Exchange between littoral and pelagic waters in a stratified lake due to wind-induced motions: Lake Kinneret, Israel

We used field data and three-dimensional numerical simulations to investigate the exchange processes between the lake boundaries (littoral) and lake interior (pelagic) due to wind-induced motions in Lake Kinneret, Israel. The field data showed large-scale metalimnion oscillations with amplitudes up to 10 m in response to westerly diurnal winds, the existence of a well-defined turbidity intrusion into the metalimnion of the lake and a benthic boundary layer (BBL). The observed vertical and horizontal movements of the turbidity were explained by the vertical and horizontal advection associated with basin-scale wave motions and a residual circulation set up by the basin-scale motions and the wind field. The horizontal advective transport in the metalimnion, associated with the velocities induced by the basin-scale mode-two Poincaré wave, provided the primary control for the exchange between the lake boundaries and lake interior on daily time scales. Detailed comparison of simulation results with both temperature and velocity profiles revealed excellent agreement for time scales from hours to days. The validated numerical model was used to extract the residual motions that provided the secondary exchange flows, in the various regions of the lake on time scales from days to weeks. The residual motion in the surface layer consisted of a combination of an anticlockwise gyre in the western half of the lake and a weak clockwise gyre along the eastern boundary that were forced by the curl of the wind field. In the metalimnion, residual motions were predominantly forced by the basin-scale internal wave motions and an anticlockwise gyre established itself throughout the whole basin. Lastly in the BBL, residual motions consisted of an anticlockwise gyre, but the geographic distribution was patchy. Together these results provide new insight into a proper characterization of the processes underlying the flux paths of water and particles between the lake boundaries and lake interior. Handling editor: L. Naselli-Flores     

Effect of benthic boundary layer transport on the productivity of Mono Lake, California

The significance of the transport of nutrient-rich hypolimnetic water via the benthic boundary layer (BBL) to the productivity of Mono Lake was studied using a coupled hydrodynamic and ecological model validated against field data. The coupled model enabled us to differentiate between the role of biotic components and hydrodynamic forcing on the internal recycling of nutrients necessary to sustain primary productivity. A 4-year period (1991–1994) was simulated in which recycled nutrients from zooplankton excretion and bacterially-mediated mineralization exceeded sediment fluxes as the dominant source for primary productivity. Model outputs indicated that BBL transport was responsible for a 53% increase in the flux of hypolimnetic ammonium to the photic zone during stratification with an increase in primary production of 6% and secondary production of 5%. Although the estimated impact of BBL transport on the productivity of Mono Lake was not large, significant nutrient fluxes were simulated during periods when BBL transport was most active.     

Analysis of diurnal boundary layer development in boreal forests: measurements and simulations

Aims Combining field data analysis and modeling, this study investigates factors influencing the diurnal boundary layer (BL) development in boreal forest.Methods Field data analysis used both air sounding and surface flux measurements collected during the Boreal Ecosystem–Atmosphere Study field campaigns in central Canada. Model study applied a non-local transilient turbulence theory (TTT) to simulate the impact of the heterogeneous boundary conditions together with initial conditions on the BL development at the Candle Lake and Thompson release sites over boreal forests. Boundary conditions were characterized by the integrated surface flux measurements from different forest stands. The lake effect was included in constructing the surface fluxes at Candle Lake release site.Important findings Analyses of serial upper air sounding data and tower flux data indicate strong linear impacts of surface sensible heat forcing on the diurnal BL development above boreal forests. The regression slopes on the relationship between the BL development and the surface fluxes reflect the influences of initial boundary conditions to the BL developments. Both the modeled and the measured diurnal BLs show that lakes reduce sensible heat flux, leading to a shallower boundary in Candle Lake than in Thompson. Comparison of the model results and field measurements on the BL profiles indicates that the TTT model has the capability to simulate the BL development above boreal forests for sunny, rainfall or cloudy days. This study demonstrates the importance of lake on surface fluxes and BL development. The modeling effort shows the potential to couple the transilient theory with a land surface process model to study land surface and atmosphere interaction in boreal forest.     

Basin-scale internal waves in the bottom boundary layer of ice-covered Lake Müggelsee, Germany

We performed high-resolution temperature measurements under ice cover in Lake Müggelsee, Germany, during the winter of 2005–2006. Intense seiche-like temperature oscillations developing after the ice-on have been encountered in a thin water layer above the sediments. The oscillations were initiated immediately after lake freezing by the release of the potential energy of the thermocline slope and existed for several weeks without appreciable external forcing. The oscillations were associated with a basin-scale internal waves existing in the lower stratified part of the water column. The weakness of the density stratification under ice ensured the long wave periods, exceeding the period of geostrophic inertial oscillations at the lake’s latitude. As a result, two frequency peaks were present in the oscillations corresponding to two rotational waves, one of Kelvin-wave type and another of Poincaré type wave. The rotational character provided long dissipation times of the waves and allowed the oscillations to persist in lake several weeks. Temperature measurements in the upper several centimeters of the sediment demonstrated that oscillations of the near-bottom temperature produced vertical density instability and pore-water convection in the upper sediments.     

Basin-scale control on the phytoplankton biomass in Lake Victoria, Africa

The relative bio-optical variability within Lake Victoria was analyzed through the spatio-temporal decomposition of a 1997–2004 dataset of remotely-sensed reflectance ratios in the visible spectral range. Results show a regular seasonal pattern with a phase shift (around 2 months) between the south and north parts of the lake. Interannual trends suggested a teleconnection between the lake dynamics and El-Niño phenomena. Both seasonal and interannual patterns were associated to conditions of light limitation for phytoplankton growth and basin-scale hydrodynamics on phytoplankton access to light. Phytoplankton blooms developed during the periods of lake surface warming and water column stability. The temporal shift apparent in the bio-optical seasonal cycles was related to the differential cooling of the lake surface by southeastern monsoon winds. North-south differences in the exposure to trade winds are supported by the orography of the Eastern Great Rift Valley. The result is that surface layer warming begins in the northern part of the lake while the formation of cool and dense water continues in the southern part. The resulting buoyancy field is sufficient to induce a lake-wide convective circulation and the tilting of the isotherms along the north-south axis. Once surface warming spreads over the whole lake, the phytoplankton bloom dynamics are subjected to the internal seiche derived from the relaxation of thermocline tilting. In 1997–98, El-Niño phenomenon weakened the monsoon wind flow which led to an increase in water column stability and a higher phytoplankton optical signal throughout the lake. This suggests that phytoplankton response to expected climate scenarios will be opposite to that proposed for nutrient-limited great lakes. The present analysis of remotely-sensed bio-optical properties in combination with environmental data provides a novel basin-scale framework for research and management strategies in Lake Victoria.     

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.     

Modelled influences of non-exchanging trichomes on leaf boundary layers and gas exchange

The two main resistances in the exchange of gases between plants and the atmosphere are stomatal and boundary layer resistances. We modeled boundary layer dynamics over glabrous and pubescent leaves (assuming non-exchanging trichomes) with leaf lengths varying from 0.01 to 0.2 m, and windspeeds of 0.1-5.0 m x s(-1). Results from theoretical and semi-empirical formulae were compared. As expected, boundary layer thickness decreased with decreasing leaf length and increasing windspeed. The presence of trichomes increased leaf surface roughness, resulting in lowered Reynolds numbers at which the boundary layer became turbulent. This effect is especially important at low windspeeds and over small leaves, where the Reynolds number over glabrous surfaces would be low. We derived a new simple dimensionless number, the trip factor, to distinguish field conditions that would lead to a turbulent boundary layer based on the influence of trichomes. Because modeled rates of CO2 and H2O(v) exchange over turbulent boundary layers are one or more orders of magnitude faster than over laminar boundary layers, a turbulent boundary layer may lead to increased carbon uptake by plants. The biological trade-off is potentially increased transpirational water loss. However, in understory habitats characterized by low windspeeds, even a few trichomes may increase turbulence in the boundary layer, thus facilitating photosynthetic gas exchange. Preliminary field data show that critical trip factors are exceeded for several plant species, both in understory and open habitats.     

Interannual and between-site variability in the occurrence of clear water phases in two shallow Mediterranean lakes

It is widely accepted that clear water phases constitute a regular stage in the seasonal succession of plankton in dimictic lakes and reservoirs (i.e. PEG Model). The occurrence of such a phenomenon in Mediterranean shallow lakes is characterised by a marked interannual variability, which makes it difficult to establish reliable predictions on the dynamics and functioning of plankton in these ecosystems. In the present paper we analyse the factors influencing the occurrence of the clear water phases in the two shallow lakes of the Albufera of Adra, a coastal wetland region of south-eastern Spain: Lake Honda and Lake Nueva. Despite their geographical proximity, both lakes depicted large hydrological and limnological differences. Lake Honda is an epigenic and recharge lake that is strongly influenced by the hydrological conditions in its watershed, while Lake Nueva can be classified as a hypogenic and discharge lake and, as such, is less affected by the hydrological regime. In contrast, the morphometry, exposure and fetch of Lake Nueva make this ecosystem especially sensitive to wind forcing. Clear water phases in these shallow lakes were linked with periods of low thermal stability and the dominance of small-edible algae in the phytoplankton community, both of which allowed a Daphnia magna population to grow up and induce the algae collapse by grazing. In Lake Honda, those conditions were met during the spring of 2002 under the influence of intense rainfall-events, while in Lake Nueva the clear water phase was induced in the spring of 2003 by the occurrence of strong and frequent wind events. In both lakes, a relatively high water column thermal stability and the abundance of cyanobacteria early in the spring prevented the development of the Daphnia magna population and the occurrence of the clear water phase.     

Sulfide and methane evolution in the hypolimnion of a subtropical lake: a three-year study

The differential impact of microbial sulfate reduction and methanogenesis on the mineralization of particulate organic carbon (POC) in warm monomictic Lake Kinneret (LK), Israel was studied during three consecutive lake cycles. The hypolimnetic accumulation of total sulfide and dissolved methane was examined in relation to the physical forcing of the water column and the settling flux of particulate matter. With the on-set of thermal stratification in spring, both solutes increased concomitantly with the depletion of oxygen, first in the benthic boundary layer, followed by the upper hypolimnion. Methane production was restricted to the sediments as emphasized by the persistently linear concentration gradient in the hypolimnion. Sulfate reduction occurred both in the sediments and the water column as revealed by the hypolimnetic distribution of sulfide and recurring metalimnetic sulfide peaks. Annual differences in the accumulation pattern of both solutes appeared to be primarily linked to the settling flux of POC and the length of the stratified season. Relatively lower hypolimnetic concentrations of dissolved methane during the stratified season of 2000 coincided with increased ebullition of gaseous methane, likely as the result of a nearly a 2 m drop in the lake level. Overall, sulfate reduction accounted for more than 60% of the POC settling flux, a finding that differs from similar studies made in temperate lakes where methanogenesis was shown to be the primary mode of terminal carbon mineralization. Intensive organic carbon turnover at the sediment water interface and comparatively high sulfate concentrations in LK are the most likely reason.     

On the dynamical response of Lake Chapala, Mexico to lake breeze forcing

Fluctuations in the atmospheric characteristics, as well as variations in the water level of Lake Chapala are discussed. Field measurements of the atmospheric characteristics and lake level during December 1996 through January 1997 are described; using spectrum analysis of synchronous time series. The findings suggest that the variability is due to the diurnal cycle of atmospheric elements. Lake breeze circulation plays an important role in the area of Lake Chapala; since it was registered in 83% of the data. Periodic fluctuations in atmospheric pressure and wind generate significant seiche amplitudes in the lake, with the periods of about 6 h. With the help of a simple model, the seiche parameters are estimated. The amplitude of one-nodal seiches on one of the edges of the lake; is on average equal to 18 mm. This wave should generate currents of approximately 0.012 m s^–1 at the lake`s centre in the area of the nodal line. The experimental results on the thermal regime and circulation of Lake Chapala are discusssed as well. Surface temperature variations were registered at the eastern part of the lake. In all cross-sections, typical spatial variations of 3 °C were registered, over a distance of 100–300 m. A bouy station registered movements of an internal thermal front in the body of the water. The leading edge of the front was accompanied by intense internal waves, in the form of internal KdeV solitones. The front near the buoy station was produced by the movement of a warm body of water travelling from the shallow eastern part of the lake and trigered by morning breeze.     

Spatial and temporal variability of particulate phosphorus fractions in seston and sediments of Lake Kinneret under changing loading scenario

Over a period of three years, the flux of particulate phosphorus to the sediment–water interface of Lake Kinneret was monitored by using seston traps deployed near the bottom of both accumulation and resuspension zones. The trap material was subjected to sequential phosphorus extraction. The obtained data set was compared to the phosphorus distribution in the surface layer of bottom sediments. Due to the sequence of drought years less allochtoneous phosphorous is reaching the lake resulting in a continuous decline of total particulate phosphorus (TPP) in the upper sediment layer. The observed decline in sedimentary TPP in spite of increased TPP sedimentation can be seen as a dilution effect due to the sedimentation of material with a relatively lower P content. The change in sedimentation can be seen as the result of increased resuspension at low lake levels. With sedimentary P in the littoral zone being unaffected by the drop in the external P load, the changes observed in the profundal zone appear to be driven by internal wave activity.     

The Consequences of Internal Waves for Phytoplankton Focusing on the Distribution and Production of Planktothrix rubescens

Consequences of internal wave motion for phytoplankton and in particular for the distribution and production of the harmful and buoyant cyanobacterium Planktothrix rubescens were investigated based on data from two field campaigns conducted in Lake Ammer during summer 2009 and 2011. In both years, P. rubescens dominated the phytoplankton community and formed a deep chlorophyll maximum (DCM) in the metalimnion. Internal wave motions caused vertical displacement of P. rubescens of up to 6 m and 10 m, respectively. Vertical displacements of isotherms and of iso-concentration lines of P. rubescens from the same depth range coincided, suggesting that P. rubescens did not or could not regulate its buoyancy to prevent wave-induced vertical displacements. Diatoms dominated the phytoplankton community in the epilimnion and were vertically separated from P. rubescens. The thickness of the diatom layer, but not the diatom concentrations within the layer, changed in phase with the changes in the thickness of the epilimnion caused by internal wave motions. Seiche induced vertical displacements of P. rubescens caused fluctuations in the light intensity available at the depth of the P. rubescens layer. The interplay between seiche induced vertical displacements of the P. rubescens layer and the daily cycle of incident light lead to differences in the daily mean available light intensity between lake ends by up to a factor of ∼3. As a consequence, the daily mean specific oxygen production rate of P. rubescens differed by up to a factor of ∼7 between lake ends. The horizontal differences in the specific oxygen production rate of P. rubescens were persistent over several days suggesting that the associated production of P. rubescens biomass may lead to phytoplankton patchiness. The effect of internal seiches on the spatial heterogeneity and the persistence of horizontal differences in production, however, depend on the timing and the synchronization between internal wave motion and the daily course of incident light intensity. Vertical displacements caused by internal waves could be distinguished from other factors influencing the distribution of P. rubescens (e.g. active buoyancy control, production, vertical mixing) by a temperature-based data transformation. This technique may be of general use for separating wave-induced transport from other processes (e.g. sedimentation, vertical mixing) that affect the distributions of dissolved substances and suspended particles.     

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.     

Numerical modeling of vertical stratification of Lake Shira in summer

A one-dimensional numerical model and a two-dimensional numerical model of the hydrodynamic and thermal structure of Lake Shira during summer have been developed, with several original physical and numerical features. These models are well suited to simulate the formation and dynamics of vertical stratification and provide a basis for an ecological water-quality model of the lake. They allow for the quantification of the vertical mixing processes that govern not only the thermal structure but also the nutrient exchange, and more generally, the exchange of dissolved and particulate matter between different parts of the lake. The outcome of the calculations has been compared with the field data on vertical temperature and salinity distributions in Lake Shira. Lake Shira is meromictic and exhibits very stable annual stratification. The stratification is so stable because of the high salinity of the water. If the water in Lake Shira were fresh and other parameters (depth, volume, and meteorology) were the same, as now, the lake would be mixed in autumn. Using the newly developed models and using common meteorological parameters, we conclude that Lake Shira will remain stratified in autumn as long as the average salinity is higher than 3‰.     

Nutrient fluxes from upwelling and enhanced turbulence at the top of the pycnocline in Mono Lake, California

Time series measurements of temperature at 15 depths and profiles of temperature-gradient microstructure were obtained during a period with strong wind forcing and subsequent calm in Mono Lake, California. The wind forcing increased the amplitude of basin-scale internal waves and energy at all wave frequencies relative to the calm period. Rates of dissipation of turbulent kinetic energy, , were high ( > 10^–6 m² s^–3) at the top of the pycnocline at both an inshore and an offshore site on a day when winds reached 10 m s^–1 and on the following two days at an inshore site ( > 10^–7 m² s^–3). The enhanced turbulence occurred at the depth of a subsurface temperature maximum (z _TM) and coincidentally with elevated concentrations of NH₄, reduced concentrations of chlorophyll a and particulate carbon, and increased abundance of the macrozooplankter Artemia monica. The NH₄ at z _TM was more dispersed and of lower concentration inshore than offshore and indicated greater turbulent transport inshore. Over the course of 4 days, chlorophyll a concentrations increased in the upper mixed layer, and C:N and C:Chl ratios decreased. Offshore, the change in C:N ratio indicated a relaxation of moderate nutrient deficiency. We hypothesize that excretion by A. monica and turbulent transport of the NH₄ from the subsurface temperature maximum led to improved physiological status of phytoplankton in the upper mixed layer.     

Factors controlling planktonic size spectral responses to autumnal circulation in a Mediterranean lake

1. Studies of planktonic size spectra have been common in recent years, but few concerning the effects of autumnal lake processes on those distributions have been reported. We carried out such a study for 93 days during early circulation in a small, mesotrophic, seepage lake with only benthivorous fish. Two distinct mixing periods occurred before full circulation in Las Madres lake (Spain). As a proxy of overall planktonic‐ and phytoplanktonic size distributions, the shape of a Pareto I power function was traced over time. The phytoplanktonic size spectrum was also used to test the hypothesis that phytoplankton might show a trophic cascade during early circulation. 2. Our results demonstrated that the overall size spectrum changed smoothly during the autumnal mixing, with sequential changes controlled by a combination of biotic and abiotic variables, albeit experiencing different temporal delays, always shorter than a week. A first combined, autogenic process, driven both by algal competition and a crustacean trophic effect, and a second physical‐forcing process, driven by the combined effect of convective cooling and decreasing irradiance on mixed layer dynamics, may be related to the overall planktonic spectrum in the first and second periods of mixing, respectively. 3. The phytoplanktonic size spectrum did not appear to be dictated by a trophic cascade in Las Madres lake, their dynamics being mostly controlled by physical forcing, along with some effect of non‐edible primary producers and cladocerans in the first and second periods of mixing, respectively. 4. Our results and others covering early circulation with longer sampling periods suggest that planktonic size spectral dynamics during lake circulation is context‐dependent (i.e. varying from one lake to another), thus preventing generalisation. However, when further studies with finer temporal resolution have been carried out, it is likely that clear‐cut patterns in the planktonic size spectrum will emerge, arising from the interplay of autogenic plankton dynamics, implying some resistance to community change because of external physical forcing, and the velocity of autumnal mixing.     

The ice cover on small and large lakes: scaling analysis and mathematical modelling

Lake ice cover is described by its thickness, temperature, stratigraphy and overlying snow layer. When the ratio of ice thickness to lake size is above ～10^?5, the ice cover is stable; otherwise, mechanical forcing breaks the ice cover, and ice drifting takes place with lead-opening and ridging. This transition enables a convenient distinction to be made between small and large lakes. The evolution of the ice cover on small lakes is solved by a wholly thermodynamic model, but a coupled mechanical–thermodynamic model is needed for large lakes. The latter indicates a wide distribution of ice thickness, and frazil ice may be formed in openings. Ecological conditions in large lakes differ markedly from those in small lakes because vertical mixing and oxygen renewal may take place during the ice season, and the euphotic zone penetrates well into the water column in thin ice regions. Mesoscale sea ice models are applicable to large lakes with only minor tuning of the key parameters. These model systems are presented and analysed using Lake Peipsi as an example. As the climate changes, the transition size between small and large lake ice cover will change.     

The influence of leaf size and shape on leaf thermal dynamics: does theory hold up under natural conditions?

Laboratory studies on artificial leaves suggest that leaf thermal dynamics are strongly influenced by the two‐dimensional size and shape of leaves and associated boundary layer thickness. Hot environments are therefore said to favour selection for small, narrow or dissected leaves. Empirical evidence from real leaves under field conditions is scant and traditionally based on point measurements that do not capture spatial variation in heat load. We used thermal imagery under field conditions to measure the leaf thermal time constant (τ) in summer and the leaf‐to‐air temperature difference (?T) and temperature range across laminae (T_range) during winter, autumn and summer for 68 Proteaceae species. We investigated the influence of leaf area and margin complexity relative to effective leaf width (w_e), the latter being a more direct indicator of boundary layer thickness. Normalized difference of margin complexity had no or weak effects on thermal dynamics, but w_e strongly predicted τ and ?T, whereas leaf area influenced T_range. Unlike artificial leaves, however, spatial temperature distribution in large leaves appeared to be governed largely by structural variation. Therefore, we agree that small size, specifically w_e, has adaptive value in hot environments but not with the idea that thermal regulation is the primary evolutionary driver of leaf dissection.     

Release of metals from polluted sediments in a shallow lake: quantifying resuspension

The contribution of erosion of bed sediment to the load of metals leaving Lake Ketelmeer, a shallow lake in the Netherlands fed by the IJssel branch of the River Rhine, is reported. Transport of suspended matter and associated trace metals was measured using both centrifuges and sediment traps at several locations in the lake. Mass balances of suspended matter and heavy metals were calculated using data from these field measurements.Metal/scandium-ratios were used to identify the source of the suspended matter in the lake. Since the bed sediment is more polluted than the suspended sediment entering the lake, higher metal/scandium-ratios were found for bottom sediment in the lake compared with those for suspended matter entering the lake from the River IJssel. Using the metal/scadium-ratio in suspended matter from the lake, it was calculated that bottom sediments made up 43% of the suspended matter leaving the lake. This implies an erosion flux of bottom sediment of 16 g m^–2 d^–1. For cadmium, mercury, chromium and zinc, this erosion process accounts for more than 50% of the pollutant load leaving Lake Ketelmeer and entering Lake IJsselmeer.