Through all kinds of weather on Sloterplas: What works against blue-green algae?
What is the best way to combat blue-green algae? UvA students embark on fieldwork to determine the difference in blue-green algae between the Sloterplas and the Nieuwe Meer. But then the wind picks up.
‘Hold on,’ warns UvA researcher Yuri Sinzato as the measuring equipment almost gets swept off the raft. It is Wednesday morning and Sinzato is standing with six master’s students in Biological Sciences on a raft the size of a dorm room on the Sloterplas lake. High waves inundate the raft, piloted by a student with an outboard motor. The students - some wearing knee-high boots, others flip-flops - are now ankle-deep in water.
Today’s goal is to determine the amount of blue-green algae at different depths in the Sloterplas and compare those results with the Nieuwe Meer, the lake where the students conducted the same experiment a week earlier in much sunnier conditions. What is the difference in blue-green algae between those two comparable lakes?
Jacuzzi
‘Blue-green algae - not actually algae but cyanobacteria - have been a problem on the Sloterplas for 30 years,’ aquatic biologist Petra Visser, program director in the Master of Biological Sciences, told me by phone later that day. ‘Blue-green algae can be recognized by a greenish-blue, oily layer floating on the water. They thrive in warmer temperatures and in phosphate and nitrogen-rich water.’
Visser has been researching ways to combat blue-green algae for years. He did his doctorate on the mixing facility in the Nieuwe Meer that uses bubbles to drive out the bacteria. Says Visser: ‘Air bubbles up from the pipes, mixing the water layers in the lake. This turns the lake into a kind of jacuzzi.’
Blue-green algae benefit from the layers of water in lakes and ponds. This is because the bacteria need the surface for sunlight and the deeper water layers for nutrients. Tiny air bubbles - gas vacuoles - in the cell allow the bacteria to float to the surface of the water, where there is enough sunlight for the bacteria to make sugars. The weight of the sugars then causes the bacteria to sink deeper into the water where they can take on nutrients. When the bacteria use up the sugars, the blue-green algae floats back up. If those water layers are disturbed, as they are in the Nieuwe Meer, then blue-green algae do indeed appear to disappear.
Milk bottles
What do the students expect to be different in Sloterplas this week? ‘First of all, we expect more blue-green algae here,’ says student Bodil (23). ‘And we want to measure stratification: the difference in temperature between different water layers.’
To do this, they lower a measuring instrument called a Hydrolab into the water, explains student Eva (25). The Hydrolab records the temperature and light intensity at different depths. The students also take water samples at different depths. The samples go into a crate of glass milk bottles. Later that afternoon, they will analyze the blue-green algae colonies in the lab under the microscope.
But first they have to go quite a distance up the lake, where the water reaches a depth of 30 meters. According to the owner of the water sports center De Duikelaar, where the students rented the raft, this is perfectly possible in this wind. ‘The raft really does float.’
The raft is ankle-deep in water and students do their best to keep the floats and measuring equipment on board.
As the unwieldy raft turns onto the lake, half-meter waves hit the deck. The raft ends up ankle-deep in water and students do their best to keep the floats and measuring equipment on board. Research leader Sinzato determines that taking measurements is going to be very tough. So it’s back to square one.
The students are then obliged to take measurements in the harbor, but at a depth of two meters, that’s not nearly deep enough. So the blue-green algae yield at the end of the morning is not high. Says Visser: ‘Because of the wind, the measurements are not very representative. The blue-green algae can blow in all directions.’
Non-native mussel
The amount of blue-green algae in the Sloterplas has already decreased in recent years. Visser says: ‘Five to ten years ago, when the city was ready to begin tackling blue-green algae in the Sloterplas, the bacteria suddenly largely disappeared.’
Research by Waternet, the water board’s executive organization, the city of Amsterdam, and the University of Amsterdam showed that the quagga mussel was responsible, a non-native mussel that purifies the water, giving blue-green algae less of a chance. ‘This is not to say that the solution is to introduce this mussel into other lakes,’ Visser says. ‘Quagga mussels are an invasive species that can clog pipes and be a nuisance.’
Couldn’t the mixing facility in the Nieuwe Meer also help in the Sloterplas? ‘That choice is now up to Waternet,’ Visser explains. ‘As far as I know, they prefer a more sustainable solution that uses various techniques to prevent nutrients such as phosphate and nitrogen from entering the Sloterplas.’
Visser says: ‘The best solution, of course, is to remove the food source for blue-green algae. That means finding a solution to the nitrogen and phosphate problem. But that’s not so easy in a densely populated country like the Netherlands.’
