DOI: 10.3990/2.179

 

Jubilee Conference Proceedings, NCK-Days 2012

Erodibility of soft fresh water sediments: the role of bioturbation by meiofauna

M. A. de Lucas1, M. Bakker1, J. C. Winterwerp1, T. van Kessel2 and F.Cozzoli3 1Environmental Fluid Mechanics, Delft University of Technology, 2628CN, Delft, The Netherlands, m.a.delucaspardo@tudelft.nl 2Sediment Transport and Morphology, Deltares, 2629HD, Delft, The Netherlands 3Netherlands Institute of Sea Research (NIOZ) 4400 AC Yerseke, The Netherlands

ABSTRACT Markermeer is a large and shallow fresh water lake in The Netherlands. It has a 680 km2 surface and a 3.6 m mean water depth.

Markermeer is characterized by its high turbidity, which affects the lake ecosystem seriously. As part of a study that aims to mitigate this high turbidity, we studied the water bed exchange processes of the lake’s muddy bed. The upper cm’s – dm’s of the lake bed sediments mainly consist of soft anoxic mud. Recent measurements have proved the existence of a thin oxic layer on top of the soft anoxic mud. This oxic layer is believed to be responsible for Markermeer high turbidity levels. Our hypothesis is that the oxic layer develops from the anoxic mud, and due to bioturbation. In particular we will refer to bioturbation caused by meiobenthos. The objective of this study is to determine the influence of the development of the oxic layer on the water-bed exchange processes, as well as the role of bioturbation in this processes. This is done by quantifying the erosion rate as a function of bed shear stresses, and at different stages of the development of the oxic layer. Our experiments show that bioturbation increases the erosion rate of Markermeer sediments, and therefore affects the fine sediment dynamics of the lake.

INTRODUCTION Markermeer is a large artificial fresh water lake located in the

centre of The Netherlands. Together with the northern IJselmeer it is the largest natural fresh water reservoir of Europe. This area is known as the IJselmeer Region. During the last decades, the lake has experienced a decrease in its ecological values. [Noordhuis & Houwing, 2003; van Eerden & van Rijn, 2003]. This is may be caused by a food web that is not functioning optimally. Fine sediments, which are in significant concentrations in the water column, are considered to be an important pressure on the food web of the lake [Van Kessel et al, 2008]. Moreover, water quality problems are often related to sediment composition and transport in Markermeer [Van Duin, 1992]. Therefore, fine sediments in the system seem to be a key factor towards an explanation of the negative trend over the last decades. As a part of a study that aims to mitigate Markermeer high turbidity, we studied the water bed exchange processes of the lake’s muddy bed.

The upper cm’s – dm’s of the lake bed sediments mainly consist

of soft anoxic mud. Recent measurements have proved the existence of a thin oxic layer on top of the soft anoxic mud. Thin oxic layers on the mud surface exert a pronounced influence upon the exchange of substances across the mud water interface [Mortimer, 1942]. In fact, the sediment concentration in Markermeer’s water column is dominated by erosion and sedimentation of this oxic layer [Vijverberg, 2008]. Our hypothesis is that the oxic layer develops from the anoxic mud. The main mechanism responsible for the development of the oxic layer would be bioturbation.

Bioturbation includes the processes of feeding, burrowing and

locomotory activities of sediment dwelling benthos [Fisher &

Lick, 1980]. The activity of this benthic biota severely affects sediment dynamics [Le Hir et al., 2007]. Previous researchers have measured the effect of bioturbation in the erodibility of sediments [Willows et al., 1998; Widdows et al., 1998 and 2000; Amaro et al., 2007]. The erodibility of sediments was characterized through the turbidity of the water in an annular flume. Our approach focuses on quantifying the erosion rate as a function of bed shear stresses. We quantified those erosion rates at several time stages within the development of the oxic layer. Our aim is a better understanding of the physics associated to bioturbation driven erosion.

MARKERMEER PHYSICAL DESCRIPTION Lake Markermeer did not existed before the 20th century. The

IJsselmeer Region used to be the Zuiderzee, a shallow inlet from the North Sea of about 5000 km2. During the Zuiderzee era there was a landward fine sediment flux, caused by tide and estuarine circulation. Thick layers of clay and loam were deposited as a result of this flux. Then in the 20th century, the Zuiderzee works took place, and the morphology of the region changed significantly. Figure 1 illustrates the differences in bottom composition between the two periods, as well as the differences in morphology. Markermeer was created in the upper reaches of the old sea inlet, with finer bottom sediments and smaller depths than the northern IJsselmeer. The dike separating the lakes, known as the Houbtrijdijk, does not allow for the fine sediments to be transported outside of Markermeer anymore.

Markermeer is a shallow lake, with a mean water depth of 3.6

m. About 90% of the lake has a water depth between 2 and 5 m [Vijverberg, 2008]. The total surface of water, including Lake IJmeer, is 691 km2 [Coops et al, 2007]. The volume of stored

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K-Days 2012

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Conference Proc

ries of erosion r several bottomments. The samzed layer deve

was also studiedwith Gamma Delft. The erosust Erosion MicDeltares. The rAn OSLIM turthe microcosmer was measusamples of d

tested. Equationom the turbidity

E c Q

eroded mass (glowing water, Qon outlet, and Δthe turbidity me

by means of dicrocosm bed, hear step.

erosion experithe bioturbationplied for the def

ESULTS ANs the results of wo days-old oxc layer. This hThe erosion raterosion rate of ast of the testedof a six days-ofour days-old oby the high mo

ment. Finally theer than the erofor most of the t

thickness was 1after six days, ae defaunated saery bottom she

An oxic layer dHowever, its me. We beliee was caused by

ceedings, NCK

experiments wm shear stresee

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sion experimencrocosm Systemresulting calibrrbidity meter w

m, with which tured. The turbdifferent concn 1 shows how

y data:

t

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dividing the eroand by the n

iments at 2, 4, n process. The sfaunated sampl

ND ANALYSour first set of

xic layer, is largholds for moste of a four dayan two days-ol

d bottom shear old oxic layer oxic layer only ortality of Tubife erosion rate oosion rate of a tested bottom s

1 mm after two and 2.5 mm aftample was alwear stress, and idid developed thickness was

eve that the oy the diffusion o

-Days 2012

was executed. es was determied at several staess. A defaunability. This samnt in the Nucnts were performm. This microcoration curve canwas installed athe turbidity ofbidity meter entrations of

w the eroded m

ment concentraarge (l/s) of wterval between on rate (g/m2 s) oded mass by

number of seco

6 and 8 days sisame measureme.

SIS f experiments. ger than the ero of the bed shs-old oxic layeld oxic layer. Tstresses. Howeis larger than for 0.8 Pa. Thifex in the six d

of an eight dayssix days old o

hear stresses.

days, 1.3 mm after eight days. ays between 0 t did not show on the defauna1 mm only,

oxic layer in of oxygen only

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results has imics of Marke

n of the bed, abe possible und

larger than 0.8m shear stress (g

d50 of 80 μm, aonly under storis that Marke

fore, and givermeer (e.g. shaing a major con

ACKauthors thanks

en, Marinus Hoce on this paper

e 3. Calibraticosm System. T90 (instead of daccording to Sh

DISCUSSION

nclude that biotuediments underthe bioturbationdefaunted samte over time un

oxidation canstrength.

mportant implicrmeer. Withou

and therefore a der bed shear 8 m would be ngiven a wave pand a depth of 3rm conditions irmeer is charaen the very llowness, mudd

ntribution for th

KNOWLED to Peter Herm

om and Ruurd Nr and related wo

ion curve of The error bars d50) for the cahields-van Rijn.

D

N

turbation increar bottom shear n time, the larg

mple did not exnder any of the n not be resp

cations for the ut biota effec

a turbid water cstresses larger

needed for prodperiod of 5 s, a3 meters). Thosin Markermeeracterized by a particular chady bottom), the

he characteristic

DGEMENT man, Gerard KrNoordhuis for tork.

the UMCES-were calculate

alculation of the.

De Lucas et al.

113

ases the erosionstresses of 0.4

ger the increasexperienced anystudied bottomonsible of the

fine sedimentcts, significantcolumn, wouldr than 0.8 Pa.ducing a 0.8 Paa sediment bedse wave heights. However, thehigh turbidity.

aracteristics ofe benthic faunac turbid state.

ruse, Bram vanthe support and

-Gust Erosiond by using d10e critical shear

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oordhuis R, Driehoeksmossen Kessel T, de Bsediment modeln Eerden M, vanpopulation in thn Duin E H S (1n the Markerm

shallow lake. Phverberg, T. (20hesis Delft Uni

ortimer, C. H. (Between Mud 30.1: 147-201. her, J. B. and

sediments by tu85: 3997--4006.r, P. L., Y. Monn sediment tra

effects? Continellows, R., J. wnfaunal bivalv

sediment: A fOceanography 4ddows J., Brin

Influence of bioerodability and The Netherland

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