The use (and misuse) of sediment traps in coral reef environments: theory, observations, and suggested protocols

Abstract

Sediment traps are commonly used as standard tools for monitoring ''sedimentation'' in coral reef environments. In much of the literature where sediment traps were used to measure the effects of ''sedimentation'' on corals, it is clear from deployment descriptions and interpretations of the resulting data that information derived from sediment traps has frequently been misinterpreted or misapplied. Despite their widespread use in this setting, sediment traps do not provide quantitative information about ''sedimentation'' on coral surfaces. Traps can provide useful information about the relative magnitude of sediment dynamics if trap deployment standards are used. This conclusion is based first on a brief review of the state of knowledge of sediment trap dynamics, which has primarily focused on traps deployed high above the seabed in relatively deep water, followed by our understanding of near-bed sediment dynamics in shallow-water environments that characterize coral reefs. This overview is followed by the first synthesis of near-bed sediment trap data collected with concurrent hydrodynamic information in coral reef environments. This collective information is utilized to develop nine protocols for using sediment traps in coral reef environments, which focus on trap parameters that researchers can control such as trap height (H), trap mouth diameter (D), the height of the trap mouth above the substrate (z o ), and the spacing between traps. The hydrodynamic behavior of sediment traps and the limitations of data derived from these traps should be forefront when interpreting sediment trap data to infer sediment transport processes in coral reef environments.

SedPods: a low-cost coral proxy for measuring net sedimentation

“…5. The round shape limits bias caused by shifting current directions; the flat surface limits turbulence by flows (see Storlazzi et al 2011 for more details on this topic). 6.…”

“…As summarized by Storlazzi et al (2011), the devices most commonly used to gather this information, sediment tube traps, provide commonly misinterpreted results. We have sought to address the need for a more realistic representation of net particle accumulation on coral surfaces using recoverable sediment pods (SedPods).…”

“…Sediment traps provide some useful information, particularly in settings where wave resuspension is not significant, but they have been shown to give misleading results in certain situations about the quantity and character of sediment that is actually being deposited on coral surfaces. Storlazzi et al (2011) and Risk and Edinger (2011) summarize the physics of sediment trap processes, discuss the errors that occur in trap design, and emphasize the general inappropriateness of traps for reliable, quantifiable results in energetic environments, such as those found on many coral reefs. A key concern is that sediment traps do indeed ''trap'' sediment and thereby particles that might be only temporarily resuspended from the adjacent seabed-or coral surface-by passing waves or near-bottom currents.…”

Continuous in situ monitoring of sediment deposition in shallow benthic environments

“…Because of their simple design (open-ended tubes) and construction (e.g. PVC pipes) they have frequently been used for describing sedimentation rates on reefs (English et al 1997;Storlazzi et al 2011;Browne et al 2012). 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).…”

“…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).…”