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Abstracts of the IAHR President's Webinar 1: Saving Lakes from Nutrient Enrichment and Global Warming

Three reports will be given at the IAHR President's Webinar 1: Saving Lakes from Nutrient Enrichment and Global Warming, which will be organized on February 25 (Feb. 24 in the Americas), 2021. Below are the abstracts of these reports:

A Design Strategy for Saving Lakes from the Impact of Nutrient Enrichment and Global Warming

Speaker: Prof. Jörg Imberger
Abstract: The aquatic environment in a lake is the home for the ecology in the lake and the existence of a close connection between the hydrodynamics, together with ethe trophic level and the species that are found in the lake has long been assumed. What is perhaps less well understood is that many species modify the hydrodynamic environment to suite their own life style, giving them an evolutionary advantage, just as humans build shelters and grow food on farms and transport the food back to their homes. The two length scales of the water motion in the lake, that have the greatest influence over species selection and growth rate, given a certain trophic level and mean water temperature, are the smallest length scales that are determined by the rate of turbulent dissipation that determine the home conditions for the pathogens, bacteria, phytoplankton and zooplankton, the foundation of the food chain. At the other end of the motion length scale spectra are the basin scales circulations that distributes the nutrients that enter the lake via the rivers, the surface runoff and groundwater seeps, throughout the lake. The fauna and flora in a particular lake have evolved as those that can thrive and maybe even modify these two length scales to their advantage. Over the last 50 years the water column stability has been increased in most deep lakes by global warming and increased algal growth rendering the water more turbid, thus causing the incoming solar radiation to absorbed closer the water surface. The changes in the water column stability ,in turn, change the above two length scales. The survival of the present species in the lake depends on their ability to accommodate these changes.  Africa has some of the world's largest, deepest and most beautiful lakes. Lake Victoria is the second largest by area in the world and Lake Tanganyika is world's second largest freshwater lake by volume and depth. Both these lakes are shared by a number of different countries making them extremely important in terms of political harmony. Pollution from one country can easily be convected to the neighbouring country’s shoreline and in extreme cases can change the whole ecology of the Lake by altering the two above length scales. This can very quickly start a blame game and then even lead to conflict in some instances. In the next 20 years or so the aquatic ecology in all deep lakes around the world will be under threat due to global warming and nutrient loadings increasing water column stability, preventing overturns occurring on a regular basis an example of an extreme change of the above two length scales. Given that many of the lakes in Africa have multi country boundaries, the larger lakes in Africa could quite easily become a major foci for national conflict. This also provides an opportunity for IAHR, an international science and engineering associations with a good functioning, international network of water researchers and practitioners to assist in resolving transboundary issues before they become a source of stress.

I shall start by describing, in lay terms, the various dynamical regimes in a lake such as Lake Victoria and the strength of the horizontal connection. Then I shall show how adaptive real-time, self-learning, management strategies can be used to avoid blame games from taking hold, saving counties from getting angry with each other. In the final few views will give an overview of the global extent of the problem.

This report, to be given on February 25, is one of the three related lectures Prof. Jorg Imberger has prepared. For more details about these three talks, please visit Improving Water Security Through Connecting With Nature

Water Quality Restoration from Increasing Nutrient Enrichment: A Review of 30 Years’ Efforts for Hyper-Eutrophic Lake in China

Speaker: Prof. Yong Liu
Abstract: Eutrophication of shallow lakes is a great challenge for China. It has experienced a period from rapid pollution in the 1980s to strict pollution control since late 1980s. Lake Dianchi is among the three most polluted large shallow lakes in China. Its restoration is a demonstration for the tradeoff between economic growth and water quality improvement for the less developed regions. The mitigation measures for Lake Dianchi include wastewater treatment plant (WWTP) construction and pollution interception, transboundary water transfer, riverine pollution control, agricultural non-point source pollution control, wetland restoration, and sediment dredging. In recent years, the water quality of Lake Dianchi is recovering rapidly, however the algal bloom is still very serious. A combination of multiple models was applied to analyze the temporal variations of 30-year water quality change and explore the joint effect of external nutrient input and internal cycling.

Flow through Living Roughness

Speaker: Prof. Stephen Monismith

Abstract: Kelp forests, seagrass beds, and coral reefs are among the most productive, bio-diverse, and beautiful of marine ecosystems. Hydrodynamics plays a central role in their function, controlling mass transfer between the organisms and the overlying flow, setting patterns and rates of transport of larvae, and affecting the extent to which the environment (temperature, pH, etc.) in these nearshore systems are different from that of the adjacent ocean. Central to this interplay of physics and ecology is the fact that the resistance to flow provided by kelp plants, seagrasses, or corals is determined by the geometry and density of these “ecosystem architects”, behavior that is described by the dynamics of complex, canopy flows. I will present field observations of these flows drawn from a coral reef in American Samoa, a seagrass bed in Palau, and a kelp forest in Baja. In the first case, drag can be related explicitly to the reef geometry at cm scales. In the second case, drag varies strongly with velocity reflecting the re-configuration of the drag elements due to bending, behavior that can be modeled with some accuracy. In the final case, I will show velocity measurements taken over the span of several years during which time the condition of the kelp forest varied between non-existent and dense. During this time rms depth-averaged velocities varied by a factor of 3, showing the importance of kelp-dependent drag to flow. However, this last case seems substantially more complicated in that drag on flow through a kelp forest is strongly influenced by surface waves, has multiple drag sources, and involves substantial inhomogeneity in the distribution of the kelp plants themselves. Nonetheless, I will present a simplified approach to inferring drag through kelp.

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