Sediment run-off from land has been recognised as a threat to the biodiversity of shallow estuarine and coastal areas. Extreme rainfall events can cause flooding and landslides, which may result in rapid deposition of fine terrigenous sediments and have serious impacts on benthic communities. We designed a field experiment to study the response of intertidal benthos to such depositions of terrigenous clay. The experiment was conducted at 2 contrasting intertidal sites: a sheltered muddy sand habitat and an exposed sand habitat influenced by wind waves. Terrigenous clay (50% water content) was deposited in replicated experimental plots (2 m diameter) at each site in layers 0, 3, 6 or 9 cm thick. The initial response of the resident macrofauna and subsequent recolonisation was monitored over a period of 408 d. Physical and chemical properties of the experimental plots and wave climate at each site were also measured. The experiment demonstrated highly deleterious effects of catastrophic terrigenous clay deposition on estuarine macrobenthic communities. At both sites following clay deposition, the numbers of individuals were reduced by more than 50% after 3 d and by more than 90% after 10 d, irrespective of clay thickness. Mud crabs Helice crassa were the only animals able to emerge through the clay layer. They also exhibited elevated densities in clay treatments over the course of the experiment. After 28 d, a storm occurred (maximum wave height 0.4 m, period = 6 s) which dispersed the clay deposits at the exposed sandy site. Recolonisation of the surficial sediments was rapid at the exposed site following this wind-wave disturbance. However, deeper-dwelling animals such as large bivalves had not recovered to levels observed in the control plots by the end of the experiment. At the more sheltered muddy sand site, the clay deposition resulted in long-lasting habitat change; Although the clay was gradually broken up and re-colonised by small crabs and surface dwelling macrofauna, recovery was still incomplete 408 d after deposition. The results emphasise the role of wind-wave disturbance and transport of sediments and macrofauna with bedload, and the importance of bioturbation by crabs as facilitators of macrobenthic recovery after disturbance.

[1] A series of controlled laboratory experiments have been conducted to investigate the backscatter of high frequency sound (3-5 MHz) from suspensions of fine sediment in its unflocculated (primary) state and at various levels of flocculation. The size and fall-velocity distributions of the flocs were determined using an optical system and a settling tube, thus allowing floc density to be determined. The measurements have conclusively demonstrated that the acoustic properties of the flocculated particles are not solely controlled by the primary particles; some aspect of the floc structure is influencing the scattering characteristics. The overall trend is for the form function (K s ) to increase as the degree of flocculation increases. This trend was also observed in the total scattering cross section ( t ) but this result is dependent on the assumption that viscous absorption for flocculated particles is negligible. The measured scattering properties are compared to the predicted values from two theoretical models, the elastic (ES) and fluid sphere (FS) models. While the results show that, in their current form, neither model is capable of adequately representing the scattering characteristics of a suspension of flocculated particles, the two models did provide upper (ES) and lower (FS) bounds to the measurements. In terms of the operational use of acoustics to measure the concentration of flocculated sediments, empirical relationships could be fitted to the observations but, until a better theoretical understanding of how sound interacts with flocculated particles is achieved, the fitting of such empirical relations may be somewhat premature.

29The development of a theoretical description of how sound interacts with flocculating 30 sediments has been lacking and this deficiency has impeded sound being used to extract 31 quantitative suspended sediment parameters in suspensions containing flocs. As a step 32 towards theoretically examining this problem a relatively simple heuristic approach has been 33 adopted to provide a description of the interaction of sound with suspensions that undergo 34 flocculation. A model is presented for the interpretation of acoustic scattering from 35 suspensions of fine sediments as they transition from primary particles, through an 36 intermediate regime, to the case where low density flocs dominate the acoustic scattering. 37The approach is based on modified spherical elastic solid and elastic fluid scatterers and a 38 combination of both. To evaluate the model the variation of density and compressional 39 velocity within the flocs as they form and grow in size is required. The density can be 40 estimated from previous studies; however, the velocity is unknown and is formulated here 41 using a fluid mixture approach. Uncertainties in these parameters can have a significant effect 42 on the predicted scattering characteristics and are therefore investigated in the present study.

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