Sedimentary losses of phosphorus in some natural and artificial Iowa lakes

Phosphorus sedimentation in four natural and four artificial Iowa lakes was measured by using sediment traps to determine if sedimentary phosphorus losses were greater in artificial lakes than in natural lakes and the limnological factors influencing phosphorus loss rates. Mean phosphorus sedimentation rates ranged from 13.3 to 218 mg · m^–2 day^–1. Although phosphorus sedimentation rates for the natural lakes as a group did not differ significantly from the rates for artificial lakes, there were significant differences among individual lakes. Phosphorus sedimentation rates also varied significantly during different seasons at different locations within a lake and at different depths within a location. Despite the variance, phosphorus sedimentation rates were strongly correlated with inorganic sediment concentrations and inorganic matter sedimentation rates, thus suggesting that inorganic sediments influence phosphorus sedimentation rates. When Iowa data were combined with data from published studies, mean sedimentation rates were directly correlated with mean chlorophyll a concentrations of the lakes. These data strongly suggest that sedimentation rates as measured by sediment traps are strongly influenced by the trophic status of a lake. Though sedimentation rates were higher in the more productive lakes, it is suggested that these rates represent only gross sedimentation rates rather than net sedimentation rates because of resuspension and resedimentation of bottom sediments.     

Sediment resuspension: rescue or downfall of a thermally stratified eutrophic lake?

The aim of this study was to estimate the effect of sediment resuspension, a common phenomenon in many lakes, on the phosphorus budget of a eutrophicated lake. We used two different approaches, mass balance calculations and spatially comprehensive resuspension measurements, to determine the level of phosphorus loading from which rehabilitation action is started in a dimictic north temperate lake. The effect of resuspension was assumed to be significant, since it often is a governing process for cycling of material in lakes. Internal loading was multifold to that of external loading as determined by the budget calculation. Spatially comprehensive sedimentation and resuspension measurements were necessary, since deep site versus spatially comprehensive measurements had a marked difference in their results. Resuspension of P slightly exceeded the internal loading assessed by budget calculations and thereby proved its significance as a governing in-lake process that influences P cycling strongly. The shallow areas were of importance, since most of the total P load originated from there. The fate of P after resuspension depends on the retention capacity of resuspended particles in addition to prevailing biological and physico-chemical conditions. Therefore, sediment resuspension can either strengthen or diminish internal nutrient load and the processes of the shallow zones are of importance.     

Temporal variability of particulate organic carbon,nitrogen and phosphorus in the Northern Adriatic Sea

The particulate organic carbon (POC), nitrogen (PN) and phosphorus (PP) vertical distribution along the water column and temporal variability in coastal and offshore waters of the Northern Adriatic Sea were related to the hydrodynamic conditions and biological processes. Fresh water inputs from the Po and Adige rivers enhance primary production, resulting in high POC, PN and PP concentrations at the surface. In offshore waters, POC and PN concentrations were about 3–4 times less than in the coastal waters, while PP were up to 10 times lower, highlighting a marked phosphorus depletion. In the bottom layer, the POC content decreases due to the strong density gradients which separate bottom waters with prevailing degradation processes. Short term 48 h-variability of POC, PN and PP in the coastal waters was determined to a great extent by variations in the spreading of river plumes at the surface and by nepheloid layers and resuspension processes in the bottom waters. The particulate matter in the Adriatic offshore waters is extremely depleted as regards particulate phosphorus and is characterised by C_org:P and N:P ratios higher than the Redfield ratio.     

Sediment resuspension in a monomictic eutrophic reservoir

During the mixing period sediment traps were placed at 9 different levels of the water column in La Concepción reservoir (Málaga-Spain). During the exposure time a benthic nepheloid layer with high suspended matter was detected. Seston vertical flux profile shows an exponential increase with depth, suggesting that a large fraction of the collected particulate matter comes from bottom resuspension, due to an intermittent complete mixing process. It was estimated that resuspension 1 m above the bottom increased the fluxes of seston, particulate organic carbon, particulate nitrogen and particulate phosphorus by factors of 4.1, 1.5, 2.1 and 4.8 respectively.The, fitting to an exponential function allowed us to obtain a resuspension coefficient and the scale vertical length for this event, 36 m, which is higher than those estimated for other aquatic environments. From the above results the calculated vertical eddy diffusivity, suggesting a high turbulence in the water reservoir during the turnover period. Resuspension is considered important in order to estimate net downward fluxes and to understand different processes such as redistribution of sediment and fertilization of the water column.     

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%.     

Summer sedimentation in six shallow arctic lakes

The movement of sediment between the lake bottom and water column of shallow lakes can be sizeable due to the large potential for resuspension in these systems. Resuspended sediments have been shown to alter phytoplankton community composition and elevate water column production and nutrient concentrations. We measured the summer sedimentation rates of two lakes in 2003 and six lakes in 2004. All lakes were shallow and located in the Alaskan Arctic. In 2004, turbidity, light attenuation, total sediment:chlorophyll a mass in the sediment traps, and thermal stratification were also measured in each of the lakes. The sediment:chlorophyll a mass was much greater than if the sediment was derived from phytoplankton production in all of the lakes, indicating that the source of the sedimenting material was resuspension and allochthonous inputs. Consistent with these findings, the temporal variation in sedimentation rate was synchronous between most lakes, and sedimentation rate was positively related to wind speed and rainfall suggesting that sedimentation rate was strongly influenced by landscape-scale factors (e.g., wind and rain events). Two of the lakes are located on deposits of loess that accumulated during past glacial periods. These two lakes had sedimentation rates that were significantly greater and more variable than any of the other lakes in the study, as well as high turbidity and light attenuation. Our results indicate that sedimentation in these shallow arctic lakes is supported primarily by allochthonous inputs and resuspension and that landscape-scale factors (e.g., weather and geology) impact on the transport of materials between the lake bottom and water column. Handling editor: J. Saros     

Measurement and prediction of sedimentation in small Swedish lakes

Sediment traps were placed in 29 small lakes in south and central Sweden at 2 m below the surface of the lakes and at 2 m above the lake bottom. Traps were exposed for approximately 120 days during the summer months before collection. Rates of sedimentation in both top and bottom traps were compared to 32 catchment, morphometric and water column parameters in an attempt to identify the processes which influence sediment accumulation. Using only lake water pH, maximum lake depth (D_max) and lake surface area (A_o), 67% of the variance in the bottom trap sedimentation rates was explained. Only pH and A_o were useful predictors for the top traps. Using the bottom traps as a measure of gross sedimentation and the top traps as a measure of net sedimentation (plus periphyton growth in the traps), resuspension was separated from net sedimentation in the bottom traps. Resuspension calculated from these data is compared with more conventional methods of calculation.     

The benthic boundary layer approach and its application to Lake Päijänne,Finland

Resuspension of bottom sediments is the net result of a wide variety of different fluid mechanical processes with characteristic time and length scales that extend over six orders of magnitude. The sum of these effects is most heavily concentrated in a layer adjacent to the bottom called the benthic boundary layer (BBL). The physics of BBL must be understood before improved solutions to the resuspension problem are possible. Traditionally in lakes, sedimentation and resuspension have been modelled with the aid of equations which ignore the time and space variations of near bottom processes. This can lead to wrong estimates of material transport. With the exception of few recent studies, benthic boundary layer approach has been mainly applied to marine environments. The instrumentation has been a major problem for development of the theory and its applications, but during recent years some new instruments have been giving promising results. This paper discusses the applicability of the theory in Finnish lakes and presents results from Lake Päijänne.     

Nutrient metabolism (C,N, P,and Si) in the trophogenic zone of a meromictic lake

The dynamics of seston and dissolved elements in a meromictic lake with high concentrations of manganese and iron in the monimolimnion were studied through an annual cycle. This publication presents results for assimilation, sedimentation and recovery of nutrients (C, N, P, and Si) in the trophogenic zone. Phosphorus deficiency kept the productivity of the diatom dominated phytoplankton at an oligotrophic level. High concentrations of iron in influent streams and redistribution followed by precipitation of iron during periods of partial turnover removed phosphorus from the water. High concentrations of manganese and sulfate did not have the anticipated fertilizing effect, and recovery of nutrients from the depth of the lake was negligible. Mass balance calculations indicate that liberation of phosphorus from the sediments in the trophogenic zone was most important for the maintenance of primary production. 75% of carbon, 80% of nitrogen and 25% of phosphorus assimilated by the phytoplankton was mineralized in the trophogenic zone. Silica was effectively regenerated from the littoral zone during the decline of diatom blooms. Nitrogen and silica retention was 45% of the external load compared to 66% for phosphorus.Dept. of Limnology University of Oslo     

The effect of herbivorous crustacean zooplankton on epilimnetic carbon and phosphorus cycling

Plastic limnocorrals (LCs; volume 70 m³) in oligo-mesotrophic Lake Lucerne, Switzerland were used to manipulate planktonic communities by (1) removing large zooplankton at the start of the experiment, and (2) adding phosphate during experiments of two weeks duration.Primary production (¹⁴C-assimilation), chlorophyll, standing crops and sedimentation of particulate organic carbon (POC) and of particulate phosphorus, as well as plankton composition were assessed simultaneously in two to four differently treated LCs. Carbon and phosphorus mass balances were calculated from assimilation, temporal change in standing crop of particulate matter, and sedimentation.A quick elimination of crustaceans by screening hardly increased primary production, but decreased sedimentation and mineralisation of particulate organic carbon, and thus significantly enhanced the standing crop of POC, but decreased POC turnover. The exclusion of crustaceans increased the mean residence time of total phosphorus by a reduced P loss by sedimentation.Increased grazing pressure during the experiment showed little grazing induced effect on plankton composition and primary production except at the very end.We conclude that in an oligo-mesotrophic lake, buffering mechanisms attenuate the impact of changing grazing pressures on primary production.     

Phosphorus pools and internal loading in a eutrophic lake with gradients in sediment geochemistry created by land use in the watershed

Spatial variations in phosphorus (P) fractionation, sediment geochemistry, and sorptive properties for P are assessed to test the hypothesis that these sediment properties vary within the lake and are governed by different land uses in the watershed. The dynamic equilibrium between P in sediment and water is investigated using sorption–desorption isotherms. Sediments in the littoral zone were rich in iron (Fe), aluminium (Al), and clay material in comparison to sediments from the lake proper and thus had better abilities to sorb and retain P. In the limnetic zone, there was an increasing abundance of primary minerals, and the fraction of apatite-P was high, while the level of total P was low. The amount of labile adsorbed P (LAP) in the littoral sediments varied because of contrasting land use in the sub-catchments draining into different parts of the lake. Sediments in areas where forest streams enter the lake contained significantly more LAP than sediments in areas impacted by agricultural influenced streams. Internal P loading from sediments predominantly originating from forest streams is mainly governed by sediment resuspension. The dominant P pool in sediments near the inlets of agriculturally influenced streams was non-apatite inorganic P, of which the Fe-bound is a potentially important source of P under anoxic conditions.     

Modelling sediment transport in shallow lakes — interactions between sediment transport and sediment composition

In shallow, wind exposed lakes, the light conditions, the cycling of nutrients, heavy metals and organic micro-pollutants and changes in the local composition of the sediment top layer can be dominated by resuspension/erosion of bottom sediment and sedimentation of suspended solids. A 2 dimensional model for Sediment Transport, Resuspension and Sedimentation in Shallow lakes (STRESS-2d), based on an existing transport model, is discussed. In the model, mass balance equations for the water compartment and the bottom sediment are solved numerically. Up to 7 sediment fractions can be taken into account, each having a specific set of resuspension/erosion and sedimentation parameter values. Several options for modelling the changes in the bottom sediment composition are available.A simulation experiment for Lake Veluwe (The Netherlands), in which model options with and without the distinction of sediment fractions were used, showed that using sediment fractions to account for the variability in the sediment composition leads to an improvement of the model results, particularly the simulated phosphorus sediment-water exchange fluxes. For Lake Ketel (The Netherlands) two options for modelling changes in the bottom sediment composition are compared. It is shown that an option in which a thin water-sediment layer on top of the more consolidated bottom sediment is simulated provides an improvement in the simulation of the suspended solids concentration.     

The trophic history of Enäjärvi,SW Finland,with special reference to its restoration problems

The eutrophicated Enäjärvi was studied by paleolimnological analyses and sediment mapping. The sedimentary record indicates that the lake nutrient balance had deteriorated due to lowering of the lake water level in the year 1928. From that event onwards Chironomus plumosus and Cyclotella astraea characterize the chironomid and diatom communities. The concentrations of sedimentary total and mobile phosphorus show that since then the internal load of phosphorus has controlled the nutrient cycle of the lake. The areal distribution of mobile phosphorus can be explained by dominant wind directions and wind resuspension of the sediment is the key factor in the nutrient cycle. Restoration of Enäjärvi must be based on actions which stabilize the surface sediment and improve its natural phosphorus-binding capacity. They include the regulation of lake water to as high a level as possible and the removal of the majority of roach.     

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.     

Contribution of mathematical modeling to lake ecosystem understanding: Lake Bourget (Savoy,France)

This paper demonstrates how mathematical modeling can contribute to improve understanding of lake behavior. Since the 60's Lake Bourget, one of the largest in France, had been suffering from eutrophication which was checked in 1980 by the diversion of the main sewers entering the lake. A research program was implemented between 1987 and 1990, including an on-site sampling campaign conducted concurrently with thermal and biogeochemical modeling of lake behavior. The model helped provide a better understanding of the ecosystem, displaying some processes hitherto misunderstood: (1) Winter overturn does not reach the bottom of the water column when the weather is mild. This leads to a incomplete reoxygenation of the hypolimnion and to redox conditions inducing the release of orthophosphate from the sediment, (2) Grazing by herbivorous zooplankton is getting more important in the control of spring algal growth as eutrophication of the lake regresses, (3) Settling of particulate phosphorus seems a complex and very important process in Lake Bourget, showing high sedimentation rates for particulate mineral phosphorus.     

Modeling Transport of Fluff Layer Material in the Baltic Sea

To estimate impacts of nutrients and pollutants discharged by river inflow on the ecosystem it is essential to quantify the sedimentation, resuspension and transport processes of fine particulate matter. The present study aims at the modeling of basic transport processes of fine material in the western Baltic. A three-dimensional model of the Baltic Sea, that is based on the Modular Ocean Model (MOM-3.1) (Pacanowski & Griffies 2000), was applied to study maximum bottom shear velocities. Comparing the calculated shear velocities with the critical value for resuspension of fluff layers allows an identification of potential areas of deposition and accumulation. Further, model experiments were conducted to study transport paths of fluff layers in the southwestern Baltic. The model results give information about the resuspension and accumulation areas of fine material and could provide indications of potential accumulation areas of diapause eggs or cysts.     

Surficial sediment phosphorus fractions along a biogeochemical gradient in Nyanza (Winam) Gulf, northeastern Lake Victoria and their possible role in phosphorus recycling and internal loading

Different phosphorus fractions and metal element composition of surficial sediments were measured on three occasions in 2005 and 2006 along a transect between Nyanza Gulf and offshore Lake Victoria, in order to assess the potential for sediments to contribute to the water column P concentrations in Lake Victoria. Total phosphorus (TP), apatite phosphorus (AP), inorganic phosphorus (IP) and organic phosphorus (OP) increased in sediments along the gulf towards the main lake while the non-apatite inorganic phosphorus (NAIP) increases were less defined. The longitudinal gradient of sediment TP and its fractions in Nyanza Gulf is a result of high rates of terrigenous input and resuspension and transport of the light, phosphorus rich inorganic and organic matter towards the main lake. TP in the sediment ranged from 812.7 to 1,738 mg/kg dry weight (DW) and was highest in the Rusinga Channel, the exchange zone between the gulf and the main lake. AP was the most important TP fraction, contributing between 35 and 57.3% of TP. Ca content in the sediment was strongly associated with TP and AP in the sediment (r² = 0.92 and 0.98, respectively) in the gulf and the channel, indicating the importance of apatite in controlling P availability in these zones. In the gulf and the Rusinga Channel, the less bioavailable apatite phosphorus dominated, whereas in the deeper main lake OP was the major fraction illustrating the importance of anaerobic release of P from sediments and acceleration of internal P loading in the main lake.     

Nitrogen sedimentation in a lake affected by massive nitrogen inputs: autochthonous versus allochthonous effects

1. Monthly changes to N loadings, in‐lake particulate organic nitrogen (PON), planktonic PON, and PON sedimentary flux were studied in a Spanish flowthrough, seepage lake subject to massive nitrogen inputs from June 2003 to December 2004 when water renewal was very rapid (0.09–0.17 year). 2. The distribution of in‐lake PON did not show a seasonal trend. Total nitrogen input flux ranged from 1.23 to 4.83 g N m^?2 day^?1, 71–76% of which is nitrate while PON represents 6–10%. PON sedimentation rates ranged from 9 to 90 mg N m^?2 day^?1 and fluctuated on a seasonal basis, reaching a minimum in winter and early spring and a maximum after thermal turnover had occurred. 3. This fluctuation was not related to either autochthonous planktonic production or allochthonous inputs. Since charophyte populations in Colgada Lake underwent a seasonal pattern of growth and decomposition, and ¹⁵δN values of settling material peaked at the end of that decomposition process, we suggest that PON sedimentary flux could be partly driven by decomposed charophyte particles. 4. However, the picture of PON sedimentation in this lake was more complex than anticipated because water residence time partly explained PON variability, albeit with a 1 month lag. Water residence time explained 40% of the overall variance of yearly averaged PON sedimentary flux in a meta‐analysis of 13 lakes worldwide. However, the factors such as phytoplankton composition, trophic structure, bottom communities, nutrient loading or productivity levels may also be influential on PON settling dynamics.     

Regularities in Primary Production,Secchi Depth and Fish Yield and a New System to Define Trophic and Humic State Indices for Lake Ecosystems

General relationships between phytoplankton production, chlorophyll, total, dissolved and particulate phosphorus, Secchi depth, humic level, trophic level, fish production and latitude are described by regression equations using an extensive “Soviet” data base covering a wide domain of lake characteristics and a European data base. New systems for defining lake trophic and humic status are presented. The results may be used for more precise estimates of fundamental lake properties and for many practical issues of lake management, e.g., predictions of fish catch. We have used strict chlorophyll‐a concentrations for every trophic class and we have omitted Secchi depth from the trophic classes, since Secchi depth and other variables strongly related to water clarity (like suspended particulate matter and particulate organic carbon) depend on autochthonous production, allochthonous influences and resuspension. We have used the Secchi depth as a simple operational measure of the effective depth of the photic zone. It has also been shown that among these lakes there exist a very strong relationship between primary production and latitude. In fact, 74% of the variability among the lakes in mean summer primary production can be statistically related to variations in latitude. These data also show a strong relationship between primary production and fish yield, which can be used to address many fundamental issues in lake management, like “normal and abnormal fish production”.     

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.