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Optical properties are used to understand the spatial and temporal variability of particle properties and distribution within the Columbia River Estuary, especially in the salinity transition zone and in the estuarine turbidity maximum region. Observations of optical properties in the Columbia River Estuary are consistent with the established model that the river water brings more organic, smaller particles into the estuary, where they flocculate and settle into the salt wedge seaward of the density front. Large tidal currents resuspend mineral‐rich, larger aggregates from the seabed, which accumulate at the density front. Optical proxies for particle size (beam attenuation exponent γ and backscattering exponent γbb) are compared to conventional measurements. The γ and γbb are different to the expected trend with Sauter mean diameter Ds of suspended particles from low‐ to medium‐salinity waters (LMW). Ds increases in the LMW as does the γ derived from a WET Labs ac‐9, which indicates that the particle population dominating the ac‐9 is decreasing in size. The most likely explanation is that flocculation acting at LMW transfers mass preferentially from medium‐sized particles to large‐sized particles that are out of the size range to which the ac‐9 is most sensitive; γbb shows no trend in the LMW. Since γbb is a proxy of proportion of fine particles versus large flocs, the variation of γbb may be insensitive to changes in the medium‐sized particles. The overall results demonstrate that γbb is a reliable proxy for changes in particle size in a stratified environment. Plain Language Summary: Suspended particles delivered by rivers affect water quality in estuaries, so effective and economical monitoring of the variability of particles is particularly important in environmental management. Direct measurements of suspended particle size, concentration, and composition from water samples require substantial time and labor, which has driven efforts to develop indirect, yet accurate, alternative measurements of particle properties. In this study, we used optical instruments to measure particle variability within the Columbia River Estuary. The observations demonstrate that the river water delivered smaller, organic particles to the estuary, where they aggregated with one another and deposited to the seafloor. Large tidal currents resuspended mineral‐rich, larger aggregates from the seafloor, and these aggregates accumulated in the region where river waters encountered coastal ocean waters that flowed into the estuary. We observed that optical measurements related to particle size were different from the expected trend in the transition zone between fresh, river water and salty, coastal water. This unexpected trend requires more study, but we propose that it was caused by preferential aggregation of medium‐sized organic‐rich particles into large particles. This process would affect the processing of carbon, nutrients, and contaminants in estuaries. Key Points: Optical properties provide a good representation of particle dynamics in the Columbia River EstuaryVariations of alternative optical proxies for particle size are different to the expected trend at salinity transition zoneObservations are consistent with a model of preferential flocculation of medium‐sized particles [ABSTRACT FROM AUTHOR]

In most aquatic environments, suspended sediment is composed of loosely packed particle aggregates, termed flocs that have variable apparent densities. The apparent density of flocs, which is defined as particle dry mass over wet volume, is an important variable because it affects settling velocity and vertical sediment flux. Two established methods exist for measuring apparent density. One method uses physical measurements of sediment mass concentration combined with measurements of particle volume concentration from optical instruments to estimate apparent density. This method is laborious because it requires the collection of water samples, so it is not conducive to construction of high-resolution time series of density. Another method uses video observations of particles in a settling column to measure particle size and settling velocity. These measurements are used to solve for apparent density according to Stokes Law. The goal of this study is to develop a new method that uses the ratio of particulate beam attenuation to particle volume to estimate apparent density of sediment in suspension. Data from five studies are used to compare density estimates with the new method to the previous methods. The new optical method produces apparent densities that are correlated linearly with measurements of the ratio of dry mass to wet volume. However, the new optical method produces density estimates that do not correlate with video estimates of apparent density. This lack of correlation is due to sampling bias of the video method, which has a relatively large lower limit of resolution in particle size. Development of a higher resolution camera would eliminate the current bias in particle size and would enable further assessment of the new optical method as an accurate proxy for apparent density. [ABSTRACT FROM AUTHOR]

Seasonal variability in the sediment dynamics of a Bay of Fundy tidal creek and salt marsh system was analyzed to better understand the ecomorphodynamics of a high suspended sediment concentration intertidal habitat. Data were collected over 62 tides for velocity, suspended sediment concentration, deposition, and grain size at four stations from the creek thalweg to the marsh surface. Five topographic surveys were also conducted throughout the 14-month study. Deposition rates per tide varied spatially from 56.4 g m -2 at the creek thalweg to 15.3 g m -2 at the marsh surface. Seasonal variations in deposition in the creek and marsh surface were from 38.0 g m -2 to 97.7 g m -2 and from 12.2 g m -2 to 19.6 g m -2 respectively. Deposition and erosion were greatest in late fall and winter. This seasonal change, led by higher suspended sediment concentrations, was observed in the creek and at the marsh bank but notably absent from the marsh edge and marsh surface. Sediments were predominantly deposited in floc form (76–83%). Because of high floc content, higher suspended sediment concentrations led to more rapid loss of sediment from suspension. With increasing sediment concentration, deposition increased in the tidal creek and at the marsh bank but not at the marsh edge or marsh surface. This suggests that in highly flocculated environments the water column clears fast enough that very little sediment remains in suspension when the water reaches the marsh and that the sediment concentration during marsh inundation is independent of the initial concentration in the creek. [ABSTRACT FROM AUTHOR]

Abstract: Across a limited depth range (5–10m) on many continental shelves, the dominant sediment size changes from sand to mud. This important boundary, called the sand–mud transition (SMT), separates distinct benthic habitats, causes a significant change in acoustic backscatter, represents a key facies change, and delimits more surface-reactive mud from less surface-reactive sand. With the goal of improving dynamical understanding of the SMT, surficial sediments were characterized across two SMTs on the Adriatic continental shelf of Italy. Geometric mean diameter, specific surface area (SSA), mud fraction (<63μm) and heavy metal concentrations were all measured. The SMT related to the Tronto River is identified between 15 and 20m water depth while the SMT associated with the Pescara River varies between 15 and 25m water depth. The sediment properties correlate with a new, process-based sedimentological parameter that quantifies the fraction of the sediment in the seabed that was delivered as flocs. These correlations suggest that floc dynamics exert strong influence over sediment textural properties and metal concentrations. Relative constancy in the depth of the SMT along this portion of the margin and its lack of evolution over a period during which sediment input to the margin has dramatically decreased suggest that on the Adriatic continental shelf energy is the dominant control on the depth of the SMT. [Copyright &y& Elsevier]

Abstract: Measurements of floc sizes, floc settling velocities and effective floc densities were obtained at three locations on the inner Adriatic shelf (Italy) in February and May/June 2003 using the in situ size and settling column tripod INSSECT (Mikkelsen et al., 2004. INSSECT—an instrument platform for investigating floc properties close to the seabed. Limnology and Oceanography: Methods 2, 226–236). Measurements were carried out in a water depth of 8–12m and the final data set comprised 2491 flocs. Relationships of observed floc size vs. floc settling velocity and floc size vs. effective floc density were similar to those observed by other authors working in similar settings. The raw data showed significant scatter around mean trends, indicating that only a relatively small fraction of variability in floc settling velocity and effective floc density was explained by floc size. For bin-averaged data, however, much of the variability in settling velocity and density was explained by floc size. On the Adriatic shelf mean floc settling velocities varied from 0.48 to 1.35mms−1, while mean effective floc densities varied from 8.1 to 27.5kgm−3; within the range reported by other authors. Analysis of variance showed significant differences in floc settling velocities, effective floc densities and floc size in space and time (seasons). Thus, floc settling velocities, effective floc densities and floc size on the inner Adriatic shelf could not be characterized by a common mean, but were more appropriately characterized by a range of values, varying in time and space. [Copyright &y& Elsevier]

Abstract: Fine-grained (<63 μm) particles in the aquatic environment flocculate into larger, porous entities (flocs). Flocculation models have been proposed wherein the particles in the water exist either as single grains or as part of flocs. This approach, however, contradicts the idea put forward by Eisma [Eisma, D., 1986. Flocculation and de-flocculation of suspended matter in estuaries. Neth. J. Sea Res. 20, 183–199.] that a flocculated suspension would consist of large porous, fragile flocs, macroflocs (>∼125 μm), made up of smaller, more sturdy flocs (microflocs, <∼125 μm) and single grains. This paper combines results from a LISST-100 laser diffraction particle sizer and a digital floc camera in order to produce full-size spectra covering in situ particle sizes >2.5 μm, the smallest size resolvable by the LISST-100. For the first time, this allows a detailed investigation of changes in floc volume during break-up and flocculation. In accordance with Eisma [Eisma, D., 1986. Flocculation and de-flocculation of suspended matter in estuaries. Neth. J. Sea Res. 20, 183–199.], flocs are divided into macroflocs (here >133 μm) and microflocs (36–133 μm). In addition, a single-grain fraction (<36 μm) is introduced as particles of this size are of importance for the optical properties of the water column. The variation of these three fractions over a range of forcing conditions is examined using a particle volume size range from 2.5 to 9900 μm. When stress increases, the volume occupied by macroflocs decreases while the volume occupied by microflocs increases. It is also demonstrated that when stress decreases, the volume occupied by macroflocs first increases and then decreases as flocs settle out during quiescent conditions. However, in general no overall relationship between stress and floc size was found because parameters other than stress influence floc size, most notably resuspension, settling, advection, and biological activity. On average, macroflocs make up 40–65% of the total suspended volume, whereas single grains and microflocs each make up 15–34% of the total suspended volume. Because an inverse relationship between floc size and density generally exists, this means that occasionally the majority of the suspended mass can be found in single grains and microflocs. This has implications for the optical properties of the water column as scattering is dominated by the smallest particles in suspension. The findings also have implications for existing flocculation models that would probably benefit from inclusion of the microfloc fraction. [Copyright &y& Elsevier]

Abstract: A LISST-100 in situ laser particle sizer was deployed together with a digital floc camera during field work in the Newark Bay area (USA) and along the Apennine margin (the Adriatic Sea, Italy). The purpose of these simultaneous deployments was to investigate how well in situ particle (floc) sizes and volume concentrations from the two different instruments compared. In the Adriatic Sea the two instruments displayed the same temporal variation, but the LISST provided lower estimates of floc size by a factor of 2–3, compared to the DFC. In the Newark Bay area, the LISST provided higher values of floc size by up to a factor of 2. When floc size was computed using only the overlapping size bins from the two instruments the discrepancy disappeared. The reason for the discrepancy in size was found to be related to several issues: First, the LISST measured particles in the 2.5–500μm range, whereas the camera measured particles in the 135–9900μm range, so generally the LISST should provide lower estimates of floc size, as it measures the smaller particles. Second, in the Newark Bay area scattering from particles >500μm generally caused the LISST to overestimate the volume of particles in its largest size bin, thereby increasing apparent floc size. Relative to the camera, the LISST generally provided estimates of total floc volume that were lower by a factor of 3. Factors that could explain this discrepancy are errors arising from the accuracy of the LISST volume conversion coefficient and image processing. Regardless of these discrepancies, the shapes of the size spectra from the instruments were similar in the regions of overlap and could be matched by multiplying with an appropriate correction coefficient. This facilitated merging of the size spectra from the LISST and the DFC, yielding size spectra in the 2.5–9900μm range. The merged size spectra generally had one or more peaks in the coarse end of the spectrum, presumably due to the presence of flocs. The fine end (<100μm) of the spectrum displayed a flat tail with equal concentration of particles in all size classes. Size spectra with this shape indicate that the classical Junge model for description of in situ particle size spectra is reasonable for particles smaller than 100μm but not for larger particles. Floc fraction was computed for the merged spectra by using a diameter-to-mass conversion and found to vary between 0.34 and 0.95, within the range reported by other authors. [Copyright &y& Elsevier]

Petroleum exploration on continental shelves raises concerns over potential impacts it may have on ecologically sensitive and valuable fishing grounds. Several studies demonstrate that drill mud discharged from platforms exhibit negative impacts on surrounding benthic communities. However, the dispersal and deposition mechanisms of such mud are poorly understood. This study investigates the evolution of disaggregated particle size distributions of bentonite enriched water-based drill mud in a suspension flowing over a flat sand bed using a laboratory flume. Observed size-specific decay rates of suspended sediment concentrations indicate that suspended grains deposit as single grains and flocs. A simple model that considers deposition of particles by gravitational settling of single grains and flocs yields estimates of decay rates in good agreement with the measurements. The proposed model provides a grain size-specific analysis of parameters characteristic of a poorly sorted aging suspension that have previously been undocumented. [Copyright &y& Elsevier]

Abstract: Entropy analysis has been used to classify in situ particle (floc) size spectra of suspended particles into groups based on similar distribution characteristics. Results revealed that the in situ spectra sorted into groups that reflected different forcing conditions (e.g. variations in turbulence). The different forcing conditions were not necessarily reflected in other commonly used distribution measures such as median floc diameter. This suggests that entropy analysis may be an effective approach for investigating the effect of changes in forcing conditions on floc size. It is hypothesized that it may be possible to derive the average shape of floc size spectra from measurement of the forcing conditions alone and subsequently derive parameters such as floc fraction, floc density, floc settling velocity and the optical properties of the water column from the average spectra. [Copyright &y& Elsevier]

Abstract: In situ technologies were employed to monitor suspended particle flocculation and floc settlement and utilization by a cohort of sea scallops (Placopecten magellanicus) during the 2000 spring phytoplankton bloom in Bedford Basin, Nova Scotia, Canada. The objectives were to determine the effect of bloom flocculation and settling on food acquisition and utilization by scallops, and to assess the potential role of flocculation in enhancing the bioavailability of trophic resources and particle-reactive contaminants to bivalve filter feeders. The development and flocculation of the phytoplankton bloom were monitored within the surface layer (10 m depth) by in vivo chlorophyll fluorescence and silhouette camera observations. Sedimentation rate, seston abundance and composition, and sea scallop functional responses were monitored at 20 m depth (below the bloom) to provide insight into the potential forcing of feeding and digestion processes by changes in the abundance, composition and properties of the ambient food supply. The bloom began in mid-March and median floc diameter at 10 m depth increased rapidly from 200 μm to greater than 400 μm between 21 and 28 March. Flocs were observed to be abundant in the surface layer up to 4 April. Daily vertical particle flux was high during the last week of March and declined to near zero by 1 April. Clearance rates of scallops held at 20 m depth were relatively high (average±S.D.; 11.7±4.0 L h−1) during the period of bloom settlement and declined rapidly to low levels (0.4±0.9 L h−1) after 31 March. Average absorption efficiency also declined (0.88±0.01 to 0.78±0.05) after bloom settlement. Daily biodeposition rates by scallops were poorly correlated with temporal variations in the quantity (total particulate matter and chlorophyll a concentration) or quality (organic content) of seston available to the scallops, but were significantly correlated with sedimentation rate. Comparison of disaggregated inorganic particle size distributions for suspended particulate matter, settled particles, and scallop feces indicated that fine-grained particles (1 to 4 μm) were effectively ingested by sea scallops—an indication of whole floc ingestion. The settlement of flocs produced during the spring bloom appears to be important in regulating this species physiological energetics and for enhancing the bioavailablility of fine particles (including picoplankton) and particle-reactive contaminants. [Copyright &y& Elsevier]