Samenvatting

The turbidity in Lake Markermeer limits the primary production by phototrophic algae and the growth of plants. In turn, this limits the amount of fish and other organisms that live in and around Lake Markermeer in its food sources. To determine if there are organisms that limit the resuspension, a survey was conducted in which (filamentous) sulphur bacteria were found. These bacteria were assumed to have a positive impact on the resuspension of the sediments in Lake Markermeer.

To determine if this was true, multiple samples were taken and tested to determine the spatial distribution, the effects on sediments, the impacts on nutrients and the contribution to the primary production. Beforehand however, a literature study was conducted to determine possible problems and solutions these bacteria could have. This showed that the bacteria would most likely positively impact amount of sediment that was resuspended but could also negatively influence the system as the bacteria are not available as a food source. The bacteria are also able to store large amounts of nitrate and elemental sulphur in their vacuoles which could influence the nutrient balance in Lake Markermeer.

In addition, literature stated that there could be an effect of the amount of oxygen that is present in the substrate by a form of microstratification. After analysing the samples, it became clear that the bacteria do have significant impacts on the resuspension of sediments, especially when there are a lot of bacteria present. It was also found that the effects on the nutrient balance in Lake Markermeer where not of significant impact. Relations where found between the nutrient concentrations in the soil and surface water, but the values were not different than expected.
The availability as food source has not been fully tested. Initial results of research done by Witteveen&Bos however, shows that the bacteria do not serve as a food source available for some of the more common fish in Lake Markermeer. This becomes of greater importance as the bacteria form a significant amount of biomass in the lake. This biomass has been found to be higher than other benthic species such as mussels. Furthermore, no evidence has been found in this research of microstratification caused by the bacteria. It has therefor been recommended that further research should be conducted to determine the exact workings of these bacteria within Lake Markermeer. In addition to this, it should be investigated if these bacteria can be cultivated and if they are used as a food source for other organisms.