Sulphur bacteria on the bed of Lake Marken
Published: 28 November 2019
To get to grips with how the sediment in Lake Marken interacts with nature development on and around the Marker Wadden, it is important to learn more about how living organisms affect sediment behaviour. Ecologist Ruurd Noordhuis of Deltares is one of the experts working on this area. ‘Rijkswaterstaat uses a sediment model for the management of the Markemeer-IJmeer waters but it doesn’t include biological effects on the sediment like the filtration of suspended matter by mussels, plants that further sedimentation or fish that churn up the bed.’ Algae or bacteria in the bed can also play a role when they capture high densities of sediment and stop it being re-suspended. ‘We found two types of sulphur bacteria in the samples we took this spring: Thioploca and Beggiatoa.’
Thioploca forms white filaments that are visible to the naked eye. Off the coast of South America, they form mats measuring thousands of square kilometres that can be up to 30 centimetres deep. You can also find them in larger lakes such as Lake Constance, says Ruurd. ‘In 2008, a Rijkswaterstaat colleague working on mussel research in the Lake Marken also found the characteristic white filaments. And when we also found mats like this around the Marker Wadden, the question was how important this is in terms of the effects on sediment and in relation to the primary production around the Marker Wadden. The sulphur means these bacteria are not a good source of food,’ explains Ruurd. ‘They are a dead end in the food web. Which means they don’t make a direct contribution to food production for the birds that we would like to see on the Marker Wadden.’ So there were plenty of reasons to look further at how these bacteria affect resuspension and primary production.
Thioploca bed, photo Ruurd Noordhuis
Graduate Luc Kauhl (Hogeschool Zeeland) and Gerlinde Roskam (Deltares) have been taking soil samples every three weeks since September at 29 locations in the Lake Marken. They are working with Harm van der Geest of the University of Amsterdam, who is taking samples weekly for a study of the water quality in Lake Marken. Researchers analyse the samples in the laboratories of Deltares and the University of Amsterdam.
Spread throughout Lake Marken
The sulphur bacteria have now been found at all the sampled locations, says Ruurd. ‘At the moment, they are also the most common filamentous organisms in the sediment by far. We hope to know more soon about how Thioploca and Beggiatoa affect the system.’