Lake Ecosystem

Overview of Lakes as Ecosystems

Lakes are relatively large, standing bodies of inland water that form distinct ecosystems. Unlike rivers, conditions in lakes change mainly with depth and distance from shore rather than along a flow direction. Lakes are especially useful “model systems” in ecology because many processes are easy to observe and measure: layering of water, nutrient cycles, food webs, and ecosystem changes over time.

In this chapter, the focus is on what is specific to lake ecosystems: their physical structure, typical zones, characteristic communities, seasonal dynamics, and typical developmental changes (eutrophication, aging of lakes).

Physical Structure of a Lake

Depth, Surface Area, and Shape

The physical shape (morphology) of a lake strongly influences its biology.

Important parameters include:

• Maximum depth: affects whether the lake can fully mix from top to bottom.
• Mean depth: influences light penetration relative to total volume.
• Surface area: controls the effects of wind (mixing, waves) and heat exchange.
• Shoreline shape: complex, indented shorelines provide more habitat diversity than smooth, circular lakes.

Shallow lakes warm up faster, often support dense plant growth, and may mix completely several times a year. Deep lakes can develop strong thermal stratification and large deep-water regions that may become oxygen-poor.

Thermal Stratification

In many temperate lakes, water does not remain at a uniform temperature throughout the year. Instead, it forms layers (strata) with different temperatures and densities, which strongly affect mixing and oxygen availability.

Three Main Layers in Summer

In a thermally stratified lake during summer (in temperate climates), three layers are typically distinguished:

1. Epilimnion
• Upper, warm, well-lit layer.
• Mixed by wind; temperature is relatively uniform.
• Usually rich in oxygen due to contact with the air and photosynthesis.
2. Metalimnion (Thermocline)
• Middle layer where temperature changes rapidly with depth.
• The thermocline is the zone of the steepest temperature gradient.
• Acts as a barrier to mixing: water above and below exchanges slowly.
3. Hypolimnion
• Deep, cold bottom layer.
• Not directly influenced by wind; isolated from surface in summer.
• Light is strongly reduced; photosynthesis is low or absent.
• Oxygen may progressively decline, especially in nutrient-rich lakes, because of decomposition of sinking organic matter and little re-supply from the surface.

Seasonal Mixing Types

Depending on climate and lake depth, different mixing regimes occur:

• Dimictic lakes (typical in many temperate regions):
• Two mixing periods (spring and autumn), separated by summer stratification and winter stratification (under ice).
• In spring and autumn, cooling or warming makes the water column similar in temperature from top to bottom, allowing full mixing by wind (“turnover”).
• Monomictic lakes:
• Mix once a year (either only in winter or only in summer), usually found in somewhat warmer or colder climates with less seasonal temperature contrast.
• Polymictic lakes:
• Shallow lakes that can mix frequently or almost continuously; strong stratification either does not develop or is short-lived.
• Meromictic lakes:
• Deep basins or lakes with special chemistry where the deepest water layer never mixes with the surface water.
• Permanent deep-water layer (monimolimnion) often remains anoxic and can accumulate dissolved substances (e.g., salts, hydrogen sulfide).

Thermal stratification and mixing cycles are key for understanding nutrient availability, oxygen dynamics, and habitat conditions for organisms in lakes.

Spatial Zonation of Lakes

Lakes have distinct horizontal and vertical zones, each with characteristic environmental conditions and communities.

Horizontal Zones

Littoral Zone

• Shallow, near-shore region where light reaches the bottom.
• Often supports abundant rooted plants (macrophytes) and algae.
• High habitat diversity: stones, sand, mud, plant stands.
• Typically the most productive part of the lake and critical nursery habitat for many animals.

Limnetic Zone (Pelagic Zone)

• Open-water area away from the shore, above the effective light penetration depth for photosynthesis.
• Dominated by plankton (free-floating organisms) and open-water swimming animals (nekton, in fish-ecology usage).
• Subdivided vertically (see below).

Profundal Zone

• Deep-water region beyond effective light penetration, on or above the bottom.
• No photosynthesis; organisms depend on organic material sinking from above.
• Often oxygen-poor in summer in stratified, nutrient-rich lakes.

Benthic Zone

• Lake bottom (sediment) including shallow and deep regions.
• Habitat for numerous invertebrates and microbes that break down organic matter.
• Important for nutrient regeneration and long-term storage (sedimentation) of material.

Vertical Light and Productivity Zones

Light decreases with depth, creating zones with different potential for photosynthesis:

• Euphotic zone:
• Upper layer where light intensity is sufficient for net photosynthesis (photosynthesis > respiration).
• Main area of primary production.
• Compensation depth:
• Depth at which photosynthetic oxygen production equals oxygen consumption by respiration; net production is zero.
• Aphotic zone:
• Deep zone below the compensation depth; light is too weak for photosynthesis.
• Organisms rely on organic matter produced in upper layers.

The depth of the euphotic zone depends on water transparency, which is influenced by dissolved substances, phytoplankton, suspended particles, and colored humic substances.

Typical Communities of Lake Ecosystems

Each zone of the lake is inhabited by characteristic organism groups adapted to the prevailing conditions.

Plankton: Drifters of the Open Water

Plankton are organisms that live freely in the water column and cannot actively overcome water currents on a large scale.

Phytoplankton

• Microscopic, photosynthetic organisms (mainly algae and cyanobacteria).
• Form the basis of the pelagic food web.
• Composition changes seasonally:
• In cooler, nutrient-rich conditions (spring), diatoms may dominate.
• In warmer, stable, nutrient-rich waters (summer), green algae and cyanobacteria may increase.
• Blooms can color the water and strongly affect light penetration and oxygen dynamics.

Zooplankton

• Animal plankton (e.g., water fleas such as Daphnia, copepods, rotifers).
• Grazes on phytoplankton, bacteria, and detritus.
• Many exhibit vertical migration:
• During the day, they stay in deeper water (protection from visual predators).
• At night, they ascend to surface layers to feed.

Nekton: Active Swimmers

In lake ecology, nekton generally refers to larger, free-swimming organisms such as fish and some active invertebrates.

• Fish species often partition the lake vertically and horizontally:
• Some prefer the littoral zone with vegetation.
• Others occupy the open water (pelagic) zone.
• Cold-water species may favor deeper layers in summer.
• Feeding types include:
• Planktivorous fish (eating plankton).
• Benthivorous fish (feeding on organisms in or on sediment).
• Piscivorous fish (predators of other fish).

Benthic Organisms: Life on and in the Sediment

The benthos includes all organisms living at the bottom or in the sediment:

• Macrozoobenthos: visible invertebrates such as insect larvae (e.g., mayflies, caddisflies, chironomids), snails, bivalves, worms, crustaceans.
• Meiobenthos and microbenthos: small invertebrates and microorganisms (e.g., nematodes, protozoa, bacteria).
• Plants in the littoral zone: rooted macrophytes (submerged, floating-leaved, emergent species).

Functions:

• Break down and remineralize organic matter sinking from upper layers.
• Serve as food for bottom-feeding fish and other predators.
• Modify sediment structure and chemistry (bioturbation).

Periphyton and Biofilms

Periphyton are communities of algae, bacteria, fungi, and small invertebrates that grow attached to surfaces: stones, plants, wood, artificial structures.

• Contribute significantly to primary production in shallow lakes.
• Provide food for grazing invertebrates and some fish.
• Influence nutrient exchange at surfaces.

Energy Flow and Nutrient Dynamics in Lakes

The general principles of energy flow and nutrient cycles apply to lakes, but some features are particularly important for lake ecosystems.

Primary Production and Food Webs

• Primary producers:
• Phytoplankton in the open water.
• Periphyton and macrophytes in the littoral zone.
• Primary consumers:
• Zooplankton grazing on phytoplankton.
• Herbivorous invertebrates and fish feeding on plants and periphyton.
• Higher trophic levels:
• Predatory invertebrates.
• Fish that eat plankton, benthic invertebrates, or other fish.
• Birds, mammals, and humans as top consumers.

Detrital pathways are very important: a large fraction of primary production does not get eaten directly but sinks as detritus or is excreted and then decomposed by microbes and benthic organisms.

Internal Nutrient Cycling

Nutrients such as nitrogen and phosphorus are cycled within the lake between water, organisms, and sediment.

Key processes:

• Uptake of dissolved nutrients by phytoplankton and macrophytes.
• Regeneration of inorganic nutrients from organic matter by bacteria and decomposers.
• Sedimentation of particles (dead organisms, feces, mineral particles) to the sediment.
• Release from sediment:
• Under oxic conditions, some nutrients may be trapped in the sediment.
• Under anoxic conditions, phosphorus and other substances can be released back into deep water, especially in stratified lakes.

This internal loading of nutrients can sustain high productivity even if external nutrient inputs are reduced.

Oxygen Dynamics

Oxygen availability in lakes is tightly linked to physical structure, biological activity, and nutrient levels.

• In the epilimnion:
• Exchange with the atmosphere plus photosynthesis usually ensure oxygen saturation or even supersaturation during the day.
• At night, respiration by all organisms consumes oxygen.
• In the hypolimnion:
• Isolated from the atmosphere and photosynthetic oxygen production during stratification.
• Organic matter sinking from above is decomposed, consuming oxygen.
• If decomposition demand is high, oxygen can become strongly reduced or depleted (hypoxia or anoxia).
• In shallow or frequently mixing lakes, such oxygen deficits are less likely because mixing repeatedly replenishes deep-water oxygen.

Oxygen conditions in bottom waters are key indicators of lake health and suitability as habitat for sensitive organisms (e.g., salmonid fish).

Lake Types by Nutrient Status and Productivity

Lakes can be characterized by trophic state, a concept describing nutrient availability, algal biomass, and general productivity.

Oligotrophic Lakes

• Low nutrient levels (especially phosphorus).
• Low phytoplankton biomass; water is clear, deep light penetration.
• Oxygen-rich from surface to bottom throughout the year (in deep lakes, sometimes except very deep zones).
• Typically low organic matter in sediment.
• Often found in mountainous regions, on nutrient-poor bedrock.
• Characteristic fauna may include cold-water fish like trout or char.

Mesotrophic Lakes

• Intermediate nutrient levels and productivity.
• Moderate phytoplankton concentrations; water clarity is reduced compared to oligotrophic lakes.
• Oxygen generally adequate, but some depletion may occur in deeper water in late summer.
• Often show higher biodiversity in macrophytes and invertebrates than extreme oligotrophic or hypereutrophic systems.

Eutrophic Lakes

• High nutrient levels, especially phosphorus and nitrogen.
• High phytoplankton biomass; water often turbid, light does not penetrate deeply.
• Frequent algal blooms, including potential blooms of cyanobacteria.
• Strong oxygen depletion in deep water during stratification; in extreme cases, anoxia.
• Thick organic-rich sediments, high decomposition rates.
• Species sensitive to low oxygen are reduced; tolerant or opportunistic species dominate.

Hypertrophic Lakes

• Extreme form of eutrophication.
• Very high nutrient and algal levels, frequent and intense blooms, often dominated by cyanobacteria.
• Oxygen depletion can occur even in surface waters at night or after bloom collapses.
• Serious ecological problems and strong limitations for water use (drinking water, recreation).

Natural and Human-Driven Eutrophication

Natural Lake Succession

Lakes are geologically temporary features. Over long time scales, they undergo a natural aging process:

1. Initially deep, clear, oligotrophic conditions in young lakes (e.g., newly formed glacial lakes).
2. Gradual accumulation of nutrients and organic matter from the catchment and internal processes.
3. Increasing productivity and biomass of plants and algae.
4. More sedimentation, reduction of mean depth, expansion of littoral vegetation.
5. Transition toward wetland or bog-like conditions and eventual terrestrialization.

This long-term lake succession is governed by climate, geology, hydrology, and biology.

Cultural Eutrophication

Human activities can greatly accelerate eutrophication, a process called cultural (anthropogenic) eutrophication:

Major sources of nutrient input:

• Agricultural runoff:
• Fertilizers (nitrates, phosphates) washed into rivers and lakes.
• Manure and slurry from livestock operations.
• Wastewater:
• Insufficiently treated sewage.
• Detergents containing phosphates (historically important).
• Urban and industrial runoff.
• Atmospheric deposition of nitrogen compounds from combustion processes.

Consequences:

• Increased growth of phytoplankton and aquatic plants.
• Frequent algal and cyanobacterial blooms (potential toxin production).
• Reduced water transparency and changes in species composition.
• Enhanced oxygen consumption in deeper layers due to high decomposition rates.
• Occurrence of hypoxia or anoxia, fish kills, and loss of sensitive species.
• Release of nutrients from anoxic sediments, reinforcing eutrophication.

Measures to counteract cultural eutrophication include reducing external nutrient loads, improving wastewater treatment, controlling agricultural practices, and sometimes in-lake measures (e.g., removal of nutrient-rich sediments, artificial aeration).

Human Use and Management of Lakes

Lakes are used for multiple purposes and are strongly affected by human activities.

Ecosystem Services of Lakes

Lakes provide:

• Drinking water and industrial water supply.
• Recreation and tourism (swimming, boating, fishing).
• Fisheries and aquaculture.
• Flood regulation and storage of water.
• Climate regulation at local and regional scales via heat storage and evaporation.
• Biodiversity and habitat for many rare or specialized organisms.

Impacts and Management Challenges

Key human-induced impacts include:

• Nutrient enrichment (eutrophication).
• Chemical pollution (pesticides, heavy metals, organic contaminants).
• Physical alteration:
• Shoreline construction, embankments, harbors.
• Dredging or artificial depth changes.
• Biological invasions:
• Introduction of non-native fish, invertebrates, or macrophytes.
• Overfishing and disruption of natural food webs.
• Water level manipulations (for hydropower, irrigation, or flood control).

Management aims to:

• Maintain or restore good ecological status.
• Balance multiple uses without compromising ecosystem integrity.
• Monitor key indicators such as nutrient concentrations, algal biomass, oxygen levels, and biological communities.

Lakes in the Context of the Biosphere

Within the larger structure of the biosphere, lakes:

• Act as nodes in regional water and nutrient cycles, connecting terrestrial and aquatic environments.
• Influence and are influenced by their catchments (watersheds); land use, soil type, and climate in the surrounding area strongly shape lake conditions.
• Serve as climate archives: sediment layers accumulate over time and preserve information about past environments (e.g., pollen, fossils, chemical signatures).
• Provide model systems for understanding general ecological principles such as:
• Energy flow and trophic cascades.
• Nutrient cycling and feedbacks.
• Succession and regime shifts (e.g., clear-water vs. turbid-water states).

Thus, lake ecosystems are not isolated; they are integral components of landscapes and the global biosphere, linking local processes to larger-scale ecological and biogeochemical dynamics.