(BIO 386) Lec 11 - Lakes

Significance of lakes

• Part of network of water connecting terrestrial environment and the sea

• Can be thought of as large pools in the stream

• Foci of processing because of long retention time

• Locations of endemism

Lakes make up…

52% of the population

Societal roles of lakes

• Resources

  • Drinking water

  • Food resources

  • Recreation

• Hydrography

  • Water Storage

  • Flood control

• Climate

  • Source of moisture

  • Landscape heat capacity

Lake chemistry reflects…

chemistry of sources waters

Degree of alteration of lake chemistry depends on

• Biological activity (productivity)

• Residences time

Three sources of water

1) Precipitation (rain, snow)

• very dilute

• usually acidic

• affected by aerosols (sea spray, dust)

2) Overland flow (Rivers/streams, runoff)

• Dissolved composition

  • Reflects geological material in watershed

  • Presence of wetlands in watershed

  • Depends on flow path to stream

• Particle Loads

  • Amount depends on flow of stream and parent material

  • Position in river continuum may affect type of particles

3) Groundwater

• Very few particles

• High in dissolved substances

  • Products of slower weathering - depends on time

  • Depends on materials encountered along flow path

Closed Basin lakes

Lake with no outlet

• Water evaporates, minerals stay behind

• some lakes drainage lakes → closed lakes

• response to climate change recorded in sediments

Types of closed basin lakes

1) Soda lakes - bicarbonates (minerals from weathering)

2) Saline lakes - chloride (minerals from precipitation)

Lake classification

• Water color, reflects:

  • abundances of photosynthetic

  • inputs or organic matter from watershed

• Physical structure - density stratification

  • Temperature change with depth

  • Effects of solutes on density

  • Seasonality of density

Wavelengths of sunlight are…

absorbed differently by pure water

• Red light and UV are absorbed the most

• Blue penetrates the furthest and is most commonly scattered back to eye, why clear water appears blue

What is CDOM?

Colored dissolved organic matter

• absorbs blue, reflects in yellow, green and red

• looks brown

Algal biomass affects…

water clarity

• Water always absorbs same light, but algae vary in abundance

• transparency measures algal biomass

Water clarity lake types

• Dystrophic (Brownness)

(Goes from blue to most green)

• Oligotrophic

• Mesotrophic

• Eutrophic

Implications of light for lake structure

Light radiation absorbed by water, DOM or particles

• This energy heats water

Heating occurs most in surface water

• Water becomes lighter than deep water

  • The difference affects chemistry and biology

Temp and light declines exponentially with depth. This is stable stratification because…

warmer water is less dense than colder water

The effect of wind on lakes

Acts against buoyancy forces resulting from density differences related to temp.

Physical structure of deeper lakes

In deeper lakes, wing can only mix the surface to a certain depth. Erosion of the boundary is slowed by stronger buoyancy forces and the growing inertia of the surface mixed layer.

Resulting temperature zonation is…

Reversed in the summer and winter

Three layers of deeper lakes

1) Epilimnion

• Warm, low-density, surface waters

2) Thermocline

• Zone of rapid temperature change

3) Hypolimnion

• Cold, high-density, deep water

Deep water and thermocline are isolated from atmosphere

Density gradient prevents…

mixing of deep water with surface water

Primary producers

1) Macrophytes

• Submerged in aquatic vegetation

• In freshwater, usually higher plants

• Much less structural tissue than on land

2) Phytoplankton

• Free floating

• Microscopic single celled plants

• Can form colonies

Cyanobacteria (Phytoplankton)

• Many fix N2

• Can float due to gas vesicles

• Can be toxic in freshwater

Methods for measuring production

• Bottle incubations

• Whole lake studies

Euphotic zone

Zone where light is great enough to support some photosynthesis

Compensation depth

The depth at where compensation light level is reached, where NPP = 0

Critical mixing depth

The mixing depth where NPP over a day = 0

Short term limitations of primary production

• What the current plankton community needs now

• Assessed by growth in bottles after nutrient addition

Long term limitations of primary production

• What determines total ecosystem biomass

  • Ecosystem processes can alter nutrients in response to additions

  • Algal communities can change, altering needs

• Assessed by

  • Accumulation of biomass in whole ecosystem experiments

  • comparisons between lakes

CO2 limiting ONLY short term

CO2 is removed by photosynthesis replenished

1) Carbonate buffer system - increases pH

2) Carbon concentrating mechanisms (CCMs)

3) Diffusion from atmosphere - decreases pH

Sources of nutrients to lakes

• Dust - mineral particulate P

• Riverine inputs

  • Mostly particulate P

  • Dissolved P where pollutants are important

    • Agricultural P

    • Sewage P - detergents

• Groundwater - mostly dissolved phosphate

• Animal Vectors

  • Can be important in lakes

  • Oceans are too large

Short term N is often…

Limiting

• System adjusts N pools based on available P

• Denitrification can remove excess nitrate

• N-fixation can be limited by MO, light, and other factors

70% of aquatic algae are eaten by…

herbivores

Export affects O2 depletion in deep waters

• organic matter fuels respiration by bacteria

• can lead to fish kills

• O2 sensitive species especially at risk

• Also cold-loving species must stay deep where O2 is low

Ecosystem heterotrophy

Where ecosystem respiration exceeds ecosystem NPP in most lakes