CONTENTS
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1

Residence time and physical processes in lakes

Walter AMBROSETTI, Luigi BARBANTI and Nicoletta SALA

The residence time of a lake is highly dependent on internal physical processes in the water mass conditioning its hydrodynam­ics; early attempts to evaluate this physical parameter emphasize the complexity of the problem, which depends on very different natural phenomena with widespread synergies. The aim of this study is to analyse the agents involved in these processes and arrive at a more realistic definition of water residence time which takes account of these agents, and how they influence internal hydrody­namics. With particular reference to temperate lakes, the following characteristics are analysed: 1) the set of the lake's caloric com­ponents which, along with summer heating, determine the stabilizing effect of the surface layers, and the consequent thermal stratifi­cation, as well as the winter destabilizing effect; 2) the wind force, which transfers part of its momentum to the water mass, generat­ing a complex of movements (turbulence, waves, currents) with the production of active kinetic energy; 3) the water flowing into the lake from the tributaries, and flowing out through the outflow, from the standpoint of hydrology and of the kinetic effect generated by the introduction of these water masses into the lake. These factors were studied in the context of the general geographical properties of the lake basin and the watershed (latitude, longitude, morphology), also taking account of the local and regional climatic situa­tion. Also analysed is the impact of ongoing climatic change on the renewal of the lake water, which is currently changing the equi­librium between lake and atmosphere, river and lake, and relationships

2

The application of models to find the relevance of residence time in lake and reservoir management

Sven E. JØRGENSEN

The residence time is among the most important factors that determine the water quality of lakes and reservoirs. Models are useful tools to reveal the relationship between the residence time and the water quality. Three case studies are presented to illustrate the application of models to determine the importance of the residence time for the water quality. It was found that manipulation of the residence time, i.e. ecohydrology, may be a very useful environmental management tool.

3

Using a continuous function for residence time to quantify the impact of climate change on the dynamics of thermally stratified lakes

David Glen GEORGE and Margaret Anne HURLEY

The residence time of a lake is usually expressed as an annual average. This takes no account of the short-term fluctuations in the rainfall which result in a pronounced seasonal variation in the effective residence time. In this paper, we describe how we com­bine the regional output of an established Global Climate Model (GCM) with a new definition of residence time to quantify the effect of future changes in the weather on the dynamics of thermally stratified lakes. Illustrative examples are taken from a series of lakes in the English Lake District and demonstrate that some lakes respond to changes in the rainfall that extend over two, three or even four seasons. The critical factors influencing the hydraulic responses of the selected lakes were the size of each basin and the ratio of epilimnetic to hypolimnetic volume. In the smaller lakes, the effective residence time was just a few days whilst the large lakes had residence times that were close to one year. This division of lakes into single season and multi-season lakes is functionally very im­portant since the projected changes in the weather may well be confined to a particular time of year.

4

Regional assessment of lake water clarity using satellite remote sensing

Stacy A.C. NELSON, Patricia A. SORANNO, Kendra Spence CHERUVELIL, Sam A. BATZLI and David L. SKOLE
 

Lake water clarity as measured by Secchi disk transparency (SDT) is a cost-effective measure of water quality. However, in re­gions where there are thousands of lakes, sampling even a small proportion of those lakes for SDT year after year is cost prohibitive. Remote sensing has the potential to be a powerful tool for assessing lake clarity over large spatial scales. The overall objective of our study was to examine whether Landsat-7 ETM+ could be used to measure water clarity across a large range of lakes. Our spe­cific objectives were to: 1) develop a regression model to estimate SDT from Landsat data calibrated using 93 lakes in Michigan, U.S.A., and to 2) examine how the distribution of SDT across the 93 calibration lakes influenced the model. Our calibration dataset included a large number of lakes with a wide range of SDT values that captured the summer statewide distribution of SDT values in Michigan. Our regression model had a much lower r² value than previously published studies conducted on smaller datasets. To ex­amine the importance of the distribution of calibration data, we simulated a calibration dataset with a different SDT distribution by sub-sampling the original dataset to match the distribution of previous studies. The sub-sampled dataset had a much higher percent­age of lakes with shallow water clarity, and the resulting regression model had a much higher r² value than our original model. Our study shows that the use of Landsat to measure water clarity is sensitive to the distribution of water clarity used in the calibration set.

5

Vertical mixing as a determinant of trophic status in deep lakes: a case study from two lakes south of the Alps (Lake Garda and Lake Iseo)

Nico SALMASO, Rosario MOSELLO, Letizia GARIBALDI, Fabio DECET, Maria Cristina BRIZZIO and Paolo CORDELLA

The identification of the factors and mechanisms determining a particular lake’s typology is crucial for the correct interpretation of trophic evolution. Nutrient concentrations are not the only properties which determine the trophic characteristics of lakes; others, such as morphometry, hydrology and climatic conditions, also have a major influence on the development of algal biomass and spe­cies composition. Large, deep lakes belong to a well defined typology. Their trophic status depends not only on algal nutrient loads, but also on the extent of the spring vertical mixing and renewal time. These factors are closely interrelated, for the actual renewal time approaches the theoretical renewal time only during the complete overturn. This paper compares the influence of different cir­culation patterns on the trophic status of two deep subalpine lakes with contrasting mixing characteristics. Lake Garda is the largest lake in Italy (S=368 km², V=49 km³, Z_max=350 m). It is characterised by irregular circulation events (oligomixis); during the 1990s, complete homogenisation of the water column occurred in 1991 and 1999-2000. The years between these episodes showed an in­crease of hypolimnetic temperatures and a progressive vertical stratification of nutrients. Full overturn episodes were characterised by a sudden decrease of temperature and complete homogenisation of the chemical and physical variables along the water column, with a corresponding increase of nutrients and algal biomasses at the surface. In contrast, the last mixing involving the deepest waters in Lake Iseo (S= 60,9 km², V=7,6 km³, Z_max=251 m) occurred at the beginning of the 1980s, after which the reduced water renewal and the high trophic level resulted in a marked decrease in oxygen concentrations and in the establishment of conditions of anoxia during the 1990s. At present the lake is meromictic. In 1999 and 2000, when a complete overturn was observed in lakes Maggiore and Como as well as in Lake Garda, Lake Iseo experienced a spring mixing which was still limited, but able to determine an increase in the algal biomass.

6

Over twenty years trend of chloride ion concentration in Lake Biwa

Yasuaki AOTA, Michio KUMAGAI and Kanako ISHIKAWA

Recent increase of chloride ion concentration in Lake Biwa was considered. Over the past 20 years' data at the North Basin of Lake Biwa showed that chloride ion concentration has been continuously increasing from 7.4 to 9.9 mg l^-1 at 0.5 m depth from lake surface and from 7.3 to 9.9 mg l^-1 above the bottom (depth of over 80 m from lake surface). This low level salinity indicated, how­ever, about 35% increase through 20 years. In this paper, we reported the trend and the tendency of chloride ion concentration at some locations and the change of climatic data through 20 years in Lake Biwa. In a short period within one year, chloride ion con­centration clearly fluctuated in the upper water layer. This fluctuation was mostly influenced by precipitation. Similar trend of chlo­ride ion concentration could be seen in the South Basin of Lake Biwa with much higher concentration than that in the North Basin. We also discussed the long-term changes of chloride concentrations in 5 major rivers with large catchment area, water level and precipitation. The data of river discharge indicated that some rivers in the southern part of the North Basin in Lake Biwa contain a relatively high chloride concentration compared with others. Furthermore, we proposed a simple conceptual model with variable outflow and inflow fluxes. This conceptual model expressed an amount of chloride budget. We estimated the average chloride flux flowing from the watershed into Lake Biwa using the data of chloride ion concentration in the lake and the data of water discharge from the lake. Comparing the results of the model analysis with the data of 12 major rivers contributing large flux to Lake Biwa, we suggested that river inflow is one of the major sources to induce the increase of chloride ion concentration in the lake, and the inflow from non-point source can be also important sources.

7

Classification of the ecological status of volcanic lakes in Central Italy

Fiorenza G. MARGARITORA, Marcello BAZZANTI, Ornella FERRARA, Luciana MASTRANTUONO, Marco SEMINARA and Daria VAGAGGINI
 

A synthesis is made of biological data collected in the last three decades on five volcanic lakes in Central Italy with the aim of evaluating their environmental status by means of biological parameters related to zooplankton, littoral and profundal zoobenthos frequently used to detect water quality changes in lakes. A number of bioindicators and bioindices were selected for this purpose, as far as possible following an integrated approach using information drawn from physical and chemical variables. Our results allowed the lakes to be classified according to their biological quality level, which proved to be in good agreement with physical and chemi­cal diagnoses. Lake Bracciano exhibited the best environmental quality, due to the presence of a ring waste water collecting system, to its large size and depth, and to its strong hydrodynamism. Lake Martignano followed, with some symptoms of stress in the hypolimnion due to a marked summer deoxygenation. Oxygen depletion in the profundal characterized also Lake Vico, which showed a mesotrophic condition in the sixties, and 20 years later now shows clear signs of increased trophy (meso-eutrophy). Lake Albano seemed meso-eutrophic with total absence of fauna below a depth of 120 m due to a meromictic status. Finally Lake Nemi, exposed to domestic wastes in the 70s, suffered a heavy eutrophication with dramatic algal blooms and fish kills. Following the diversion of discharges that occurred in the early 90s, today this lake is characterized by the partial improvement of many ecological features. The need for integration between physical and chemical analyses and biological data in order to obtain a reliable evaluation of lake environmental quality is stressed, especially as far as routine implementation in managing and recovery procedures is concerned.

8

Analysis of eutrophication state and trend for lakes in China

JIN XIANGCAN

The article analyzes the present state and trend of eutrophication of lakes in China and concludes that lakes throughout the country are commonly undergoing the process of eutrophication: most of urban lakes are facing hypertrophication, many medium-sized lakes are of eutrophic state, some lakes even approaching to hypertrophic level. The five large freshwater lakes are in the con­dition of eutrophication, especially Lake Caohu and Lake Taihu are already in the state of eutrophication, water quality is deterio­rating and ecosystem is destroyed. According to domestic and foreign experiences of the successful demonstrations in eutrophication and pollution treatment, this article puts forward the theory of combining source control with ecological restoration, which as the guidance for eutrophication control of lakes in China.

9

Lake contamination models for evolution towards steady state

Johan C. VAREKAMP

Most lakes are in an average steady state for water but contaminants may not yet have reached steady state or are gradually being flushed out in a clean-up program. The evolution towards steady state for fully mixed or stratified lakes can be described by basic equations of mass flow. The time-concentration paths for fully mixed lakes are asymptotic toward a steady state concentration, which is reached in about 6 contaminant residence times (and clean-up also takes about 6 residence times). Stratified lakes also evolve towards a whole-lake steady state concentration but show oscillating patterns of concentration versus time, with the amplitude and dampening period depending on the volume ratio of epilimnion to total lake volume. In most natural lakes, the compositional contrast between epilimnion and hypolimnion will become almost erased in 2-4 residence times. An acid lake in North-Patagonia is used as an example of contamination of a thermally stratified lake by volcanic effluents.

10

Technologies for lake restoration

Helmut KLAPPER

Lakes are suffering from different stress factors and need to be restored using different approaches. The eutrophication remains as the main water quality management problem for inland waters: both lakes and reservoirs. The way to curb the degradation is to stop the nutrient sources and to accelerate the restoration with help of in-lake technologies. Especially lakes with a long retention time need (eco-) technological help to decrease the nutrient content in the free water. The microbial and other organic matter from sewage and other autochthonous biomasses, causes oxygen depletion, which has many adverse effects. In less developed countries big reservoirs function as sewage treatment plants. Natural aeration solves problems only partly and many pollutants tend to accu­mulate in the sediments. The acidification by acid rain and by pyrite oxidation has to be controlled by acid neutralizing technologies. Addition of alkaline chemicals is useful only for soft waters, and technologies for (microbial) alkalinization of very acidic hardwater mining lakes are in development. The corrective measures differ from those in use for eutrophication control. The salinization and water shortage mostly occurs if more water is used than available. L. Aral, L. Tschad, the Dead Sea or L. Nasser belong to waters with most severe environmental problems on a global scale. Their hydrologic regime needs to be evaluated. The inflow of salt water at the bottom of some mining lakes adds to stability of stratification, and thus accumulation of hydrogen sulphide in the moni­molimnion of the meromictic lakes. Destratification, which is the most used technology, is only restricted applicable because of the dangerous concentrations of the byproducts of biological degradation. The contamination of lakes with hazardous substances from industry and agriculture require different restoration technologies, including subhydric isolation and storage, addition of nutrients for better self-purification or anaerobic technologies, to eliminate the pollutant nitrate by microbial denitrification. The retention time is an important parameter for modellers and limnologists, that allows them to decide, which technologies – hydromechanical, chemical or biological – have to be applied alone or in combination to cope best with each specific problem. The technologies have to be economical and ecological safe.

11

Water quality management in Lake Kinneret (Israel): hydrological and food web perspectives

Moshe GOPHEN

Long term (1969-2001) data record of nutrient and plankton temporal distribution, and hydrological parameters in Lake Kinneret, combined with metabolic parameters of zooplankton, which were experimentally measured, were statistically (ANOVA) analyzed. Trophic relations between food web compartments were quantitatively considered to evaluate directional combination of ecological forces. Monthly data of inflow discharges, and lake volume were used to calculate residence time values and the data were incorporated into the ecological analysis. The seasonal fluctuations of the hydrological parameters, nutrients, and plankton in­ventories represent typical subtropical climate conditions: high level in winter and low in summer months. It was found that nitrogen inventories in the lake declined and the biomass of grazable phytoplankton was enhanced since early 1980’s. Dissolved phosphorus was decreased mostly in summer months when the lake is nutrient limited, as a result of phytoplankton uptake. Zooplankton was de­clined until 1993 and increased later. Zooplankton preferably feed on chlorophytes and diatoms with supplemental resources of detritus, bacteria and protozoa. The most abundant zooplanktivorous fish, Lavnun (Bleak, Acanthobrama spp.) populated the lake very densely during 1993-95 and biomanipulation management of subsidized fishery caused lowering of predation pressure resulted in zooplankton enhancement and suppression of additional primary produced matter. It is concluded that zooplankton in Lake Kinneret is not food limited and fishery management (Lavnun removal) might be efficient to enhance zooplankton grazing capacity and algal suppression if phosphorus flux is reduced. Long term changes of nano-phytoplankton are affected by both phosphorus availability and zooplankton grazing and fish predation has a significant impact on zooplankton density. Fishery management aimed at algal suppression might be efficient if phosphorus supply is reduced. The similarity between seasonal fluctuations of food web components and residence time is a result of the natural subtropical conditions. The major effect on nutrient loads is due to inputs from the catchment and to a lesser extend to internal processes. Consequently, management implications aimed at lake water quality protection should be mostly directed towards nutrient removal in the drainage basin and loads reduction by pumping water for sup­ply. When such a management is successfully implemented biomanipulation (fish removal) might be efficient.

12

Experience from ILEC's project on Environmental Education

Sven E. JØRGENSEN

The experience of the ILEC’s educational project is presented. The main conclusion from the project is that lakes can be applied as good educational illustrations of environmental issues due to the following seven points: lakes are typical ecosystems, lakes illus­trate very well the hydrological cycles,lakes have a clear borderline, openness of an ecosystem is well illustrated by lakes, the rec­reational value of lakes is very clear, damage by pollutants can easily be illustrated by lakes, the social-economic value of lakes is very obvious.