T.J. Tolhurst

A comparison and measurement standardisation of four in situ devices for determining the erosion shear stress of intertidal sediments

Abstract Predictive modelling of estuarine sediment erosion and transport requires a description of the erosional properties of the bed. The two main variables of interest are the critical erosion shear stress (τcr) and the erosion rate (e). A number of different erosion devices exist to measure the erosion shear stress of intertidal sediments in situ. These devices apply different strategies to induce and measure erosion, and the area over which erosion is integrated varies greatly. In addition, the definition of erosion threshold differs between workers. This makes comparison of data collected from different devices very difficult. Four different types of erosion device, Microcosm system, In Situ Erosion Flume (ISEF), SedErode and cohesive strength meter (CSM) were used during the July 1997 EC INTRMUD Humber estuary (UK) field campaign. These devices were deployed simultaneously on the Skeffling intertidal mudflat to allow comparison of the data generated. This involved the comparison of suspended particulate matter (SPM) time series, the nature of the applied shear stress (τo) and the area over which erosion was integrated. The initial goal was to develop a standard analysis procedure for comparison of stability measurements. The erosion threshold calculated from area normalised suspended particulate matter (SPMn) time series was relatively comparable between devices especially between the Microcosm and ISEF. However, device size and natural sediment spatial heterogeneity affected the results. The erosion rate varied by orders of magnitude between the different devices. This variation seemed to be due to the considerable differences in device deployment time. In conclusion, SPM data from different devices are broadly comparable, whilst erosion rates are only comparable if the shear stress steps are of the same duration.

In situ versus laboratory analysis of sediment stability from intertidal mudflats

Abstract Measurements of intertidal mudflat erosion thresholds from in situ and laboratory erosion devices were contrasted. Alteration of the critical erosion shear stresses was potentially brought about either by physical disturbance (vibration, compaction and water loss), ongoing biological activity or changes in the behaviour of infaunal organisms during transport and resting of the excavated cores. In an initial experiment, box cores were collected from the Humber estuary Skeffling mudflats (April 1995) and transported back to the laboratory for measurement in a linear flume. These cores suffered visible disturbance during transport to the laboratory and their erosion thresholds were considerably higher than in situ data obtained by the Sea Carousel erosion device. In the main study, cylindrical cores collected in the Sylt-Romo Bight (June 1998) were collected and transported in a manner that minimised disturbance. The stability of these cores was measured with the EROMES laboratory erosion device and compared to near-by in situ measurements taken with the cohesive strength meter (CSM) erosion device. These devices use different criterion to calculate the erosion threshold (erosion rate and attenuation threshold, respectively), resulting in differences in the calculated erosion threshold. However, when an attenuation threshold was used for both devices the erosion thresholds were comparable. When disturbance of cores was minimised, in situ and lab erosion thresholds were comparable. However, user bias in site selection can influence results where there is spatial variation in sediment properties. Stability measurements should therefore be made on randomly selected sediment areas.

The influence of an extracellular polymeric substance (EPS) on cohesive sediment stability

Extracellular polymeric substances (EPS) are secreted by a variety of benthic organisms including diatoms and bacteria. These substances are often considered the primary mechanism by which intertidal benthic organisms increase sediment stability. The stabilisation is caused by physico-chemical interactions between clay minerals and EPS and physical strengthening and gluing by EPS strands. The effects of an EPS (xanthan gum) on the erosion characteristics (threshold and rate) of cleaned cohesive sediments were investigated in the laboratory for the first time using two erosion devices, and compared to in situ measurements on natural sediments. The addition of EPS was found to increase the erosion threshold and to decrease the erosion rate of the cohesive sediment, even at the lowest EPS content. Low-temperature scanning electron microscopy (LTSEM) images of the sediment revealed differences in the microstructure and EPS distribution between the five laboratory treatments and natural sediments. This study supports the contention that EPS secretion is partly responsible for the increases in sediment stability caused by many intertidal benthic organisms.

Estimation of surface chlorophyll‐a on an emersed mudflat using field spectrometry: accuracy of ratios and derivative‐based approaches

The accuracy of spectral indices and derivative‐based approaches for quantifying chlorophyll‐a on emersed intertidal sediments is explored. Reflectance measurements (350 nm–1050 nm) and collocated samples of sediment were collected during low tide. In the laboratory, the amount of chlorophyll in each sample was measured spectrophotometrically. Because the sediment grain‐size influences the brightness of the sediment and this is known to have an influence on vegetation indices, the proportion of sediment with a grain‐size of <63 μm was determined. Several spectral indices were evaluated, including ratios of visible bands, near‐infrared (NIR)/red bands and NIR/green bands. Two new vegetation indices are proposed: R562/R647 and R400/R500. Several derivative‐based approaches were explored, including derivative reflectance at individual wavelengths, the wavelength position of the red‐edge inflection point (REIP), and the area under the derivative curve. Indices constructed from NIR/red bands were weakly correlated with chlorophyll‐a (r 2 = 0.45–0.50). NIR/green ratios had the weakest correlation with chlorophyll‐a (r 2 = 0.03–0.18). Ratios of visible bands had the strongest relationships with chlorophyll‐a (r 2 = 0.65–0.68). The derivative measure with the strongest relationship to chlorophyll was the first derivative reflectance at 607 nm (r 2 = 0.78). The spectral indices and derivative‐based methods were evaluated by applying them to a test set of spectral data. R562/R647 was superior to other indices and estimated chlorophyll, on average, to within +/−1.71 μg cm−2 (9% of the observed range of chlorophyll present). Sediment grain‐size did not appear to have a consistent impact on any of the analytical methods tested.

The effects of rain on the erosion threshold of intertidal cohesive sediments

Intertidal sedimentary environments are complex systems governed by interactions between physical, chemical and biological processes and parameters. Tidally induced flow and wave action are known to be an integral driving force behind the erosion, transport, deposition and consolidation cycle (ETDC) of intertidal sediments. Whilst considerable advances have been made in understanding both the physical and biological processes and their interactions in these systems, it is clear that there are gaps in our understanding. One factor that has been largely ignored to date is that of rain. Visual observations in the field and associated data indicated that rain showers during low tide are correlated with a reduction in the erosion threshold of intertidal cohesive sediments. This paper presents preliminary field and laboratory data showing the importance of rain in reducing the erosion threshold of cohesive intertidal sediments. The implications for our knowledge of, and modelling of the ETDC cycle of cohesive intertidal sediments are discussed.

Characterizations of how species mediate ecosystem properties require more comprehensive functional effect descriptors

The importance of individual species in mediating ecosystem process and functioning is generally accepted, but categorical descriptors that summarize species-specific contributions to ecosystems tend to reference a limited number of biological traits and underestimate the importance of how organisms interact with their environment. Here, we show how three functionally contrasting sediment-dwelling marine invertebrates affect fluid and particle transport - important processes in mediating nutrient cycling - and use high-resolution reconstructions of burrow geometry to determine the extent and nature of biogenic modification. We find that individual functional effect descriptors fall short of being able to adequately characterize how species mediate the stocks and flows of important ecosystem properties and that, in contrary to common practice and understanding, they are not substitutable with one another because they emphasize different aspects of species activity and behavior. When information derived from these metrics is combined with knowledge of how species behave and modify their environment, however, detailed mechanistic information emerges that increases the likelihood that a species functional standing will be appropriately summarized. Our study provides evidence that more comprehensive functional effect descriptors are required if they are to be of value to those tasked with projecting how altered biodiversity will influence future ecosystems.

The effects of tidally-driven temporal variation on measuring intertidal cohesive sediment erosion threshold

Accurate measurement of intertidal sediment erodibility is essential for the development of meaningful and accurate models of sediment dynamics. Despite considerable advances in technology and methodology, the measurement of cohesive intertidal sediment erosion remains problematic. Sediment erodibility varies according to both physical and biological properties and processes. These cannot be considered in isolation, as they can interact to create both positive and negative feedbacks, resulting in seemingly idiosyncratic responses in the system. If working models of estuarine sediment dynamics are to be made, it is essential that the influence of these processes on both the measurements and the system itself be considered. Recent developments in measurement technology enable rapid measurement of sediment stability allowing temporal and spatial variability to be measured on a time scale of minutes. This paper reports temporal variability in cohesive intertidal sediment erosion threshold related to immersion and emersion, and the concomitant responses in selected sediment properties (carbohydrates, water content and chlorophyll). Erosion threshold tended to increase over emersion and decrease over immersion, although the patterns of change varied depending upon local conditions, and in one case there was no temporal trend. Temporal changes resulted in a range of measured erosion threshold, dependant upon the erosion device used. Modifications to existing methodology, in order to account for this variation, are proposed and implications for modelling erosion processes are considered.

Pigment fingerprints as markers of erosion and changes in cohesive sediment surface properties in simulated and natural erosion events

Abstract The pollutant dynamics of estuaries is closely associated with the transport of particulate matter and one problematic area is the quantification of the erosion, deposition and transport of intertidal sediment. The primary event in sediment erosion is the resuspension of surficial layers which are often densely colonized by assemblages of microphytobenthos and bacteria. The dominant microphytobenthos (Bacillariophyceae or diatoms) contain characteristic marker pigments, chlorophyll c and fucoxanthin. To date, few marker compounds have been used to characterize particulate matter and to monitor its transport. This study identifies pigments as useful marker compounds in monitoring erosive events, particularly at low erosion rates. Erosion can either be considered as occurring when the stress at the surface exceeds the threshold strength of the bed or as one side of a deposition/entrainment equilibrium always occurring at the bed surface. The present analysis indicates that an erosion threshold exists and that pigments derived from surface biofilms appear in the water column before the general failure of the bed. The result is a lower yield stress (U*crit) for biofilm components than for the sediments themselves, supporting the contention that the ‘yield stress’ of natural sediments has a strong biological component, in addition to purely physical control. Pigment fingerprinting is shown to provide information on the origin of suspended matter, including the suspension and deposition of particles. The potential for using microphytobenthic pigments as indicators of erosion events in laboratory simulations (erosion and respiration chambers) and in the field is discussed.

Lack of correlation between surface macrofauna, meiofauna, erosion threshold and biogeochemical properties of sediments within an intertidal mudflat and mangrove forest

This article describes the relationship between 10 selected properties of the sediments (chlorophyll a and b, colloidal and total carbohydrate, water concentration, sediment type, organic matter, erosion threshold and erosion rate) and meio- and macrofauna within and among three different habitats in an urbanized intertidal mudflat/mangrove forest in Tambourine Bay, Sydney Harbour, Australia. Many of the biogeochemical variables were significantly different among habitats, often grading from mudflat to mangrove canopy. In contrast to previous studies, patterns of distribution of macrofauna among habitats were weak. For the meiofauna, only copepods showed any significant difference among habitats, with the greatest numbers in the open mudflat habitat and least under the mangrove canopy. There was a gradient in fauna among the habitats; overall macrofauna abundances were greatest under the mangrove canopy and least on the mudflat, while meiofauna abundance was greatest in the pneumatophore habitat and least under the canopy. Correlations between fauna and properties of sediment were generally weak. When the habitats were analysed separately, some correlations were strengthened but relationships were inconsistent. Thus, while some taxa vary significantly among habitats there was not a strong relationship between biogeochemical properties and either macro- or meiofauna. This suggests that localised factors other than the measured properties of the sediments are driving patterns in fauna at these small scales, which requires further investigation to be unravelled.

High-resolution computed tomography reconstructions of invertebrate burrow systems

The architecture of biogenic structures can be highly influential in determining species contributions to major soil and sediment processes, but detailed 3-D characterisations are rare and descriptors of form and complexity are lacking. Here we provide replicate high-resolution micro-focus computed tomography (μ-CT) data for the complete burrow systems of three co-occurring, but functionally contrasting, sediment-dwelling inter-tidal invertebrates assembled alone, and in combination, in representative model aquaria. These data (≤2,000 raw image slices aquarium−1, isotropic voxel resolution, 81 μm) provide reference models that can be used for the development of novel structural analysis routines that will be of value within the fields of ecology, pedology, geomorphology, palaeobiology, ichnology and mechanical engineering. We also envisage opportunity for those investigating transport networks, vascular systems, plant rooting systems, neuron connectivity patterns, or those developing image analysis or statistics related to pattern or shape recognition. The dataset will allow investigators to develop or test novel methodology and ideas without the need to generate a complete three-dimensional computation of exemplar architecture.