💧Limnology Unit 11 – Anthropogenic Effects on Freshwater Ecosystems

Key Concepts and Definitions

• Anthropogenic effects refer to the impacts caused by human activities on the environment, including freshwater ecosystems
• Limnology studies the biological, chemical, and physical features of freshwater ecosystems (lakes, rivers, streams, wetlands)
• Eutrophication occurs when excessive nutrients (nitrogen and phosphorus) lead to algal blooms and oxygen depletion in water bodies
• Bioaccumulation is the gradual accumulation of pollutants in an organism's tissues over time, often magnifying up the food chain
• Invasive species are non-native organisms that cause ecological or economic harm when introduced to new environments
• Riparian zones are the vegetated areas along the banks of rivers and streams, providing critical habitat and ecosystem services
• Hydrological alterations include changes to water flow, quantity, and connectivity due to human activities (dams, diversions, groundwater pumping)

Freshwater Ecosystem Basics

• Freshwater ecosystems cover less than 1% of Earth's surface but support a disproportionately high level of biodiversity
• Lakes, rivers, streams, and wetlands are the main types of freshwater ecosystems, each with distinct characteristics and ecological roles
• Freshwater ecosystems provide essential services such as water purification, flood control, nutrient cycling, and recreation
• The littoral zone in lakes and streams is the shallow, near-shore area where light penetrates to the bottom, supporting diverse aquatic life
• Stratification occurs in lakes when distinct layers form due to temperature and density differences, influencing nutrient and oxygen distribution
• Wetlands (marshes, swamps, bogs) act as natural filters, trapping sediments and pollutants while providing habitat for many species
• Rivers and streams transport water, nutrients, and organisms across the landscape, connecting terrestrial and aquatic ecosystems

Major Anthropogenic Threats

• Water pollution from agricultural runoff, industrial discharges, and sewage introduces excess nutrients, toxins, and pathogens into freshwater systems
• Habitat loss and fragmentation result from land-use changes (urbanization, deforestation, wetland drainage), disrupting ecosystem functions and biodiversity
• Overexploitation of freshwater resources for irrigation, industry, and human consumption leads to water scarcity and ecological degradation
• Invasive species, often introduced through human activities, can outcompete native species and alter ecosystem dynamics
• Climate change exacerbates existing threats by altering temperature, precipitation, and hydrological patterns, affecting freshwater ecosystems
• Dams and other hydrological modifications disrupt natural flow regimes, sediment transport, and fish migrations, fragmenting river systems
• Microplastics and other emerging contaminants pose new challenges for freshwater ecosystems, with largely unknown long-term effects

Pollution Types and Sources

• Point source pollution originates from a single, identifiable source (factories, wastewater treatment plants, oil spills)
• Non-point source pollution comes from diffuse sources spread over a large area (agricultural runoff, urban stormwater, atmospheric deposition)
  • Agricultural runoff often contains excess nutrients (nitrogen and phosphorus) from fertilizers, leading to eutrophication
  • Urban stormwater carries pollutants (heavy metals, oil, chemicals) from roads, parking lots, and lawns into nearby water bodies
• Organic pollutants include pesticides, herbicides, and pharmaceuticals that can persist in the environment and harm aquatic life
• Inorganic pollutants such as heavy metals (mercury, lead, cadmium) can accumulate in sediments and bioaccumulate in aquatic organisms
• Thermal pollution from power plants and industrial cooling systems raises water temperatures, affecting oxygen levels and species distributions
• Plastic pollution, including microplastics, can be ingested by aquatic organisms and transfer up the food chain, with largely unknown consequences
• Atmospheric deposition of pollutants (acid rain, mercury) can originate from distant sources and impact freshwater ecosystems

Habitat Alteration and Destruction

• Urbanization leads to the loss and fragmentation of freshwater habitats through land conversion, channelization, and increased impervious surfaces
• Deforestation in riparian zones removes natural buffers, increasing erosion, sedimentation, and water temperature fluctuations
• Wetland drainage for agriculture and development eliminates critical habitats for many aquatic and terrestrial species
• Dams and reservoirs alter natural flow regimes, disrupt sediment transport, and create barriers for fish migrations
  • Reduced downstream sediment delivery can lead to coastal erosion and delta subsidence
  • Changes in water temperature and oxygen levels below dams can affect aquatic species composition and productivity
• Channelization and stream straightening for flood control or navigation simplify habitat complexity and reduce biodiversity
• Groundwater depletion from overexploitation can lower water tables, affecting surface water levels and dependent ecosystems
• Invasive species can outcompete native species for resources, alter habitat structure, and disrupt food webs

Climate Change Impacts

• Rising water temperatures due to global warming can lead to reduced oxygen levels, altered species distributions, and increased harmful algal blooms
• Changes in precipitation patterns (droughts, floods) affect water availability, quality, and ecosystem functions
  • More frequent and intense droughts can concentrate pollutants, increase salinity, and stress aquatic organisms
  • Increased flood frequency and magnitude can cause erosion, habitat destruction, and the spread of pollutants and invasive species
• Shifts in hydrological regimes (earlier snowmelt, reduced ice cover) can disrupt the timing of ecological processes and species interactions
• Ocean acidification, caused by increased atmospheric CO2 absorption, can impact freshwater ecosystems through altered chemical processes and species migrations
• Climate change can exacerbate the spread and impacts of invasive species by creating new suitable habitats and reducing native species' resilience
• Sea-level rise can lead to saltwater intrusion into coastal freshwater systems, altering water chemistry and species composition
• Increased frequency and intensity of extreme events (heatwaves, storms) can cause sudden and long-lasting impacts on freshwater ecosystems

Ecological Consequences

• Eutrophication leads to algal blooms, reduced water clarity, oxygen depletion, and fish kills, disrupting aquatic food webs and biodiversity
• Habitat loss and fragmentation reduce species populations, limit gene flow, and increase vulnerability to local extinctions
• Invasive species can alter community structure, disrupt ecosystem functions, and cause cascading effects through food webs
• Bioaccumulation of pollutants in aquatic organisms can lead to reproductive disorders, developmental abnormalities, and increased mortality rates
  • Biomagnification of toxins up the food chain can pose health risks to top predators and humans who consume contaminated fish
• Changes in water temperature and chemistry can alter species distributions, phenology, and interspecific interactions, affecting community composition
• Reduced connectivity between freshwater habitats can limit species dispersal, gene flow, and population resilience to disturbances
• Degradation of freshwater ecosystems diminishes their capacity to provide essential services (water purification, flood control, recreation)

Mitigation and Conservation Strategies

• Establishing protected areas and conservation easements to safeguard critical freshwater habitats and their surrounding watersheds
• Implementing best management practices in agriculture (precision farming, cover crops, riparian buffers) to reduce nutrient and sediment runoff
• Improving wastewater treatment and stormwater management infrastructure to minimize point and non-point source pollution
• Restoring degraded habitats through wetland reconstruction, stream bank stabilization, and riparian zone reforestation
  • Prioritizing restoration efforts in areas with high ecological value or potential for connectivity and ecosystem services
• Promoting sustainable water use and allocation through efficient irrigation techniques, water pricing, and stakeholder collaboration
• Developing and enforcing regulations on pollutant discharges, land-use practices, and invasive species introductions
• Monitoring and early detection of invasive species, coupled with rapid response and eradication efforts
• Incorporating climate change adaptation strategies into freshwater conservation planning, such as protecting climate refugia and enhancing ecosystem resilience
• Engaging local communities and stakeholders in conservation efforts through education, citizen science, and participatory decision-making
• Investing in research to better understand the complex interactions and cumulative effects of anthropogenic stressors on freshwater ecosystems