MEPS 278:205-223 (2004)  -  doi:10.3354/meps278205

Interacting effects of Hydrobia ulvae bioturbation and microphytobenthos on the erodibility of mudflat sediments

Francis Orvain1,3,*, Pierre-Guy Sauriau1, Angélique Sygut1, Lucette Joassard1, Pierre Le Hir2

1CREMA (CNRS-IFREMER UMR 10), Centre de Recherche sur les Ecosystèmes Marins et Aquacoles de L¹Houmeau, Place du séminaire BP5, 17137 L¹Houmeau, France
2IFREMER, Centre de Brest, DEL/EC-TP, BP 70, 29280 Plouzané, France
3Present address:
LBBM Laboratoire de Biologie et de Biotechnologie Marine, Université de Caen, Esplanade de la Paix, 14032, Caen Cedex, France

ABSTRACT: Microphytobenthos-macrofauna sediment interactions and their effects on sediment erodability were examined in laboratory experiments. Sediment beds were manipulated in a tidal mesocosm to produce diatom mats in exponential or in stationary phases of development after 6, 8 or 11 d of culture. These sediment beds were used in flume experiments to investigate the influence of bioturbation by the gastropod Hydrobia ulvae on both sediment and pigment resuspension as a function of the physiological state of the microphytobenthic mats. In most experiments, only a surface layer was resuspended. A model was used to analyse in detail the contribution of each variable to this surface-layer erosion. Bioturbation was the major factor controlling resuspension, and its extent was influenced by sediment density and the growth stage of the microphytobenthos. The amount and extent of bioturbation is assumed to be influenced by sediment density and chlorophyll a content. Snail bioturbation can, in turn, influence the amount of microalgal resuspension. The quantity of pigment resuspended due to bioturbation increased by a factor of 15 when the diatom mats were in exponential growth stages. However, as the age of the mat increased, the influence of bioturbation on pigment resuspension declined. When the mats became senescent, Type I erosion occurred with erosion rates high enough to obscure any effects of bioturbation. To summarise, we assume that there are 2 causes of microphytobenthos resuspention, depending on the physiological state of the mat: (1) in the exponential phase, bioturbation substantially affects the resuspension of pigments which are present in the surface layer (the biogenic fluff layer) and (2) in the senescent phase, the increase in bed roughness and water content renders the mat fragile, leading to bed erosion.

KEY WORDS: Resuspension · Microphytobenthos · Physiological state · Hydrobia ulvae · Model · Erosion · Bioturbation · Sediment

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