Determining the Optimal Biomass of Macrophytes during the Ecological Restoration Process of Eutrophic Shallow Lakes

Many studies have shown that macrophytes play a significant role in controlling eutrophication; however, only a few of these are based on macrophyte biomass. Based on the growth characteristic of macrophytes, we propose an approach for the assessment of the optimal biomass of macrophytes in the decay and growth periods in Lake Datong (a shallow lake), using a lake ecological model. The results showed that the pollution load of the lake should be reduced by 50% while conforming to the Environmental Quality Standards for Surface Water (EQSSW) Class Ⅲ. In contrast, with an increase in the pollution load of 5%, the results indicate that the lake may deteriorate to a turbid state over the next few years. The macrophyte biomass should be harvested during the decay period, when 80% biomass is beneficial to the water quality of the eutrophic shallow lake. Based on macrophyte simulation from 2020–2024, the wet biomass of macrophytes should be controlled at 5.5 kg/m2. The current macrophyte biomass in Lake Datong is four-fold higher than the simulated optimal biomass. This study provides a reference for the adequate ecological restoration of the lake and its subsequent maintenance, as well as scientific support for improving the comprehensive evaluation standard of healthy lakes and the theoretical basis of lake ecological restoration.

Relationship between bream (Abramis brama) activity and water turbidity in a shallow lake under different season conditions

The foraging activity of large-bodied benthivorous fish has been suggested to be of key importance for maintaining shallow lakes in a turbid state. Since especially the spring ecosystem dynamics determines the successive state of shallow lakes, such impact could depend on seasonality in benthivore activity patterns. However, we do not yet know to what extent the activity of large-bodied benthivorous fish affects lake turbidity across the year. In order to investigate seasonal dynamics in bream activity and its impact on water turbidity under natural conditions, bream daily activity was studied in a small (39 ha) shallow Danish lake using passive biotelemetry technology, i.e. a modified Passive Integrated Transponder (PIT)-tag antenna system. We tracked the activity of 448 benthivorous bream over a period of four years (2012 to 2016) and during the same period wind conditions, water turbidity and temperature was measured. Results showed a clear relationship between bream activity and water turbidity at water temperature below 15°C indicating that winter season activity of benthivorous bream may play an important role for maintaining lake ecosystems in a turbid state. Also wind speed and wind direction affected water turbidity, suggesting that wind induced resuspension can be important even in small shallow lakes. This is to our knowledge the first full-scale study under natural conditions to describe how bream activity influence lake turbidity on a day-to-day basis. Our findings also add a seasonal component to previous findings by showing that benthivorous feeding bream have the potential to increase water turbidity also in the winter season and thereby, ultimately, impact ecosystem functioning within shallow lakes.

Intrinsic processes causing periodic changes in stability in a shallow biomanipulated lake

Shallow lakes experience alternative states in their biotic organisation over time. In this study, we analysed a long-term dataset on the recovery from eutrophication of a shallow hypertrophic lake (Lake Major, Hungary) following fish manipulation. Disturbances in the food web triggered a shift, markedly affecting both abiotic and biotic variables. Clear and turbid states of Lake Major over the period 1999–2009 were defined by concentrations of chlorophyll-a. Lake Major in a clear-water state had higher transparency and submerged macrophyte cover, lower turbidity and fish biomass, and total phosphorus was relatively low compared to that shown during the turbid-water state. It follows from our study that a regime shift from a turbid state to a clear state can be expected in response to fish manipulation. Reduction in fish biomass increases light conditions, decreases internal nutrient loading and promotes macrophyte dispersion. Food-web effects appear to be considerable in these shallow lakes containing benthic fishes. The roles of phosphorus and submerged macrophytes are essential in maintenance of alternative states of vegetation in shallow lakes of various climatic zones.

DEVELOPMENT AND ANALYSIS OF A NOVEL KIND OF SMART THERMOTROPIC MATERIAL

Thermally induced switching temperature and spectral transmittance of a novel kind of smart thermotropic material developed by a different mixing proportion of hydroxypropyl methyl cellulose (HPMC), sodium chloride ( NaCl ) and pure water was measured. Radiation transmittance measurements were carried out on a thermotropic double glazing window sample, a double glazing window and a low-E double glazing window. Results show that the thermotropic double-glazed window with optimum mixing proportion of HPMC, NaCl and pure water of 2:10:100 by mass-reduces radiation transmittance at fully turbid state by up to 72% and 32% respectively, compared to the ordinary double-glazed window and low-E double-glazed window which do not have adjustable radiation transmittance; its radiation transmittance changed from transparent state to light scattering state up to 60%, indicating a high performance on switching solar radiation and a great potential for energy efficient windows.

Measurement of Optical and Radiation Transmittance of Thermotropic Material

Thermally induced switching temperature, spectral transmittance of a new kind of thermotropic material developed by a different mixing proportion of hydroxypropyl methyl cellulose (HPMC), Sodium Chloride (NaCl) and pure water was measured. The optimum mixing proportion of samples for application was determined by solar radiation and outdoor temperature measurements. Radiation transmittance was tested on samples of a thermotropic double glazing window, double glazing window and Low-E double glazing window. Results show that thermotropic double-glazed window with the optimum mixing proportion of HPMC, NaCl and pure water of 2:10:100 by mass reduces radiation transmittance at fully turbid state by up to 72% and 32%, respectively, compared to the ordinary double-glazed window and Low-E double-glazed window which do not have adjustable radiation transmittance, and its radiation transmittance changed from transparent state to light scattering state is up to 60%, indicating a high performance on switching solar radiation and a great potential for energy efficient windows.

Climate change and the EU Water Framework Directive: how to deal with indirect effects of changes in hydrology on water quality and ecology?

In order to set ecological goals and determine measures for the European Water Framework Directive, the effects of climate change on lake ecosystems should be estimated. It is thought that the complexity of lake ecosystems makes this effect inherently unpredictable. However, models that deal with this complexity are available and well calibrated and tested. In this study we use the ecosystem model PCLake to demonstrate how climate change might affect the ecological status of a shallow peaty lake in 2050. With the model PCLake, combined with a long-term water and nutrient balance, it is possible to describe adequately the present status of the lake. Simulations of future scenarios with increasing precipitation, evaporation and temperature, showed that climate change will lead to higher nutrient loadings. At the same time, it will lead to lower critical loadings. Together this might cause the lake to shift easier from a clear water to a turbid state. The amount of algae, expressed as the concentration Chl-a, will increase, as a consequence turbidity will increase. The outcome of this study; increasing stability of the turbid state of the lake, and thus the need for more drastic measures, is consistent with some earlier studies.

Possible effects of climate change on ecological functioning of shallow lakes, Lake Loenderveen as a case study

Possible effects of climate change on ecological functioning of shallow lakes, Lake Loenderveen as a case study The European Water Framework Directive (WFD) requires all inland and coastal waters to reach "good ecological status" by 2015. The good ecological status of shallow lakes can be characterised by clear water dominated by submerged vegetation. The ecological response of shallow lakes on nutrients largely depends on morphological and hydrological features, such as water depth, retention time, water level fluctuations, bottom type, fetch etc. These features determine the "critical nutrient load" of a lake. When the actual nutrient load of a lake is higher than the critical nutrient load, the ecological quality of this lake will deteriorate, resulting in a turbid state dominated by algae. Climate change might lead to changes in both environmental factors and ecosystem response. This certainly will have an effect on the ecological status. As an illustration the results of a multidiscipline study of a shallow peaty lake (Loenderveen) are presented, including hydrology, geochemistry and ecology. Ground- and surface water flows, nutrient dynamics and ecosystem functioning have been studied culminating in an application of the ecological model of the lake (PCLake). Future scenarios were implemented through changing precipitation, evaporation and temperature. Climate change will lead to higher nutrient loads and lower critical nutrient loads. As a consequence lakes shift easier from clear water to a turbid state.

Alternative Attractors of Shallow Lakes

Ponds and shallow lakes can be very clear with abundant submerged plants, or very turbid due to a high concentration of phytoplankton and suspended sediment particles. These strongly contrasting ecosystem states have been found to represent alternative attractors with distinct stabilizing feedback mechanisms. In the turbid state, the development of submerged vegetation is prevented by low underwater light levels. The unprotected sediment frequently is resuspended by wave action and by fish searching for food causing a further decrease of transparency. Since there are no plants that could serve as refuges, zooplankton is grazed down by fish to densities insufficient to control algal blooms. In contrast, the clear state in eutrophic shallow lakes is dominated by aquatic macrophytes. The submerged macrophytes prevent sediment resuspension, take up nutrients from the water, and provide a refuge for zooplankton against fish predation. These processes buffer the impacts of increased nutrient loads until they become too high. Consequently, the response of shallow lakes to eutrophication tends to be catastrophic rather than smooth, and various lakes switch back and forth abruptly between a clear and a turbid state repeatedly without obvious external forcing. Importantly, a switch from a turbid to a stable clear state often can be invoked by means of biomanipulation in the form of a temporary reduction of the fish stock.