Text: René Lipmann Photos: Rob Idema

As an ecologist and nature guide, Rob Idema likes to make rowing trips through the Botshol, a lake and marsh area next to the Vinkeveense Plassen. In search of bluethroat, spoonbill and reed orchid. He is just as happy to dive at the popular diving location of Vinkeveen. The lake holds few secrets for Rob.

Next to the Vinkeveense Plassen lies a peat lake area Botshol, unknown to many divers. What is special about this nature reserve?

Botshol is an old peat bog. An area where peat was extracted for the local population and the village of Abcoude. When peat could be extracted better and easier in, for example, the Vinkeveense Plassen and brown coal and coal became available, extraction stopped in Botshol. Botshol was part of the municipality of Abcoude for a very long time. Even though it is quite a distance away from there. That is one of the reasons why the lakes and open water that were created by the peat extraction slowly but surely grew over again and the current marsh area was created. It was not until 2011 that it was added to the municipality of De Ronde Venen (and Vinkeveen) by merging Acoude and Rond Venen.

The area is known for its special flora above and below water. It gained international fame, especially for its stoneworts. Nowadays, Botshol is managed as a marshland by Natuurmonumenten. You can enter the area with a rowing boat or with the boat of Natuurmonumenten. Officially, swimming is not allowed, but this is tolerated as long as it is on a small scale. Spoonbills and cormorants breed in colonies in Botshol. You can also regularly see the osprey, marsh harrier and hawk flying and you can find various water birds and reed birds there. For example, the tufted duck, the mustached duck, bluethroat and bearded tits.

Can you also dive or snorkel in Botshol? What is the difference with the Vinkeveense Plassen? 

No, in Botshol it is officially forbidden to swim or to go on the water with your own boats or floating bodies. Diving and snorkeling are also not allowed. Although this was different in the past. Back then, the Vinkeveense plassen were an opaque green soup. Back then, diving was done in Botshol. Fortunately, the Vinkeveense Plassen have become much clearer due to the construction of sewage and a dephosphating plant, and we can now enjoy the underwater life in the clear, more accessible water. The Vinkeveense plassen are also much deeper. The maximum depth in Botshol is approximately 3.5 meters.

How many lakes are there actually? The sand islands, these are not formed naturally?

There are 3 Vinkeveense lakes, the Noordplas, Zuidplas and the small lake. The Noord and Zuidplas are separated by the Baambrugse zuwe, which connects Eiland 4. The Zuidplas and the small lake are separated by the N201, which runs right through them. The lakes were created by peat extraction or peat cutting. In the Vinkeveense lakes, peat extraction started late and continued for a long time. In 1896, a peat extraction concession was issued for the area that is now the Vinkeveense lakes and this was valid for 90 years. Peat extraction could therefore continue until 1986. This did not happen, peat extraction stopped around 1975, whereby in recent years the peat was no longer used for peat but was used as peat moss in the Westland. Before peat extraction was stopped, another method of exploitation had been devised. Sand for the construction of new residential areas was extracted along the beaches until around 1960. With the flood disaster in mind, coastal protection became increasingly important and it was decided that sand could no longer be extracted along the coast. Sand therefore had to come from another location. A large sandbank appeared to lie beneath the peat layer of Vinkeveen. Vinkeveen sold its sand to Amsterdam. The suction of sand began in the Vinkeveen lakes. In the Noordplas in particular, sand was sucked away to a depth of 50 metres. Because a large lake was created with a lot of wave action, it was decided to construct a ring of 12 sand islands to protect the hinterland from erosion by waves.

How did the unique structure of the Vinkeveense legakkers come about?

In Vinkeveen a peat cutting concession had been issued for 90 years. However, the industry had not stood still. Peat cutting machines had been developed, enormous floating vessels with a steam engine that cut the peat to a depth of 5 metres and mixed it with water and then sprayed it on the island from which the peat was cut. There the peat mud was dried, tamped down and cut into blocks. In this way, increasingly narrow long legakkers were created. To prevent the island from being washed away by the waves, it had to have a minimum width.

What makes the Vinkeveense Plassen so attractive to you as a diver? 

I am always happy to be in or on the Vinkeveense Plassen. Whether it is at Eiland 4 in the diving area or at Eiland 1, shallow between the water plants. Unfortunately, the stairs were removed there last year. Diving has become a bit more of a challenge there. You can also go a bit deeper at Eiland 1 along the slope, where you can see the water pipe running to the other side (Eiland 8) when visibility is good. You can also dive very deep to the zoo at 40 meters in front of Eiland 4. The depth is also a very special experience. Other places where there is not much diving, such as the fish forests in the Zuidplas, are also special. You can sometimes find sponges on the branches there that extend 40 to 50 cm into the water column. What I also like to do is snorkeling along the northwest side of the Vinkeveense Plassen, looking for snails and caddis flies or looking at stoneworts along Eiland 8. The Vinkeveense Plassen have something special for everyone to discover. Special encounters for me were the first specimens of various types of invasive American crayfish. Or multiple encounters with burbot. Other nice observations are, as one of the first, black-mouthed gobies and the Caucasian dwarf goby.

Islands 1,2,4 and 5 are accessible from shore. Do you have your own boat to sail and dive to Island 8? 

No, I don't have my own boat. With our diving club Scubadoe 'de Venen' we did have our own boat for a while. However, you can always dive on the Vinkeveense Plassen by joining the boat dives of Airdiving, Scuba Academie, GoDive Woerden or Scuba Diving Vinkeveense plassen. It is possible to go on the water almost every day. Although it is of course more pleasant in the summer.

One of the places that is most appreciated by many divers is the pike garden at Island 8. That is a fairly shallow hill, where at a depth of about 6 meters, almost always a group of pike-perch, perch and pike is 'relaxing'. You can hang there relaxed while the big fish hang quietly just on or just above the bottom. 

You are an ecologist and nature guide. What is the nature value of the Vinkeveense Plassen?

Well, that is quite a difficult question. Because the more you know, the more you realise what you don't know yet. It also depends on the angle from which you look at the natural value. What I really like about the Vinkeveense Plassen at the moment is that the visibility is excellent. However, that is not ideal for someone who fishes for bream or carp, for example. These are fish that do well and occur in large numbers in nutrient-rich murky waters. The clarity ensures that water plants can grow again from a certain depth. In places where there is not much danger, this can cause larger water plants such as pondweed to grow to the surface and then get stuck in a boat propeller or around a rudder. Many macrofauna, snails and small fish find shelter between these water plants. So you would say that it is good for nature. But for the experience of the Vinkeveense Plassen it is less so. A species that always causes a bit of a stir is the freshwater jellyfish. People who are not familiar with the fact that they can also occur in fresh water are sometimes shocked by it. The temperature of the water is very important for these jellyfish, they only appear when the water temperature has been above 20 degrees for a long time. However, that is also the water temperature at which blue-green algae do well. You don't want blue-green algae because you can get very sick from it. In addition, we also have a special rare fish in the Vinkeveense Plassen, the loach. This fish cannot withstand water temperatures that are above 18 degrees for a long time. So when the top layer gets warm, the loach has to retreat to the cooler depths while all its prey is in the warmer top layers. The warmer summers of recent years are therefore good for some species, but bad for others. 

Are these species doing well in Vinkeveen? Are there any current developments?

Many species are doing well in the Vinkeveense Plassen. Some are really not, for example the bullhead. A fish of about 10 cm long that makes nests early in the spring under a piece of wood or stone. The one who makes the nest is the male, he tries to seduce a female to lay eggs against the roof of his nest and then fertilizes them and guards them until they hatch. It is fun to watch the fish busy and fun to keep track of how many of those nests there are. Now there are two exotics that have the same way of reproducing. The round goby and the marble goby. The round goby and the marble goby are larger and more aggressive than the bullhead and 'our' bullhead is being pushed out of its nesting places. I have not seen them in the past two years. The same actually applies to the burbot, this fish has also hardly been seen in recent years. What I find special about 2024 is that I hardly see any crayfish anymore. And that there are a number of reports of catfish and sturgeon. Perhaps released specimens, but who knows, a population may establish itself.

Should we accept these new species? Or fight them?

There are quite a few known cases where an exotic species has become so dominant that it has taken over an entire ecosystem for many years, and people in particular are affected. For example, rabbits in Australia or Nile perch in Lake Victoria. Often, the exotic species come as a result of human action and people are unable to control the population. For example, the American crayfish has also come to Europe. In the meantime, these crayfish have become an indispensable part of our waters. Three species have established themselves in the Vinkeveense Plassen, successively the spotted American crayfish, then the red American crayfish and then the knobbed American crayfish. All three are still there, but the numbers of knobbed crayfish have been the highest in recent years. Combating the crayfish is very difficult. In 2024, the numbers seem to have decreased, but the cause has not yet been discovered. The local fisherman thinks that it is because a lot of eel has been released and that they have learned to eat the crayfish. In any case, he has seen that his eels grow very quickly. A major disadvantage of combating exotic species is that you are often also combating your local native species because most trapping techniques, diseases and hunting are not very specific and are disruptive.

You love night diving at Island 4, what makes such a dive attractive to you?

Night diving is great fun because you can look even better at the subjects you catch in your lamp and you can often get a bit closer to the species. In addition, you see species that are hidden during the day. For example, the burbot is a nocturnal animal that hunts at night, but pike-perch also like to hunt at night. The large pike-perch, but also the smaller perch, have even developed adapted behavior. They use the diver's lamp to surprise their prey. This sometimes also results in comical scenes of buddies who are not yet used to this and are quite startled when a large pike-perch suddenly pops up from diagonally behind their lamp and grabs a fish.

Rob, you are a guest speaker at Duikvaker on February 2nd at 11:00. What can I expect from your presentation?

I try to outline the origin of the Vinkeveense Plassen and the entire ecosystem and food web. I also give a look into the future. What do I think will change, now that the zoning plan Vinkeveense Plassen will soon be adopted. What can we as divers see in future plans and what threats are still lurking for divers, but also for the animals and plants that live in the Vinkeveense Plassen.