Important Freshwater Ecosystems

Why This Matters

Freshwater ecosystems cover less than 1% of Earth's surface yet support roughly 10% of all known species—this disproportionate biodiversity makes them a cornerstone of limnological study. You're being tested on more than just definitions; exams expect you to understand how water movement, nutrient cycling, thermal stratification, and connectivity shape these systems and the organisms within them.

Each ecosystem type demonstrates different principles: why lakes stratify while rivers mix continuously, how wetlands function as biogeochemical processors, and what happens when groundwater connects to surface systems. Don't just memorize which species live where—know what physical and chemical processes make each ecosystem unique and how human activities alter these dynamics.

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Lentic Systems: Standing Water Ecosystems

Standing water bodies develop distinct physical and chemical gradients because water residence time allows for stratification, nutrient accumulation, and stable habitat zones. Thermal and chemical layering drives much of what makes these systems ecologically distinct.

Lakes

• Thermal stratification—seasonal layering into epilimnion, metalimnion, and hypolimnion creates distinct habitat zones and drives nutrient cycling patterns
• Long water residence time allows for internal nutrient processing, making lakes sensitive to eutrophication from external loading
• Littoral, pelagic, and benthic zones support different communities, with light penetration determining the extent of primary production

Ponds

• Shallow depth prevents persistent stratification—most ponds mix completely, creating more uniform conditions than lakes
• High surface-to-volume ratio makes ponds highly responsive to atmospheric inputs and temperature changes
• Breeding habitat for amphibians and invertebrates; small size makes them vulnerable to rapid nutrient loading and algal blooms

Reservoirs

• Artificial impoundments alter natural hydrology by storing water and controlling downstream flow regimes
• Dendritic morphology (branching shape from flooded river valleys) creates different conditions than natural lake basins
• Trap sediments and nutrients behind dams, affecting downstream ecosystems and disrupting natural sediment transport

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Lotic Systems: Flowing Water Ecosystems

Moving water creates fundamentally different conditions than standing water. Continuous unidirectional flow prevents stratification, delivers nutrients and oxygen, and shapes physical habitat through erosion and deposition.

Streams

• Headwater systems represent the smallest and most numerous freshwater habitats, often comprising 70-80% of total river network length
• High oxygen levels from turbulent flow support cold-water species like trout and sensitive macroinvertebrates
• River Continuum Concept—streams transition predictably from heterotrophic headwaters dependent on leaf litter to autotrophic mid-reaches with more sunlight

Rivers

• Longitudinal zonation creates distinct communities from headwaters to mouth based on temperature, flow, and substrate changes
• Sediment transport and deposition shape channel morphology and create diverse microhabitats (riffles, pools, runs)
• Migratory corridors for anadromous and catadromous fish; dams fragment these pathways with significant ecological consequences

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Transitional and Wetland Systems

These ecosystems exist at boundaries—between terrestrial and aquatic environments, or between fresh and salt water. Fluctuating water levels and mixing zones create high productivity and unique biogeochemical processing.

Wetlands

• Biogeochemical hotspots—anaerobic sediments drive denitrification, removing nitrogen from water; vegetation traps sediments and uptakes phosphorus
• Hydroperiod (duration and timing of flooding) determines plant community composition and wildlife use patterns
• Carbon storage in organic soils makes wetlands significant in global carbon cycling; drainage releases stored carbon as $CO_2$

Floodplains

• Flood pulse concept—periodic inundation delivers nutrients and connects aquatic and terrestrial food webs, driving high productivity
• Lateral connectivity with main river channels creates spawning and nursery habitat during high water periods
• Nutrient-rich alluvial soils support agriculture, but levees disconnect floodplains from rivers, eliminating ecological benefits

Estuaries

• Salinity gradients from freshwater to marine conditions create physiological challenges that limit species but support high biomass
• Nursery function for commercially important fish and shellfish; mixing of nutrients from rivers and ocean drives exceptional productivity
• Sediment deposition at the freshwater-saltwater interface creates dynamic, shifting habitats

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Groundwater-Dependent Systems

Subsurface water connects to surface ecosystems in ways that aren't always visible but profoundly influence hydrology and water chemistry. Groundwater provides baseflow, thermal stability, and chemical inputs to many surface waters.

Groundwater Systems

• Aquifers store 30% of global freshwater and supply baseflow that maintains rivers and wetlands during dry periods
• Long residence times (years to millennia) mean contamination persists and recovery is extremely slow
• Groundwater-surface water exchange occurs in hyporheic zones beneath streams, supporting unique microbial communities and nutrient processing

Springs

• Constant temperature and chemistry create stable environments that support endemic species found nowhere else
• Groundwater discharge points indicate aquifer conditions; changes in flow or chemistry signal aquifer stress
• Thermal springs support chemosynthetic communities in some cases, similar to deep-sea hydrothermal vents

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Quick Reference Table

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Self-Check Questions

1. Which two ecosystems best demonstrate the importance of periodic flooding for ecological function, and what concept explains this relationship?

2. Compare and contrast how lakes and rivers process nutrients differently based on their fundamental physical differences (lentic vs. lotic).

3. A stream maintains cool temperatures and steady flow even during summer drought. What groundwater-related process explains this, and which ecosystem type serves as the source?

4. An FRQ asks you to explain how wetlands improve water quality. Which three biogeochemical processes would you describe, and what conditions in wetlands make them possible?

5. Why might a reservoir support different fish communities than a natural lake of similar size, even if both thermally stratify? Consider at least two structural or hydrological differences.