Tuesday, November 19, 2024

Introduction

Video lecture replacement due to lost lecture. Review material from previous class and connect to Thursday's lecture.

Lake Systems Overview

Transition from river systems to lake systems is crucial in understanding freshwater ecology. Lake systems are often referred to as Atlantic River systems due to their geographic distribution and water flow characteristics.

Distinction:

• Low technology (Low tech): Characterized by running water typical of river systems.

• Land technology (Land tech): Typically refers to still water, but it can be misleading as lakes exhibit vertical movements in water loops.

Characteristics of Lakes

Three fundamental characteristics of lakes:

1. Distinct Edges: The delineation of lakes is defined by their shoreline, where terrestrial and aquatic ecosystems interact.

2. Homogenous Bottoms: Lake bottoms display a more uniform composition compared to the varied substrates found in river systems.

3. Well-Mixed Water: Seasonal mixing occurs in lakes, which influences nutrient distribution and the aquatic food web.

Bottom Materials

• Lakes may consist of varying materials: sand, silt, rock, gravel.

• The consistency of these materials is generally more uniform than what is commonly observed in river beds.

Freshwater Sources

• A significant amount of the world's freshwater is trapped in glaciers, emphasizing their importance in global water supplies.

• Lakes and rivers contain limited freshwater volumes in comparison, making their management and conservation vital due to the increasing rarity and high value of water resources.

Types of Lakes

1. Tectonic Lakes:

• Formed through geological shifts, creating faults in the Earth's crust (e.g., the Great Rift Valley).

• Result from drop-out falls, leading to Caribbean lakes or rift lakes, which can be rich in biodiversity.

2. Volcanic Lakes:

• Created by volcanic activities, including collapsed calderas or lava flows that block river paths.

• Notable locations include Hawaii, where volcanic lakes contribute to the unique ecosystem.

3. Glacial Lakes:

• Predominantly found in temperate zones, shaped by glacial activities that carve the land.

Subtypes:

• Cirque Lakes: Formed by glaciers at high ridgelines, often circular in shape.

• Valley Lakes: Result from glacier movements that leave depressions filled with water.

• Kettle Lakes: Created when glaciers melt, resulting in depressions filled with water fragmented from the ice.

4. Other Lake Types:

• Artificial Lakes: Constructed for water supply, recreation, or hydroelectric power, often by damming rivers.

• Sinkhole Lakes: Occur when subsurface material collapses, creating natural depressions.

• Oxbow Lakes: Formed when rivers cut off meanders, trapping water in old river bends.

• Floodplain Lakes: Developed from rivers breaching their levees, leading to seasonal inundation.

Lake Structure and Zones

Utilization of morphometric routes and bathymetric maps is essential for understanding lake morphology, revealing insights into depth and landscape.

Basic Zones in Lakes:

1. Photic Zone: The upper layer where light penetrates and supports photosynthetic organisms.

2. Benthic Zone: The lake bed, where light cannot reach, leading to different forms of life adapted to these conditions.

3. Aphotic Zone: The layer above sediments too deep for light penetration, creating unique ecological challenges.

4. Thermocline: A critical layer with a steep temperature gradient that separates warmer upper waters from colder layers below, influencing aquatic life and mixing processes.

Water Motion and Mixing

• Wind Influence: Wind plays a vital role in driving water motion, leading to mixing events that affect temperature distribution and nutrient availability in lakes.

• Vertical Motion: Seasonal changes can result in significant stratification, influencing the distribution of oxygen and nutrients, crucial for sustaining life.

Lake Substratum

• The substratum in lakes tends to be more homogenous than in rivers, often shaped by specific geological and hydrological conditions, such as those found in the Hauraki Basin.

Dissolved Oxygen Profiles

• Stratified Profile: Higher oxygen levels at the surface with a sharp decline in the hypolimnion, due to limited mixing, which can lead to anaerobic conditions.

• Well-Mixed Profile: Characteristics of healthy lakes where oxygen levels remain consistent from top to bottom, supporting diverse aquatic life.

Salinity and its Effects

• Freshwater fish are hypo-osmotic and struggle to adapt to increased salinity, leading to elevated mortality rates in environments with higher salt concentrations.

• Increased salinity levels require additional energy expenditure for osmoregulation, affecting the overall fitness of freshwater species.

Acidity Effects on Organisms

• The acidity curve indicates that extreme pH levels can severely damage aquatic organisms' tissues.

• Organisms face challenges with osmoregulation and overall health in environments with extreme pH levels.

Comparison of Lake Types

Oligotrophic Lakes:

• Characterized by low nutrient levels, resulting in clear waters and limited biological productivity. Typically, these lakes are deep and round in shape.

Eutrophic Lakes:

• High nutrient levels lead to turbid waters, rich in plant and animal life, supporting diverse trophic levels. These lakes are often shallower with complex, dendritic shapes.

Lake Biota Zones

1. Pelagic Zone: Refers to the open water, not deep enough for photosynthesis.

2. Profundal Zone: Comprises areas too deep for light to penetrate, where different organisms thrive.

3. Littoral Zone: The shallow area where light supports photosynthesis, crucial for plant life.

Energy Flow in Different Zones

• Pelagic Zone: Energy flows from phytoplankton to zooplankton, progressing to small fish and then larger fish.

• Profundal Zone: Detritus forms the base, providing energy for primary consumers, like worms, which are then consumed by fish.

• Littoral Zone: Complex interactions with various primary producers enhance energy flow and ecological dynamics.

Complexity in the Littoral Zone

• The littoral zone represents the most complex marine environment due to the interaction of multiple energy sources, including macrophytes, benthic algae, and phytoplankton, enhancing ecological interactions and energy flow dynamics.

Food Webs vs Food Chains in Lakes

• Functional Complexity: Food webs dominate in lake ecosystems due to the multitude of species interactions, including the presence of omnivores and several energy flow pathways counteracting simplified food chains.

Taxonomy of Algae

• Various algae play critical ecological roles in lake systems, including:

  • Cyanobacteria: Known for nitrogen fixation, can produce toxins affecting water quality.

  • Chlorophyta: Contributing to primary production, providing oxygen and food.

  • Diatoms: Important indicators of environmental conditions, highly diverse and significant in aquatic food webs.

Conclusion

• A comprehensive understanding of major ecological patterns, algal succession, and nutrient dynamics within various lake zones is essential for effective ecosystem management. Prepare for an assessment focused on detailed understanding of lake ecology, highlighting interactions and nutrient dynamics that support lake health.