Effects of low tide rainfall on the erodibility of intertidal cohesive sediments

Low tide rainfall may represent an important but little studied process affecting sediment fluxes on intertidal mudflats. In this study, we simulated rainfall events on an intertidal mudflat (median grain size=18.4 μm) of low slope (1 in 300) then quantified effects on sediment erodibility. Treatments consisted of a high (4.1 mm min⁻¹ for 6 min) and low (0.36 mm min⁻¹ for 60 min) rain intensity, chosen to match naturally occurring events and experiments were conducted seasonally (May and August) to encompass variations in ambient sediment stability. Changes in bed elevation due to rainfall were estimated using marked rods and sediment erodibility parameters (mass of sediment eroded at a flow velocity of 0.3 m s⁻¹ (ME-30, g m⁻²) and critical erosion velocity (U_crit, m s⁻¹)) were determined in annular flumes (bed area=0.17 m²). Ambient/control sediment erodibility in May (ME-30=211 g m⁻², U_crit=0.18 m s⁻¹) was higher than in August (ME-30=30 g m⁻², U_crit=0.26 m s⁻¹) and was correlated with changes in biological activity. In May, surface sediment was influenced by high densities of the bioturbating snail Hydrobia ulvae (1736 ind. m⁻²) and low biomass of the sediment stabilising microphytobenthos (5.7 μg chlorophyll a cm⁻²). In contrast, in August H. ulvae densities were low (52 ind. m⁻²) and microphytobenthic biomass higher (9.2 μg chlorophyll a cm⁻²). The high rain treatment caused a decrease in bed elevation of between 1.5 mm (May) and 4.4 mm (August) and significantly reduced sediment organic content and microphytobenthic biomass. Rainfall increased sediment erodibility; compared to ambient sediments ME-30 increased by a factor of 1.4× in May and 8.8× in August and caused a 10–30% decline in U_crit. The seasonal difference in treatment effect was due to the change in ambient sediment stability. The low rain treatment in August had no effect on bed elevation, microphytobenthic biomass or sediment erodibility. In May, the same treatment caused a reduction in bed elevation (0.5 mm) and microphytobenthic biomass but counter-intuitively, a decrease in sediment erodibility (ME-30 was reduced by 40%, U_crit increased by 5%) compared to controls. We attribute this result to removal by rainfall of easily eroded surface flocs and biogenic roughness which resulted in an underlying sediment with a smoother surface and greater resistant to erosion. Results suggest that high intensity rain events may destabilise intertidal sediments making them more susceptible to erosion by returning tidal currents and that the sediment eroded during such events may represent a considerable fraction (up to 25%) of the seasonal variation in shore elevation. The impact of natural rain events are likely to vary considerably due to variations in droplet size, intensity and duration and the interaction with ambient sediment stability.     

Critical erosion profiles in macro-tidal estuary sediments: Implications for the stability of intertidal mud and the slope of mud banks

Vertical profiles of the critical erosion threshold (τ_crit) in sediment have been measured at 11 stations along the axis of the Tamar Estuary and at a single station in a tributary of the Tamar at St. John's Ford. The τ_crit of surface sediment increased from 0.04 Pa in the upper, brackish estuary to 0.09 Pa in the lower estuary. In the upper estuary τ_crit only increased slightly with depth whereas in the marine estuary τ_crit increased rapidly from 0.09 Pa at the surface to 0.25 Pa at 15 cm below the sediment surface. The results showed that the relationship between τ_crit and bulk density (ρ_b) obtained previously for surface sediment was also applicable to sediments from depths of 10–15 cm and probably deeper. Profiles of ρ_b were measured to depths of 70 cm using a corer. In the lower (marine) estuary ρ_b increased with depth in the sediment from 1580 kg m⁻³ at the surface to 1720 kg m⁻³ at 70 cm. In the upper estuary ρ_b values were lower at 1170–1200 kg m⁻³ and profiles were almost homogeneous indicating that consolidation was not occurring. The mid-estuary was transitional between these two situations. These results are consistent with the seasonal accumulation and loss of ‘mobile’ sediment observed previously in the upper estuary with changes in river flow, and with the apparent stability of intertidal mud in the lower marine estuary deduced from historical bathymetric survey records. The slopes of the intertidal mud banks ranged from 1–2% in the lower estuary to 20–25% in mid-estuary but, instead of continuing to increase in steepness towards the head as the estuary became narrower, the measured slopes actually decreased. It is speculated that the lack of consolidation through continual mobilisation and settlement cycles combined with an increase in silt content in the upper estuary resulted in sediment that lacked the mechanical strength to maintain steep slopes.     

What drives the evolution of the sedimentary phosphorus cycle?

The ‘Chicken Creek’ artificial catchment area, Welzow-South, E Germany, created to study processes and structures of initial ecosystem development, discharges into a small experimental lake (A=3805 m², V=3992 m³, z_max=2.4 m). This lake was man-made in 2005 and filled by natural surface runoff until January 2006. In summer 2006 and 2008, the actual development of sediments and the evolution of the phosphorus (P) cycle were studied. 19.7% of the original lake volume was filled by sediment within the first 3 years. A fine-grained sediment representing silt (6.3-63 μm) accumulated at high accretion rates at the deepest point (200 mm a⁻¹, 0-24 mm week⁻¹) due to massive erosion in the catchment. The sediment is low in organic matter (2.5-5.2%) and total P (TP, 0.31-0.97 mg g⁻¹). Low amounts of P associated with degradable organic matter and surplus of metal hydroxides (Fe:P∼40, Al:P∼20) favor an efficient P binding and low dissolved P concentrations in pore water (1-107 μg l⁻¹). Hence, the mineral sediment quality and the low rates of P release (0.06 mg m⁻² d⁻¹) revealed that a lake at an initial stage of development has essentially no sedimentary P cycle compared to eutrophic shallow lakes. However, the increasing emersed and submersed macrophyte growth will control further lake succession by intensifying the internal nutrient cycling. The macrophytes drive the evolution of a sedimentary P cycle by mobilizing and translocating P, by accumulating carbon and thus by stimulating microbial and redox processes.     

The effect of wave-induced turbulence on intertidal mudflats: Impact of boat traffic and wind

Semi-diurnal and fortnightly surveys were carried out to quantify the effects of wind- and navigation-induced high-energy events on bed sediments above intertidal mudflats. The mudflats are located in the upper fluvial part (Oissel mudflat) and at the mouth (Vasière Nord mudflat) of the macrotidal Seine estuary. Instantaneous flow velocities and mudflat bed elevation were measured at a high frequency and high resolution with an acoustic doppler velocimeter (ADV) and an ALTUS altimeter, respectively. Suspended particulate matter concentrations were estimated by calibrating the ADV acoustic backscattered intensity with bed sediments collected at the study sites. Turbulent bed shear stress values were estimated by the turbulent kinetic energy method, using velocity variances filtered from the wave contribution. Wave shear stress and maximum wave–current shear stress values were calculated with the wave–current interaction (WCI) model, which is based on the bed roughness length, wave orbital velocities and the wave period (T_S). In the fluvial part of the estuary, boat passages occurred unevenly during the surveys and were characterized by long waves (T_S>50 s) induced by the drawdown effect and by short boat-waves (T_S<10 s). Boat waves generated large bottom shear stress values of 0.5 N m⁻² for 2–5 min periods and, in burst of several seconds, larger bottom shear stress values up to 1 N m⁻². At the mouth of the estuary, west south-west wind events generated short waves (T_S<10 s) of H_S values ranging from 0.1 to 0.3 m. In shallow-water environment (water depth <1.5 m), these waves produced bottom shear stress values between 1 and 2 N m⁻². Wave–current shear stress values are one order of magnitude larger than the current-induced shear stress and indicate that navigation and wind are the dominant hydrodynamic forcing parameters above the two mudflats. Bed elevation and SPM concentration time series showed that these high energy events induced erosion processes of up to several centimetres. Critical erosion shear stress (τ_ce) values were determined from the SPM concentration and bed elevation measurements. Rough τ_ce values were found above 0.2 N m⁻² for the Oissel mudflat and about 1 N m⁻² for the Vasière Nord mudflat.     

Interannual (1999-2005) morphodynamic evolution of macro-tidal salt marshes in Mont-Saint-Michel Bay (France)

This paper provides a detailed study on the sedimentation patterns and the recent morphodynamic evolution affecting the macro-tidal salt marshes located west of the Mont-Saint-Michel (France). Twenty-two stations along three transects on the marshes were seasonally monitored for marsh surface level variations from 1999 to 2005, using a sediment erosion bar. The corresponding erosion/accretion rates were obtained together with data on topography, vegetation cover, and grain size of surface sediment. To examine the mechanisms contributing to the salt marsh sedimentation, the data and their evolution were treated with respect to tides, relative mean regional sea level, and wind speed/frequency variations.From 1999 to 2005, the marsh was globally accreting (from 3.45 to 38.11 mm yr⁻¹ in the low marsh, up to 4.91 mm yr⁻¹ in the middle marsh, and up to 1.35 mm yr⁻¹ in the high marsh), while the study was conducted during a window of decreasing trend in mean regional sea level (−2.45 mm yr⁻¹ according to regional-averaged time series). These sedimentation rates are one of the highest recorded worldwide; however, the sedimentation was not found to be continuous over the period in question. This pattern is illustrated by the strong extension of the marshes from 1999 to 2002, and the relative stability observed from 2003 to 2005. The imported and reworked sediments are trapped and fixed by the dense vegetation (Puccinellia maritima, Halimione portulacoides), inducing the general seaward extension of the marshes. The processes governing sediment budget (accretion/erosion) show annual, seasonal, and spatial variability on the marsh. Spatial variations display contrasted patterns of erosion/sedimentation between the low, middle, and high marsh, and between the different transects. These patterns are a result of distance from sediment sources, strong heterogeneity in vegetation cover (human induced or not), and contrasting topographic and micro-topographic characteristics. The higher accretion rates are observed in distal settings in the low marsh, and strongly decrease toward the middle and high marsh. This evolution results from a decrease in accommodation space/water column thickness, and frequency of inundation coupled with an increase in station elevation, but also from the cumulated effects of vegetation cover and micro-topography. The vegetation cover of the low and middle marsh enhance the settling and fixing of fine sediments imported through tides or dispersed by flood and ebb currents.The seasonal evolution of the marshes is marked by contrasting effects of water storage in the sediment. The overall seasonal sediment budget is controlled by the variation of the frequency of inundation relative to tidal range and marshes topography. Autumns are influenced by the tide (equinoxes), relative mean regional sea level, and variations in wind speed/frequency. Winter wind speed and frequency in relation with tidal variations appear to be the main parameters regulating winter marsh evolution. Summers are predominantly under the influence of local variations in water storage (desiccation) while external parameters generally display a low influence. Although it is not governed by any one parameter, springtime sediment budget seems to result from strong interaction between the above-cited parameters, despite the significant frequency of inundation (equinoxes).     

The erosion rates of cohesive sediments in Venice lagoon,Italy

The stability of cohesive sediments from Venice lagoon has been measured in situ using the benthic flume Sea Carousel. Twenty four stations were occupied during summertime, and a sub-set of 13 stations was re-occupied during the following winter. Erosion thresholds and first-order erosion rates were estimated and showed a distinct difference between inter-tidal and sub-tidal stations. The higher values for inter-tidal stations are the result of exposure that influences consolidation, density, and organic adhesion. The thresholds for each state of sediment motion are well established. However, the rate of erosion once the erosion threshold has been exceeded has been poorly treated. This is because normally a time-series of sediment concentration (C) and bed shear stress (τ₀(t)) is used to define threshold stress or cohesion (τ_crit,z) and erosion rate (E). Whilst solution of the onset of erosion, τ_crit,0, is often reported, the evaluation of the erosion threshold variation through the process of erosion (eroded depth) is usually omitted or not estimated. This usually leads to assumptions on the strength profile of the bed which invariably has no credibility within the topmost mm of the bed where most erosion takes place. It is possible to extract this information from a time-series through the addition of a step in data processing. This paper describes how this is done, and the impact of this on the accuracy of estimates of the excess stress (τ₀(t)–τ_crit,z) on E.     

Parameterization of near-bed processes under collinear wave and current flows from a two-phase sheet flow model

Sediment transport models require appropriate representation of near-bed processes. We aim here to explore the parameterizations of bed shear stress, bed load transport rate and near-bed sediment erosion rate under the sheet flow regime. To that end, we employ a one-dimensional two-phase sheet flow model which is able to resolve the intrawave boundary layer and sediment dynamics at a length scale on the order of the sediment grain. We have conducted 79 numerical simulations to cover a range of collinear wave and current conditions and sediment diameters in the range 210–460 μm$\mathrm{\mu }\mathrm{m}$. The numerical results confirm that the intrawave bed shear stress leads the free stream velocity, and we assess an explicit expression relating the phase lead to the maximum velocity, wave period and bed roughness. The numerical sheet flow model is also used to provide estimates for the bed load transport rate and to inspect the near-bed sediment erosion. A common bed load transport rate formulation and two typical reference concentration approaches are assessed. A dependence of the bed load transport rate on the sediment grain diameter is observed and parameterized. Finally, the intrawave near-bed vertical sediment flux is further investigated and related to the time derivative of the bed shear stress.     

Geomorphological effects of catastrophic flooding on a small subtropical Estuary

The Mgeni Estuary is situated on the east coast of South Africa. Tidal salinity changes commonly extend 2.5 km upstream from the mouth. The subtropical climatic regime causes seasonal flooding, the geomorphological effects of which are increased by a steep river gradient. A 120-150 year flood event during September 1987, with an estimated peak discharge of 10800 cumecs, resulted in the erosion of 1.86 × 10⁶ m³ of sediment from the estuary. The normal high tide volume of the estuary (0.35 × 10⁶ m³) increased to 1.85 × 10⁶ m³ after the flood. The post-flood intertidal volume (tidal prism) of the estuary was 0.43 × 10⁶ m³ compared to 0.19 × 10⁶ m³ before the event. The nature and extent of erosion in the estuary was controlled by the composition of the estuary banks and bed material. Gravel and sand substrates were preferentially eroded from the estuary bed before the cohesive bank materials were undercut in sections of the estuary. Mud and fine sand were deposited on overbank areas when the flood waned and a lag of coarse sand and gravel was produced on the estuary bed. Approximately 46 per cent of the bedload sediment supplied to the estuary since 1917 was retained in the estuary until the 1987 flood. The remainder was transported through the system and into the Indian Ocean. Results show that catastrophic floods play an important role in sedimentation in small estuaries.     

Numerical modeling of bedload and suspended load contributions to morphological evolution of the Seine Estuary (France)

This numerical modeling study (i) assesses the influence of the sediment erosion process on the sediment dynamics and subsequent morphological changes of a mixed-sediment environment, the macrotidal Seine estuary, when non-cohesive particles are dominant within bed mixtures (non-cohesive regime), and (ii) investigates respective contributions of bedload and suspended load in these dynamics. A three dimensional (3D) process-based morphodynamic model was set up and run under realistic forcings (including tide, waves, wind, and river discharge) during a 1-year period. Applying erosion homogeneously to bed sediment in the non-cohesive regime, i.e., average erosion parameters in the erosion law (especially the erodibility parameter, E₀), leads to higher resuspension of fine sediment due to the presence of coarser fractions within mixtures, compared to the case of an independent treatment of erosion for each sediment class. This results in more pronounced horizontal sediment flux (two-fold increase for sand, +30% for mud) and erosion/deposition patterns (up to a two-fold increase in erosion over shoals, generally associated with some coarsening of bed sediment). Compared to observed bathymetric changes, more relevant erosion/deposition patterns are derived from the model when independent resuspension fluxes are considered in the non-cohesive regime. These results suggest that this kind of approach may be more relevant when local grain-size distributions become heterogeneous and multimodal for non-cohesive particles. Bedload transport appears to be a non-dominant but significant contributor to the sediment dynamics of the Seine Estuary mouth. The residual bedload flux represents, on average, between 17 and 38% of the suspended sand flux, its contribution generally increasing when bed sediment becomes coarser (can become dominant at specific locations). The average orientation of residual fluxes and erosion/deposition patterns caused by bedload generally follow those resulting from suspended sediment dynamics. Sediment mass budgets cumulated over the simulated year reveal a relative contribution of bedload to total mass budgets around 25% over large erosion areas of shoals, which can even become higher in sedimentation zones. However, bedload-induced dynamics can locally differ from the dynamics related to suspended load, resulting in specific residual transport, erosion/deposition patterns, and changes in seabed nature.     

Clearance rates of Cerastoderma edule under increasing current velocity

Estimates of clearance rates (CR) of Cerastoderma edule (300 ind. m⁻²) as a function of free-stream current velocity (U) (from 5 to 40 cm s⁻¹) were compared between a small annular (60 l) and a large racetrack (8850 l) flume with different hydrodynamic conditions. Results showed that the flumes differ considerably in their hydrodynamic characteristics. The relationship between CR and U is different in the two flume tanks, however there appears to be a straightforward unimodal trend between CR and shear velocity (U_*). It was found that the cockles themselves influence the benthic boundary layer (BBL) characteristics, by causing steeper velocity gradients and increasing the mixing over the cockle bed compared to bare sediment. This provides new evidence on how endobenthic organisms can affect the BBL. However, the influence of CR on U_* could not be quantified because these parameters have interactive effects that cannot be dissociated.     

Sedimentation in small-scale irrigation schemes in Ethiopia: Its sources and management

Numerous irrigation schemes in sub-Saharan Africa (SSA) exhibit excessive sedimentation, resulting in underperformance and high maintenance costs. In the current study, a participatory monitoring program was used to investigate sediment causes and sources, measure the annual sediment load, and monitor desilting campaigns in two small scale irrigation schemes in Ethiopia, Arata-Chufa (100 ha) and Ketar (430 ha), for three years (2016–2018). Sedimentation quantities were huge, where the annual river sediment influx ranged from 220 m³ for the Arata-Chufa scheme to 1,741 m³ for the Ketar scheme. On average 0.3 m³/m of sediment were removed from the main canal for Arata-Chufa costing 794 days of labor per year. In Ketar, sediment quantities were even greater: 1.1 m³/m was removed requiring 3,118 days of labor per year. The sediment influx from the river source amounts to up to 95% for Arata-Chufa and moderately reaches 46% for Ketar, with the remainder of the sediment entering with overland erosion flows. Farmers reported increased sedimentation over time and difficulty paying operation and maintenance fees instead preferring to contribute labor for the desilting campaigns. Sedimentation management is fragile and mainly involves frequent desilting campaigns and unharmonized efforts to reduce overland sediment inflows. Factors contributing to sediment deposition include mild longitudinal bed slopes, the location of the intake, canal layout, and lack of canal banks for protection against surface water inflow in addition to sub-optimal canal operations. Excessive sedimentation is a major challenge resulting in underperformance of numerous irrigation schemes in SSA, and the stakeholders’ lack of awareness of the sources of sedimentation is an underlying factor aggravating sedimentation problems. It is concluded that investigating the sources, extent, and types of sedimentation entering a small-scale irrigation scheme is the basis for reducing maintenance costs and for effective management of sedimentation problems.     

Benthic oxygen fluxes and sediment properties on the northeastern New Zealand continental shelf

We examined spatial variations in benthic remineralisation (measured as sediment oxygen consumption (SOC)) and sediment properties on the northeastern New Zealand continental shelf and slope to assess the importance of benthic mineralisation in this ecosystem and to provide data for more complete global carbon budgets. SOC measured in dark incubations conducted in early summer ranged from 128 μmol m⁻² h⁻¹ at the deepest (360 m) to 1222 μmol m⁻² h⁻¹ at the shallowest (4.2 m) site and decreased significantly with water depth (p<0.001, r²=0.78, SOC=1222.8−456.3×log₁₀[water depth], n=14 sites). These rates were in the range found on continental shelves elsewhere (64–1750 μmol m⁻² h⁻¹, n=30 studies) and had a very similar distribution with water depth. SOC was also measured in light incubations at seven sites (4.2–35 m water depth) to examine the effects of microphytobenthos and accounted for 42–106% of rates measured in the dark. Measurements of near-bed light intensities suggested that microphytobenthos production was not solely regulated by light intensity but evidently influenced by other factors. A two-dimensional PCA ordination of surface sediment properties accounted for 83.3% of the total variance in the data and divided the study area into three clusters that corresponded well to its spatial division into the shallow (<30 m) Firth of Thames, the Hauraki Gulf (30–50 m) and the northern shelf-slope region. In the Firth of Thames sediments were very fine-grained with low CaCO₃ and high total organic matter and pigment content, and low C:N ratios. The northern shelf-slope sediments showed the opposite trends to the Firth of Thames and those in the Hauraki Gulf had mostly intermediate values. Dark SOC was significantly correlated with sediment organic matter, carbon, nitrogen, pigments and silt/clay content (p<0.05, r=0.55–0.85) but a multiple linear regression revealed that water depth was the only significant predictor. Calculations suggest that approximately 13%, 10% and 34% of primary production is remineralised in the sediments of the northern shelf-slope region, Hauraki Gulf and Firth of Thames, respectively, indicating a strong benthic–pelagic coupling on the northeastern New Zealand continental shelf that was particularly pronounced in the Firth of Thames due to its shallow depth and significant terrestrial and riverine inputs.     

Seasonal contribution of living phytoplankton carbon to vertical fluxes in a coastal upwelling system (Ría de Vigo, NW Spain)

The aim of this study is to explore the contribution of living phytoplankton carbon to vertical fluxes in a coastal upwelling system as a key piece to understand the coupling between primary production in the photic layer and the transfer mechanisms of the organic material from the photic zone. Between April 2004 and January 2005, five campaigns were carried out in the Ría de Vigo (NW Iberian Peninsula) covering the most representative oceanographic conditions for this region. Measurements of particulate organic carbon (POC), chlorophyll-a (chl a), phaeopigments (phaeo), and identification of phytoplankton species were performed on the water column samples and on the organic material collected in sediment traps.The POC fluxes measured by the sediment traps presented no seasonal variation along the studied period ranging around a mean annual value of 1085±365 mg m⁻² d⁻¹, in the upper range of the previously reported values for other coastal systems. The fact that higher POC fluxes were registered during autumn and winter, when primary production rates were at their minimum levels points to a dominant contribution of organic carbon from resuspended sediments on the trap collected material. On the contrary, fluxes of living phytoplankton carbon (C_phyto) and chl a clearly presented a seasonal trend with maximum values during summer upwelling (546 mg m⁻² d⁻¹ and 22 mg chl a m⁻² d⁻¹, respectively) and minimum values during winter (22 mg m⁻² d⁻¹ and 0.1 mg chl a m⁻² d⁻¹, respectively). The contribution of C_phyto to the vertical flux of POC ranged between 2% and 49% in response to the pelagic phytoplankton community structure. Higher values of C_phyto fluxes were registered under upwelling conditions which favour the dominance of large chain-forming diatoms (Asterionellopsis glacialis and Detonula pumila) that were rapidly transferred to the sediments. By contrast, C_phyto fluxes decreased during the summer stratification associated with a pelagic phytoplankton community dominated by single-cell diatoms and flagellates. Minimal C_phyto fluxes were observed during the winter mixing conditions, when the presence of the benthic specie Paralia sulcata in the water column also points toward strong sediment resuspension.     

Phytoplankton biomass and primary production responses to physico-chemical forcing across the northeastern New Zealand continental shelf

Phytoplankton biomass and primary production were monitored in the Hauraki Gulf and on the northeastern continental shelf, New Zealand - using ship surveys, moored instruments and satellite observations (1998-2001) - capturing variability across a range of space and time scales. A depth-integrated primary production model (DIM) was used to predict integrated productivity from surface parameters, enabling regional-specific estimates from satellite data. The shelf site was dominated by pico-phytoplankton, with low chlorophyll-a (<1 mg m⁻³) and annual production (136 g C m⁻² yr⁻¹). In contrast, the gulf contained a micro/nano-phytoplankton-dominated community, with relatively high chlorophyll-a (>1 mg m⁻³) and annual production (178 g C m⁻² yr⁻¹). Biomass and productivity responded to physico-chemical factors; a combination of light, critical mixing depths and/or nutrient limitation—particularly new nitrate-N. Relatively low biomass and production was observed during 1999. This coincided with inter-annual variability in the timing and extent of upwelling- and downwelling-favourable along-shelf wind-stress, influencing the fluxes of new nitrate-N to the shelf and gulf. Relationships with the Southern Oscillation Index are also discussed. Our multi-scaled sampling highlighted details associated with stratification and de-stratification events, and deep sub-surface chlorophyll-a not visible to satellite sensors. This study demonstrates the importance of multi-scaled sampling in gaining estimates of regional production and its responses to physico-chemical forcing.     

Remarkable geochemical changes and degassing at Voui crater lake,Ambae volcano,Vanuatu

Ambae (also known as Aoba), is a 38 × 16 km² lozenge-shaped island volcano with a coastal population of around 10 000. At the summit of the volcano is lake Voui — one of the largest active crater lakes worldwide, with 40 × 10⁶ m³ of acidic water perched 1400 m a.s.l. After more than 300 years of dormancy, Ambae volcano reawakened with phreatic eruptions through Voui in 1995, and culminating in a series of surtseyan eruptions in 2005, followed by a rapid and spectacular colour change of the lake from light blue to red in 2006. Integrating lake water chemistry with new measurements of SO₂ emissions from the volcano during the 2005–2006 eruptive period helps to explain the unusual and spectacular volcanic activity of Ambae — initially, a degassed magma approached the lake bed and triggered the surtseyan eruption. Depressurization of the conduit facilitated ascent of volatile-rich magma from the deeper plumbing system. The construction of a cone during eruption and the high degassing destabilised the equilibrium of lake stratification leading to a limnic event and subsequently the spectacular colour change.     

Methanogenesis in the sediment of the acidic Lake Caviahue in Argentina

The biogeochemistry of methane in the sediments of Lake Caviahue was examined by geochemical analysis, microbial activity assays and isotopic analysis. The pH in the water column was 2.6 and increased up to a pH of 6 in the deeper sediment pore waters. The carbon isotope composition of CH₄ was between − 65 and − 70‰ which is indicative for the biological origin of the methane. The enrichment factor ε increased from − 46‰ in the upper sediment column to more than − 80 in the deeper sediment section suggesting a transition from acetoclastic methanogenesis to CO₂ reduction with depth. In the most acidic surface layer of the sediment (pH < 4) methanogenesis is inhibited as suggested by a linear CH₄ concentration profile, activity assays and MPN analysis. The CH₄ activity assays and the CH₄ profile indicate that methanogenesis in the sediment of Lake Caviahue was active below 40 cm depth. At that depth the pH was above 4 and sulfate reduction was sulfate limited. Methane was diffusing with a flux of 0.9 mmol m^− 2 d^− 1 to the sediment surface where it was probably oxidized. Methanogenesis contributed little to the sediments carbon budget and had no significant impact on lake water quality. The high biomass content of the sediment, which was probably caused by the last eruption of Copahue Volcano, supported high rates of sulfate reduction which probably raised the pH and created favorable conditions for methanogens in deeper sediment layers.     

Sedimentary records of natural and artificial Huanghe (Yellow River) channel shifts during the Holocene in the southern Bohai Sea

Two gravity cores collected off the modern Huanghe (Yellow River) delta in the southern Bohai Sea were analyzed for grain size, the total organic carbon (TOC)/total nitrogen (TN) ratio, color diffraction, magnetic susceptibility, ¹⁴C dating and ¹³⁷Cs and ²¹⁰Pb isotope contents to clarify changes in the sedimentary environment during the Holocene. In particular, the effect of natural and artificial river-course shifts of the Huanghe on the Bohai Sea sediment was investigated. A peat layer, scouring surface and sharp changes in the grain size, TOC/TN ratio, sediment color (L^?, a^?) and magnetic susceptibility were identified and are likely to be due to the early-Holocene sea-level rise resulting in environmental changes from coastal to shelf environments in the Bohai Sea. After the sea level reached its maximum at 6-7 ka BP, the lateral shifts in the river course of the Huanghe formed 10 superlobes, and superlobe 7 (11-1048 AD) and superlobe 10 (1855-present) of the Huanghe delta affected the core sites. The northern site of BH-239 has been more affected by the Huanghe since the middle Holocene. Notably, in the superlobe 10 period, the reshaping of the northern Huanghe delta due to an artificial river-course shift from northward to eastward in 1976 (e.g., a∼10 km shoreline retreat due to coastal erosion) was recorded in the core sediments, particularly in terms of the TOC/TN ratio, sediment color (L^? and a^?) and magnetic susceptibility, owing to the huge sediment supply from coastal erosion of the former river mouth area.     

Export fluxes of biogenic matter in the presence and absence of seasonal sea ice cover in the Chukchi Sea

Drifting sediment traps were deployed at 9 stations in May-June (ice-covered conditions) and July-August (ice-free conditions) 2004 in the Chukchi Sea to investigate the variability in export fluxes of biogenic matter in the presence and absence of sea ice cover. Measurements of chlorophyll-a (Chl-a), particulate organic carbon (POC), particulate nitrogen (PN), phytoplankton, zooplankton fecal pellets, and the stable carbon isotope composition (δ¹³C) of the sinking material were performed along Barrow Canyon (BC) and a parallel shelf-to-basin transect from East Hanna Shoal (EHS) to the Canada Basin. POC export fluxes were similarly high in the presence (378±106 mg C m⁻² d⁻¹) and in the absence of ice cover (442±203 mg C m⁻² d⁻¹) at the BC stations, while fluxes were significantly higher in the absence (129±98 mg C m⁻² d⁻¹) than in the presence of ice cover (44±29 mg C m⁻² d⁻¹) at the EHS stations. The C/N ratios and δ¹³C values of sinking organic particles indicated that POC export fluxes on the Chukchi continental shelf were mostly composed of freshly produced labile material, except at the EHS stations under ice cover where the exported matter was mostly composed of refractory material probably advected into the EHS region. Chl-a fluxes were higher under ice cover than in ice-free water, however, relatively low daily loss rates of Chl-a and similar phytoplankton carbon fluxes in ice-covered and ice-free water suggest the retention of phytoplankton in the upper water column. An increase in fecal pellet carbon fluxes in ice-free water reflected higher grazing pressure in the absence of ice cover. Elevated daily loss rates of POC at the BC stations confirmed other indications that Barrow Canyon is an important area of carbon export to the basin and/or benthos. These results support the conclusion that there are large spatial and temporal variations in export fluxes of biogenic matter on the Chukchi continental shelf, although export fluxes may be similar in the presence and in the absence of ice cover in highly productive regions.     

Estuarine mud flocculation properties determined using an annular mini-flume and the LabSFLOC system

Most entrained estuarine sediment mass occurs as flocs. Parameterising flocculation has proven difficult as it is a dynamically active process dependent on a set of complex interactions between the sediment, fluid and the flow. However the natural variability in an estuary makes it difficult to study the factors that influence the behaviour of flocculation in a systematic manner. This paper presents preliminary results from a laboratory study that examined how floc properties of a natural estuarine mud from the Medway (UK), evolved in response to varying levels of suspended sediment concentration and induced turbulent shearing. The experiments utilised the LabSFLOC floc video camera system, in combination with an annular mini-flume to shear the suspended sediment slurries. The flows created in the mini-flume produced average shear stresses, at the floc sampling height, ranging from 0.01 N m⁻² to a peak of 1.03 N m⁻². Nominal suspended particulate matter concentrations of 100, 600 and 2000 mg l⁻¹ were introduced into the flume. The experimental runs produced individual flocs ranging in size from microflocs of 22.2 μm to macroflocs 583.7 μm in diameter. Average settling velocities ranged from 0.01 to 26.1 mm s⁻¹, whilst floc effective densities varied from 3.5 up to 2000 kg m⁻³. Low concentration and low shear stress were seen to produce an even distribution of floc mass between the macrofloc (>160 μm) and microfloc (<160 μm) fractions. As both concentration and stress rose, the proportion of macrofloc mass increased, until they represented over 80% of the suspended matter. A maximum average macrofloc settling velocity of 3.3 mm s⁻¹ was attained at a shear stress of 0.45 N m⁻². Peak turbulence conditions resulted in deflocculation, limiting the macrofloc fall velocity to only 1.1 mm s⁻¹ and placing over 60% of the mass in the microfloc size range. A statistical analysis of the data suggests that the combined influence of both suspended concentration and turbulent shear controls the settling velocity of the fragile, low density macroflocs.