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 Building with Nature Guideline > Projects > Knowledge - Sediment and ecology in delta lakes

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Knowledge - Sediment and ecology in delta lakes


Abstract: How to restore the ecological quality of turbid lakes

Technological readiness level: 6 (technology demonstrated in relevant environment)

Environment: Lakes and rivers
Topics: Alternative stable states, ecological quality, nutrient dynamics, sediment dynamics, lakes

 

Clear lakes are often characterised by the presence of aquatic vegetation

 

Delta lakes can be found scattered across the globe. In spite of large differences, such as their climate zones, similarities can be found. Of all 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 naturally. Despite differences in location, surface area and depth, there are recurring issues when it comes to the ecological status of lakes. Many shallow lakes in delta areas across the globe are characterised by a deteriorating water quality, caused by high nutrient loading and turbidity. This project knowledge page gives background information on this change and provides insight into Building with Nature interventions to influence sedimentation and ecological processes. The topics that are being discussed, are: 


  1. Clear and turbid lakes;
  2. Nutrient dynamics;
  3. Turbidity and sediment; and 
  4. Restoring ecological states.


1. Clear and turbid lakes

In general, two types of shallow lakes can be distinguished. One is characterised by high water transparency, dominance of aquatic vegetation, small amounts of nutrients and a diverse fish community. The other type is characterised by turbid water, with no or small amounts of aquatic vegetation, large amounts of nutrients, large seasonal algal blooms and a fish community that 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.

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2. Nutrient dynamics

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.

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3.Turbidity and sediment dynamics

The stable state theory states that aquatic ecosystems can switch between clear and turbid conditions. 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.

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4.Restoring ecological states

The ecological status of many 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. Using the Building with Nature philosophy, 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.


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. 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).

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References

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