Continuous in situ monitoring of sediment deposition in shallow benthic environments

Abstract

Sedimentation is considered the most widespread contemporary, human-induced perturbation on reefs, and yet if the problems associated with its estimation using sediment traps are recognized, there have been few reliable measurements made over time frames relevant to the local organisms. This study describes the design, calibration and testing of an in situ optical backscatter sediment deposition sensor capable of measuring sedimentation over intervals of a few hours. The instrument has been reconfigured from an earlier version to include 15 measurement points instead of one, and to have a more rugose measuring surface with a microtopography similar to a coral. Laboratory tests of the instrument with different sediment types, colours, particle sizes and under different flow regimes gave similar accumulation estimates to SedPods, but lower estimates than sediment traps. At higher flow rates (9-17 cm s -1 ), the deposition sensor and SedPods gave estimates[109 lower than trap accumulation rates. The instrument was deployed for 39 d in a highly turbid inshore area in the Great Barrier Reef. Sediment deposition varied by several orders of magnitude, occurring in either a relatively uniform (constant) pattern or a pulsed pattern characterized by short-term (4-6 h) periods of 'enhanced' deposition, occurring daily or twice daily and modulated by the tidal phase. For the whole deployment, which included several very high wind events and suspended sediment concentrations (SSCs)[100 mg L . For the first half of the deployment, where SSCs varied from\1 to 28 mg L -1 which is more typical for the study area, the deposition rate averaged only 8 ± 5 mg cm. The capacity to measure sedimentation rates over a few hours is discussed in terms of examining the risk from sediment deposition associated with catchment run-off, natural wind/wave events and dredging activities.

Damselfishes alleviate the impacts of sediments on host corals

“…The sediment levels used in the present study were designed to reflect severe, prolonged sediment deposition. As such, the levels used herein are higher than average background sedimentation levels that are often reported [55,57]. However, prior research has documented that select natural coral populations experience sediment deposition rates exceeding 200 mg cm −2 d −1 [33], which is considerably higher than the sedimentation levels used in the current experiment.…”

“…The above point also highlights the ongoing knowledge gaps in our basic understanding of sediment dynamics on coral reefs (i.e. understanding links between turbidity levels, sedimentation rates and sediment accumulation on the benthos) [55,57,105] and how best to quantify these processes [56,106]. Indeed, while sediment traps have been the mainstay of measuring sedimentation for decades, a burgeoning body of evidence is highlighting that they do not measure sedimentation, but instead quantify a 'trapping rate' and only trap a subset of the sediment passing over coral reef ecosystems [56,57,107].…”

“…In some circumstances high levels of sediment deposition on reefs is associated with increased turbidity levels [55], and such high turbidity levels can also alter fishes' behaviours [93,94]. For example, reduced foraging distances exhibited by damselfishes in high water flow conditions [43,95] or turbid water [96,97], will alter the nature of damselfish-coral interactions, resulting in variable amounts of sediment removed during sedimentation events [16,43].…”

“…In addition, a level of 100 mg cm −2 d −1 was chosen to facilitate comparison with previous research that has explored the impacts of sediment deposition on corals under deposition rates ranging from 0.5 to 600 mg cm −2 d −1 in natural and controlled ex situ aquaria conditions [13,33,[52][53][54]. While maximum sedimentation rates of greater than 100 mg cm −2 d −1 have been reported from around Magnetic Island, near Orpheus Island on the inner-shelf of the GBR [55], sedimentation rates of 100 mg cm −2 d −1 over prolonged periods are generally considered severe within the context of coral reef ecosystems [33,53]. However, many published studies have relied on sediment trap data to set their experimental treatments, and while this method is commonly used to quantify sediment accumulation rates it can royalsocietypublishing.org/journal/rsos R. Soc.…”

Sediment deposition and coral smothering

“…mg cm -2 day -1 [5, 16–19], and in the project deposition was estimated using optical back scatter techniques [17, 20, 21], in some of the first measurements of their kind for a large dredging project. Some problems were encountered with the deposition sensors, which have since been modified and redesigned as described in [22]. Although deposition could not be measured in absolute terms (as mg cm -2 day -1 ), relative levels of deposition with distance from dredging could be calculated.…”