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.     

Empirical evidence of the importance of sediment resuspension in lakes

This paper reviews the empirical evidence for bottom sediment resuspension, which has been documented in many aquatic systems, and attempts to put those observations into a predictive framework. The evidence suggests that resuspension is episodic in all systems with the spatial extent of resuspension being determined by the wind speed, duration of disturbance, effective lake fetch and lake morphometry. The rates of deposition of resuspended material vary greatly with measurements recorded between 0.5 and 50 g m^?2 d^?1. In many systems and at many times of year resuspended material comprises the majority of the flux of particles to lake bottoms. An order-of-magnitude estimate of the long-term contribution of resuspended material to the total flux of particulate matter is 85%.     

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.     

Identifying resuspended particles using isotope ratios

Three simple methods were developed to estimate the proportion of particles in lake water derived from resuspended material. These techniques use the different distributions of long and short-lived radioisotopes sorbed onto particles and were tested in the three basins of Lake Erie using ⁷Be, ¹³⁷Cs and ²¹⁰Po/²¹⁰Pb.While the concentration of ²¹⁰Po on particles did not vary significantly in the lake, resuspended particles were characterized by high concentrations of ¹³⁷Cs and low concentrations of ⁷Be. The distribution of these radioisotopes is consistent with a simple mixing model in which the fraction of particles in the lake water derived from resuspension ranged from 8% to about 100%. Higher concentrations of resuspended particles were found in deeper samples from the nepheloid layer and in the shallow western basin where thermal stratification was very weak.     

Nonlinear climate change and Andean feedbacks: an imminent turning point?

A 370 000‐year paleoecological record from Lake Titicaca provides a detailed record of past climate change in which interglacial periods are seen to have some elements of commonality, but also some key differences. We advance a conceptual feedback model to account for the observed changes that includes previously ignored lake effects. Today Lake Titicaca serves to warm the local environment by about 4–5 °C and also to increase rainfall. We observe that as water levels in the lake are drawn down due to warm, dry, interglacial conditions, there is a possible regional cooling as the lake effect on local microclimates diminishes. Positive feedback mechanisms promote drying until much of the lake basin is reduced to salt marsh. Consequently, the usual concept of upslope migration of species with warming would not be applicable in the Altiplano. If, as projected, the next century brings warmer and drier conditions than those of today, a tipping point appears to exist within ca. 1–2 °C of current temperatures, where the relatively benign agricultural conditions of the northern Altiplano would be replaced by inhospitable arid climates. Such a change would have profound implications for the citizens of the Bolivian capital, La Paz.     

The effect of resuspension on algal production in a shallow lake

Waves cause erosion and resuspension of bottom sediments. In shallow lakes resuspension can take place over most of the lake area and the resuspended matter can stay in suspension for such a long time that the mean light intensity in the lake is reduced, causing reduced algal growth. The increase of suspended matter and light attenuation in the water of lake Tämnaren, Sweden, was found to be proportional to wind velocity to the third power. After each storm increased attenuation of light lasted for a week. The algal production was estimated to be reduced to only 15 % of what it would have been without increased turbidity due to resuspension.     

Effects of sunlight on the production of dissolved organic and inorganic nutrients from resuspended sediments

A series of laboratory-based and field experiments was conducted to address the effects of sunlight-exposed resuspended sediments on dissolved nutrient fluxes in two different water bodies. In suspensions of tidal creek sediments in 0.2 μm-filtered creek water, measurable increases in dissolved nutrients occurred after only 2 h of exposure to simulated sunlight. During a 6-h irradiation, nutrient release rates for total dissolved nitrogen (TDN) and phosphate were 2.2 ± 0.5 (standard error; S.E.) μmol g^?1 h^?1 and 0.09 ± 0.005 μmol g^?1 h^?1 (S.E.), compared to no significant changes in dark controls. The majority of nitrogen was released as dissolved organic nitrogen (87% on average) with lesser amounts of ammonium (13%). Continental shelf sediments resuspended in unfiltered seawater also released phosphate and TDN when exposed to sunlight, suggesting that this process can occur in a variety of marine and estuarine environments. The source material for inorganic nutrients appears to be associated with sediments rather than dissolved organic matter, as no significant changes in nutrient concentrations occurred in experiments with 0.2 μm-filtered creek water or seawater alone. Results suggest that photoproduction of dissolved nutrients from resuspended sediments could be an episodically significant and previously unrecognized source of dissolved organic and inorganic nutrients to coastal ecosystems. This process may be especially important for continental margins where episodic resuspension events occur, as well as in regions experiencing high riverine sediment fluxes resulting from erosion associated with deforestation and desertification.     

Byglandsfjorden. Primary production and other limnological features in an oligotrophic Norwegian lake

Byglandsfjorden is a lake situated in southern Norway, about 80 km north of Kristiansand. The catchment area consists of archean rocks. The river Otra flows through the lake. Water samples were taken from the lake 4 times during the summer of 1971, and primary production was measured three times. The chemical analyses show an oligotrophic lake. The lake is acid and poor in electrolytes. pH-values vary between 5.3 and 6.0, and the specific conductivity lies at about 10. Primary production is rather high, and per m² lake surface the values are found to be about the same as in Svinsjøen, a rather eutrophic lake near Oslo. However, an index used by Rodhe (1965) shows that the lake is of a more oligotrophic type.     

Relations between resuspension and the content of 137Cs in freshwater fish in some Swedish lakes

In many Swedish lakes, the fallout of ¹³⁷Cs from the Chernobyl accident in April 1986 has largely accumulated in the sediments. The availability and transfer of deposited ¹³⁷Cs to biota is influenced by factors such as resuspension. The frequency of resuspension and the ¹³⁷Cs-content of different fish species was studied in three shallow lakes in Uppland, central Sweden, and in one deeper lake in northern Sweden. Resuspension was measured by the use of sediment traps. Sedimentation rates measured from the traps in the shallower lakes were 5–10 times higher than normal for this type of lake, indicating that resuspension was an important factor. The decrease of the ¹³⁷Cs-content in muscle tissue of pike, perch and roach was slow in each of the shallow lakes. ¹³⁷Cs decreased by about 30% over a period of 2 years in the shallowest lake (maximum depth 4 m), whereas ¹³⁷Cs decreased by 50% in the deeper lakes (maximum depth 10 m). The slower rates of decline of ¹³⁷Cs in biota from the shallow lakes, are probably a function of sediment dynamics (mainly influenced by lake morphometry, wind direction and strength). They may be influenced, also, by bioavailability of resuspended sediment material. In the deepest northern lake, much of the ¹³⁷Cs-containing material collected in the sediment traps originated from the catchment area. Resuspension was minimal, and the high activity of ¹³⁷Cs in the sediment had no effect on content or decline of ¹³⁷Cs in lake fish.     

Sedimentation and distribution of gamma-emitting radionuclides in bottom sediments of southern Lake Päijänne,Finland, after the Chernobyl accident

The rates of sedimentation of fallout nuclides were determined by means of sediment traps during 28 months after the Chernobyl accident in southern Lake Päijänne, Finland. The spatial distribution of the radionuclide content of the lake bottom was studied on 35 sediment cores in winters 1987/88 and 1988/89. The results were compared with simultaneous observations of the radionuclide content of the water and seston. The role of different transfer mechanisms in the elimination of radionuclides from the water column is discussed.The values recorded for the flux to the lake sediments were on average of the same order of magnitude as the initial deposition on the lake surface (Cs-137 65 kBqm^–2). The radionuclide flux to the sediments was rapid during the first months after the accident. After that the elimination of dissolved nuclides from the water mass became significantly slower. The highest flux rate was that of Ce-144 and the lowest that of Rh-106 (Ru-106). Of the radiocesium, about half of the initial inventory was transferred to the sediments after the first observation year.The content of radionuclides showed considerable spatial variation on the lake bottom (Cs-137 7–280 kBqm^–2). Direct adsorption of radiocesium explains unexpectedly high concentrations on shallow erosion bottoms. There was a clear tendency for the concentrations to increase with depth, as a result of the focusing effect. Sediment resuspension had a significant impact on the total flux of radionuclides to deepwater sediments. Estimates were made of the resuspended flux of radionuclides.     

The response of the sedimentological regime in Lake Kinneret to lower lake levels

The surface level of Lake Kinneret is regulated to remain between –209 m and –213 m msl. During the stratified period, soluble reactive phosphorus (SRP), ammonium (NH _inf4 ^sup+ ) and dissolved sulphide (H₂S) accumulate in the hypolimnion. The concentration of these solutes, which are direct and indirect products of the decomposition of organic matter, increase considerably in summers with lower lake levels. A numerical model describing depth-averaged hypolimnion and epilimnion current velocities for high and low lake levels was adapted for Lake Kinneret. Simulated hypolimnetic currents were shown to be stronger for low lake levels as a result of the fact that low lake levels are characterized by a thinner hypolimnion while the thickness of the epilimnion remains unchanged. We suggest that the stronger hypolimnetic currents have the following consequences: 1. turbulence is induced, 2. the enhanced turbulence results in higher resuspension, 3. because SO⁴= is available to bacteria on resuspended particles, mineralization rates are enhanced, and 4. focusing of fine sediments and associated organic matter to the pelagic zone is enhanced.     

Role of climate and agricultural practice in determining matter discharge into large,shallow Lake Võrtsjärv,Estonia

Understanding the dynamics of fine sediment transport across the upper intertidal zone is critical in managing the erosion and accretion of intertidal areas, and in managed realignment/estuarine habitat recreation strategies. This paper examines the transfer of sediments between salt marsh and mudflat environments in two contrasting macrotidal estuaries: the Seine (France) and the Medway (UK), using data collected during two joint field seasons undertaken by the Anglo-French RIMEW project (Rives-Manche Estuary Watch). High-resolution ADCP, Altimeter, OBS and ASM measurements from mudflat and marsh surface environments have been combined with sediment trap data to examine short-term sediment transport processes under spring tide and storm flow conditions. In addition, the longer-term accumulation of sediment in each salt marsh system has been examined via radiometric dating of sediment cores. In the Seine, rapid sediment accumulation and expansion of salt marsh areas, and subsequent loss of open intertidal mudflats, is a major problem, and the data collected here indicate a distinct net landward flux of sediments into the marsh interior. Suspended sediment fluxes are much higher than in the Medway estuary (averaging 0.09 g/m³/s), and vertical accumulation rates at the salt marsh/mudflat boundary exceed 3 cm/y. Suspended sediment data collected during storm surge conditions indicate that significant in-wash of fine sediments into the marsh interior can occur during (and following) these high-magnitude events. In contrast to the Seine, the Medway is undergoing erosion and general loss of salt marsh areas. Suspended sediment fluxes are of the order of 0.03 g/m³/s, and the marsh system here has much lower rates of vertical accretion (sediment accumulation rates are ca. 4 mm/y). Current velocity data for the Medway site indicate higher velocities on the ebb tide than occur on the flood tide, which may be sufficient to remobilise sediments deposited on the previous tide and so force net removal of material from the marsh.     

Vegetation death and rapid loss of surface elevation in two contrasting Mississippi delta salt marshes: The role of sedimentation, autocompaction and sea-level rise

From 1990 to 2004, we carried out a study on accretionary dynamics and wetland loss in salt marshes surrounding two small ponds in the Mississippi delta; Old Oyster Bayou (OB), a sediment-rich area near the mouth of the Atchafalaya River and Bayou Chitigue (BC), a sediment-poor area about 70 km to the east. The OB site was stable, while most of the marsh at BC disappeared within a few years. Measurements were made of short-term sedimentation, vertical accretion, change in marsh surface elevation, pond wave activity, and marsh soil characteristics. The OB marsh was about 10 cm higher than BC; the extremes of the elevation range for Spartina alterniflora in Louisiana. Vertical accretion and short-term sedimentation were about twice as high at BC than at OB, but the OB marsh captured nearly all sediments deposited, while the BC marsh captured <30%. The OB and BC sites flooded about 15% and 85% of the time, respectively. Marsh loss at BC was not due to wave erosion. The mineral content of deposited sediments was higher at OB. Exposure and desiccation of the marsh surface at OB increased the efficiency that deposited sediments were incorporated into the marsh soil, and displaced the marsh surface upward by biological processes like root growth, while also reducing shallow compaction. Once vegetation dies, there is a loss of soil volume due to loss of root turgor and oxidation of root organic matter, which leads to elevation collapse. Revegetation cannot occur because of the low elevation and weak soil strength. The changes in elevation at both marsh sites are punctuated, occurring in steps that can either increase or decrease elevation. When a marsh is low as at BC, a step down can result in an irreversible change. At this point, the option is not restoration but creating a new marsh with massive sediment input either from the river or via dredging.     

Nitrogen transformations at the sediment–water interface across redox gradients in the Laurentian Great Lakes

The capacity of a lake to remove reactive nitrogen (N) through denitrification has important implications both for the lake and for downstream ecosystems. In large oligotropic lakes such as Lake Superior, where nitrate (NO₃ ^?) concentrations have increased steadily over the past century, deep oxygen penetration into sediments may limit the denitrification rates. We tested the hypothesis that the position of the redox gradient in lake sediments affects denitrification by measuring net N-fluxes across the sediment–water interface for intact sediment cores collected across a range of sediment oxycline values from nearshore and offshore sites in Lake Superior, as well as sites in Lake Huron and Lake Erie. Across this redox gradient, as the thickness of the oxygenated sediment layer increased from Lake Erie to Lake Superior, fluxes of NH₄ ⁺ and N₂ out of the sediment decreased, and sediments shifted from a net sink to a net source of NO₃ ^?. Denitrification of NO₃ ^? from overlying water decreased with thickness of the oxygenated sediment layer. Our results indicate that, unlike sediments from Lake Erie and Lake Huron, Lake Superior sediments do not remove significant amounts of water column NO₃ ^? through denitrification, likely as a result of the thick oxygenated sediment layer.     

Bioturbation enhances the aerobic respiration of lake sediments in warming lakes

While lakes occupy less than 2% of the total surface of the Earth, they play a substantial role in global biogeochemical cycles. For instance, shallow lakes are important sites of carbon metabolism. Aerobic respiration is one of the important drivers of the carbon metabolism in lakes. In this context, bioturbation impacts of benthic animals (biological reworking of sediment matrix and ventilation of the sediment) on sediment aerobic respiration have previously been underestimated. Biological activity is likely to change over the course of a year due to seasonal changes of water temperatures. This study uses microcosm experiments to investigate how the impact of bioturbation (by Diptera, Chironomidae larvae) on lake sediment respiration changes when temperatures increase. While at 5°C, respiration in sediments with and without chironomids did not differ, at 30°C sediment respiration in microcosms with 2000 chironomids per m² was 4.9 times higher than in uninhabited sediments. Our results indicate that lake water temperature increases could significantly enhance lake sediment respiration, which allows us to better understand seasonal changes in lake respiration and carbon metabolism as well as the potential impacts of global warming.     

Benthic boundary mixing and resuspension induced by internal seiches

The effect of internal seiches on horizontal hypolimnetic bottom currents and on the stationary well-mixed benthic boundary layer (BBL) induced by these currents was studied for 2 weeks in a small prealpine lake using thermistor strings, an acoustic current meter and a CTD (C: conductivity, T: temperature, D: depth) equipped with a transmissometer. 150 profiles of temperature, conductivity and transmissivity taken during two days clearly indicate the existence of a well-mixed BBL 2 to 7 m thick. This is the result of intense mixing in the zone of high shear above the sediment associated with seiching motion. The concentration of suspended or resuspended particles, mainly of organic nature, within the BBL, was 2 to 4 times greater than that measured directly above the BBL. Resuspension is thought to be associated rather with high-frequency burst-like currents with measured speeds ranging up to 7 cm s^–1 than with the average bottom current speed of about 2 cm s^–1.     

Removal of Fast Flowing Nitrogen from Marshes Restored in Sandy Soils

Groundwater flow rates and nitrate removal capacity from an introduced solution were examined for five marsh restoration designs and unvegetated plots shortly after planting and 1 year post-planting. The restoration site was a sandy beach with a wave-dampening fence 10 m offshore. Simulated groundwater flow into the marsh was introduced at a rate to mimic intense rainfall events. Restoration designs varied in initial planting density and corresponded to 25%, 50%, 75% and 100% of the plot area planted. In general, groundwater flow was slower with increasing planting density and decreased from year 0 to year 1 across all treatments. Nevertheless, removal of nitrate from the introduced solution was similar and low for all restoration designs (3–7%) and similar to the unvegetated plots. We suggest that the low NO₃ ⁻ removal was due to sandy sediments allowing rapid flow of groundwater through the marsh rhizosphere, thereby decreasing the contact time of the NO₃ ⁻ with the marsh biota. Our findings demonstrate that knowledge of the groundwater flow regime for restoration projects is essential when nutrient filtration is a target goal of the project.     

Temporal dynamics and spatial variation of algae in relation to hydrology and sediment characteristics in the Okefenokee Swamp,Georgia

In a 20-month study, phytoplankton and periphyton chl a, and dry mass of macroscopic algal aggregates in four marshes and a lake within the Okefenokee Swamp (Georgia, USA) were comparable to other wetlands and lake littoral areas. Chlorophyll levels in two marshes were inversely related to water level and phytoplankton at three marshes developed unimodal maxima following macrophyte dieback. Standing stocks in a vernally inundated marsh were greater than a nearby marsh which was permanently inundated; chlorophyll levels displayed longer blooms in the inundated marsh during periods of low rainfall or after drought. Field dynamics, sediment sorption characteristics and algal bioassays suggest that evaporative drawdown stimulates algae by release of nutrients from exposed peat, while high water levels reduce nutrient release from sediments and disperse phytoplankton through flushing. Equilibrium phosphate concentrations of sediments and algal levels were higher at an abandoned rookery than a nearby non-rookery area, indicating nutrient enrichment from residual guano deposits.     

Mixis strategies and resting eeg production of rotifers living in temporally-varying habitats

The organic phosphate pool of some Camargue sediments (South of France) was studied, after removal of inorganic phosphate, with Ca-NTA/dithionite (Fe bound phosphate) and Na-EDTA (Ca bound phosphate). The organic phosphate was divided into an acid soluble organic phosphate fraction (ASOP) and a residual organic phosphate fraction (ROP). The extraction of organic matter with 2.0 M NaOH (90 °C) from ROP yielded considerable quantities of Org-P. In this extract the presence of phytate (inositol hexa phosphate) could be demonstrated using phytase to hydrolyse the phytate. Phytate was shown to account for a considerable part of organic phosphate in sediments of freshwater marsh sediments as well as in the sediment of the brackish/salt water lake ‘Etang de Vaccares’. In laboratory experiments phytate was found to precipitate with all poly-valent cations tested. Furthermore, phytate was found to be strongly adsorbed onto Fe(OOH), which may explain its accumulation and its stability in sediments. Considerable quantities of ASOP were found; the chemical stucture of this pool remains unknown.     

Phosphorus dynamics in shallow eutrophic lakes: an example from Zeekoevlei,South Africa

Zeekoevlei is the largest freshwater lake in South Africa and has been suffering from hyper-eutrophic conditions since last few decades. We have used total P (TP), dissolved phosphate (PO₄ ³⁻), organic P (OP), calcium (Ca) and iron (Fe) bound P fractions to investigate the relevant physical, chemical and biological processes responsible for sedimentation and retention of P and to study phosphorus (P) dynamics in this shallow lake. In addition, redox proxies (V/Cr and Th/U ratios) are used to study the prevailing redox conditions in sediments. Adsorption by CaCO₃ and planktonic assimilation of P are found to control P sedimentation in Zeekoevlei. Low concentration of the labile OP fraction in surface sediments restricts the release of P by bacterial remineralisation. Low molar Ca/P and Fe/P ratios indicate low P retention capacity of sediments, and P is most likely released by desorption from wind-induced resuspended sediments and mixing of pore water with the overlying water column. Handling editor: J. Saros