m deep the water will get stratified during the summer season. This temperature stratification often leads to anoxic conditions in the deeper parts, which has a large effect on fauna. When in the winter season the water gets mixed, again, the deep pits can provide wintering habitat for fish species.
The ecological effect of the sediment traps varies. Preliminary computations show that in the close vicinity of the pit visibility nearly always improves, with a noticeable effect up to 3 km downstream. If algae growth is light-limited, algal blooms may occur in the clearer water, but it is also possible for zooplankton to flourish. Piscivorous birds that rely on eye sight can profit from the newly created gradients. The effect on bottom-dwelling species is less clear. The adjacent figure shows the suitability for sediment traps based on the suitability of the area for aquatic vegetation, zebra mussels and waterfowl. The green zone is not suitable for mussels, the blue zone not for aquatic vegetation, whereas mussel banks would be located too deep to be reached by waterfowl. This means under certain conditions there is some potential for the creation of sediment traps in the blue zone. Red is the area with the highest ecological potential; suitable for aquatic vegetation and mussels within reach of feeding birds.
- Hydrological consequences. Depending on the location, creating sediment pits can have severe hydrological consequences for the surrounding area. For example, creating a pit in lake Markermeer with a depth of ten to fifteen meters could impair the integrity of specific water-impermeable soil layers. This could affect ground water tables, and therefore indirectly increase seepage in a region far beyond the lake’s edge.
Of course, money is also an important determinant for the construction of sediment traps. The most cost-effective locations are often found near sites where sand or clay is needed. So, in determining a good location for sediment traps, effectiveness, ecology, hydrology and costs need to be taken into account. Another important characteristic of sediment traps, is that they are of a temporary nature. As time progresses the created depressions will gradually fill up with sediment. Even though this measure may have helped removing sediment from the system, new supplies of sediment can increase turbidity again as the pit fills up. It was shown that depending on local conditions, sediment traps can fill up rapidly in lake Markermeer – the smaller ones up to 2 m per year (Natuurmonumenten 2012). Removing the accumulated sediment may therefore involve considerable and recurring costs.
The primary objective of shelter structures is to reduce sediment concentrations in specific parts of a lake. They function in two fundamentally different ways;
- By acting as a breakwater structure reducing wave action, a sheltering structure can reduce resuspension of sediment in its lee.
- By guiding currents, the structure can divert currents and reduce the transport of suspended solids from the open water into the sheltered area. If applied on a large enough scale, applying this measure can influence the current and transport patterns in the entire lake.
A sheltering measure should not only reduce local resuspension, but also the influx of suspended solids in order to be successful. Like other turbidity-reducing measures, shelter structures may serve multiple purposes. If successful in reducing turbidity, aquatic vegetation is enabled to recover, which results in a better and more diverse food supply for waterfowl. The submerged vegetation will eventually provide habitat and spawning grounds for fish, which will attract pescivores.
The effect on the sediment concentration in the lee of a shelter structure can be considerable, and – if constructed in the right location – it can serve a large area. Impact size and impact area are determined by location, positioning, shape and local hydrological conditions.
The adjacent figure shows the results of a model study on the effects of different shelter structures – in this case a series of islands or a large dam – on the turbidity of the Markermeer (Visser and Vijverberg, 2010). It shows the outcome of a model study on the impact of small islands (left) or a large dam (right) in the Markermeer. The colours represent the change in water turbidity as a result of the structures, ranging from red (high turbidity) to blue (low turbidity) (Visser and Vijverberg, 2010). This figure shows that location, size and shape are important factors determining the effect of shelter structures. Analysis has shown that these types of structures can be effectively applied all over lake Markermeer. However – as the structures aim at increasing transparency and therefore improving the ecological status of parts of the lake – the most interesting locations can be found in the more shallow zones of (fourth figure, left) where waters are already less turbid. Adjacent figure shows the depth of lake Markermeer (left) and vegetation cover of the lake (right).
Clear waters do not automatically yield aquatic vegetation; depth, substrate and limited disturbance – from birds and recreational activities – are equally important factors. During the summer months lake Markermeer and IJmeer have about 30.000 ha of shallow water with a maximum depth of 2.7 m. In terms of depth, this is an important zone for aquatic vegetation. Shelter structures should be used to shield off these zones and improve water clarity there.
Plant growth in zones recently shielded from sediment is quickest if in the area there is already some vegetation from which colonisation can start. The fourth figure (right) shows locations alongside the coast that already have some aquatic vegetation. The west side of the lake is therefore the best location for shelter structures.
The Markermeer – IJmeer area already has a number of shelter structures (Gouwzee, Hoornse Hop, Pampushaven, Muiden, ice breakers along the Houtribdijk, breakwaters Oostvaardersdijk and natural developments in lake Eemmeer). They can provide valuable information on their effects. Monitoring of turbidity, aquatic vegetation, zoobenthos and fish species in these sheltered areas has shown that water transparency is good and that submerged vegetation often flourishes, providing habitat to fish. Vegetation composition heavily depends on the depth gradient in combination with water turbidity. Mussel populations settle usually in deeper parts with less vegetation. The sheltered zones are used as resting or breeding areas by several bird species.
Like many shallow lakes in densely populated delta areas, the Markermeer area is not only valuable for its nature: recreation is also an important factor. Abundance of aquatic vegetation can cause a conflict of interest with recreation like sailing and surfing. Others do not want the open character of the area to be disrupted by – for example – trees growing on small islands. Therefore it is recommended to try and combine shelter structures with recreational facilities like boat docks or small restaurants. Such combinations can increase public appreciation and support for these measures. Another option, of course, is to create submerged structures, which can have a similar effect on turbidity, but are not visible at the water surface. But even then, aquatic vegetation may still be a problem for boating and surfing activities.
Another way of reducing sediment concentrations is by covering the source with a less fluffy substrate. This can have two major impacts:
- By capping the sediment, the source of sediment is isolated and kept from continuous resuspension. Capping also prevents the creation of new fluffy material by erosion or bioturbation of the clayey lake bed.
- An additional positive effect is that a less fluffy substrate improves habitat suitability for mussels (Kleijn et al. 2006).
In theory capping is a very effective measure to reduce turbidity, especially if the entire lake is tackled at the same time. From a more practical perspective, however, this measure is difficult to execute. The sheer size of most delta lakes makes it impossible to construct the cap everywhere at the same time, which means that a capped area will soon be covered with fine sediment from the vicinity. On top of that, the costs of such a system-wide measure would be large. From a technical and financial perspective, sediment capping in delta lakes is therefore not feasible.
Yet, smaller parts of a lake could be suitable for capping, especially areas that are sheltered from sediment input from the vicinity. Sediment capping in these parts can cut off the local source of turbidity and improve local conditions. Local-scale sediment capping may lead to drastic changes of the local ecosystem. Some species will disappear, while others may flourish. It is believed that sediment capping with a sandy layer will have a net positive impact on ecosystem quality, especially for mussels (Klijn et al., 2006).
Increasing habitat diversity
In creating robust lake ecosystems, besides focusing on reducing sediment concentrations in the water column, it is possible to focus on increasing habitat diversity. Two measures have been proposed for the Markermeer region that aim at increasing diversity and dynamics of the natural habitat of flora and fauna; creating marshes and creating foreshores.
Creating the Markermeer marsh
One idea to restore the ecological value of the Markermeer is the creation of a large marsh close to the Houtribdijk, the dam separating lake Markermeer from lake IIsselmeer. The goal of constructing this marsh is to create large-scale land-water transitions to increase habitat diversity. By strengthening and restoring ecosystems, one attempts to increase the lake’s attractiveness to humans as well as flora and fauna. Several sub-goals can be served by the plans:
- Creating a large area of gradual land-water transitions: current land-water transitions are mostly abrupt and fixed with hard structures, such as dike revetments. This and the meticulously regulated (and therefore practically fixed) water level during the last decades have yielded low habitat diversity.
- Adding new nature values: marshes provide suitable spawning grounds to several species of fish and nesting and resting grounds to waterfowl, reed and marsh birds.
- Improving ecological robustness: creating a more diverse habitat with high species diversity will improve the robustness of the entire lake ecosystem.
- Stabilizing and improving existing nature values: the marshlands will provide shelter for breeding birds and wintering birds.
- Strengthening spatial diversity: because of the large scale the marshes offer plenty of opportunity to species that are not served by the smaller-scale projects along the coast.
An interesting approach to design marshes from scratch is to use building blocks that constitute the important components of a large-scale marsh (see the adjacent figure; source: Royal Haskoning DHV). Each building block describes the environmental conditions, key species of that specific part of a marsh and the minimum and optimum size (adjacent figure). The marsh can be composed of different building blocks. If – for instance due to financial constraints – the design of the marsh has to be less ambitious, some building blocks can be adjusted or removed without destroying the integrity of the design.
If a large-scale marsh is created in lake Markermeer, the ecological value of the area will increase drastically. The Houtribdijk is considered to be a good location, taking physical, ecological and political aspects into account:
- In this part of the lake the water level fluctuates most, as the prevailing south-westerly winds set up the water against the dam. This water level variation evokes dynamic processes that are beneficial to marsh development
- The dam marks the transition of lake Markermeer to lake IIsselmeer, both important foraging areas for several bird species. Improving the ecological value of the area can help support bird populations.
- As wave energy is absorbed by marsh and vegetated foreshores, wave attack on the Houtribdijk can be reduced, which saves costs of maintenance and strengthening.
- As recreation activities in this area are limited, there is less chance of conflicts with water sports.
The exact shape and location depend on many factors. Several designs have been made for a marsh area alongside the Houtribdijk (for an example see fig. 8), but others concern an island at some distance from the dike. Each design has different pros and cons. Based on physical, ecological and political considerations a final design needs to be made. Below arguments are listed which need to be considered when designing the marsh:
- A round shape has relatively small circumference and therefore a small area that is directly attacked by wind, waves and currents. An important disadvantage of this design is that it is more difficult to construct the marsh in phases.
- If a more elongated marsh adjacent to the Houtribdijk is built (see the adjacent figure), the total area of the marsh and length of the shoreline can be larger against the same costs. Yet, a certain width will be needed in order to prevent external disturbances (from the dam or recreational activities).
- The location needs to be chosen such that the marsh can act as a stepping stone between other valuable nature areas.
- The northeast corner of lake Markermeer has the highest water level variation. This is important to keep the marsh healthy and to create the right environment for specific flora and fauna.
- Creating a marsh of this size can have an important impact on the sediment concentrations in the area, by extracting large amounts of fine sediment during construction, by capping part of the sediment and by diverting currents.
- A wider zone of water between the marsh and the dam would increase the sheltered area and may thus increase sedimentation in the new nature area.
- If the marsh extends alongside the Houtribdijk, it will reduce wave attack on the dike and thus the costs of maintenance and strengthening.
- When determining size and location, attention needs to be paid to the existing functions of the region, like fishing grounds or (historical) shipping routes.
Adjacent figure shows an artist impression of a marsh in lake Markermeer along the Houtribdijk
Creating a marsh in lake Markermeer aims at decreasing turbidity, improving the lake’s ecological status (and resilience), enhancing recreational possibilities and improving flood safety whilst at the same time reducing costs to maintain or strengthen the Houtribdijk. It is expected that this large-scale measure will be effective in achieving at least some of these goals.
The marsh will add new biotopes and habitats that are non-existent in the Markermeer nowadays. Introduction of new habitats is not for everybody acceptable. Experiences with similar sized marshes in the Oostvaardersplassen have shown that these types of measures can have an important impact on surrounding areas. The new nature area, at the crossroads between IJsselmeer, Markermeer, northwest Overijssel, Flevoland and North-Holland, will constitute an important stepping stone for various nature types, experience with the Oostvaardersplassen has taught a number of lessons to be taken into account when creating the new marsh. An important factor determining the future dynamics of the marsh is the reed growth. Large-scale destruction of budding reed by grazing geese will hamper the natural growth of the marsh. Another risk is the permanent desiccation of the marsh, which often leads to unbridled and difficult-to-control growth of willows. Nature islands created in the mouth of the River IJssel have shown willow growth to be much more successful than reed growth. By felling trees one has attempted to tip the balance towards reed growth, but one growing season allowed the willows to take over again.
The effect on turbidity in the entire region remains to be seen. To what extent creating marshes will permanently reduce sediment concentrations has not yet been calculated. Anyhow, a sheltered zone between the marsh and the dam is likely to yield clearer water there.
Another important point to keep in mind is that, if a stepwise approach is taken to create the marsh, older parts are not disturbed by building new ones. Therefore it may be wise to build out the marsh in rings, with new parts constructed from the water rather than from the existing marsh.
The last figure shows the relation between costs (area) and ecological effectiveness of a marsh alongside the Houtribdijk.
Ongoing research shows that the ecological improvement of the system would not be significant with a marsh with an area less than 1500 ha. Species that will flourish in such a relatively small marsh area are already well represented in the system. A larger area allows for the settlement of new species, resulting in a higher biodiversity. From an ecological perspective one may state: the bigger the better. Cost effectiveness, however, will decrease if certain ecological goals have been achieved. At the moment, it is still unclear at what area, and which costs, this point will be reached (marked with a question mark in the adjacent last figure).
The budget available is probably the biggest constraint for the size of the marsh. If bought at market price, the amount of clay needed will push the budget for a marsh of about 4500 has to something in the order of 500 M€. An important cost-reducing strategy can be to make use of material remaining from other projects in the region (making work with work). This would mean, however, that projects in the area need to be co-ordinated and phase over a long period of time. Realistically, the project can only be realised if it is (partly) privately funded and can be connected to existing projects like sand extraction or strengthening of the Houtribdijk.
Creating foreshores at the Lepelaarsplassen
To increase habitat diversity of lake Markermeer, it has been suggested to create shallow foreshores in the coastal zone of the so-called (lepelaarsplassen)(Eng.: spoonbill ponds), near the city of Almere. At this location the lake bed is rich of fine sediment, relatively deep and the shores are protected by rock revetments resulting in a low habitat value. By creating a shallow vegetated foreshore similar goals can be achieved as by creating shelter structures (see above):
- Wave attack on the flood defence system is reduced;
- Wave action on the bed is reduced, thus reducing turbidity in the lea of the foreshore;
- Currents can be guided so that sedimentation takes place at favourable locations strengthening the flood defence infrastructure
- New room is created for nature to develop
In the design of these foreshores, location and dimensions are important. First, the distance between the foreshore and the coastline needs to be determined. The width of the water body in between cannot be too large, in order to keep wind and wave action from getting a grip on the area and creating turbidity, again. Currents can be reduced by connecting the foreshore with the coastline at one point, either in the middle, creating two openings, or at the end, yielding one opening. Important considerations here are nutrient and water exchange and sediment input into the sheltered area.
Increasing habitat diversity with mudflats, shallow reed beds and nutrient-absorbing marshes can be beneficial for species like the spoonbill, bluethroat, bearded reedling and target species of Natura2000 like diving ducks, piscivorous and herbivorous birds. The foreshore can also serve as a resting ground for waterfowl. As the area is a relatively isolated part of lake Markermeer, however, the contribution to the entire lake ecosystem will be limited.
Cost indicates for creating the foreshore vary between 50 to 150 M€, depending on size, materials used and construction methods. These costs are relatively high, due to the necessity to create a wave-breaking structure in the deepest part of lake Markermeer and the creation of shallow areas with clay. If sand is used, costs will rise quickly.
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