Assessing the impacts of sediments from dredging on corals

Abstract

There is a need to develop water quality thresholds for dredging near coral reefs that can relate physical pressures to biological responses and define exposure conditions above which effects could occur. Water quality characteristics during dredging have, however, not been well described. Using information from several major dredging projects, we describe sediment particle sizes in the water column/seabed, suspended sediment concentrations at different temporal scales during natural and dredging-related turbidity events, and changes in light quantity/quality underneath plumes. These conditions differ considerably from those used in past laboratory studies of the effects of sediments on corals. The review also discusses other problems associated with using information from past studies for developing thresholds such as the existence of multiple different and inter-connected cause-effect pathways (which can confuse/confound interpretations), the use of sediment proxies, and the reliance on information from sediment traps to justify exposure regimes in sedimentation experiments.

Continuous in situ monitoring of sediment deposition in shallow benthic environments

“…The deposition sensor uses the OBS principle which is well known to be sensitive to the number, size, colour, density and shape of suspended particles and refractive index (Conner and De Visser 1992;Gibbs and Wolanski 1992;Thackston and Palermo 2000;Gray and Gartner 2010;Storlazzi et al 2015). The calibration experiments showed the instrument was reasonably insensitive to sediments of different texture and colour, in part because the PSDs used were quite similar and typical of those in dredging plumes (Jones et al 2016). The instrument was most sensitive to the light grey sediment from Davies Reef which is probably due to a higher level of reflection of the infra-red, indicating that, as with the use of nephelometers, site-specific calibration is needed depending on the expected type of sediment PSD (see Fig.…”

“…The suspended sediments can have a range of effects on filter-feeders and, by changing the quantity and quality of light, on photoautotrophs such as corals and seagrasses (Rogers 1990;Anthony and Connolly 2004;Erftemeijer and Lewis 2006;Foster et al 2010;Jones et al 2016). The sediments can also subsequently fall out of suspension, increasing sedimentation rates and making benthic organisms expend energy self-cleaning.…”

“…The sediments can also subsequently fall out of suspension, increasing sedimentation rates and making benthic organisms expend energy self-cleaning. Many studies have highlighted the significance of sedimentation on coral reefs (reviewed by Rogers 1990;Jones et al 2016), and sedimentation is '…considered one of the most Communicated by Geology Editor Prof. Eberhard Gischler widespread contemporary, human-induced perturbations on reefs… ' (ISRS 2004).…”

“…Sediment deposition, however, has proven difficult to measure at scales which are relevant to the physiology of local organisms (mg cm -2 d -1 , see review by Thomas and Ridd 2004). This is unfortunate, as at least for dredging, increased sedimentation and smothering is one of the key causal pathways leading to mortality of marine organisms in the short term (Jones et al 2016).…”

“…However, there are many problems associated with using sediment traps to quantify the vertical flux of material in shallow energetic environments that are typical of coral reef environments (Wolanski 1994;Larcombe et al 1995;Ogston et al 2004;Storlazzi et al 2011). These problems have been discussed many times in many different marine environmental settings (Hargrave and Burns 1979;Reynolds et al 1980;Butman et al 1986;Kozerski 1994;Jürg 1996;Buesseler et al 2007), including coral reefs (Ridd et al 2001; Thomas and Ridd 2004;Bothner et al 2006;Risk and Edinger 2011;Storlazzi et al 2011;Browne et al 2012;Jones et al 2016). Essentially, sediments entering traps have a lower chance of resuspension than sediment settling on the adjacent seabed, and traps therefore provide an estimate of gross rather than net sedimentation rate (see Cortes and Risk 1985).…”