The use (and misuse) of sediment traps in coral reef environments: theory, observations, and suggested protocols
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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 (zo), 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.
KeywordsSediment trap Coral reefs Sedimentation Waves Currents Shear stress
Coral reefs typically grow in relatively clear, oligotrophic waters. Land-use practices such as overgrazing and coastal development can increase the supply of terrestrial sediment to the nearshore zone. This fine-grained terrestrial sediment can smother corals and increase turbidity, which in turn will decrease the light available for photosynthesis and can modify coral growth rates and forms, create physiological stress, and even cause coral mortality (Dodge et al. 1974; Acevedo et al. 1989; Fortes 2000; Nugues and Roberts 2003; Crabbe and Smith 2005; Mallela and Perry 2007). Dissolved heavy metals and other toxic substances often adhere to fine-grained sediment, with which they are then transported within the nearshore reef ecosystem (Dickson et al. 1987; Saouter et al. 1993; Bastidas et al. 1999). The potential impacts of sediment accumulation on coral reef health include the expenditure of energy by the coral to remove sediment particles, the loss of hard substrate for new coral recruitment, and the death of coral colonies that become buried (e.g., Rogers 1990; Fabricius 2005). For these reasons, measurements of sedimentation and turbidity have become important components of coral reef studies, and sediment traps have become a standard measurement tool.
Sediment traps are containers deployed in the water column for the purpose of (a) acquiring a representative sample of the material settling vertically through the water column and (b) providing an integrated particle collection rate and particle properties over the time of deployment. The first use of containers to collect settling particulate matter in lacustrine and marine environments occurred in the first half of the 20th century (see reviews in Bloesch and Burns 1980; Reynolds et al. 1980; Butman et al. 1986). Starting in the 1970s, sediment traps were increasingly used in relatively shallow (<40 m) hermatypic coral reef environments to measure the effects of “sedimentation” on corals (e.g., Maragos 1972; Aller and Dodge 1974; Randall and Birkeland 1978). Owing to their simple construction and relatively broad use, sediment traps are now used as a standard method for monitoring “sedimentation” in coral reef environments. Traps are also suggested as an environmental monitoring tool to determine the impact or effectiveness of land-use practices (Pernetta 1993; Rogers et al. 1994; Almada-Villela et al. 2003; Wilkinson et al. 2003; Hill and Wilkinson 2004; Jordan et al. 2010).
In much of the literature in which sediment traps were used to measure the effects of “sedimentation” on corals, it is clear from descriptions of deployments and interpretations of the resulting data that information derived from sediment traps has very frequently been misinterpreted or misapplied. These errors appear to result from (1) a lack of understanding of the history of sediment trap design and implementation and (2) a lack of published data and understanding of sediment and sediment trap dynamics in environments under the hydrodynamic conditions characteristic of coral reefs. Therefore, despite their widespread use, sediment traps have the potential for providing misleading and inaccurate information about particle behavior on shallow coral reef substrates. In this paper, we will briefly review the state of knowledge of sediment trap dynamics and their trapping efficiency, which have been primarily focused on small traps deployed high above the seabed in relatively deep water. This information will then be put into the context of our state of understanding of near-bed sediment dynamics in shallow-water environments that characterize coral reefs. This overview will be followed by the first synthesis of near-bed sediment trap data collected with concurrent hydrodynamic information in coral reef environments. Finally, we will discuss the implications of the older deep-water studies and the new measurements in coral reef environments presented here to potential protocols for the deployment of sediment traps in shallow (<40 m) coral reef environments and the interpretation of the resulting data.
Basic dimensions of each variable:
mass (M, e.g.—kg, lbs), time (T, e.g.—s, h), and length (L, e.g.—cm, ft)
P = mass of particles trapped per unit area per unit time (M L−2 T−1)
H = trap height (L)
D = trap mouth diameter (L)
ρf = fluid density (M L−3)
μf = fluid viscosity (M L−1T−1)
uf = horizontal flow velocity at the height of the top of the trap (L T−1)
d = particle diameter (L)
ρp = particle density (M L−3)
Np = number of particles in the fluid per unit volume (L−3)
g = acceleration due to gravity (L T−2)
Settling velocities of sediment that characterize coral reef environments
Grain diameter (mm)
Carbonatea (cm s−1)
Volcanicb (cm s−1)
Influence of parameters that control wave-orbital velocities in coral reef environments
Parameter to vary
Wave height (m)
Wave period (s)
Water depth (m)
Elevation above bottom (m)
Wave-orbital velocity (cm s−1)
Because of the turbulent nature of flow over the trap mouth, eddies are shed from the top of the trap and new ones can form. The intensity of eddies and their frequency of shedding increases with increases in flow velocity toward and over the trap (Butman et al. 1986). The research by Gardner (1980a), Butman (1986), and Bale (1998) suggested that symmetric traps are the most efficient, since in multidirectional currents they have the same geometric properties relative the flow in all directions. Not only is the shape of the trap important, but also, as Gardner (1980a) and Nodder and Alexander (1999) show, individual traps deployed on the same array can affect the relative trapping efficiency of the adjacent traps by disturbing the mean flow field around them. They proposed a minimum of 3 D separating traps in the cross-stream direction (assuming uf direction is known and unidirectional) and 10 D separating traps in the downstream direction to eliminate flow disruption by the adjacent traps.
Using the above 9 basic independent parameters describing trap geometry, the fluid, and the particles entrained in the fluid, Gardner (1980a, b), Butman (1986), and Butman et al. (1986) defined trapping efficiency (E) as the ratio of actual trapping to the trapping rate when uf = 0. They also suggest that E should be a function of six dimensionless parameters: S, Fr, Rt, Tr, Rp, and Pr. Those authors also provide comprehensive reviews of published and laboratory data addressing a number of these dimensionless parameters. These parameters are as follows:
Four of the six parameters that govern E are functions of the environment where the trap is deployed. However, there are two parameters that researchers can control: D and Tr (Eq. 7). Based on previous research addressing flow over traps (e.g., Gardner 1980a; Butman 1986; Baker et al. 1988; Gust et al. 1996), Jurg (1996) suggested a minimum D on the order of 5 cm. Gardner (1980a, b) considered Tr as the controlling factor in P, with the most efficient trap having a Tr of 3:1; White (1990) suggested the use of Tr of at least 3:1, and preferably 5:1 in energetic environments. Gust et al. (1996), investigating E in turbulent flows that more closely approximate the conditions observed over coral reefs, suggested that the unsteady turbulent eddies that form in the flow boundary layer that develops over the mouth of the trap can propagate down to more than 7 D into the trap (Fig. 3) and thus suggested a Tr > 7:1. In contrast to earlier studies, Gardner et al. (1983) and Baker et al. (1988) worked in more energetic environments and suggested that traps were likely to preferentially collect sediment with coarser d. This is because sediment with larger d has higher ws than finer particles for a given grain shape and density; particles with slow ws relative to the circulation within the trap have a greater chance to escape with the exiting turbulent flow. This results in an underrepresentation of finer particles (silts and clays, d < 0.063 mm) in the trapped sample.
Finally, Gardner (1980b), Butman et al. (1986), and others make the point that sediment traps often only vaguely approximate the amount and type of sediment that is actually deposited on the seabed. In energetic environments that typically characterize coral reefs, resuspension is common (e.g., Ogston et al. 2004; Storlazzi et al. 2004, 2009b), resulting in inferred “sedimentation rates” that may greatly exceed those from the influx of new particles actually depositing and remaining on the seabed. Further, the flux of particles past a site (q > 0) should result in some accumulation in traps, even if the particles never actually settle on the seabed. Gardner (1980b) stated “the flux of new particles to the sediment surface is not necessarily equal to the net sedimentation rate of the region”.
The data presented here come from four studies in two island chains in the north Pacific Ocean: the Hawaiian Islands in the north-central Pacific Ocean (20–22° N, 156–160° W), and from Guam in the Mariana Islands in the northwestern Pacific Ocean (13° N, 145° E). The physical environment in both island chains are characterized by seasonal 5–10 m s−1 northeasterly trade winds that generate wave heights of 1 to 3 m with periods of 5 to 8 s and intermittent storms that generate wave heights of 3 to 6 m with periods of 10 to 18 s. Both island chains have mixed, semi-diurnal microtidal regimes, with the mean daily tidal range of approximately 0.6 m; the reefs that line these island’s shores range in depth from 1 to 40 m and host coral communities that vary considerably in terms of health. The study areas of both Hawaii and Guam are considered wave-dominated sites, in that the mean horizontal wave-orbital velocities and resulting shear stresses were greater than the mean current velocities and stresses during the study periods. Thus, while these results might not be applicable to reef passages with very energetic currents or some back-reef lagoons and deep embayments protected from waves, they likely characterize most fore reefs that are exposed to storm waves and trade-wind waves. For more information on the study areas, please see Storlazzi et al. (2004, 2009a, b, 2010) and Bothner et al. (2006).
Two types of sediment traps were used to collect suspended-sediment samples from the water column over coral reefs. Simple tube traps (STT), consisting of a plastic tube with D = 6.7 cm and H = 30 or 60 cm, were deployed with zo = 40 cm or 80 cm, respectively; while H and zo were varied between experiments, all traps for a given field experiment had the same H and zo. Additional STTs were sometimes deployed with zo = 140 cm to evaluate vertical gradients in trap collection rates. An 8-cm high baffle was placed in the top of each tube trap to reduce turbulence and minimize disturbance by aquatic organisms (Bothner et al. 2006). Programmable rotating sediment traps (RST) were deployed with zo = 140 cm. Each rotating trap consisted of a cylinder with dimensions of D = 20 cm and H = 75 cm equipped with a funnel in the lower 15 cm of the cylinder to direct settling sediment into one of 21 plastic bottles (500 ml). Sampling bottles were mounted on a carousel that rotated a new bottle under the funnel every ~4.5 days. The average daily trap collection rate (P) for both the STTs and RSTs was calculated by measuring the total mass of sediment in the trap or bottle and dividing by A and the duration of the collection period. The STTs and RSTs were generally deployed on the order of 80–110 days during the different experiments. It should be noted that these sediment trap deployments were designed to acquire suspended-sediment samples for compositional information and/or suspended-sediment flux calculations at specific locations and thus were not optimized for investigating the impacts of hydrodynamics and sediment dynamics on sediment trap dynamics.
Sediment grain size analysis was conducted on wet aliquots of the trap samples using sieving and Coulter counter techniques described by Poppe et al. (2000). Total carbon and carbonate carbon measurements were made using a Perkin Elmer CHN analyzer and a UIC coulometer, respectively. Total organic carbon (TOC) was determined by difference between total carbon and calcium carbonate (CaCO3). Critical shear stresses for the different types of sediment were calculated using the modified Shield parameter method of Madsen (1999).
Instruments were deployed concurrently at sediment trap locations to collect hourly in situ time-series measurements of tides, waves, currents, and turbidity. In order to compare the combined effect of mean near-bed currents (ucurrent) and horizontal wave-orbital velocities (uwave) to the data from the sediment traps, the combined horizontal flow velocity (uf) from the in situ data was calculated using Eq. 11. In order to determine whether the shear stress (force per unit area) applied by the waves and currents was a significant contributor to the sediment trap data, the total shear stress imparted on the seabed (τbed) was computed from the uwave and ucurrent data using the method presented by Ogston et al. (2004) that accounts for the combined effects of waves and currents. Because of biofouling, high-resolution turbidity data for the duration of sediment trap deployments are limited.
Owing to the different goals of the various experiments, uf and turbidity were not always collected nor were the sediment samples always processed for both d and sediment composition. Furthermore, it is important to highlight the large discrepancy in resolution of the datasets. The sediment traps provided one integrated sample of sediment that encompassed a range of d and grain compositions, which resulted in variations in ρp and ws for the period of deployment; the oceanographic instrumentation, on the other hand, provided hourly measurements of waves, current, flow, and sometimes turbidity. Thus, for the STTs, there were on average 2160 measurements of oceanographic parameters to compare to one sediment sample, and 108 oceanographic measurements for each of the samples from the RSTs (deployed for ~90 days and ~4.5 days, respectively). For more information on the sediment traps, oceanographic instruments, or data processing, please see Storlazzi et al. (2004, 2009a, b, 2010) and Bothner et al. (2006).
Controls on trap collection rate (P)
Using the RST data, at both of the exposed (higher uf or τbed) sites (Fig. 5b), P increased exponentially with increasing uf and τbed; the correlation was greater (r2 = 0.681; P < 0.001) for the south Kauai embayment and lower for the slightly less energetic south Molokai reef (r2 = 0.255; P < 0.02). Both relatively quiescent (lower uf or τbed) areas, however, show no significant trends in the data, with p values for the correlations between P and uf or τbed from both islands above 0.05.
Controls on grain size and composition of trapped sediment
A significant positive relationship between percent CaCO3 and uf and τbed (r2 = 0.590; P < 0.05; Fig. 9b) was found in the energetic (high uf or τbed) area off Kauai. No significant relationship was found in the energetic area off Molokai (r2 = 0.135) or at the relatively quiescent sites. Similar to relationship between CaCO3 and uf and τbed at the more energetic sites, the RST in the more energetic area off Kauai displayed a significant inverse relationship between the percent TOC and uf and τbed (r2 = 0.645; P < 0.05; Fig. 9c). When the uf and τbed were low, more low-density TOC matter (with slower ws) settled into the traps relative to CaCO3; at higher uf and τbed, the RSTs showed bias against slow ws particles as the TOC was diluted by resuspended CaCO3. As stated earlier, the south Molokai reef is relatively continuous and not impacted by major river discharge, whereas the Kauai settings were in close (<0.5 km and 2 km for the quiescent and energetic sites, respectively) proximity to a major river, and thus the linkages between the fluvial and coral reef systems were more direct.
Sediment grain size and composition: traps versus the seabed
Overall, our results show some patterns that are critical for understanding the information provided by sediment traps in coral reef settings. In more energetic areas, there appear to be positive relationships among P, uf, τbed, d, turbidity, and q and possibly relationships between d, percent CaCO3, and percent TOC and uf and τbed. These relationships suggest resuspension of material on the seabed may be an important contributor to P, d, and composition of the material collected in near-bed sediment traps in energetic areas. Conversely, in more quiescent areas, there are no clear relationships between P and uf, τbed, turbidity, d, or composition, suggesting that advection of material from elsewhere might influence P, d, and grain composition more than resuspension of material from the seabed. Although our data are limited, it also appears that P in both more energetic and more quiescent areas is inversely related to zo, similar to the observations made by Gardner et al. (1983). Together, this suggests that hydrodynamics, which vary significantly over relatively short distances in coral reef environments in both space and time (e.g., Wolanski 1994; Storlazzi et al. 2009b), strongly influence P and the composition of the material collected in sediment traps on hermatypic coral reefs.
Sediment are traps expected to preferentially collect coarser d because of their higher ws than finer particles, especially farther above the seabed where uf is greater (e.g., Gardner et al. 1983; Baker et al. 1988). The presence of significant volumes of finer (small d) material in sediment traps, especially when the seabed is substantially coarser, suggests that significant volumes (high Np) of small d sediment had advected through the area even though these particles do not reside on the seabed. Although small d material may not be observed on the seabed during the sampling period, it is likely suspended in waters over the reef, with the potential consequence of decreasing photosynthetically active radiation (PAR) and desorbing nutrients and/or contributing toxicants.
Interestingly, the influence of sediment trap design and energetics on sediment trap collection rates were first addressed about three decades ago (e.g., Tooby et al. 1977; Gardner 1980a, b; Butman et al. 1986), yet sediment traps are still commonly used in shallow coral reef environments that typically are much more energetic than the deep sea. Many authors have suggested that sediment traps should not be employed in uf greater than 20 cm s−1, yet typical current speeds in many coral reef environments often exceed 20 cm s−1 (e.g., Wolanski 1994; Lugo-Fernandez et al. 1998; Storlazzi et al. 2004). The complications of trap dynamics discussed by Gardner et al. (1983), White (1990), Jurg (1996), and others, however, do not include the effects introduced by waves that not only have wave-orbital velocities that generate uf greater than 20 cm s−1, but also because these orbital motions are flattened near the seabed, they result in fluid accelerations and decelerations over the mouth of the trap. Hydrodynamic models that describe trapping efficiency under accelerating and decelerating wave-induced motions and predict collection rates for a range of grain sizes are currently not available.
Sediment traps should be cylindrical, have D greater than 5 cm, and Rt greater than 5:1, preferably greater than 7:1 in areas where high collection rates in the trap reduce the effective Rt during the deployment period (e.g., Gardner 1980a, b; Butman 1986; Baker et al. 1988; White 1990; Gust et al. 1996; Jurg 1996; and, Bale 1998).
Sediment traps should be deployed with their trap mouths as the highest point in the flow so the trap mouths are not downstream of turbulent wakes caused by the structure (e.g., posts, rebar) used to secure the trap in its location (e.g., Tooby et al. 1977; Gardner 1980a, b; Butman 1986; Butman et al. 1986; and, Gust et al. 1996).
In experiments that use multiple traps, all traps should be deployed with their mouths at the same zo (data provided here; theory)
If possible, a sample of the surrounding seabed material should be processed using the same methods used on the material collected in the sediment trap. This will aid in identifying the source (e.g., resuspension of seabed material or advected from elsewhere) of the trapped material (data provided here).
The amount of particles collected in the trap over the duration of the deployment should be properly referred to as a “trap collection rate” or “trap accumulation rate” rather than a “sedimentation” rate. Sediment traps do not measure net sedimentation in a shallow, energetic marine system (theory).
In locations where the instantaneous combined current and wave-orbital speeds (and thus resulting uf) are greater than 10–20 cm s−1 or sufficient to resuspend the adjacent seabed material (often as evident by a rippled seabed or noticeable wave surge at the sea floor), the trapped material should only be used to provide samples of suspended sediment for physical and chemical analyses to compare to seabed samples if hydrodynamic data are unavailable (theory).
Comparison of “trap collection rate” or “trap accumulation rate” from sediment traps of different design (H, D, or Rt), different deployment parameters (zo), or different locations (d) should not be made without conducting a specific calibration experiment. The experiment should include traps of different designs in a single location where comprehensive hydrodynamic properties are simultaneously measured. Further, it should be made clear that sediment traps can, at best, provide a relative indication of corals’ exposure to suspended sediment (theory).
Even with such standardization and with a thorough understanding of the hydrodynamic processes involved, variability in the wave field, currents, and sediment distribution over a range of spatial scales, as well as poorly understood trapping dynamics in coral reef environments, makes interpretation of sediment trap results complicated. The data presented here, in conjunction with past studies, suggest that sediment trap collection rates are much more apt to record information on suspended-sediment dynamics than to provide any useful data on sedimentation. If sediment traps are to be used in energetic coral reef environments, the limitations discussed in this paper must be considered when the sediment trap results are interpreted. In light of our analyses, we recommend that prior research results in the literature be interpreted carefully and with recognition that there may be irregularities in the trapping technique or in the application to understanding coral reef processes.
This work was carried out as part of the US Geological Survey’s Coral Reef Project as part of an effort in the United States and its trust territories to better understand the effects of geologic processes on coral reef systems. Joshua Logan, Rick Rendigs, Kathy Presto, Thomas Reiss, and David Gonzales (USGS) assisted with the fieldwork and instrumentation. Michael Torresan, Michael Casso, Sandra Baldwin, Olivia Buchan, Harland Goldstein, Kate Mc Mullen, Jiang Xiao (USGS) and Steve Manganini (WHOI) performed the laboratory analyses on the sediment samples. We would also like to thank Amy Draut (USGS), Jon Warrick (USGS), and two anonymous reviewers who contributed numerous excellent suggestions and a timely review of our work.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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