Suspended sediment at the watershed scale has played a critical role in sediment pollution, water-quality degradation, and the impairment of riparian ecosystems, and thus has been widely studied in many disciplines. This paper synthesizes a variety of methods adopted in suspended sediment monitoring, estimation and modelling for understanding sediment transport processes and determining the suspended sediment load. Methods for sediment monitoring are described in terms of direct and indirect approaches. Estimation of suspended sediment load is commonly achieved by establishing a sediment rating curve. Different approaches toward the establishment of a sediment rating curve are examined thoroughly. Techniques of sediment modelling are summarized via depiction of various hydrological and sediment models at the watershed scale. The paper ends with the discussion of future developments in suspended sediment studies at the watershed scale.

Understanding watershed suspended sediment transport

Optical turbidity sensors have been successfully used to determine suspended sediment flux in rivers, assuming the relation between the turbidity signal and suspended sediment concentration (SSC) has been appropriately calibrated. Sediment size, shape and colour affect turbidity and are important to incorporate into the calibration process. This study evaluates the effect of SSC and particle size (i.e. medium sand, fine sand, very fine sand, and fines (silt + clay)) on the sensitivity of the turbidity signal. Three different turbidity sensors were used, with photo detectors positioned at 90 and 180 degrees relative to the axis of incident light. Five different sediment ratios of sand:fines (0:100, 25:75, 50:50, 75:25 and 100:0) were also evaluated for a single SSC (1000 mg l-1). The photo detectors positioned at 90 degrees were more sensitive than sensor positioned at 180 degrees in reading a wide variety of grain size particles. On average for the three turbidity sensors, the sensitivity for fines were 170, 40, and 4 times greater than sensitivities for medium sand, fine sand, and very fine sand, respectively. For an SSC of 1000 mg l-1 with the treatments composed of different proportions of sand and fines, the presence of sand in the mixture linearly reduced the turbidity signal. The results indicate that calibration of the turbidity signal should be carried out in situ and that the attenuation of the turbidity signal due to sand can be corrected, as long as the proportion of sand in the SSC can be estimated.

Effects of suspended sediment concentration and grain size on three optical turbidity sensors

Although particulate organic carbon (POC) in sediments is derived from diverse sources, characteristics of bulk POC are frequently used as indicators of the ‘quality’ of organic matter potentially available to sedimentary microbial communities. In order to investigate the extent to which characteristics of POC relate to sedimentary metabolism, the rates of the initial and terminal steps of organic carbon remineralization (extracellular enzymatic hydrolysis, and sulfate, iron, and manganese reduction, respectively) were compared at three sites in Skagerrak dominated by different terminal remineralization processes. In parallel, dissolved organic carbon (DOC) inventories and bulk POC characteristics were assessed. At all three sites, bulk characteristics of POC were similar, with C/N close to 12, low sedimentary amino acid content, and moderate concentrations of total hydrolyzable carbohydrates. On average, just 12% of POC was characterizable as carbohydrates or amino acids. These characteristics are frequently considered typical of unreactive or ‘low quality’ organic matter. At all three sites, however, organic carbon remineralization (measured as CO 2 production and sediment O 2 uptake) was quite high relative to other locations with similar bulk characteristics. A comparison of DOC inventories with rates of terminal remineralization demonstrated that at the three sites, on average 27, 8, and 31% of the sedimentary DOC pool must be turned over on a daily basis in order to support terminal respiration. Extracellular enzymatic activity, calculated as potential carbon turnover, was sufficient to support these rates. At these sites, standard chemical characterization of bulk POC does not reflect the reactivity and availability of substrates to the sedimentary microbial community. Carbon remineralization is likely fueled by a small fraction of POC, not distinguishable by measurement of bulk parameters, which is rapidly cycled through the DOC pool.

Carbon cycling in a continental margin sediment: contrasts between organic matter characteristics and remineralization rates and pathways

Bio-optical spectral properties were determined on fresh suspensions of Trichodesmium spp. collected in a tropical lagoon and put in seawater tanks (total chlorophyll concentrations range between 0.1 and 3.8 mg m -3 ). The spectrum of the backscattering coefficient was a hyperbolic function with a slope of 1.2, often showing troughs at 440, 550 and 676 nm, due to absorption peaks of chlorophyll and phycoerythrin. The absorption spectrum computed with a specific beta correction for Trichodesmium, showed a blue to red ratio (B/R) equivalent to the one of a single colony (B/R=2), and also showed the double peak of mycosporine-like amino acids (MAA's, 330 and 360 nm). The CDOM absorption spectrum showed minor MAA peaks when cyanobacterial concentrations were above 1 mg Chl a m -3 . The chlorophyll a-specific backscattering and absorption coefficients at 442 nm were respectively 0.0126 m

Bio-optical properties of the marine cyanobacteria Trichodesmium spp.

Although sound has been applied to the study of sediment transport processes for a number of years, it is acknowledged that there are still problems in using the backscattered signal to measure suspended sediment parameters. In particular, when the attenuation due to the suspension becomes significant, the uncertainty associated with the variability in the scattering characteristics of the sediments in suspension can lead to inversion errors which accumulate as the sound propagates through the suspension. To study this attenuation propagation problem, numerical simulations and laboratory experiments have been used to assess the impact unpredictability in the scattering properties of the suspension has on the acoustically derived suspended sediments parameters. The results clearly show the commonly applied iterative implicit inversion can lead to calculated sediment parameters, which become increasingly erroneous with range, as the sound propagates through the suspension. To address this problem an alternative approach to the iterative implicit formulation is investigated using a recently described dual frequency inversion. This approach is not subject to the accumulation of errors and has an explicit solution. Here the dual frequency inversion is assessed and calculated suspended sediment parameters are compared with those obtained from the iterative implicit inversion.

/pdf/acoustic-inversions-for-measuring-boundary-layer-suspended-85en8v6pki.pdf

Acoustic inversions for measuring boundary layer suspended sediment processes

Spectral optical backscatter of sand in suspension: effects of particle size, composition and colour

Des recherches sur la consommation d'oxygene des sediments, les productions de dioxyde de carbone et la reduction des sulfates ont ete menees dans les sediments vaseux de la marge continentale de Nouvelle Ecosse, afin d'examiner les variations saisonnieres et temporelles de la mineralisation du carbone. Les differences dans la texture des sediments sont causees principalement par les variations de taille et d'abondance des pelotes fecales, et les contenus en carbone et en azote sont plus bas au niveau du talus que sur le plateau continental. Les flux mesures sur un navire a partir d'incubations de sous-echantillons provenant d'un carottier-boite sont compares aux valeurs deja publiees obtenues au cours des memes campagnes mais avec des microelectrodes, par analyses de profiles verticaux realises sur l'eau interstitielle et par incubations d'echantillons provenant d'un carottier multitubes. La mineralisation du carbone mesuree soit par la consommation d'oxygene soit par la production de CO 2 n'est generalement pas significativement differente pour une meme station. La consommation d'oxygene est plus elevee a la station la moins profonde (Emerald Basin, 230 m) et moindre sur le talus (800 m), avec des valeurs intermediaires dans la Laurentia Trough (500 m), mais les differences temporelles dans les flux ne sont pas les memes a toutes les stations. Les taux de consommation d'oxygene mesures a bord a partir d'incubations de sous-echantillons d'un carottier-boite sont generalement identiques a ceux obtenus par incubations d'echantillons provenant d'un carottier multitubes. Les taux de consommation d'oxygene determines par microelectrodes et les flux de CO 2 obtenus pour l'eau interstitielle sont plus bas que ceux issus d'incubations, probablement en raison de la non prise en compte de la bioturbation dans les calculs realises sur les incubations. Le metabolisme benthique total est domine par la reduction des sulfates a toutes les stations jusqu'a au moins 35 cm au-dessous de la surface du sediment, avec a nouveau des taux plus importants sur le plateau que sur le talus. Malgre une plus grande profondeur au niveau du talus, comparee a Emerald Basin, la dimution de la mineralisation du carbone est faible, ce que suggere un apport lateral significatif au talus en provenance du plateau de Nouvelle Ecosse. En comparaison avec les sables qui recouvrent la plus trande partie du plateau, l'importance relative des sediments fins de celui-ci est accentuee car les processus de mineralisation dans ces substrats (particulierement la reduction des sulfates) impliquent une plus grande hauteur de la colonne de sediment que dans les sables, pauvres en matiere organique. En outre, la mineralisation relativement elevee sur le talus doit etre prise en compte dans les budgets de carbone de la marge continentale, en particulier dans le cas d'un transfer significatif en dehors du plateau continental.

Sulfate reduction and total benthic metabolism in shelf and slope sediments off Nova Scotia

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Spectral optical backscatter of sand in suspension: effects of particle size, composition and colour

Sulfate reduction and total benthic metabolism in shelf and slope sediments off Nova Scotia