Smothering of estuarine sandflats by terrigenous clay: the role of wind-wave disturbance and bioturbation in site-dependent macrofaunal recovery

Abstract

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.

Terrestrial deposits on intertidal sandflats: sediment characteristics as indicators of habitat suitability for recolonising macrofauna

“…Although the plots stayed proud of the surrounding sandflat, as time progressed their diameters shrank due to build-up of marine sediments at the plot edges. Unlike a previous experiment (Norkko et al 2002), they did not become covered by sediment moving with the bedload, nor were they eroded by wave action. The only visible change in the terrestrial sediment plots was the appearance of numerous tiny burrows in the clay layers on Day 23 (up to 142 burrows 0.1 m -2 ), caused by a large influx of juvenile crabs.…”

“…The terrestrial sediments induced hypoxia and anoxia on the sandflat, killed all fauna resident under the deposits, affected biogeochemical fluxes, and essentially altered the habitat. While recovery was still incomplete at the end of the experiment, Norkko et al (2002) were able to illustrate that remobilisation of the terrestrial sediments via bioturbation by crabs, and transport of ambient sediments on to the surface of the deposits with bedload, were important facilitators of macrobenthic recovery.…”

“…Slow recovery has also been noted following other naturally occurring large-scale deposition events (Foster & Carter 1997, Burd et al 2000. In an experiment designed to mimic the deposition of terrestrial sediments on an intertidal sandflat, Norkko et al (2002) monitored the response and subsequent recovery of the macrofaunal community for 13 mo. The terrestrial sediments induced hypoxia and anoxia on the sandflat, killed all fauna resident under the deposits, affected biogeochemical fluxes, and essentially altered the habitat.…”

“…microbial composition, nitrogen [N] and phosphorus [P] content, nutritive value to benthos) composition. They frequently contain a large proportion of silt and clay-sized particles, are enriched with organic matter, and levels of high-quality food (estimated by chl a content) are significantly depleted (Norkko et al 2002). Once deposited on a sandflat, their properties will be further modified.…”

Subtle and negligible effects of rainfall on estuarine infauna: evidence from three years of event-driven sampling

“…Previous studies also report changes in assemblages attributable to freshwater pulses (Hoese 1960, Tettelbach et al 1985 sedimentation events (Peterson 1985) or wind-driven waves (Thrush et al 1996, Norkko et al 2002. Experimental work has shown that deposition of terrestrial sediments, mimicking storm-induced depositions, can cause decreased diversity in estuaries (Norkko et al 2002, Lohrer et al 2004). Conversely, Dobbs & Vozarik (1983) reported no significant effect of a single storm event on the macrobenthos at a shallow subtidal study site, although the authors postulated that effects may have been more pronounced at more exposed sites.…”