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The Effect of Bioturbation on the Erodibility of Fine Sediments in Lake Markermeer

Lake Markermeer is a large and shallow freshwater lake in the center of the Netherlands. The lake was originally part of the Zuiderzee, a sea inlet of the North Sea. The lake is an important ecosystem, especially for water birds. The main problem in Lake Markermeer is the decreasing ecological value. High turbidity levels are believed to be one of the reasons for the alarming ecological state. Wind-induced waves easily stir up the bed sediments due to the shallowness of the lake. Next, a layer of soft mud is present at the bed that is easily resuspended, especially the oxidized top layer. The goal of this study is to discover the effect of crawling and burrowing animals, so called ‘bioturbation’, on the erodibility of bed sediments in the lake. The effect of bioturbation was studied by performing erosion experiments on several mud samples with and without biota. Biota was obtained by sieving the natural top oxidized layer, Tubifex worms and Ostracoda were the main biota found. The defaunated sample was created by by gamma radiation. The U-GEMS microcosm was used for the erosion experiments, a combination of a central suction and rotating disk provides a uniform bed shear stress on the sediment bed. A series of experiments was performed for every sample; erodibility was tested at the start and after a bioturbation period of 2, 4, 6 and 8 days respectively. The results showed that bioturbation indeed increases the erodibility of the mud. Erodibility increased after n days of bioturbation, while for the control experiment, the defaunated sample, erodibility decreased after n days. The results also showed that the effect of bioturbation on erodibility is a function of time, the total eroded mass increases with increasing bioturbation period. The effect of bioturbation on erodibility was shown for the soft mud layer and also for the Zuiderzee deposits, the stronger mud layer below the soft mud layer. Moreover, observations showed that bioturbation increases the thickness of the oxidized top layer. However, the oxic state of the sediment is not responsible for increased erosion and can only be used as an indication for bioturbation history. An important bioturbation mechanism is the production of fecal pellets by Tubifex worms, these worms feed at a certain depth in the black anoxic layer and produce easy erodible fecal pellets at the surface. Another noteworthy observation is the erosion type, decreasing erosion rates were observed for increasing shear stresses, this indicates that erosion is limited by the amount of sediments that is available for erosion. From this research we can conclude that bioturbation is an important mechanism influencing the erodibility of bed sediments in Lake Markermeer. We hypothesize that bioturbation is an explanation for the high erodibility of the oxidized top layer and bioturbation have also enhanced the erosion of the stronger Zuiderzee deposits, thereby creating the soft mud layer. However interpolation to the field is rather difficult, though a causal relationship has been demonstrated, this does not necessarily imply that bioturbation-induced erosion is a dominant mechanism.