Dynamic Limnology between Movements and Management

Abstract

The subject matter of this chapter revolves around the intersection of hydrodynamics in French lakes with the economic development of their respective territories. The aim is to understand the interactions between the dynamics of these lakes and territorial actors operating within these limnic areas. Each party is subjected to different management approaches, yet none of them take advantage of lake movements. These management strategies are organized based on the specific limnological mechanisms of each lake, including their temporal and spatial characteristics, as well as the risks they pose to human activities integrated into these lake ecosystems. This chapter will specifically focus on unused water and sediment movements as means to interact with local management practices. These phenomena serve as a foundation upon which all actors surrounding these lakes can rely to guide the protective measures implemented for their activities. This chapter aims to provide scientific insights to support decision-making for the actors operating in proximity to these lakes.

1. Introduction

During the inception of limnology, François-Alphonse Forel introduced the concept that human beings are not merely part of lake’s ecosystem. Through their activities, humans have the capacity to interact significantly with a lake and impact its ability to provide ecosystem services [1]. From a legal perspective, French lakes, including those in Europe, are theoretically required to be managed in the same manner. However, the reality is different. Despite the fact that management approaches are heavily influenced by scientific knowledge, each manager operates according to their individual needs and challenges. Every entity present along the shores of these lakes contributes to human influence on their respective areas of interest.

From a management perspective, the administrative structuring of water resources in France is guided by a range of directives, including the Water Framework Directive (WFD-2000), Water Development and Management Master Plans, and municipal decrees. The overarching objective is to achieve a state of “good condition,” which theoretically considers societal entities inhabiting the managed territory and their expectations. However, practical implementation deviates from this ideal. Despite aspirations for a systemic approach, the prevailing framework is characterized by rigidity, relying on naturalistic indicators while neglecting a diverse range of actors utilizing these aquatic environments, particularly for recreational purposes. Today, the number of stakeholders surrounding the lakes has diversified significantly and is still gradually increasing. Within the federation of municipalities of the Great Lakes, a multitude of sectors are represented, including military, oil, tourism, associations, regulatory authorities, and administrative entities. Consequently, the primacy of exclusively studying the dynamics of lakes as a reference for management indicators becomes increasingly questionable. Significant dynamic phenomena, such as flooding, water level fluctuations, navigation, erosion, or conservation of fauna and flora, come into play and shape management considerations. It is thus pertinent to explore dynamic behaviors of these lakes. Limnology facilitates a territorial approach that encompasses all relevant stakeholders integrating them into a comprehensive societal system. This approach considers the spatial and temporal dimensions, encompassing both natural and anthropogenic aspects. It involves establishing linkages between the environmental dynamics of biocenosis, biotope, and the socio-cultural practices imposed by human activities in relation to lakes.

In an approach to limnological geography [2], which places lakes at the centre of societal-environmental interactions across various spatial and temporal scales, the employ of dynamic limnology will use, in this chapter. This discipline seeks to understand the physical dynamics exerted by a lake within its own boundaries or towards its immediate surroundings. This dynamic perspective completes physical and biocenosis approach usually prefer to study water bodies to assist in management strategies in the face of global change. This study of lake dynamics (movements of water and sediments) was born with the creation of limnological sciences by F.-A. Forel, first studied the dynamics of the formation of Lake Geneva (Léman, in French), including geomorphological aspects and the appearance of “seiches” (variations in water level within the same lake) [3]. More recent researchers have furthered these investigations to enhance the quantification and mapping of lake physics at various scales [4]. We have opted for the dynamic approach due to its historical neglect in these limnic territories [5], especially by managers. The multitude of spatial approaches employed in contemporary limnology adds complexity to discerning the roles of different actors in lake management, without assigning undue importance to any perspective. To address this complexity and translate it into a diagnosis that prioritize specific parameters, our focus is on diagnosing water and sediment movements.

Our objective here is to produce a comprehensive diagnosis of the dynamic mechanisms occurring within these lakes to enhance their consideration by managers. By creating a diagnosis rooted in dynamic phenomena applied to specific territories, we aim to provide novel and readily applicable data on lake behaviours that can be easily understood by stakeholders and inform future management strategies. To achieve this, we will generate new cartographic data at the scale of the specific lake under study, irrespective of its characteristics, within the federation of municipalities of the Great Lakes located in the Landes, New-Aquitaine, France.

2. Methods

In order to develop a chapter that offers practical perspectives to managers, we will study the hydrodynamic processes of lakes using methodologies that can be easily replicated.

2.1 Presentation of studied lakes

The sites examined in this chapter concern two lakes situated in the federation of municipalities of the Great Lakes (Figure 1), within the Department of Landes, in the region of New Aquitaine (France). This study follows on from a French thesis that explores the following question: Can water movements and their effects on differentiated sediment deposits in lakes and ponds contribute to the maintenance of economic activities in their territory?

Figure 1.

Localization map.

Most lakes along the Aquitaine coast are geographically separated from the Atlantic Ocean by coastal dune barriers. The formation and development of these water bodies are closely intertwined with the history of the dunes themselves. By examining the periods of mobility of the dune belts, it has been possible to determine the specific characteristics and evolution of these lakes. The available data indicate that these bodies of water originally existed as lagoons wide open to the ocean until around 1000 BC [6]. Subsequently, these lagoons progressively became obstructed due to the accumulation of sand transported by longshore drift and finally closed definitively in Gallo-Roman era.

These lakes within the federation of municipalities of the Great Lakes are commonly referred to as ponds of Parentis-Biscarrosse and Cazaux-Sanguinet in France. Historically, these study sites were neglected in favor of their proximity to the nearby ocean, leading to their derogatory classification as “ponds” at the time. However, according to Laurent Touchart’s definition, these water bodies qualify as lakes. Locally, they are now recognized and referred to as lakes, which is technically accurate considering that their average depth exceeds six metres, as defined by the Ramsar Convention of 1971. Additionally, they cover an area exceeding 200 ha, which meets the criteria set forth by E. Jedicke for defining a lake. Therefore, in this chapter, we will refer to them as lakes. Studied lakes collectively encompass over 9340 ha, with a total volume of 750 million liters. More specifically, Lake Parentis-Biscarrosse holds an estimated volume of 250 million liters, while Lake Cazaux-Sanguinet is estimated to contain 500 million liters [7].

Parentis-Biscarrosse and Cazaux-Sanguinet lakes are located in the extreme north-west of the Landes department, close to the border with Gironde (a French department a little further north) and below Arcachon basin. These two natural lakes can be reached in around an hour’s drive from Bordeaux. Lake Parentis-Biscarrosse is slightly smaller and situated a little further south than Lake Cazaux-Sanguinet. It covers an area of approximately 3540 ha at an altitude of 19 m above sea level, with a maximum depth of 20.5 m. Lake Cazaux-Sanguinet, on the other hand, covers an area of around 5600 ha, at an altitude of 20 m above sea level, with a maximum depth of 23 m [7].

These lakes also share the characteristic of being shared by different communes. Lake Parentis-Biscarrosse is located in the municipalities of Parentis-en-Born, Biscarrosse, Gastes and Sainte-Eulalie-en-Born. Its counterpart is spread across the communes of Biscarrosse, Sanguinet and Cazaux, which come under the jurisdiction of the Gironde town of La Teste-de-Buch.

To move beyond the confines of limnology solely for naturalistic or physicochemical purposes, we have incorporated dynamics (Figure 2) of these “lentic” features to address their economic and societal significance. This diagnostic approach considers the criteria mentioned above.

Figure 2.

Illustration of lake dynamics interest.

Methodologies employed in this study encompass a combination of surveys and field observations, complemented using Geographic Information Systems (GIS).

2.2 Bathymetries

Lakes exist within basins, and when examining their dynamics, bathymetry emerges as a fundamental approach for initial investigations. The Adour-Garonne water agency conducted bathymetric surveys of lakes under study in 2014, and we will leverage their existing data. Additionally, bathymetric information was supplemented with a Digital Terrain Model generated by the French National Geographical Institute (IGN). By utilizing pre-existing data available throughout French territory, this diagnostic approach can be replicated on other water bodies (Figure 3).

• Data formatting: a dataset was prepared involving the processing of approximately 37,000 points for each of these lakes.

• Bathymetry calculation: Bathymetry was determined either through echo sounder surveys conducted on water bodies under study or by using existing point data and employing TIN interpolation techniques.

• Spatial segmentation: Data were cut based on the area of the water body.

• Spatial projection: All calculations were performed in the appropriate spatial projection system, specifically EPSG: 2154 (French work).

• Integration of bathymetric data: Bathymetric data was merged with available digital terrain models.

• Volume calculations: Quantitative assessments of the volume were conducted.

• Integration with built elements and networks: These data were combined with information pertaining to constructed features and various networks, utilizing data from IGN’s BD TOPO.

• Cartographic visualization: Cartographic representations were generated, depicting lake beds (including archeological features, sandbanks, etc.) and highlighting issues related to water levels.

Figure 3.

Sediment movement’s process.

This initial methodological approach enables us to understand dynamics of lake beds, as well as the water levels and volumetric characteristics of the water bodies studied.

2.3 Water movements on surface

Following the examination of lake depressions where certain movements occur, our attention will shift to movements generated by winds. The study of wind patterns serves to augment the previous analysis by elucidating the formation of waves on the lake surface. The investigation of these wind patterns (Figure 4) is conducted through the utilization of two distinct yet complementary methodologies.

• Potential wind races calculation [8]

• Effective fetch calculation [9]—determining the area where wind energy generates waves—for each studied water body using a regular grid.

• Calculation of wave height, period, and velocity.

• Determination of maximum fetch or wind races.

• Wind weighting based on an annual wind rose provided by MétéoFrance.

• Field verification of data accuracy, where a higher fetch or wind races correspond to increased coastal exposure to winds and erosion.

Figure 4.

Water movement process on lake’s surface.

This secondary methodological approach allows for the acquisition of findings regarding the dynamics of the lake banks or beds and the effects of wave-induced water actions on lake surfaces.

2.4 Water movements inside lakes

The movements taking place inside lakes are commonly called currents. Apparently, lakes are even less calm than we think. These currents are not visible to the naked eye but can be easily identified if we pay attention to the evolution of bathymetry, fish location or invasive plants.

The use of an acoustic Doppler current meter coupled with mapping software makes it possible to visualise currents by spatialising their dynamic cells within lakes.Their visualizations depend on whether the current metre is in a fixed station or if it moves on the water’s surface. In a fixed station, the device is fixed on a support that is immersed at a predefined point upstream. Its aim is then to go from the lake bottom to the surface. On the other hand, the other device will be guided by our boat and will detect lake currents from the surface to the bottom. To detect the direction of the currents will be used by a compass linked to the device is calibrated directly in real-time by the device in a fixed station. In the mobile position, the compass must be calibrated before any measurement is taken.

• Calibrate the device according to water’s salinity.

• Calibrate the compass.

• Prepare the field trip (weather, necessary equipment, and boats with the planned routes).

• Recovery of equipment if it operates in a fixed state.

• Export of data in a usable format under Geographic Information System.

• Assemble data in the projection system used.

• Display them with a map background.

• Interpolate all transects.

• Obtain a mesh representative of the lake (e.g., 10 m step).

• Join data (speed and direction) for each point in the mesh.

• Choose a spatial representation combining speeds and directions.

• Display data based on collected data and depth.

• Correlate data with other previously known movements and issues.

This third methodological approach enables the acquisition of results pertaining to the internal dynamics of lakes and directions of water flux.

These three methodological parts presented the dynamics that we seek to comprehend through a concise diagnosis of water and sediment movements. However, it is important to acknowledge that these dynamics do not exist in isolation from human interactions. Further societal investigations will soon complement this dynamic diagnosis of water bodies.

3. Results

To address societal concerns related to phenomena such as floods, water quality issues, erosion, and sedimentation, we will employ dynamic limnology and utilize the methodologies to assess the impacts in these two lakes with similar genesis.

3.1 Bathymetries

The bathymetric analysis of these lakes was conducted using GIS. Generated maps provide visual representations of submerged topography and features within the lakes. Through this data acquisition, updated volume calculations for lakes were obtained. Newly determined volumes exhibit a variation of approximately 5% compared to previous measurements, even for lakes with identical altimetry, as exemplified by Lake Parentis-Biscarrosse in Figure 5.

Figure 5.

Water movement process inside lakes.

Nevertheless, these bathymetric analyses of these lakes revealed irregularities that correspond to archeological findings in certain instances, and potential archeological discoveries in others. The lake basins were subsequently incorporated into a digital model, facilitating adjustments to water levels to address the various concerns related to buildings and utility network infrastructures.

Water level projections (Figure 6) have been established based on historical records of significant flooding or low-water periods, with the aim of comparing them with water management practices regulated by the lake’s altitude. The aim is to assess the need for better management of water levels.

Figure 6.

Pixel-based bathymetry analysis and volume of lake parentis-Biscarrosse.

This baseline data will then be used to analyze changes in sediment dynamics linked to the anthropization of the lake’s banks.

3.2 Water movements on surface

To study the relationship between wind and waves, we implemented the calculation method proposed by Håkanson and Jansson [9] to determine fetches. To carry out this approach, we selected points of interest in order to recreate fetch diagrams for each wind direction based on the wind rose data from MétéoFrance. Maximum fetches lengths were extracted, while the remaining values were weighted according to the frequency of the corresponding winds. The data obtained can be used to identify beaches subject to erosion due to wind-induced wave action.

The evaluation of fetches makes it possible to obtain theoretical values for the waves that reach the coast. This information helps managers to understand wave phenomena in their limnic territory. Moreover, these theoretical waves can be observed daily by anyone looking to enjoy the view from these lakes.

The analysis of lake dynamics related to wind-induced water movements has produced some interesting results, described in an article written jointly by Pascal Bartout, Laurent Touchart and myself [10]. The obtained data are presented in the form of graphs and maps (Figures 7 and 8). Maps highlight the most exposed coasts, characterized by sectorial winds, which correspond to the eroded zones (illustrated in dark blue in Figure 9). Erosion can be understood through accompanying graphs that provide insights into the impacting wave characteristics. Conversely, areas that have experienced minimal wind exposition on these maps exhibit visible sediment accumulation (illustrated by light blue zones in Figure 9).

Figure 7.

Issues highlighted by GIS visualization.

Figure 8.

Wave graphs according to fetches.

Figure 9.

Wind exposure maps by lake (weighted on the left for lake parentis-Biscarrosse and maximum on the right for lake Cazaux-Sanguinet).

As revealed by the image captured prior to the 2022 summer season (Figure 10), the southern coast of Lake Cazaux-Sanguinet exhibits significant erosion, indicated by the dark blue color (Figure 9). As a result, this beach, previously used by tourists, is now closed to the public for safety reasons. These phenomena could have been anticipated and dealt with by implementing local policies aimed at reducing these risks. By identifying areas with a high potential for erosion, managers of the federation of municipalities of the Great Lakes can use these results to prevent risks and forecast the costs of environmental works to maintain uses. This study is, therefore, essential for lake managers to better manage and safeguard these lake environments.

Figure 10.

Eroded coast.

Furthermore, it is noteworthy that invasive exotic plants, Lagarosiphon Major and Egeria Densa, present in these lakes exhibit a tendency to avoid areas characterized by intense winds and waves. These plants have a greater propensity to acclimatize themselves in protected areas from winds and waves. Additional factors and conditions play a role in the colonization of specific sectors by these plants, as it will be explored in the subsequent section.

3.3 Water movements inside lakes

Internal movements of lakes are commonly referred to as currents. Although not very visible, these currents manifest in the form of lake-specific cells moving in different directions and at speeds ranging from almost zero to several metres per second. This section highlights the results related to the correct interpretation of these currents in lake environments.

3.3.1 Water movements directions inside lakes

Movements internal to the lakes are specific because of their directions. These currents must be detected thanks to currents metre in lakes that identify water moving in cells. Different directions and highs from few centimeters to some tens are remarkable. This section highlights the results related to the correct interpretation of these current directions in lake environments.

Cells can be identified by depth. Thanks to this representation by transects and georeferencing on GIS software, it is possible to map it.

With current data, we obtain significant values and representations with speeds associated (Figure 11). This makes it possible to have a global visualization of what happens in the lakes at a time T. This particularity allows us today to confirm that lakes are not simple basins filled with water and where water circulates from upstream to downstream, or tributaries to outfalls. In fact, we can follow tributaries from their entrance into lakes to the outfall. It appeared in Figure 12 with purple water column next to the coasts. Entrance currents followed runs along the coast like an oceanic littoral drift [11], marking a new frontier that invasive plants do not overtake. Moreover, the rest is still moving as we can see a lot of different directions of water inside lakes (Figure 13).

Figure 11.

One of currents map possible.

Figure 12.

Data directions untreated.

Figure 13.

Frontier between lake and invasive plants.

Direction data is easily correlated thanks to date and time parameter with winds of the sector, easily identifiable thanks to a wind rose from MeteoFrance. Currents directions correspond to those of the winds and waves produced. Their directions seem to be affected by prevailing winds only on the first 20 cm (Figure 14).

Figure 14.

Wind rose of the directions from currents.

Wind rose of currents in one point, taken from Parentis-Biscarrosse Lake gives evidence directions exiting inside lake and extracting from a current metre. Directions are oscillating inside water column, all day long but also any day (Figure 15).

Figure 15.

Currents direction.

Moreover, these directions on a step of 10 cm is significant to distinguish all movements inside the lakes despite the gales visible in Figures 16 and 17. A limit imposed by currents metre to constate that lakes are not calm and sleeping described in the literature.

Figure 16.

Transect of speeds.

Figure 17.

Currents speeds graphic.

3.3.2 Water movements speeds inside lakes

Each current reveals a significant speed in cells detected by a current metre. Currents identified by the different measurement campaigns have speeds ranging from nearly 0.01 m per second to more than 4 m per second according to the wind (Figure 17).

As the previous figure shows speeds cells of currents, during a day without wind, in red, a tributary of Parentis-Biscarrosse that affects the entirety of the water column. The rest of the lake contains a lot of speeds in blocks that could be considered as a new typology of lake zonation.

Contrary to currents direction and homogeneity in changing, current speeds show irregularities because of gales.

With this graphic, the average speed is the same during all the period of measurement. Parentis-Biscarrosse Lake appears to be in a dynamic equilibrium situation at around a speed of 0.5 m per second, a speed corresponding to the value in metre per second of the largest tributary of the lake. Measurements over the month of June 2022 confirm this average balance at around 0.5 m per second average in this lake. Measurements on the lake, of similar genesis, Cazaux-Sanguinet, will they be able to confirm this hypothesis arises from the interpretation of the results of the lake of Parentis-Biscarrosse?

Nevertheless, balance is here of several increases in the average speed in connection with gales exceeding 60 km per hour, over the few days of measurements.

Therefore, a concise assessment of water body dynamics can be achieved utilizing open-source GIS software, requiring no specialized equipment other than for current measurements. The outcomes generated through these distinct methodologies serve as a bridge between local stakeholders and the lakes, facilitating the guidance and planning of actions on these limnic territories.

4. Discussions

These lakes seem to be full of vitality. Are they alive? And just as blood circulates in the human body, water circulates in the lakes, and in an almost random way but under the main effect of winds or rivers.

With these results, the findings obtained enable actors and managers of these lakes to comprehend most phenomena that impact their limnic territories. They can perceive these lakes or ponds as living systems, dependent on various mechanisms that require monitoring to ensure their proper functioning. Armed with this knowledge, each stakeholder can establish guidelines for the preservation of their activities. By analyzing dynamic movements of water and sediments within these water bodies, action plans can be devised to combat invasive plants and safeguard banks and infrastructure. An improved understanding of these dynamics facilitates the anticipation of water body evolution. Based on this information, what measures will the stakeholders in these limnic territories undertake next?

That section on currents brings here, a lot of future evolution. Movement inside lakes are manifold but useful to manager of this lake. These currents can guide jobs for lake as new zonation to advantage some part rather than other?

Currents are permanent, but their direction is not dependable. Could these currents help human being to energetic transition?

This concise chapter awaits further completion with available information pertaining to the subject of these water bodies. It will soon be augmented with comprehensive lake data.

5. Conclusion

This chapter presents a significant advancement in the integration of dynamic limnology by providing a diagnostic approach based on lake movements. It highlights the previously underemphasized dynamic aspects of water bodies, which have not received sufficient attention from managers due to the predominant focus on biocenosis. The inclusion of spatial criteria for dynamics now allows for a comprehensive consideration of the entire system, including human interactions, within the limnic territory. Through these studies, various lake users who previously overlooked the significance of movements occurring within these water bodies can now better comprehend the intricacies of the environment in which they operate. The understanding of erosion, sediment deposition, floods, invasive plant development, and other issues can be improved by incorporating dynamic criteria related to water and sediment movements. This dynamic perspective facilitates the creation of decision-support maps for stakeholders in their respective limnic areas. At the scale of water bodies, focusing on dynamics raises questions about the future evolution of these aquatic environments. Indeed, dynamic characteristics of a water body can vary from one region to another, as observed in these two lakes of similar origin and shape in the northern region of Landes, France. Therefore, this diagnostic approach warrants consideration for every water body due to its ease of implementation. It offers valuable insights and enhances management practices by facilitating a deeper understanding of these complex environments through the lens of lake dynamics.

Acknowledgments

The thanks for this chapter go directly to the federation of municipalities of the Great Lakes (in the Landes, France) and all users or managers of these lakes. Of course, I thank university of Orleans, my thesis directors, and university Mont Blanc – Savoy to that opportunity to study misunderstood lakes movements with materials and knowledges.