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

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 Lucas

1

, M. Bakker

1

, J. C. Winterwerp

1

, T. van Kessel

2

and F.Cozzoli

3

1_{Environmental Fluid Mechanics, Delft University of Technology, 2628CN, Delft, The Netherlands, m.a.delucaspardo@tudelft.nl} 2_{Sediment Transport and Morphology, Deltares, 2629HD, Delft, The Netherlands}

3_{Netherlands 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}

112 wat ran tem T by circ dire Fi in Fig bot rep 2 ter is about 2.5 nges between 6 mperature during

The large scale wind induced f culations of w ections in surf

igure 2. Relatio Markermeer [N gure 1. The left p ttom compositio presents clay. 5 109 _m3 _[Van 6 and 18 month g 2010 was 13. flow pattern in flow [Vijverberg water. This c face and near

onship between Noordhuis, 201 panel shows th on, after the ex

Duin, 1992]. T hs [Vijverberg, 6±5.4 °C. Markermeer is g, 2008]. Wind circulations ma the bottom, w wind speed an 10]

he sea inlet bott xecution of the The residence t 2008]. The w s mainly domina induces horizo ay have oppo which results i d suspended so om composition Zuiderzee wor time water nated ontal osite in a comple circula Kessel fine se waves may r relation Marker more im bed tha & WLD Mark cylindr contain and lig choose experim several Marker were o sieves. olids n during the Zu rks [Lenselink Jubilee ex 3D flow patt tion patterns c et al, 2008]. T ediment transp which, together resuspend sedi nship between rmeer is shown mportant effect an wind-induce Delft Hydraulics kermeer anoxi rical containers ners were kept ght conditions w e 6.5 °C and

ment was perfo l individuals of rmeer, were ad obtained by siev The oxic mud

uiderzee era. Th & Menke, 199

Conference Pr tern [Vijberverg an occur depen This water circ

ort over the s r with currents, iments from t n wind speed n in Figure 2. t on the re-susp d-currents in M s, 2006].

METHO

c mud sedime s, with Marker in a small ch were controlled no light exp ormed to every f Tubifex, a cha dded to the sam veing oxic mud

started to deve he right panel sh 95]. Green repr roceedings, NC g, 2008]. Differ nding on wind culation is resp system. Wind , induce bed she the bed of t d and suspend Wind-induced pension of sedim Markermeer [Ro

ODS

ent samples w rmeer water o hamber, in whi to mimic field position. An ry anoxic mud aracteristic ben mple. The Tubif d through 250μ elop on top of th

shows the curre resents loam a

K-Days 2012 rent large scale

direction [van ponsible for the also generates ear stresses that the lake. The ded solids in d-waves have a ments from the

oyal Haskoning were placed in on top. These ch temperature conditions. We initial erosion sample. Then nthic species of ifex individuals μm and 500 μm he anoxic mud, nt Markermeer and dark green

e n e s t e n a e g n e e e n n f s m , r n

upo ero wit wit con was Rea in a was see suc out cali sed was W (g/l thro me then sur wit the freq F ero rate stre larg also the ero pro oxi oxi lay T fou ero 0.1 tim sam con def Jubilee C on which a ser sion rate under th these experim thin the oxidiz ntrol sample w s defaunated w actor Institute D an UMCES-Gu s calibrated at n in Figure 3. ction outlet of t t-flowing wate ibrated with diments to be t s calculated fro Where E is the l) in the out-fl ough the suctio asurements of t n calculated b face of the m thin each bed sh

We performed beginning of t quency was app

RE

Figure 4 shows sion rate of a tw e of an anoxic esses applied. T ger than the er o holds for mo erosion rate o sion rate of a f obably caused b c layer experim c layer is large er. This holds f The oxic layer t ur days, 2 mm a sion rate of the (g/m2_{s) for ev}

me evolution. A mple as well. nstant over tim faunated sample

Conference Proc ries of erosion r several bottom ments. The sam zed layer deve was also studied with Gamma Delft. The eros ust Erosion Mic Deltares. The r An OSLIM tur the microcosm er was measu samples of d tested. Equation om the turbidity

E

  

c Q

eroded mass (g lowing water, Q on outlet, and Δ the turbidity me by means of d icrocosm bed, hear step. erosion experi the bioturbation plied for the def

ESULTS AN

s the results of wo days-old ox c layer. This h The erosion rate rosion rate of a st of the tested of a six days-o four days-old o by the high mo ment. Finally the

er than the ero for most of the t thickness was 1 after six days, a e defaunated sa ery bottom she An oxic layer d However, its me. We belie e was caused by ceedings, NCK experiments w m shear stresee mples were teste

elopment proce d for its erodib Rays treatmen sion experimen crocosm System resulting calibr rbidity meter w m, with which t

ured. The turb different conc n 1 shows how y data:

t

 

g), c is the sedim Q is the discha Δt (s) is the int eter. The erosio dividing the ero

and by the n

iments at 2, 4, n process. The s

faunated sampl

ND ANALYS

our first set of xic layer, is larg holds for most e of a four day an two days-ol d bottom shear old oxic layer oxic layer only ortality of Tubif e erosion rate o osion rate of a tested bottom s 1 mm after two and 2.5 mm aft ample was alw ear stress, and i did developed thickness was eve that the o y the diffusion o -Days 2012 was executed. es was determi ed at several sta ess. A defauna bility. This sam nt in the Nuc nts were perform m. This microco ration curve can was installed at he turbidity of bidity meter entrations of w the eroded m ment concentra arge (l/s) of w terval between on rate (g/m2 _s) oded mass by number of seco 6 and 8 days si same measurem e.

SIS

f experiments. ger than the ero of the bed sh s-old oxic laye ld oxic layer. T

stresses. Howe is larger than for 0.8 Pa. Thi

fex in the six d

of an eight days six days old o hear stresses.

days, 1.3 mm a fter eight days. ays between 0 t did not show on the defauna 1 mm only, oxic layer in of oxygen only The ined ages ated mple clear med osm n be t the f the was the mass (1) ation water two was the onds ince ment The sion hear er, is This ever, the is is days s old oxic after The and any ated and the y. Over rate of to 0.8 P in eros increas shear decreas The dynam erosion only b Waves bottom with a occur o reality Therefo Marker is causi The Prooije guidan Figure Microc and d9 stress

D

rall we can con f Markermeer s Pa. The larger sion rate. The se in erosion rat stresses. Thus se in bed shear results has im ics of Marke n of the bed, a be possible und larger than 0.8 m shear stress (g d50 of 80 μm, a only under stor

is that Marke fore, and give rmeer (e.g. sha ing a major con

ACK

authors thanks en, Marinus Ho ce on this paper e 3. Calibrati cosm System. T 90 (instead of d according to Sh

DISCUSSION

nclude that biotu ediments under the bioturbation defaunted sam te over time un oxidation can strength. mportant implic rmeer. Withou and therefore a der bed shear 8 m would be n given a wave p and a depth of 3 rm conditions i rmeer is chara en the very llowness, mudd ntribution for th

KNOWLED

to Peter Herm om and Ruurd N r and related wo ion curve of The error bars

d50) for the ca hields-van Rijn. D

N

turbation increa r bottom shear n time, the larg mple did not ex nder any of the

n not be resp

cations for the ut biota effec a turbid water c stresses larger needed for prod period of 5 s, a 3 meters). Thos in Markermeer acterized by a particular cha dy bottom), the he characteristic

DGEMENT

man, Gerard Kr Noordhuis for t ork. the UMCES-were calculate alculation of the . De Lucas et al. 113 ases the erosion

stresses of 0.4 ger the increase xperienced any studied bottom onsible of the fine sediment cts, significant column, would r than 0.8 Pa. ducing a 0.8 Pa a sediment bed se wave heights . However, the high turbidity. aracteristics of e benthic fauna c turbid state.

ruse, Bram van the support and

-Gust Erosion d by using d10 e critical shear n 4 e y m e t t d . a d s e . f a n d n 0 r

114 No D van s van p van in s Vij th Mo B 3 Fish s 8 Hir in e Wi in s O Wi I e T 4 oordhuis R, Driehoeksmosse n Kessel T, de B sediment model n Eerden M, van population in th n Duin E H S (1 n the Markerm shallow lake. Ph verberg, T. (20 hesis Delft Uni ortimer, C. H. ( Between Mud 30.1: 147-201. her, J. B. and sediments by tu 85: 3997--4006. r, P. L., Y. Mon n sediment tra effects? Contine llows, R., J. w nfaunal bivalv sediment: A f Oceanography 4 ddows J., Brin Influence of bio erodability and The Netherland

REFER

Houwing E el in het Marke Boer G, Boderi l Markermeer. D an Rijn S (2003 he Ijsselmeer are 1992) Sedimen meer—a two d h.D. thesis, Wa 008). Mud dyn iversity of Tech (1942). The Ex and Water in d Lick, W.J. ( ubified oligo-c .

nbet, and F. Orv ansport modell ental shelf resea widdows, and R ve on the ero flume and mo 43 (6), 1132-13 nsley M. D., Sa ota on spatial an material flux s). Marine Ecol Fig

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