Jordi Colomer

Effects of the water withdrawal in the stratification patterns of a reservoir

The change in the stratification pattern in Boadella Reservoir (Catalonia, Spain) due to the switch in water withdrawal was investigated for a 6-month period in the year 2000. A numerical one-dimensional model (DLM) was used to predict the thermal structure of the reservoir during the period of maximum water demand. The model was found to satisfactorily predict the basic trends of the thermal stratification of the water column of the reservoir. We used this model to investigate various possible water withdrawal scenarios. When thermal stratification has been completely developed, the location of the main thermocline coincides with the depth of the outlet, in the various withdrawal scenarios considered. The possible effect of the switch between outlets on the water quality of the reservoir is discussed.

The role of advection and turbulent mixing in the vertical distribution of phytoplankton

The purpose of this study is to analyse the role of the flow field on the horizontal and vertical distributions of different phytoplankton populations thriving in the water column of a shallow coastal ecosystem. Two extreme flow conditions are illustrated. The first was a low energetic flow, under calm meteorological conditions and a stratified temperature of the water column. The second flow, coincident with the passage of a storm front, was more energetic resulting in increased mixing that homogenized the temperature in the whole water column. Although the mixing level homogenized the temperature of the water column in the high-energy period, it was not enough to homogenize the temperature in the low-energy period. In contrast, in both periods, the mixing level was enough to homogenize the vertical distribution of particles. A decrease in the concentration of particles from the calm period to the high-energy period was attributed to an advection event with warmer water of lower plankton concentration that resulted in a decrease of the total concentration of suspended particles in the water column. Data are used to test a model of plankton mixing proposed by Ruiz et al. (J. Plankton Res., 18 (1996) 1727).

Bottom currents induced by baroclinic forcing in Lake Banyoles (Spain)

Because of the difference in morphometry and in the underground heating of the two lobes of Lake Banyoles there is a differential cooling that causes a density current — the denser water of the shallower lobe plunging into the deepest lobe forming a bottom current — which redistributes water between the lobes and replaces that of the northern lobe about every 5 days. This current has been studied during the mixing periods of the years 1989–1991 from temperature and current measurements and it has been found that it increases or slows down depending on the direction of the wind. Furthermore, it is observed that it mainly flows along the west shore of the lake, deflecting towards the right as an effect of the bottom morphometry. However, due to the long time scale of the density current the influence of the earths rotation should not be neglected in advance. Finally, a simple numerical model has been used to corroborate the magnitude of this current — whose velocity has been directly measured — which is found to be the most important in the lake during the mixing period and so, it dominates the lake-wide circulation

Seasonal development of a turbid hydrothermal lake plume and the effects on the fish distribution

Measurements of the suspended particle concentration in a lake demonstrate the presence of a turbid hydrothermal plume. The plume, generated from a warm source develops upward carrying a suspension of particles from the bottom. It is found that the vertical structure of the plume depends on the stratification of the water column, which in turn depends on the seasonal meteorological conditions. During the stratified period of the lake, the plume is constricted to the hypolimnion zone. At the beginning of the summer season, the plume reaches the bottom of the metalimnion meanwhile at the end of this season the plume does not. In the mixed period of the lake, the plume develops in the whole water column and reaches the surface of the lake, where particles accumulate and spread around the center. Measurements of the vertical fish distribution in the stratified period show that the plume can constrict the fish habitat due to an increase of suspended particles transported by the plume.

Filtering capacity of Daphnia magna on sludge particles in treated wastewater.

A great challenge in water reuse is the reduction of suspended particle concentration in wastewater. In particular the reduction of the presence of small particles in suspension which cause a cloudy appearance in the water and, which also make disinfection difficult. The present study evaluates the filtering capacity of a population of Cladodera (Daphnia magna) in secondary effluents from a wastewater plant. The study was performed in both a mesocosm and the laboratory, in an effort to compare the grazing on sludge particles by Daphnia versus the settling rate of those sludge particles. The particle volume concentration of small particles (with a diameter below 30 μm) was used to evaluate the efficiency of the proposed biotreatment system for small particles. Both laboratory and mesocosm results showed that the suspended particle volume concentration decreased with time due to the Daphnia filtration, with the highest reduction in experiments carried out with the highest Daphnia concentration. In the mesocosm experiments, the Daphnia diameter was also found to play an important role, with an allometric relationship between the filtering rate of Daphnia and the Daphnia nondimensional diameter. In laboratory experiments, the effect of D. magna in the suspended concentration of small particles was in the range of 10.1-29.4%, according to the range of Daphnia concentration of 10-50 ind/l. For laboratory experiments, sedimentation was responsible for 62.2% of the suspended particle concentration reduction. For the mesocosm experiments, the reduction in the particle concentration attributed to the Daphnia filtration ranged between 2.5 and 39%, corresponding to Daphnia concentrations of between 5 and 100 ind/l (i.e. biovolumes of 8-60 ind/l).

Sediment entrainment in karst basins

Abstract: In this paper, the suspension of sediments induced by groundwater jets in two basins of a karst lake (basin I and II of lake Banyoles, Catalonia) is studied. Field experiments were carried out during the period 1989-1994 to investigate the sediment dynamics within the basins. During this period, the sediment in basin I (B1) was found to be permanently in suspension while the sediment in basin II (B2) remained at the bottom of the basin, except on two occasions when the ground water discharge increased and caused resuspension. A two-dimensional k-ɛ turbulence model was used to simulate the suspension of the sediment and the formation of the lutocline (the interface of the suspended sediment). The model predicts a reduction of the turbulent kinetic energy at the lutocline due to the buoyancy flux. This is used to estimate the inflow into the basins and the maximum height that the lutocline can rise, which is found to depend on the settling velocity, the mean inflow rate and the geometry of the basin. Also, the model is used to predict the water circulation below the sediment interface.

Observations of the Particle Size Distribution and Concentration in a Coastal System using an In Situ Laser Analyzer

An in situ laser particle size analyzer (LISST-100, Sequoia Scientific, Inc.) has been used to study the particle size distribution and concentration of biological and non biological particles in the water column of a Mediterranean coastal system. Two field campaigns have been carried out during low and high energy conditions of the flow, caused by the passage of a storm front. For the low energy period, the water column remained stratified, whereas for the high energetic period the water column was warmer and well mixed. The first study dealt with the distribution of particles near the bottom of the coastal area. Here, two regions were taken into account. The first region was a seagrass meadow of Posidonia oceanica and the second region was a barren sand area. The second study dealt with the determination of the vertical distribution of suspended particles in the whole water column of the system. The results showed a decrease in the vertical concentration of suspended particles in the water column with the passage of the storm front, which was associated with advection of warm water mass rather than by vertical mixing. In contrast, vertical resuspension determined the fate of suspended particles at the bottom of the water column and an increase of their concentration was found.

DAYTIME HEAT BALANCE FOR ESTIMATING NON-RADIATIVE FLUXES OF LAKE BANYOLES, SPAIN

Combining a six-term heat balance equation and a seasonal thermocline model, a new equation to calculate non- radiative fluxes of Lake Banyoles has been determined. Mean daily measurements of global solar radiation, downward longwave radiation, wind speed, air temperature and water surface temperature were used as input data and lake temperature as the calculated output data of the model. To calibrate performance of the new equation, calculated lake temperature was compared with measured lake temperature during both the mixing and the stratified period of the lake. The new coefficients in the wind function to calculate the non-radiative fluxes at the water surface were assumed to depend on the variability of the wind speed, the air temperature of the study area and the surface temperature of the lake. In addition, the results were used to estimate the heat balance of the air–water interface of Lake Banyoles over a period of two years. The processes that have been taken into account are shortwave and longwave radiation, back-radiation emitted by the lake, sensible and latent heat and throughflow.

Localized algal blooms induced by river inflows in a canyon type reservoir

The local response of the phytoplankton community to river inflow processes was investigated with modeling and field analyses in a long and narrow, stratified reservoir in mid-summer. The river water had high concentrations of phosphorus and nitrogen (ammonium and nitrate) and temperature had large variations at diurnal scales. As a consequence of the large variation in river temperature, the level of neutral buoyancy (the depth where the river water spreads laterally in the reservoir) oscillated between the surface (overflows) during the day, and the depth of the metalimnion (interflows) during the night. The reservoir remained strongly stratified, which favoured the presence of cyanobacteria. It is shown that under these conditions, nutrient-rich river water injected during overflows into the surface layers promoted the occurrence of localized algal blooms in the zones where the overflow mixed with the quiescent water of the reservoir. A series of hydrodynamic simulations of the reservoir were conducted both with synthetic and realistic forcing to assess the importance of river temperatures and wind-driven hydrodynamics for algal blooms. The simulations confirmed that the river inflow was the main forcing mechanism generating the localized bloom.

On the presence of aggregates in the basins of Lake Banyoles

Suspension of sediment by subterranean springs is a very common phenomena in the karstic, multibasin Lake Banyoles. The flow of groundwater into the lake introduces a suspension of marly and argilaceous materials. The temperature of this slurry is nearly constant throughout the year, around 19°C. The lake is formed by 6 basins and at the bottom of these basins there are 12 sources where the presence of suspended material signifies entering groundwater (Figure 1). Suspension of bottom lake sediment by groundwater is a feature of Lake Banyoles that has not been described elsewhere. Here we present experimental results which show that the maximum height that the sediment can rise to is affected by the formation of aggregates.