The disaggregated inorganic grain size (DIGS) of bottom sediment analyzed with a Coulter Counter (CC) has been used to show that the fraction of sediment deposited in flocs (floc fraction) increased in both the near and far field after the introduction of open cage salmon aquaculture, altering benthic habitat and species composition. As a result, DIGS was identified as a potential indicator of regional environmental changes due to aquaculture. Laser diffraction is an attractive alternative to the CC because of its greater efficiency and larger size range. To determine if a laser diffraction instrument, Beckman-Coulter LS 13 320 (LS), could replace the CC within a Canadian national aquaculture monitoring program, the DIGS of 581 samples from five different regions in eastern Canada were analyzed with an LS and a CC. Results show that the LS could not be used to calculate floc fraction. Instead,

In the hyper-tidal Minas Basin located in the upper Bay of Fundy, Canada, the concentration of total suspended matter increases by an order of magnitude in winter as material from the expansive tidal flats is remobilized and storms increase cliff erosion. Tripods with a digital floc camera, LISST 100X Type B, optical backscatter sensors, settling velocity camera, ADCP and an in-situ water filtration system were deployed during winter and summer conditions to examine how this variation in concentration affects particle packaging and settling. In winter, the higher concentration leads to rapid formation of large, low density flocs with lower settling velocities during the brief periods of low stress between ebb and flood. In summer, the material in suspension is dominated by small aggregates with higher excess densities and higher settling velocities. After a 70 mm rainfall event, flocs with lower density, and lower settling velocities appear again. Flocculation rate increases with concentration leading to the rapid formation of flocs in winter. However, stresses are high, so sediment remains mobile in winter. In the transition from winter to summer concentration is initially high, floc formation and settling is rapid, and wave energy is reduced allowing material to accumulate on the flats. In summer, the concentration of the sediment remaining in suspension is too low for flocs to form and aggregates dominate the population of particles suspended by the tide. Only when an input of new sediment occurs do new flocs form.

Understanding the processes that lead to seasonal changes in grain size on muddy macro-tidal flat and channel complexes will assist efforts to predict future changes caused by climate change and construction of infrastructure like tidal power generators, wind farms, and causeways. Surficial sediment samples were collected for disaggregated inorganic grain size (DIGS) analysis every month for 1 year from a tidal flat and from a tidal channel and its banks in Kingsport, NS, Canada, which is located in the Minas Basin of the Bay of Fundy. A process-based parameterization of the DIGS distribution was used to determine floc fraction, representative of the percentage of fine-grained material deposited to the seabed in flocs. Small changes in floc fraction occurred seasonally in the channel. Values were higher in late winter and lower in late summer. A more pronounced seasonal variation in floc fraction was observed on the tidal flats. Observed changes in floc fraction correlated with elevation. Results from this study are discussed in terms of the controls on floc fraction which include suspended particulate matter concentration (SPM), turbulence, and stickiness in the water column, all of which can be used to understand the formative processes responsible for shaping the bottom sediment texture.

Floc size, density, and settling velocity were investigated in the Connecticut River estuary over 3 years spanning varying fluvial discharge regimes to determine the role of cohesive suspended particle characteristics in the sediment-transport patterns of an energetic estuary. Concurrent measurements of flow, bed stress, salinity, and suspended sediment concentration were used to identify primary controls on floc size variability. Water discharge ranged from 202 to 910 m3/s between the three sampling campaigns, and the timing of major sediment-discharge events preceding measurement periods from 23 to 162 days. Two distinct particle populations were observed under high and low sediment-discharge regimes. With abundant fluvial sediment input, flocculation occurred resulting in large, loosely packed flocs dominating the suspended signal (median sizes of 194–209 μm; median excess densities of 13–17 kg/m3). Following an extended period of low sediment discharge, small, dense aggregates resuspended from the bed were observed throughout the water column (median size of 171 μm and excess density of 60 kg/m3). The timing and partial decoupling of water and sediment discharge led to inter-annual patterns of particle packaging controlled by fresh sediment supply. When the estuary is “charged” with sediment following high discharge events, the characteristic large, less dense flocs with lower settling velocities primarily bypass the estuary. The similar disaggregated grain size distribution of the suspended material of the two regimes suggests the same source sediment is reintroduced to the estuary with the intrusion of the salt wedge, which extends farther up-estuary during low-discharge regimes. The fines repackaged as dense aggregates ultimately supply the channel margins and off-channel coves currently experiencing sediment accumulation.

Measurements of optical properties have been used for decades to study particle distributions in the ocean. They are useful for estimating suspended mass concentration as well as particle-related properties such as size, composition, packing (particle porosity or density), and settling velocity. Measurements of optical properties are, however, biased, as certain particles, because of their size, composition, shape, or packing, contribute to a specific property more than others. Here, we study this issue both theoretically and practically, and we examine different optical properties collected simultaneously in a bottom boundary layer to highlight the utility of such measurements. We show that the biases we are likely to encounter using different optical properties can aid our studies of suspended sediment. In particular, we investigate inferences of settling velocity from vertical profiles of optical measurements, finding that the effects of aggregation dynamics can seldom be ignored.

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 and backscattering exponent (bb)) are compared to conventional measurements. The and (bb) are different to the expected trend with Sauter mean diameter D-s of suspended particles from low- to medium-salinity waters (LMW). D-s increases in the LMW as does the 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; (bb) shows no trend in the LMW. Since (bb) is a proxy of proportion of fine particles versus large flocs, the variation of (bb) may be insensitive to changes in the medium-sized particles. The overall results demonstrate that (bb) is a reliable proxy for changes in particle size in a stratified environment. 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.

Optical instruments that measure the particulate beam attenuation (c(p)) and backscattering coefficients (b(bp)) have been used widely to estimate suspended particulate mass (SPM) in various environments. In pycnoclines, density differences can cause significant forward light scattering (schlieren). This fluid-induced scattering increases the cp measured by instruments with small acceptance angles that are open to the environment (not pumped). When the 1-m bin-averaged buoyancy frequency (N) exceeds 0.05 s(-1) in the Columbia River estuary, inaccurate estimates of cp are observed by such instruments. The schlieren phenomena do not affect the cp and bbp measured with a pumped transmissometer or a backscattering meter. It follows that to obtain a reliable proxy of SPM in a highly stratified environment, the use of either a backscattering sensor or a pumped transmissometer is required.

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 56.4gm-2 at the creek thalweg to 15.3gm-2 at the marsh surface. Seasonal variations in deposition in the creek and marsh surface were from 38.0gm-2 to 97.7gm-2 and from 12.2gm-2 to 19.6gm-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.

A laboratory study was carried out to examine the effect of bed sediment texture on the erodibility of salmon aquaculture waste fecal material and salmon feed pellets. Erodibility measurements of this material were made using a Gust microcosm erosion chamber and artificially composed substrates of mud, sand, sand and gravel, sand and cobble, and cobble. Results show that cumulative mass eroded (CME) and erodibility constant (M) can vary by up to an order of magnitude depending on substrate composition, with a mud substrate having higher CME and M values than those of substrates composed of sand, gravel and cobble. Findings from this study suggest that bottom sediment texture plays a major role in aquaculture waste resuspension and subsequent transport and that predictive models of the transport of aquaculture waste should include the erosion dynamics of bottom texture.

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.

JC Garwood, MSc. Thesis, 2013 The goal of this study was to investigate the effects of intertidal mudflat biofilms on sediment erosion in the Minas Basin of the Bay of Fundy, Canada. From April through November 2012, sediment cores were collected biweekly and eroded using a Gust microcosm. Half of the cores were eroded without undergoing prior treatment, while the other half were treated with bleach prior to erosion to destroy biofilms. Size-specific sediment retention by biofilms was evaluated by comparing the disaggregated inorganic grain size (DIGS) distributions of sediment resuspended from untreated and treated cores, while seasonal variation in natural sediment erodibility was assessed by focusing on the mass eroded from untreated cores only. Results show that biofilms preferentially retained clays and very fine silts (< 10 μm), and that overall sediment erodibility decreased from spring to fall. Results also indicate that abundance of the infaunal amphipod Corophium volutator and rainfall increased sediment erodibility. Abstract: JC Garwood, PS Hill, HL MacIntyre, BA Law. “Grain sizes retained by diatom biofilms during erosion on tidal flats linked to bed sediment texture” (Submitted to Continental Shelf Research, in review) JC Garwood, PS Hill, HL MacIntyre, BA Law. “Grain sizes retained by diatom biofilms during erosion on tidal flats linked to bed sediment texture” (Submitted to Continental Shelf Research, in review) Size-specific sediment retention by diatom biofilms was measured by eroding intertidal muds at increasing shear stresses (0.01-0.60 Pa) using a Gust microcosm. The grain sizes eroded from biofilm-covered sediment were compared to those from control cores from which the biofilms were destroyed using bleach. Biofilms were quantified using carbohydrate measurements. Cores from an intertidal mud flat in the Minas Basin of the Bay of Fundy (Canada) showed biofilms preferentially retained clays and very fine silts relative to fine and medium silts. In contrast, prior field observations on an intertidal sand flat indicated that fine and medium silts were preferentially retained by biofilms relative to clays and very fine silts. These contrasting results suggest a link between size-specific sediment retention and sediment texture, where sand biofilms retain coarser, noncohesive sediment, while mud biofilms retain finer, cohesive sediment. This relationship implies that biofilms contribute to a positive feedback that maintains sediment texture in a stable state. 2. Seasonal and spatial variability in grain size within the defined inshore areas (tidal flats, creeks) Abstract: BA Law, TG Milligan, PS Hill, V Zions, JC Garwood. “Temporal and spatial changes in grain size on a macro-tidal channel-flat complex: Results from Kingsport, Nova Scotia, Bay of Fundy” (Internal Review DFO, to be submitted to Continental Shelf Research after review) BA Law, TG Milligan, PS Hill, V Zions, JC Garwood. “Temporal and spatial changes in grain size on a macro-tidal channel-flat complex: Results from Kingsport, Nova Scotia, Bay of Fundy” (Internal Review DFO, to be submitted to Continental Shelf Research after review) In April 2012, a study was initiated to examine the seasonal change in grain size on a muddy macro-tidal flat and channel complex in Kingsport, N.S. Surficial sediment samples were collected for disaggregated inorganic grain size (DIGS) analysis every month for 1 year from a tidal flat and from a tidal channel and its banks. The monthly sampling was completed in March 2013. Sediment grain size on the tidal flat correlates with distance to the nearest channel, and flocculation plays a major role in sediment deposition. These results differ from those from Willapa Bay, Washington, USA, which a meso-tidal channel-flat complex that showed no relationship between sediment grain size, floc fraction and distance to the nearest channel. Findings from this study are discussed in terms of the ability of ecosystems to maintain a stable state and with regards to the development of tidal power in the Minas Passage. 3. Temporal variability in in-situ suspended sediment concentration (offshore, eg. Minas Basin) 1) Abstract: L Ashall, RP Mulligan, BA Law. “Variability in Suspended Sediment Concentrations in the Minas Basin, Bay of Fundy, and Implications of change due to Tidal Power Extraction” (In Review for submission to Coastal Engineering) The Bay of Fundy in eastern Canada exhibits the world’s largest tidal range and exchanges approximately 110 billion tonnes of water twice a day with tidal currents up to 5ms -1 making it an ideal place for tidal power extraction using Tidal In-Stream Energy Conversion (TISEC) devices in the Minas Channel. Field observations collected from ship-based and bottom-moored sensors over an 8-day period in 2013 are used to validate a 3D hydrodynamic and sediment transport model with measurements of water levels, current profiles, waves and suspended sediment concentration profiles. The sediment conditions are initialized using a bi-modal sediment distribution map and the model simulates both cohesive and non-cohesive sediments in the Minas Basin. Modelled suspended sediment concentrations are compared horizontally, vertically, and temporally to observations and the results indicate strong data-model agreement for suspended sediment concentrations (SSC) over a range of 5 to 287 mgL -1. The implications of constructing a large-scale turbine farm within the Minas Channel and the impacts on suspended sediment within the Minas Basin are investigated. The turbine farm is simulated by adding semi-permeable structures in the model that use an energy loss term in the fluid momentum equations in the hydrodynamic model. The results emphasize the sensitivity of the system to changes in flow and suggest that a large scale tidal energy farm could reduce SSC by 37% on average across the basin which would disrupt physical and biological processes particularly on the fine-grained intertidal areas around the macrotidal basin. 2) Abstract: RP Mulligan, PC Smith, PS Hill, J Tao, D van Proosdij, BA Law. “Tidal current and wind-wave controls on suspended sediment concentrations in a macrotidal basin” (In Review, submission to the Journal of Geophysical Research Oceans) A hydrodynamic-wave-sediment model is used to simulate the tidal circulation, surface waves and suspended sediment concentrations (SSC) in Minas Basin, a 70 km long tidal estuary in the Bay of Fundy, for winter and summer periods in 2009. The model hydrodynamics are validated using acoustic-Doppler current profile observations, the surface SSC predictions are compared to satellite observations, and model results indicate that strong seasonal signals in SSC can be explained in part by seasonal changes in fetch-limited surface waves generated by local winds over the basin. Other processes that may seasonally influence sediment erodability (e.g., ice scour in winter; biofilm growth in summer) are not evaluated in this study in order to isolate the effects of surface waves in comparison with tidal currents. Model predictions in the intertidal areas indicate that surface waves can increase the bed shear stress from tidal currents alone by up to 1-5 Nm -2 , causing excess bed shear stresses to be higher and result in higher SSC by 100-200 gm -3 particularly during wind events that are stronger and more frequent in winter months. Resuspension of sediments on tidal flats is driven by the combination of shear stresses from nearbed wave orbital velocities and tidal currents, and transport of the suspended materials over deeper areas of the basin is driven by the strong tidal currents. 5. Dissemination and Technology Transfer 1) Results were presented at the NS Tidal Energy Forum at Acadia University, June 2013. Title: “Seasonal Change in Grain Size and Erodibility on a Tidal Channel-Flat Complex in Kingsport, NS” (B.A. Law, P.S. Hill, T.G. Milligan, J.C. Garwood, V. Zions) Title: “Mudflat biofilms coarsen suspended sediment in the Minas Basin" (J.C. Garwood, P.S. Hill, B.A. Law) 2) Results were presented at the Coastal Estuarine Research Federation International meeting in San Diego, USA, November 2013. Title: “Temporal and spatial change in grain size and erodibility on a macro-tidal channel-flat complex in Kingsport, N.S., Canada, versus a meso-tidal channel-flat complex in Willapa Bay, Washington, USA” (B.A. Law, T.G. Milligan, P.S. Hill, P.W. Wiberg, V. Zions) Title: "Seasonal and biofilm effects on sediment erosion and sorting in an intertidal mudflat in the Bay of Fundy, Canada" (J.C. Garwood, P.S. Hill, B.A. Law) 3) Results were presented at the NS Energy Forum at world Trade and Convention Center, Halifax, N.S., May 2014. Title: “Seasonal change in grain size and erodibility on a channel-flat complex in Kingsport, N.S: Understanding natural variability and tidal power development” (B.A. Law, T.G. Milligan, P.S. Hill, J.C. Garwood, V. Zions, C. O’Laughlin) 4) MSc. Thesis of J.C. Garwood successfully defended Nov. 2013. Results of grain size, erodibility and suspended sediment concentration been broadcast to the community through FERN, MSc Thesis is available through Dalhousie University. 6. Publications 1) J.C. Garwood. MSc. Thesis, Dalhousie University, November 2013 2) BA Law, TG Milligan, PS Hill, V Zions, JC Garwood. Temporal and spatial changes in grain size on a macro-tidal channel-flat complex: Results from Kingsport, Nova Scotia, Bay of Fundy (Internal Review DFO, to be submitted to Continental Shelf Research after review) 3) JC Garwood, PS Hill, HL MacIntyre, BA Law. Grain sizes retained by diatom biofilms during erosion on tidal flats linked to bed sediment texture” (Submitted to Continental Shelf Research) 4) Ashall, L., Mulligan, R.P., and Law, BA. Variability in Suspended Sediment Concentrations in the Minas Basin, Bay of Fundy, and Implications of change due to Tidal Power Extraction. (In Review for submission to Coastal Engineerin

Past investigations at historical gold (Au) districts in Nova Scotia, Canada have identified elevated concentrations of arsenic (As) and mercury (Hg) in nearby sediments and waters. These metal(loid)s are derived from erosion of mineralized bedrock, and the disposal of mine tailings into the environment during early operations. The Wine Harbour gold district is located along the eastern shore of Nova Scotia, and produced 1329 kg of Au from 75 581 tonnes of crushed rock from 1862 to 1939. The gold occurs in arsenopyrite-bearing quartz-carbonate veins and was extracted using stamp milling and Hg amalgamation. Historical maps document tailings deposits near former stamp mill sites; however, the extent to which these mine wastes influence environmental quality in the adjacent marine environment is uncertain. In this study, we measured metal(loid) concentrations in tailings, marine sediments, and surface waters to assess the lateral and vertical extent of mining-related impacts on Wine Harbour. Chemical analyses of terrestrial and intertidal tailings reveal high concentrations of both As (86–196 000 mg/kg) and Hg (444–320 000 µg/kg). Analyses of marine sediments show a wide range in both As (4–568 mg/kg) and Hg (<5–7430 µg/kg) concentrations. In general, the highest metal(loid) concentrations in sediments were recorded down-gradient of stamp mill sites. Elevated concentrations were also detected in sediments underlying an active mussel aquaculture operation at the western end of the harbour. Results from this study have been used to help assess potential ecosystem and human health risks associated with historical gold mine wastes in the Wine Harbour area.

In the hypertidal Bay of Fundy, environmental impacts in response to commercial-scale tidal power development remain to be fully understood. The extraction of tidal energy may impact sediment dynamics in far-field environments, such as the intertidal zone, through potential alterations to tidal amplitude in the Minas Basin. Tidal conditions (e.g. current velocity, turbulence, suspended sediment concentration) were monitored in a sheltered salt marsh creek over 18 tidal cycles in various stages of the spring-neap cycle. Samples of deposited and suspended sediments were collected and analyzed for grain size using a Beckman Coulter Multisizer III. Results suggest that the flocculated component of both deposited and suspended sediment is consistently high over a wide range of tidal conditions. A routinely high incoming concentration of highly-flocculated material results in large amounts of sediment deposition in tidal creeks in response to individual tidal cycles. Resuspension and removal of newly deposited material is shown to vary with over-marsh, bankfull and channel-restricted tides. Disruption of the tidal regime due to a reduction in Minas Basin tidal amplitude may lessen the cumulative export capacity of tidal channels over time, potentially leading to gradual infilling of tidal creeks.

Scientific understanding of aquaculture interactions with the environment is limited, especially concerning the far-field transport and possible impacts of particulate wastes.A pilot study was carried out in southwest New Brunswick, Canada, in November 2008 to determine the size, settling velocity, and density of suspended particles at an active salmon aquaculture cage site.The model of Khelifa & Hill (2006) was fit to size-versus-settling velocity data to estimate the fractal dimension of flocs and the density of the component particles within flocs.Flocs had a larger fractal dimension and smaller component-particle density than in other studies, suggesting that particles from the aquaculture operation may be incorporated into suspended flocs with average settling velocities of 1 mm s -1 .Variability in particle size and packaging was interpreted in the context of near bed velocity, tidal stage, and wind speed and direction.This analysis indicated that advection dominated observed variations in particle size and packaging.Indicators of resuspension, aggregation, disaggregation, and deposition were not detected in the time series.Advection of flocs away from the study site provides a mechanism to transport wastes over distances greater than 1 km prior to deposition; thus a settling class of 1 mm s -1 should be considered in depositional models of aquaculture wastes.

: The goal of the proposed work is to investigate the accuracy of a new method for estimating the bulk density of suspended particles in situ and non-invasively. The new method uses the commercially available LISST-100x particle size analyzer paired with a digital floc camera. It would enable collection of high temporal resolution estimates of particle bulk density.

During the last week of February and first week of March in 2010, instruments for measuring current speed and suspended particle size and concentration were placed in a secondary channel and on an adjacent intertidal mudflat at the southern end of Willapa Bay on the Pacific coast of Washington State. Observations show that during spring tides, flood-tide velocity pulses occurred in the channel as water rose above the level of the banks. These pulses resuspended flocs from the channel and advected them over the adjacent flat. During transport, there was some evidence of aggregation of resuspended flocs into larger flocs. As current speeds decreased after the flood pulses, flocs that had advected over the flat deposited quickly. Freshly deposited flocs were resuspended as water levels fell over the flat, with suspended concentrations peaking as water from the flat drained back into the channel. Flocs returning to the channel deposited as currents waned after the ebb pulse. In the channel, the more energetic ebb pulses were strong enough to cause floc breakup. Resuspension and transport of flocs were reduced significantly during neap tides. During periods with high winds, seabed stresses generated by waves limited the deposition of flocs on the seabed. These observations indicate that the floors and flanks of secondary channels as well as the flats surrounding them are the sites of floc deposition and, therefore, are the most likely locations for low-strength, high-water-content muds that contain large fractions of silt and clay. The observations also provide a mechanistic explanation for why deposition rates are typically observed to be reduced at higher elevations on tidal flats and on areas of the flats remote from secondary channels.

Bottom sediments were collected on a muddy tidal flat in Willapa Bay (southwestern Washington State) and analyzed for grain size using a Coulter Multisizer. The disaggregated inorganic grain size (DIGS) distributions obtained from the sediment were then analyzed using conventional methods including median diameter, d50, the largest 25% of the grain size in the population, d75, skewness, and kurtosis. In addition, the inverse model of Curran et al. (2004) was used to provide process-based parameters for description of the size distributions. To examine small-scale spatial differences in grain size, the results of the analysis were plotted against seabed elevation, obtained from USGS LiDAR data. Results reveal a strong inverse correlation between the mass fraction deposited to the seabed as flocs, which is called the “floc fraction”, and elevation on the tidal flat but failed to show any correlation with conventional grain-size descriptors. The dependence of floc fraction on elevation arises mainly due to the differences in size distributions between secondary tidal channels and tidal flats. The extent of flocculation in sediments deposited in the channels of the Shoalwater tidal-flat system is seasonal. Greater precipitation in the winter months is associated with periods of increased suspended-sediment concentrations, which favours more extensive flocculation and the formation and maintenance of low-strength, high-water-content muds. During the summer dry season, lower suspended-sediment concentrations lead to reduced floc fractions in the channels, while the size distributions on the flats resemble those in winter.

Many contaminants in aquatic environments are associated with loosely packed aggregates of particulate material called flocs. Flocculation allows contaminants to accumulate, at the sediment water interface and it packages them in a form that is readily available for ingestion by filter feeding organisms. Unfortunately, most samplers being used for environmental assessment and monitoring suspend this material on impact and fail to sample this critical component of the seabed. In this study we use a slo-corer to collect seabed samples with an undisturbed surface layer and a Gust microcosm erosion chamber to erode the surface of the cores at increasing shear stresses. Results from two different sites, one impacted by tailings from historic gold mining and the other by open-pen salmon aquaculture, showed the levels of metals suspended at stresses below 0.24 Pa were greater than in the Underlying sediment. Sampling this highly mobile surface layer is critical for determining the total contaminant load in bottom sediments and, more, importantly, this layer represents the most readily available material for suspension. The loss of this layer during sampling could lead to inaccurate measurements of contaminant levels during environmental assessment and effects monitoring. A re-evaluation of the ISO standard for bottom sediment sampling is recommended.