Resuspension Events Research Articles

Effects of climatic variability on phytoplankton community structure and bloom development in the eutrophic, microtidal, New River Estuary, North Carolina, USA

The roles of climatically driven freshwater flow, nutrient loading, and temperature on phytoplankton community biomass and composition were examined along a downstream transect within the New River Estuary, North Carolina from 2007 to 2010. This microtidal system has a history of eutrophication symptoms, including harmful algal blooms. Riverine discharge strongly controlled nutrient loading to the estuary. Except during very high flow conditions, loads were generally assimilated near the head of the estuary and low concentrations of dissolved inorganic nitrogen and phosphate, <2 and 0.4 μmol L−1, respectively, were found throughout most of the estuary. Total phytoplankton biomass (chlorophyll a) was maximal under moderate discharge conditions when flushing time was approximately four days and salinity was in the range of 3–18 psu. The observed non-monotonic relationship between phytoplankton biomass and flushing time reflects a balance between nutrient stimulation of phytoplankton biomass and advective losses associated with river flow. Pigment indicators of major phytoplankton taxa also revealed non-monotonic responses to flushing time with some significant inter-taxa differences in slopes and peak location. These differences suggest that flushing time or environmental covariates of flushing time are important determinants of community composition. We suggest these group-specific differences are due to advantages gained by phytoflagellates through vertical migration, ability of pico-cyanobacteria to exploit low residual nutrient concentrations, and subsidies to the diatom population via sediment resuspension events. In addition to effects of flow, pico-cyanobacteria and harmful raphidophyte taxa demonstrated positive relationships with temperature. Overall, results demonstrate a strong linkage between the phytoplankton community and two important climatic drivers, flow and temperature. The observed sensitivity of phytoplankton biomass to nutrient loads suggests that load reductions (or increases) are likely to decrease (or increase) total phytoplankton biomass in this system. However, phytoplankton community composition appears to be largely determined by environmental factors other than nutrient loading, particularly temperature, stratification and sediment resuspension.

Balance of assimilative and dissimilative nitrogen processes in a diatom-rich tidal flat sediment

Abstract. Tidal flat sediments are subject to repetitive mixing and resuspension events. In a short-term (24 h) 15N-labelling experiment, we investigated reactive nitrogen cycling in a tidal flat sediment following an experimentally induced resuspension event. We focused on (a) the relative importance of assimilatory versus dissimilatory processes and (b) the role of benthic microalgae therein. 15N-labelled substrate was added to homogenized sediment, and 15N was subsequently traced into sediment and dissolved inorganic nitrogen (DIN) pools. Integration of results in a N-cycle model allowed us to quantify the proportion of major assimilatory and dissimilatory processes in the sediment. Upon sediment disturbance, rates of dissimilatory processes like nitrification and denitrification were very high, but declined rapidly towards a steady state. Once this was reached, the balance between assimilation and dissimilation in this tidal mudflat was mainly dependent on the nitrogen source: nitrate was utilized almost exclusively dissimilatory via denitrification, whereas ammonium was rapidly assimilated, with about a quarter of this assimilation due to benthic microalgae (BMA). Benthic microalgae significantly affected the nitrogen recycling balance in sediments, because in the absence of BMA activity the recovering sediment turned from a net ammonium sink to a net source. The driving mechanisms for assimilation or dissimilation accordingly appear to be ruled to a large extent by external physical forcing, with the entire system being capable of rapid shifts following environmental changes. Assimilatory pathways gain importance under stable conditions, with a substantial contribution of BMA to total assimilation.

Characterisation of the wave field and associated risk of sediment resuspension in a coastal aquaculture zone

The interaction between extreme environmental events and the marine aquaculture industry has, in the past, led to substantial economic losses for the industry. In 1996, the aquaculture industry based near Port Lincoln in South Australia experienced a sediment resuspension event linked to the passage of a vigorous ex-tropical cyclone. A key process that drives the sediment dynamics in coastal environments is the physical forcing due to surface gravity waves. The numerical model Simulating WAves Nearshore (SWAN) has been applied to hindcast two years of wave fields for the central South Australian coastline and is validated against five datasets, using both in-situ observations and satellite derived wave heights. Using the calibrated model, we investigate the spatial variability in the relative contribution of locally generated wind waves versus incoming swell to the mean wave energy budget. The potential for sediment resuspension is then assessed using theoretical thresholds for the initiation of sediment resuspension which are derived using the Shields parameter and observed sediment grain size. Linear wave theory in conjunction with the model hindcast is used to identify the conditions that could lead to a sediment resuspension event. Incoming swell from the Southern Ocean poses the greatest risk of a sediment resuspension event, while locally generated wind waves are only able to resuspend sediments in the shallow near shore regions. We have developed a simple tool that will allow policy makers and farm managers to identify the likely combination of significant wave height and mean wave period for a particular depth, that is likely to result in the mobilisation of unconsolidated particulate matter on the seafloor.

Remobilization of heavy metals during the resuspension of Liangshui River sediments using an annular flume

The release and transport of heavy metals (Ni, Cr, Cu and Pb) from Liangshui River sediments into the overlying water column during the resuspension event were determined using an annular flume with a velocity ranging from 0.15 to 0.35 m/s. It is shown that the suspended particulate matters (SPM) increased as much as nearly 25 times from 165 to 4220 mg/L as the velocity increased. Heavy metals showed an increase in dissolved phase as the velocity increased due to their desorption from the SPM. Acid-soluble heavy metals increased as the velocity increased, indicating that part of the heavy metals transformed from stable phase to labile phase during resuspension. Heavy metal concentrations in the SPM on volume normalization increased by approximately 2–6 times. However, on the mass weighted basis they decreased, approaching the bulk-sediment contents at high velocity, due to the “particle concentration effect”. The distribution coefficients (K D) of heavy metals were higher at slower velocity during the sediment resuspension, which could be attributed to the decrease of fine particles (silt/clay fraction) during resuspension.

Exposure of inshore corals to suspended sediments due to wave-resuspension and river plumes in the central Great Barrier Reef: A reappraisal

Suspended sediment in the coastal zone is an important limiting factor for the growth and health of inshore coral reefs. The Great Barrier Reef (GBR) lagoon receives sediment from a number of tropical rivers and the physical and biological effects of riverine discharge and turbidity within the lagoon are of considerable scientific and public interest. Published data from two inshore regions of the GBR are reviewed herein to evaluate the relative influence of river plumes and wave resuspension on suspended sediment concentration (SSC) around coral communities over a range of timescales. Data from Cleveland Bay and from other sites near the mouth of the Tully River show that wave resuspension is the most dominant mechanism controlling SSC at inshore reefs. At many nearshore areas today fine-grained bed sediment is abundant, consistent with millennial-scale geological evidence of sediment dispersal prior to European settlement and catchment impacts. Flocculation, particle settling and dilution occurs within the river plume, and riverine sediment concentrations at reefs directly attributable to individual flood inputs is significantly reduced, suggesting that the plume component is a relatively small contribution to the total suspended sediment mass balance over inter-annual timescales. Resuspension events can generate higher ambient SSC than that measured in flood waters (e.g. Tully River). In addition, while visually spectacular, satellite and aerial images offer limited quantitative information of total sediment load carried by hypopycnal plumes, as many of these plumes may contain algal blooms but relatively low concentrations of suspended sediment (ca. <5mg/l). Nonetheless, the cumulative effect of sediment-laden plumes may be a vector for other adsorbed contaminants of potential ecological concern, but coral smothering by hypopycnal plumes alone appears an unlikely impact particularly at inner- and middle-shelf reefs exposed to high wave energy and resuspension. Terrigenous sediment dispersal and turbidity within the GBR is governed by physical processes common to many continental shelves globally. Despite the examples examined in detail herein, the role of frequency, magnitude and duration in determining the impact or exposure of corals to elevated SSCs is poorly constrained by limited quantitative measurements during events, and our ability to place these into a broader temporal context. More high-quality observational data, at meaningful length-scales, can only enhance our ability to disentangle potential behavioural shifts in environmental responses.

Water chemistry and nutrient release during the resuspension of FeS-rich sediments in a eutrophic estuarine system

The objective of this study was to investigate the impact of resuspending FeS-rich benthic sediment on estuarine water chemistry. To address this objective, we conducted (1) a series of laboratory-based sediment resuspension experiments and (2) also monitored changes in surface water composition during field-based sediment resuspension events that were caused by dredging activities in the Peel–Harvey Estuary, Western Australia. Our laboratory resuspension experiments showed that the resuspension of FeS-rich sediments rapidly deoxygenated estuarine water. In contrast, dredging activities in the field did not noticeably lower O2 concentrations in adjacent surface water. Additionally, while FeS oxidation in the laboratory resuspensions caused measurable decreases in pH, the field pH was unaffected by the dredging event and dissolved trace metal concentrations remained very low throughout the monitoring period. Dissolved ammonium (NH4+) and inorganic phosphorus (PO4–P) were released into the water column during the resuspension of sediments in both the field and laboratory. Following its initial release, PO4–P was rapidly removed from solution in the laboratory-based (<1h) and field-based (<100m from sediment disposal point) investigations. In comparison to PO4–P, NH4+ release was observed to be more prolonged over the 2-week period of the laboratory resuspension experiments. However, our field-based observations revealed that elevated NH4+ concentrations were localised to <100m from the sediment disposal point. This study demonstrates that alongside the emphasis on acidification, deoxygenation and metal release during FeS resuspension, it is important to consider the possibility of nutrient release from disturbed sediments in eutrophic estuaries.

Effects of simulated natural and massive resuspension on benthic oxygen, nutrient and dissolved inorganic carbon fluxes in Loch Creran, Scotland

The effect of repeated natural resuspension on benthic oxygen consumption and the effect of natural and massive resuspension on oxygen consumption and fluxes of phosphate, silicate, ammonium and dissolved inorganic carbon (DIC) were studied at two stations (S1 and S2) in a Scottish sea loch. Station S11 had organically enriched sediment and station S1 had lower organic content in the sediment. The fluxes were measured in situ using the Göteborg benthic lander. Natural resuspension, simulating resuspension events due to strong wind, waves or currents, and massive resuspension, simulating resuspension due to e.g. trawling or dredging, were created inside the incubation chambers by regulating the stirring of the incubated overlying water or by retracting and shaking the incubated sediment. Natural resuspension showed clear effects on the oxygen consumption at station S11, where it increased with an average of 12.8 (standard error (s.e.) 0.17) and 7.7 (s.e. 0.12) mmolm−2d−1 during the first and second incubations, respectively. At station S1 there was no clear effect of natural resuspension on the oxygen consumption. Massive resuspension increased the oxygen consumption on S1 with an average of 608 (standard deviation (sd) 366) mmolm−2d−1 and on S11 with an average of 2396 (sd 2265) mmolm−2d−1. The fluxes of ammonium, phosphate and silicate were affected by the massive resuspension in 50, 14 and 33% of the chambers, respectively, on station S11. However, in the majority of the cases there were no effects on the nutrient and DIC fluxes of massive resuspension. The absolute concentrations of DIC, ammonium and silicate did however instantly increase with an average of 419 (sd 297), 48 (sd 27) and 6.9 (sd 3.7) μM, respectively, at S11 upon massive resuspension. The concentrations of phosphate decreased instantly with an average of 0.2 (sd 0.1) μM. On station S1 there were effects only on the ammonium and silicate concentrations, which increased with 0.8 (sd 0.3) and 1.13 (sd 0.36) μM, respectively. The large increase in oxygen consumption due to massive resuspension indicates that activities like e.g. trawling and dredging that take place in areas where water exchange occurs infrequently may lead to oxygen depletion in bottom water, which in turn might affect the ecological balance. Silicate, ammonium and DIC can be released due to massive resuspension and contribute to increased algal blooms in surface waters.

Processes controlling manganese distributions and associations in organic-rich freshwater aquatic systems: The example of Loch Bradan, Scotland

Recent increases in manganese (Mn) concentrations in surface waters, including drinking water supplies, have triggered renewed interest in its geochemical behaviour in freshwater systems. This study, involving analysis of bottom sediments and ultrafiltered water (stream, loch and sediment porewater) samples, identified changes in aqueous phase speciation of Mn upon entering the loch waters and during its transit from the inflow to the outflow of Loch Bradan, a drinking water reservoir in SW Scotland. Diffusion out of the bottom sediments during calm periods or mixing of porewaters with loch water during resuspension events also released Mn into the overlying waters. Although 65% Mn was in colloidal form (3kDa–0.2μm) in the main streamwater inflow at the western end, 57–66% was present in the <3kDa fraction in the proximal loch waters, at least partly as a result of the release of Mn2+ from the bottom sediments. Towards the outflow at the eastern end, however, the increase in the amount of Mn associated with large organic colloids (100kDa–0.2μm) correlated with the speciation of Mn in the bottom water and the bottom sediment porewaters. While the inflow waters do have some impact upon Mn speciation at the western end of the loch, it appears that within-loch processes have a greater impact on Mn speciation near the outflow. These findings emphasise the importance of understanding the geochemical controls on Mn behaviour in aquatic systems: it is clear that although Mn may be present as truly dissolved Mn2+ in some parts of the loch, it can also be associated to a significant extent (35–47%) with large organic colloids. These findings are important not only with respect to water treatment but also in terms of understanding the likely consequences of climatic change which may exacerbate losses of Mn from the bottom sediments.

Nearshore hydrodynamics as loading and forcing factors for Escherichia coli contamination at an embayed beach

Numerical simulations of the transport and fate of Escherichia coli were conducted at Chicago's 63rd Street Beach, an embayed beach that had the highest mean E. coli concentration among 23 similar Lake Michigan beaches during summer months of 2000–2005, in order to find the cause for the high bacterial contamination. The numerical model was based on the transport of E. coli by current circulation patterns in the embayment driven by longshore main currents and the loss of E. coli in the water column, taking settling as well as bacterial dark‐ and solar‐related decay into account. Two E. coli loading scenarios were considered: one from the open boundary north of the embayment and the other from the shallow water near the beachfront. Simulations showed that the embayed beach behaves as a sink for E. coli in that it generally receives E. coli more efficiently than it releases them. This is a result of the significantly different hydrodynamic forcing factors between the inside of the embayment and the main coastal flow outside. The settled E. coli inside the embayment can be a potential source of contamination during subsequent sediment resuspension events, suggesting that deposition‐resuspension cycles of E. coli have resulted in excessive bacterial contamination of beach water. A further hypothetical case with a breakwater shortened to half its original length, which was anticipated to enhance the current circulation in the embayment, showed a reduction in E. coli concentrations of nearly 20%.

Fine sediment and nutrient dynamics related to particle size and floc formation in a Burdekin River flood plume, Australia

The extreme 2010–2011 wet season resulted in highly elevated Burdekin River discharge into the Great Barrier Reef lagoon for a period of 200days, resulting in a large flood plume extending >50km offshore and >100km north during peak conditions. Export of suspended sediment was dominated by clay and fine silt fractions and most sediment initially settled within ∼10km of the river mouth. Biologically-mediated flocculation of these particles enhanced deposition in the initial low salinity zone. Fine silt and clay particles and nutrients remaining in suspension, were carried as far as 100km northward from the mouth, binding with planktonic and transparent exopolymer particulate matter to form large floc aggregates (muddy marine snow). These aggregates, due to their sticky nature, likely pose a risk to benthic organisms e.g. coral and seagrass through smothering, and also by contributing to increased turbidity during wind-induced resuspension events.

Boundary-layer-separation-driven vortex shedding beneath internal solitary waves of depression

AbstractWe investigate global instability and vortex shedding in the separated laminar boundary layer beneath internal solitary waves (ISWs) of depression in a two-layer stratified fluid by performing high-resolution two-dimensional direct numerical simulations. The simulations were conducted with waves propagating over a flat bottom and shoaling over relatively mild $(S= 0. 05)$ and steep $(S= 0. 1)$ slopes. Over a flat bottom, the potential for vortex shedding is shown to be directly dependent on wave amplitude, for a particular stratification, owing to increase of the adverse pressure gradient ($\mathrm{d} P/ \mathrm{d} x\gt 0$ for leftward propagating waves) beneath the trailing edge of larger amplitude waves. The generated eddies can ascend from the bottom boundary to as high as 33 % of the total depth in two-dimensional simulations. Over sloping boundaries, global instability occurs beneath all waves as they steepen. For the slopes considered, vortex shedding begins before wave breaking and the vortices, shed from the bottom boundary, can reach the pycnocline, modifying the wave breaking mechanism. Combining the results over flat and sloping boundaries, a unified criterion for vortex shedding in arbitrary two-layer continuous stratifications is proposed, which depends on the momentum-thickness Reynolds number and the non-dimensionalized ISW-induced pressure gradient at the point of separation. The criterion is generalized to a form that may be readily computed from field data and compared to published laboratory experiments and field observations. During vortex shedding events, the bed shear stress, vertical velocity and near-bed Reynolds stress were elevated, in agreement with laboratory observations during re-suspension events, indicating that boundary layer instability is an important mechanism leading to sediment re-suspension.

Effect of Resuspension on the Release of Heavy Metals and Water Chemistry in Anoxic and Oxic Sediments

AbstractTwo types of river sediments with contrasting characteristics (anoxic or oxic) were resuspended and the release of heavy metals and changes in water chemistry were investigated. During resuspension of the anoxic sediment, the dissolved oxygen (DO) concentration and redox potential of the water layer decreased abruptly within the first 1 min, followed by increases toward the end of the resuspension period. Heavy metals were released rapidly in the first 6 h, probably due to the oxidation of acid volatile sulfide (AVS) of the anoxic sediment, and then the aqueous phase concentrations of the heavy metals decreased due to resorption onto the sediment until the 12‐h point. During resuspension of the oxic sediment, the DO concentration and redox potential remained relatively constant in the oxic ranges. The heavy metals were released from the oxic sediment gradually during a 24‐h resuspension period. The temporal maximum concentrations of Ni, Cu, Zn, and Cd in the aqueous phases in both experiments frequently exceeded the USEPA water quality criteria or the water quality guidelines of Australia and New Zealand. This suggests that a resuspension event could bring about temporal water quality deterioration in the two sediment environments.

In situ flume measurements of resuspension in the North Sea

The in situ annular flume, Voyager II, was deployed at three sites in the North Sea in order to investigate resuspension events, to determine the physical characteristics of the seabed, to determine the threshold of resuspension of the bed and to quantify erosion rates and erosion depths. These are the first controlled, in situ flume experiments to study resuspension in the North Sea, and were combined with long-term measurements of waves and currents. Resuspension experiments were undertaken at two muddy, and one sandy site: north of the Dogger Bank (DG: water depths ∼80 m, very fine, poorly sorted, very fine-skewed sediment experiencing seasonal thermal stratification of the water column along with oxygen depletion); the Oyster Grounds (OG: ∼40 m, similar bed properties, year round water column thermal stratification, Atlantic forcing); and in the Sean Gas Field (SGF: ∼20 m, moderately sorted, very coarse-skewed sand, and well mixed water column). The erosion thresholds of the bed were found to be 0.66–1.04 Pa (DG) and 0.91–1.27 Pa (OG), with corresponding erosion depths of 0.1–0.15 mm and 0.02–0.06 mm throughout the experiments. Evaluation of a year of current velocities from 2007 indicated that at OG, resuspension of the consolidated bed was limited to on average ∼8% of the time as a result of tidal forcing alone for short (<30 min) durations, but would potentially increase during the winter as a result of wave influences. At DG, under similar conditions this would increase to 13%, and in the SGF, wave-induced resuspension events occurred throughout the year, with the potential exceedance of the threshold for suspension greater than 50% in January and March. Resuspension of bed material and erosion rates were closely related to applied bed shear stresses, and eroded depths were significantly correlated with the physical properties of the bed. Therefore, while complex variations in biogeophysical factors affected the critical threshold of erosion, once exceeded, erosion rates were related to the nature of the sediment.

Tide-induced fine sediment resuspension in the well-mixed Hangzhou Bay, East China Sea, China

ABSTRACTHangzhou Bay is a typical funnel-shaped high turbid estuary along a mesotidal coast of the East China Sea. In this paper, field studies are undertaken in an attempt to better understand tide-induced fine sediment transport processes in the Bay. Field observations were made mainly of current velocities (speeds and directions) and suspended sediment concentrations at six different relative depths and two stations in September 1992. Results show that (1) high near-bed suspended sediment concentrations varied from 1·81 to 4·00 kg m−3 during the spring tide and from 1·04 to 8·00 kg m−3 during the neap tide; (2) tide-induced near-bed fine sediment resuspension processes are indicated by several periodic suspended sediment concentration peaks; and (3) a hysteresis effect, or suspension-lag, occurs in suspended sediment concentration and resuspension events. Tide-induced fine sediment processes are mainly responsible for very high concentration suspensions of fine sediments in Hangzhou Bay. The occurrence of the suspension-lag phenomenon is likely to be common in other similar muddy tidal environments.

Faecal contamination of water and sediment in the rivers of the Scheldt drainage network

The Scheldt watershed is characterized by a high population density, intense industrial activities and intensive agriculture and breeding. A monthly monitoring (n = 16) of the abundance of two faecal indicator bacteria (FIB), Escherichia coli and intestinal enterococci (IE), showed that microbiological water quality of the main rivers of the Scheldt drainage network was poor (median values ranging between 1.4 × 10(3) and 4.0 × 10(5) E. coli (100 mL)( -1) and between 3.4 × 10(2) and 7.6 × 10(4) IE (100 mL)( -1)). The Zenne River downstream from Brussels was particularly contaminated. Glucuronidase activity was measured in parallel and was demonstrated to be a valid surrogate for a rapid evaluation of E. coli concentration in the river waters. FIB were also investigated in the river sediments; their abundance was sometimes high (average values ranging between 2.1 × 10(2) and 3.3 × 10(5) E. coli g( -1) and between 1.0 × 10(2) and 1.7 × 10(5) IE g( -1)) but was not sufficient to contribute significantly to the river water contamination during resuspension events, except for the Scheldt and the Nethe Rivers. FIB were also quantified in representative point sources (wastewater treatment plants) and non-point sources (runoff water and soil leaching on different types of land use) of faecal contamination. The comparison of the respective contribution of point and non-point sources at the scale of the Scheldt watershed showed that point sources were largely predominant.

Effect of dynamic and salinity conditions on activities of Hg during sediment resuspension event in the Yangtze Estuary

Effect of dynamic and salinity conditions on activities of Hg during sediment resuspension event in the Yangtze Estuary

Wind‐enhanced resuspension in the shallow waters of South San Francisco Bay: Mechanisms and potential implications for cohesive sediment transport

We investigated the driving forces of sediment dynamics at the shoals in South San Francisco Bay. Two stations were deployed along a line perpendicular to a 14 m deep channel, 1000 and 2000 m from the middle of the channel. Station depths were 2.59 and 2.19 m below mean lower low water, respectively. We used acoustic Doppler velocimeters for the simultaneous determination of current velocities, turbulence, sediment concentration and fluxes. Maximum current shear velocities were 0.015 m s−1 at the station further from the channel (closer to the shore) and 0.02 m s−1 at the station closer to the channel. Peak wave‐induced shear velocities exceeded 0.015 m s−1 at both stations. Maximum sediment concentrations were around 30 g m−3 during calm periods (root mean square wave height &lt;0.15 m). During wavy periods, sediment concentrations increased to 100 g m−3 and sediment fluxes were 5 times higher than in calm conditions (0.02 g m−2 s−1 versus &gt;0.10 g m−2 s−1) at the station further from the channel 0.36 m above the bed. Closer to the channel, sediment concentrations and vertical fluxes due to wind wave resuspension were persistently lower (maximum concentrations around 50 g m−3 and maximum fluxes around 0.04 g m−2 s−1). Most resuspension events occurred during flood tides that followed wave events during low water. Although wave motions are able to resuspend sediment into the wave boundary layer at low tide, the observed large increases in sediment fluxes are due to the nonlinear interaction of wind waves and the tidal currents.

Resuspension and estuarine nutrient cycling: insights from the Neuse River Estuary

Abstract. For at least the past several decades, North Carolina's Neuse River Estuary (NRE) has been subject to water quality problems relating to increased eutrophication. Research initiated in the past several years have addressed the nutrient processes of the water column and the passive diffusion processes of the benthic sedimentary environment. Resuspension of bottom sediments, by bioturbation, tides, or winds, may also have a significant effect on the flux of nutrients in an estuarine system These processes can result in the advective transport of sediment porewater, rich with nitrogen, phosphorus and carbon, into the water column. Thus, estimates of nutrient and carbon inputs from the sediments may be too low. This study focused on the potential change in bottom water nutrient concentrations associated with measured resuspension events. Previous research used short-lived radionuclides and meteorological data to characterize the sediment dynamics of the benthic system of the estuary. These techniques in conjunction with the presented porewater inventories allowed evaluation of the depth to which sediments have been disturbed and the advective flux of nutrients to the water column. The largest removal episode occurred in the lower NRE as the result of a wind event and was estimated that the top 2.2 cm of sediment and corresponding porewater were removed. NH4+ advective flux (resuspended) was 2 to 6 times greater than simply diffusion. Phosphate fluxes were estimated to be 15 times greater than the benthic diffusive flux. Bottom water conditions with elevated NH4+ and PO43− indicate that nutrients stored in the sediments continue to play an important role in overall water quality and this study suggests that the advective flux of nutrients to the water column is critical to understand estuarine nutrient cycling.

Suspended particulate matter in Lake Erie derived from MODIS aquatic colour imagery

This paper explores the use of Moderate Resolution Imaging Spectroradiometer (MODIS) wavebands in the red/near-infra-red for estimating concentrations of suspended particulate matter (SPM) in the moderately turbid, optically complex waters of Lake Erie. Observations show that at wavelengths shorter than 550 nm, more than 50% of the absorption signal is accounted for by dissolved organic matter and phytoplankton, confirming that algorithms incorporating these wavelengths may not be appropriate for these waters. Single band and band ratios at wavelengths greater than 667 nm are tested for their suitability for monitoring SPM concentrations in these waters. A simplified regional semi-analytical model is utilized which is independent of variations in dissolved organic matter and chlorophyll absorption, enabling estimates of SPM concentrations from MODIS water-leaving radiance at 748 nm with an average root mean square (RMS) error of 40%. Knowledge of the vertical distribution of particles enables estimates of total water column suspended loads which are then related to wind re-suspension events. The method is applied to MODIS water-leaving radiance at 748 nm to produce a time series of surface and total water column suspended loads in Lake Erie for the period 2003–2007.

Toxizität und Risk Assessment fluvialer Sedimente und Schwebstoffe: Eine kurze Übersicht bisheriger und neuerer Entwicklungen

Background In recent years, sediments have become a central topic of scientific and public discussion as an important factor for determining water quality. While the quality of surface waters in Germany has significantly improved during the past years, highly contaminated sediments still create a considerable threat to the quality of several European catchment areas. Main features Here, we summarize different concepts and methods for the assessment of sediment quality and report on some novel integrative test methods for assessing sediment toxicity including contaminant re-mobilization during simulated re-suspension events. Results and discussionCurrently, different approaches for assessment of sediment pollution exist. While instrumental chemical analyses are not suitable to accurately describe sediment toxicity, combinations of biological and chemical test procedures and integrated approaches, for example weight-of-evidence studies and effect-directed analysis (EDA), have the potential to identify key contaminants. Inter-disciplinary studies combining hydrodynamic and toxicological aspects coupled to real exposure of aquatic organisms to contaminants are currently being developed. ConclusionsMonitoring and assessment of sediment quality are of increasing importance, not only for national legislation but also for the implementation of the European Water Framework Directive (WFD). Integrated approaches for the determination of sediment stability play a key role in the appropriate sediment-monitoring strategies.