Wave-induced sediment resuspension in the Öre estuary,northern Sweden

Material transported by the Öre River in northern Sweden is all deposited within the estuary which means that resuspension is necessary for the transport of particles out of the estuary. Wave-induced sediment resuspension in the estuary was studied by monitoring the distribution of suspended particles during a resuspension-redeposition cycle. The particle concentration in the water mass was measured with a light scattering probe, calibrated by comparison with the amount of particles collected on a filter.After a long period with calm weather and a low river input less than 100 tonne of suspended particulate matter was present in the estuary. However, during a period with stormy conditions significant resuspension of sediment particles occurred within the estuary. Two days after the storm approximately 1125 tonne of suspended particulate matter was found in the estuary. Most (61%) of the suspended matter was found in the deepest third of the water column, although up to 17% was present in the top third of the water column. The total load of particulate matter in the water column remained constant until day four after the storm, but a significant redistribution of the particulate matter occurred both in the vertical and horizontal directions. Nine days after the storm, a significant amount of particles (c. 350 tonnes) was still in suspension.     

High resolution measurements of sediment resuspension above an accumulation bottom in a stratified lake

A detailed record of suspended particulate matter (SPM) concentrations in the benthic boundary layer (BBL) 1.5 m above an accumulation bottom and 13.5 m below the surface was obtained from frequent (30 min interval) beam attenuation measurements made with a Sea Tech transmissometer in the main basin of Lake Erken, a moderately deep (mean depth 9 m, maximum depth 21 m) dimictic lake in central Sweden. Concentrations of SPM (g m^–3) were not as strongly correlated to the beam attenuation coefficient (c, [m^–1]), as were concentrations of the inorganic SPM fraction. Apparently, this was caused by large optically inactive organic particles which significantly affected the measurements of SPM, but had little effect on the attenuation of light.When the water column was thermally stratified, SPM concentrations in the BBL showed a seasonal increase which was related to an increase in the thermocline depth. As the epilimnion deepened, there was also a marked increase in the occurrence of rapid and large changes in SPM concentration. After the loss of stratification, the amount of SPM and the temporal variability in its concentration was reduced. Since surface waves could not influence sediment resuspension at the depth of measurement, these data show the importance of internal waves in promoting sediment resuspension in areas of sediment accumulation. The relatively short period in each summer, when the thermocline reaches a sufficient depth to allow for resuspension over accumulation bottoms, can have important consequences for both the redistribution of lake sediments and the internal loading of phosphorus.     

A review of methods used to measure sediment resuspension

Resuspension of bottom sediments is an important lake-internal process with regard to particle cycling and sedimentation. Current methods to measure sediment resuspension are reviewed, such as optical and acoustical instruments, instantaneous multiple point water samplers, sediment traps, sediment cores and grabs, radiotracers such as Pb²¹⁰, Cs¹³⁷ and Be⁷, mass balance calculations, various modelling approaches, statistical methods (correlation analysis), and laboratory experiments.For the quantification of resuspension, the combined use of sediment traps, sediment cores, near bottom current meters, and turbidity meters to measure suspended and settling particulate matter in the hypolimnion of lakes is recommended; in addition, wind stress, seiches, slumping and sliding, and riverine input may be monitored to elucidate the mechanisms behind the process.     

Food availability and gut contents in the ascidian Cnemidocarpa verrucosa at Potter Cove, Antarctica

A high seasonality characterizes Antarctic environments, and generates marked differences in availability and composition of food for benthic filter feeders. During a year-round period at Potter Cove, Antarctica, algal pigment concentration (chlorophyll a) and organic matter were measured in water column and sediment samples. Chemical composition of gut contents of the ascidian Cnemidocarpa verrucosa was also analyzed. Despite the low standing stock, capture and absorption of organic matter were detected year-round, suggesting intake of other particles besides microalgae. The mechanism that provides food to the ascidians and epibenthic communities may be related to the supply of allochtonous particles, bottom resuspension events, and microbial community dynamics. Sea-ice cycles may affect food availability in terms of water-column mixing and benthic resuspension. The scarce primary production and the high amount of sedimented material are not limiting conditions in Potter Cove, which presents a rich ascidian community.     

A Preliminary Laboratory Investigation of PCB Flux from Dredge Resuspensions and Residuals

A preliminary laboratory investigation was conducted to understand the relative contributions of major dredge resuspension and residual processes on the releases of polychlorinated biphenyl (PCB) contaminants from sediments to water column. Sediments from New Bedford Harbor were used as test samples. Six sets of experiments were run for simulated resuspension and residual scenarios. During the experiments, water above the sediments was recirculated by peristaltic pumping or orbital shaking and the levels of two PCBs, Aroclor 1248 (PCB-1248) and Aroclor 1254 (PCB-1254), were monitored for 15 days. Analysis of the model predicted data indicated that resulting water column PCB concentrations differed with sediment surface, residual, and resuspension type. Highest PCB water column concentrations were observed for a condition which used a settled fluff from thin sediment slurry as a residual source and the column water was recirculated by orbital shaking. Lowest water column PCB levels were observed for a thick sediment deposit placed over clean sand. The PCB levels in the water column for all six simulated conditions were several orders higher than the USEPA ambient water quality criteria concentrations for aquatic environment and human consumption.     

Sedimentation modified by wind induced resuspension in a shallow tropical lagoon (Cote d'Ivoire)

In shallow environments, under certain conditions of fetch, wind velocity, bathymetry and bottom characteristics, resuspension can be generated by wind induced waves. In the tropical Ebrié lagoon, austral trade winds are dominant almost all year long, and their velocity shows a marked diel pattern with maximum speed between noon and midnight. Only austral trade winds with a speed >3 m s⁻¹ allow particle resuspension which is effective for depths<1.5 m. In these areas, significantly higher values of chlorophyll biomass and mineral seston are noted during the windy sequences. Granulometric and mineralogical analyses showed that only the surficial sediment (0–3 cm) was involved in resuspension. This process induces several effects: 1) an increase of the suspended matter concentration in the water and thus a light attenuation due to a higher turbidity, 2) a redistribution in the whole water column of nutrients from the pore water and 3) a removal of the finer fractions from the superficial sediment. On the contrary, for depths>1.5 m, particle sinking is permanent in depressions which are spontaneously transformed into anoxic systems. At the lagoon scale, sedimentation is significantly modified by wind induced resuspension. According to the bathymetry and the distance from a river, three sedimentary facies are recognized. Their grain size distributions are parabolic in areas where resuspension occurs, logarithmic in areas where no resuspension is possible and hyperbolic in the hollows and the main channels. Finally, a large part of the allochthonous inputs (from drainage and rivers) and autochthonous pelagic production is trapped into the Ebrié lagoon and less than 10% of the particles entering the lagoon are exported toward the Atlantic Ocean.     

Floating-leaved macrophyte (<Emphasis Type="Italic">Trapa quadrispinosa</Emphasis> Roxb) beds have significant effects on sediment resuspension in Lake Taihu,China

Trapa quadrispinosa Roxb is a common floating-leaved macrophyte in China. In this study, the effects of Trapa on sediment resuspension in Lake Taihu, a large, shallow, eutrophic lake in eastern China, were investigated using sediment traps. The study was conducted at stations with and without Trapa beds from 7 September to 18 October, 2003. Results showed that sediment resuspension rates at the station without Trapa were significantly higher than those at the station with Trapa. During the study period (41 days), 10,970 g dw m⁻² of surface sediment was resuspended within the Trapa beds and 29,903 g dw m⁻² in the pelagic zone. These equate to phosphorus fluxes of 7.4 g m⁻² to the water column at the station with Trapa and 16.1 g m⁻² at the station without Trapa. Floating-leaved macrophytes, such as Trapa, may be important in sediment resuspension and internal phosphorus loading in shallow lakes.     

Suspended particulate matter dynamics in the shallow mesotidal urdaibai estuary (Bay of Biscay,Spain)

Seston quantity and quality was measured in the Urdaibai Estuary in summer, when river discharge is low and tidal flushing is the main driving force to transport and resuspend particulate matter in the estuary. The highest seston concentrations are found in the upper estuary, where more than 90% of total suspended particulate matter is made up of particles <20 μm. There is a temporal ebb-flood asymmetry and turbulent mixing is stronger at flood tide. During dry calm periods sediment resuspension is observed for a short period of time at floods during spring tides, decreasing drastically before slack waters. In spite of this, particulate organic matter mainly originates from anthropogenic sources (domestic wastewater discharge) and plankton production.     

Impact of resuspension of cohesive sediments at the Oyster Grounds (North Sea) on nutrient exchange across the sediment–water interface

Benthic-pelagic exchange processes are recognised as important nutrient sources in coastal areas, however, the relative impact of diffusion, resuspension and other processes such as bioturbation and bioirrigation are still relatively poorly understood. Experimental ship-based data are presented showing the effects of diffusion and resuspension on cohesive sediments at a temperate shelf location in the North Sea. Measurements of diffusive fluxes in both spring (1.76, 0.51, ?0.91, 17.6 μmol/m²/h) and late summer (8.53, ?0.03, ?1.12, 35.0 μmol/m²/h) for nitrate, nitrite, phosphate and dissolved silicon respectively, provided comparisons for measured resuspension fluxes. Increases in diffusive fluxes of nitrate and dissolved silicon to the water column in late summer coincided with decreases in bottom water oxygen concentrations and increases in temperature. Resuspension experiments using a ship board annular flume and intact box core allowed simultaneous measurement of suspended particulate matter, water velocity and sampling of nutrients in the water column during a step wise increase in bed shear velocity. The resuspension of benthic fluff led to small but significant releases of phosphate and nitrate to the water column with chamber concentration increasing from 0.70–0.76 and 1.84–2.22 μmol/L respectively. Resuspension of the sediment bed increased water column concentrations of dissolved silicon by as much as 125% (7.10–15.9 μmol/L) and nitrate and phosphate concentrations by up to 67% (1.84–3.08 μmol/L) and 66% (0.70–1.15 μmol/L) respectively. Mass balance calculations indicate that processes such as microbial activity or adsorption/desorption other than simple release of pore water nutrients must occur during resuspension to account for the increase. This study shows that resuspension is potentially an important pathway for resupplying the water column with nutrients before and during phytoplankton blooms and should therefore be considered along with diffusive fluxes in future ecosystem models.     

Resuspension in a shallow eutrophic lake

The frequency and the importance of wind-induced resuspension were studied in the shallow, eutrophic Lake Arresø, Denmark (41 km², mean depth 3 m). During storm events in autumn 1988 lake water samples were collected every 2–8 hours by an automatic sampler at a mid-lake station. The concentration of suspended solids and Tot-P was found to increase markedly. During storms up to 2 cm of the superficial sediment was resuspended, and the concentration of resuspended solids in the water column rose to 140 mg l^–1. The resuspended particles had a relatively high settling velocity and on average, a relatively short residence time in the water column of 7 hours.A model which describes the concentration of resuspended solids as a function of wind velocity and of settling velocity of the resuspended particles is presented. Using additional wind velocity data from a nearby meteorological station, the model has been used to calculate the frequency of resuspension events and concentration of resuspended solids for the period from May to November 1988.These calculations show that resuspension occurred about 50% of the time. Average flux of suspended solids from the sediment to the water was 300 g m^–2 d^–1 and during 50% of the time lake water concentration of suspended solids was more than 32 mg l^–1. A relationship between concentration of suspended solids and Secchi-depth is presented. Because of resuspension, Secchi-depth in Lake Arresø is reduced to 0.5 m.Resuspension also had a marked effect on Tot-P concentration in the lake water, and P input to the lake water being totally dominated by resuspension events.     

Phosphorus in settling matter and bottom sediments in lakes loaded by agriculture

In shallow lakes, the cycling of P between water and bottom sediments is strongly influenced by wind-induced resuspension of particulate matter. The significance of this P flux as an algal nutrient source is unclear. We examined gross sedimentation in 3 open and shallow agriculturally loaded lakes. In addition, we estimated the potential P-release from settling and bottom matter by laboratory tests. The mean daily rate of gross sedimentation was 21–170 g m^–2 d^–1 of dry sediment, 0.04–0.18 g m^–2 d^–1 of P and 0.18–2.0 g m^–2 d^–1 of N; being the highest in the shallowest and most eutrophic lake. In Lake Karhijärvi, where the most intensive measurements were taken, wind explained the temporal variation in the gross sedimentation to some extent. The settling matter consisted of inorganic particles low in nutrients, especially during peak sedimentation periods. On average, 7.7 ± 3.1% (x ± 95% confidence interval) of the P in the settling matter in L. Karhijärvi was in an algal-available form according to 2–3 week bioassays. In the bottom matter of the three lakes, 3.0 ± 1.7% and 2.5 ± 3.6%, and 4.3 ± 3.7% of the P was utilized by the algae. In L. Karhijärvi, resuspension of the potentially available P exceeded 20 times the external loading during the open water season. According to sorption tests, P is released from the bottom matter only when the concentration of o-P is <2 g l^–1. Although such a low value cannot be determined with common analytical procedures, it seems probable that the P concentration allows P desorption during P-limited periods. However, the significance of resuspended matter as an algal nutrient source calls for further research.     

Phosphorus release from resuspended sediment in the shallow and wind-exposed Lake Arresø, Denmark

Wind-induced sediment resuspension occurs frequently in the shallow and eutrophic Lake Arresø, Denmark. The impact of resuspension on internal phosphorus loading was investigated by laboratory experiments studying P-release from the undisturbed sediment surface and by experiments simulating resuspension events.Phosphorus release from undisturbed sediment sampled in May and August was 12 mg and 4 mg m^–2 d^–1, respectively. During experimental simulation of resuspension, soluble reactive phosphate (SRP) increased by 20–80 µg l^–1, which indicates that a typical resuspension event in the lake would be accompanied by the release of 150 mg SRP m^–2. The internal P loading induced by resuspension is estimated to be 60–70 mg m^–2 d^–1, or 20–30 times greater than the release from undisturbed sediment.SRP release during simulation of resuspension was mainly dependent on the equilibrium conditions in the water column and was basically independent of the increase in suspended solids and the duration of resuspension. A second simulation of resuspension conducted 26 hours later, did not result in any further release of SRP from sediment sampled in May. In contrast, there was an additional SRP release from sediment sampled in August, indicating that an exchangable P pool, capable of altering equilibrium conditions, is built up between resuspension events.It is concluded that resuspension, by increasing the P flux between sediment and water, plays a major role in the maintenance of the high nutrient level in Lake Arresø. A relatively high release rate is maintained during resuspension because of the low Fe:P ratio and the high concentration of NH₄Cl-extractable P in the sediment.     

The effect of an emergent macrophyte (Typha angustifolia) on sediment resuspension in a shallow north temperate lake

1. The effects of emergent macrophytes on water turbidity and sediment resuspension in the shallow Kirkkojärvi basin of Lake Hiidenvesi were studied with sediment traps, and concomitant sediment and water samples. The study was conducted during May–August in three different zones of a stand of the emergent Typha angustifolia .
2. Within the stand (5 m from the edge), both the concentration of suspended solids and the rate of sediment resuspension were significantly lower than at the edge and outside the stand (5 m from the edge). The differences between the zones increased towards the end of summer together with the growing stem density. During the study period (82 days), 2210 g dw m⁻² of sediment was resuspended in the outer zone. At the edge and in the inner zone, the corresponding numbers were 1414 and 858 g dw m⁻², respectively.
3. With the resuspended sediment, 39.4 mg P  m⁻² day⁻¹ was brought to the water column outside the stand, 22.4 mg P  m⁻² day⁻¹ at the edge and 13.4 mg P  m⁻² day⁻¹ within the stand.
4. In early summer, the concentration of suspended solids had a highly significant positive effect on soluble reactive phosphorus (SRP) concentration in the water, whereas in late summer no effect was found. During the study period, phosphorus retention by emergent macrophyte stands corresponded to 3–5% of the present annual external phosphorus loading of the Kirkkojärvi basin.     

Sediment dynamics and estuarine circulation in the turbidity maximum of the Elbe river

Data are presented from several campaigns concerning to the residual transport of water, salt and suspended particulate matter (SPM) at different depths and at different longitudinal and cross-sectional locations in the Elbe Estuary, Northern Germany. The intratidal behaviour of SPM is analysed and explained in terms of cyclic settling and resuspension. Superimposed on these local processes are advective horizontal transports of water and SPM in different vertical layers. Tidally averaged horizontal fluxes of salinity and SPM have been calculated at different depths. For the most part, they show upstream directed transport in the bottom layers, while in the upper water column SPM is carried towards the sea. Examples of the influence of vertical salinity gradients on SPM dynamics, of the lateral variation of residual transport of water and SPM, and the effects of a runoff peak are also presented.     

Short-term variations of seston characteristics in a shallow tropical lagoon: Effect of wind-induced resuspension

In January 1993, a 24 hour survey based on an hourly sampling was conducted in a station (1 m depth) located in the Ebrié lagoon, Côte d'lvoire, in order to determine short-term changes in seston quality and quantity and relate them to physical conditions. Water samples were collected just below the surface and at 0.9m depth to assess the seston distribution in the water column. Located in an estuarine environment, the study site was submitted to periodic effect of tides with low currents. Under certain conditions, sediment resuspension by wind-induced waves was observed in the shallow parts of this ecosystem. From the analysis of hydrodynamic processes (tide, wind and sedimentation rates), four basic situations were distinguished in the course of the study: a period with no wind and high sedimentation rates, a period characterized by wind-induced resuspension, followed by a period of sedimentation and finallly, a period without wind during the second flood tide. The particle size spectra were similar throughout the study, the distributions being unimodal (mode of 4 μm diameter). The finer particles (<10 μm) were resuspended over the whole water column at the beginning of the resuspension period, whereas the larger particles were resuspended if the wind speed exceeded 3 m s^?1. Phytoplankton species were found to provide information on the physical factors controlling the pelagic system: small cells, such as those of cyanobacteria (Synechococcus sp.), were always present over the whole tidal cycles, whereas large motile cells were predominant in the water column during the resuspension period. The seston characteristics of this part of Ebrié lagoon are mainly controlled by tidal currents, but wind-induced resuspension can modify the structure of the pelagic food web.     

Phosphorus sedimentation in seasonal anoxic Lake Scharmützel, NE Germany

The seasonal course of phosphorus (P) fractions of sinking particulate matter has been studied at the deepest location of dimictic eutrophic Lake Scharmützel (29.5 m) by paired sediment traps at 9 and 27 m water depth from May 1996 to December 1997. Relatively large depositional fluxes and considerably variable P fluxes, mainly carried by allochthonous particles, diatoms in spring, and iron during overturns transport almost 60% of the average water column P pool to the sediment surface. The contribution of resuspension and sediment focusing (24–34%) is relatively small. A sequential chemical P extraction of the matter entrapped revealed that ,loosely adsorbed` P contributed to 5–14%, organic bound P to 55–68%, and Ca-associated P to 3–6% of the annual P flux, depending on season and depth. The redox-sensitive (iron bound) P ranged from only 12% of the annual P flux during anoxic sulfidic conditions to 28% during oxic conditions. On an annual basis, 16–18% of sedimenting P was recycled within the water column, and 71–75% of total primary P flux was recycled at the sediment surface. Ultimately, 10–23% of P became incorporated into recent sediments indicating the long residence time for P and a high internal resupply of P for primary production.     

Effects of wind on a shallow lake ecosystem: Resuspension of particles in the Loosdrecht Lakes

Horizontal variation of seston concentration in the shallow, eutrophic Loosdrecht Lakes (A=9.8 km²; =1.9 m) was studied in relation to windspeed and effective fetch. Simple wave theory was applied in order to predict resuspension using wind data from a nearby meteorological station. Most results were consistent with the theory, but a clear limit for the occurrence of resuspension could not be established. Generally, changes in epipelon—the particles at the sediment-water interface — were not directly related to computed frequency of resuspension at the sampling station. The frequency was estimated for 37 grid points over the entire lake. Resuspension was computed to affect high percentages of the lake area in winter. In summer the frequency was much lower, but in June and July 1984 there were days with nearly 50% of the lake area subject to resuspension. The resulting input of particulate organic carbon into the water column during these days was estimated to equal 12–25 times the daytime phytoplankton carbon fixation. Most of the resuspended matter appeared to be redeposited rapidly. The computed frequency of resuspension for the 37 locations of the lake varied between 7 and 48 days in 1984.     

Longitudinal changes in protistan bacterivory and bacterial production in two canyon-shaped reservoirs of different trophic status

The drastic interactions of weather as El Niño events with catchment and hydrological processes can cause unexpected changes in physical, chemical and biological properties of freshwater aquatic ecosystems. The severe drought during 1998–1999 in the northeastern region of Brazil induced ecological changes in numerous reservoirs as in Tapacurá reservoir, one of the biggest drinking-water suppliers in Pernambuco state. Investigations were based on monthly sampling over 2 years (May 1998–May 2000) conducted at 3 representative stations with 3 sampled depths through the water column (0.5 m, middle and 0.5 m above the bottom). Temporal changes in ecological processes, especially stratification, were driven by two major precipitation patterns, with an initial marked dry period (period 1) followed by a rainy season (period 2). Dissolved oxygen and pH variations, higher conductivity and alkalinity values, higher concentrations of particulate organic material (carbon, nitrogen and phosphorus) and higher levels of algal biomass (chlorophyll a) characterized the dry period (May 1998–May 1999). During this phase of low water level when the reservoir storage capacity reached a minimum of 3.9%, the concentrations of chlorophyll a gradually increased with a cyanobacterial bloom (Cylindrospermopsis raciborskii) noted in April 1999. The decline in chlorophyll a and particulate organic matter were observed as a result of the first rains in May–June 1999, with the drastic changes of quality of matter (higher particulate C/N ratio). After a phase characterized by the entire water column turning anoxic, a second phase in the stratification process could be identified from June 1999 with the pronounced rainfalls accompanied by an overturn event. Annual rainfall deficit and lack of reservoir water renewal in 1998–1999 linked to the 1997 El Niño consequences were important determinants of high eutrophication levels and drastic ecological modifications in Tapacurá reservoir.     

Deposition,resuspension, and decomposition of particulate organic matter in the sediments of Lake Itasca,Minnesota, USA

Sediment traps were used to investigate the settling, resuspension, and decomposition of particulate organic matter in Lake Itasca, MN (USA). Traps were deployed in the epilimnion and hypolimnion of the deepest basin during June, 1988, sampled twice during stratified conditions (August, September) and once after the lake had mixed (October). The downward flux of particulate material increased from summer to fall. The net sedimentation of organic matter ranged from 0.6 to 2.3 g m^–2 d^–1 at 4 m and increased to 2.1 to 3.2 g m^–2 d^–1 two meters above the bottom sediment indicating that resuspended sediment was at least 33% of the settling mass during all periods. The C:N ratios of captured particles (6.8–9.5) were between the ratios of plankton (5.8 to 6.8) and the sediments (9.9 to 10.2) but smaller than the ratios of terrestrial organic materials (13.5 to 222). The monosaccharide compositions of the entrapped particles were similar to plankton samples and different from the distinct composition of the sediments. Capture of rebound particles similar to the primary flux and not decomposition may have been responsible for this similarity. Total monosaccharide concentrations were lower in the sediments than in entrapped particles. Individual sugars exhibited different patterns of accumulation in the sediments. Glucose was lowest in sediments when the relative concentrations were compared to those in source materials and entrapped particles. In contrast, sediments had the highest rhamnose and fucose concentrations. Bacterial biomass could only account for small portions of these sugars in the sediment. The distinct monosaccharide composition of resuspended sediments was not strongly recorded in materials captured by the sediment traps even after the lake had mixed.     

Possibilities and limitations of sediment traps to measure sedimentation and resuspension

Settling flux of particulate matter measured by sediment traps in shallow and turbulent waters considerably exceeds settling flux calculated from mass balances. Theoretical consideration on sinking distance and particle settling leads to the conclusion that traps overtrap particles in even moderately turbulent waters. The disappearance of turbulent diffusion and bottom shear stress within the traps causes the high rates of trapping, whereas their presence in natural water bodies hampers sedimentation.The use of sediment traps may be restricted to the identification of rapidly sinking particles, measurement of sinking velocities of well defined types of particles, and estimation of resuspension.