Lake Ecology

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Water Cycle:

Water flows in from piped underground streams beginning at 2 outer-city freshwater lakes. The water enters the corner of Peblinge Lake and flows down and up to the other basins. The water spends approximately 1 year cycling through, and is discharged via pipes to several of the old fortification water systems, finally ending up in the Øresund Strait: see diagram above. 

Lake ecology:

The City Lakes in Copenhagen have served as drinking-water reservoirs from 1723, in addition to their function as part of the defence of the city. The two upper lakes, Peblinge and Sortedams, (meaning, roughly, 'Student' and 'Black Pond') were abandoned as reservoirs due to unclean water in the mid 19th century, and the southern lake, Sankt Jorgens ('St. George's Lake) was used instead because of its better quality. In the early 20th century, a water treatment/filtration area was set up beside Skt. Jorgens Lake, and it served as emergency water reservoir until 1959.

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You can see the filtration ponds next to Sankt Jorgens Lake in the photo from 1954.

In 1929 Poul Helsøe, the city architect, designed a promenade/edge around the two top lakes, but left Sankt Jorgens Lake relatively natural as it was the emergency reservoir. The difference this has made is evident in the second satelite photo from 2001, where the two top lakes are clouded and lots of sediment is visible, and Sankt Jorgens Lake is still clear despite not having the water filtration ponds anymore.

From 2002-2006 there was a large restoration project on the lakes. The project was financed by the city of Copenhagen (costing ~12-13 million Kroner) and involved several activities: the purification of inflowing water, biomanipulation, and stocking of fish. There were also several experiments on habitat restoration of the littoral zone--the land-water interface and shallow areas outside the interface--to provide better spawning conditions for fish.

The main objective of the project was to improve the environmental quality of the water and to create a natural (less 'anthropogenic') lake ecosystem.

One of the biggest problems of the city lakes is the lack of a natural, or at least semi-natural, littoral zone.

The drawing on the left shows a natural lake edge, where the minimum water depth is between 2-5cm and there is a cover of water plants. This is the littoral zone, the area where fish can safely spawn and grow without risk of being eaten by larger pescivorous fish (or even their brothers and sisters). This is also the area frogs and toads tend to inhabit, and provides a safe shallow zone as a sort of lake nursery.

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The drawing on the right is a typical section through the edges of Peblinge and Sortedams Lake. You can see that the edge is made of stone and concrete, and has a depth of approximately 30-50cm, with no water plants growing. This means it is very difficult for the fish population to become established, as young fish are always at risk of being preyed on before they have a chance to grow.

The reason it is important to have pescivorous fish (fish that eat other fish) in a lake, is to maintain the proper biomass and diversity. I never took biology, so I had to seek out all this information from experts working in Copenhagen on this. Here is the explanation, as simplistic as I can make it related to the foodchain:

The combination of fish species and the relative magnitude of different groups of fish is very important for the environmental quality of the water. 'Turbid' lakes (those with large amounts of algae) are almost always dominated by fish species that eat a lot of zooplankton ('Cypranids' such as bream and roach). This has the effect that the density of zooplankton is low, because it is always eaten. Zooplankton eat algae, but if the zooplankton population is always being eaten, the algae density becomes high and the water therefore turbid. The turbid water has the effect that submerged macrophytes (plants) have a hard time because the algae blocks sunlight and the plants can't grow. Turbid water and a loss of macrophytes therefore has a negative effect on biodiversity.

Lakes with clear water, on the other hand, have a fish community dominated by predatory or piscivorous fish, e.g. pike and perch. These fish prey on the cypranids, so the density of zooplankton is high (because they are no longer eaten as much) and the algae is low. The water becomes clear. Submerged macrophytes can grow and biodiversity increases.

The key factor that can change a clear-water lake into a turbid one is the concentration of plant nutrients in the water. Nutrients are things like Phosphorous and Nitrogen, and to some degree Chlorophyll. In freshwater lakes, Phosphorous is the most important one. While the term 'nutrient' may make you think they are good for water, it is in fact the opposite. A high concentration of nutrients will encourage algal blooms, and the water becomes deprived of oxygen. Nutrient rich (eutrophic) lakes are always turbid; nutrient poor (ologotrophic) lakes are always clear.

So, what does it boil down to?

Turbid Lakes = Bottom feeding cypranidic fish = A lot of algae + A lot of nutrients = loss of oxygen = loss of biodiversity = BAD.

Clear Lakes = Mixture of predatory fish = less bottom feeding fish = more zooplankton to eat the algae = more oxygen = clear water where light can reach the bottom = more plants = better conditions for biodiversity = GOOD.

Okay, so what does this have to do with design?

Copenhagen municipality envisions the city lakes as a place where people will be able to swim and fish--and this requires that the lakes become healthy in a self-sustaining way (i.e., with little or no costly mainenance as there is now).

So, there are two main factors to deal with:

1. Nutrients entering the city lakes - phosphorous and nitrogen are a high risk because of agricultural runoff entering the source lakes. However, these are being treated/cleansed before entering the city lakes, so the risk is not as high anymore. The city is still concerned. One of the best ways to filter excess nutrients is through reed belts and wetlands.

2. Lack of Pescivorous fish to maintain balanced diversity - The pescivorous fish need a shallow nursery zone to safely grow before they can manage on their own in the deeper water. Hard edges must become naturalized edges.

The image below is from google earth and shows an experimental littoral zone made during the remediation. It is tiny compared to the size of the lakes, but was meant to provide a nursery for pike fry so they could maintain their population and help to keep the lakes clear.

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Any proposal for the city lakes will have to address the main objectives set forward at the beginning of the remediation: to improve the environmental quality. And the best way to do that is by changing the edge conditions.

Now you know the ecological background for my project.

My next post will discuss the theoretical context and the relationship between humans and nature.