Lecture 17 - Lake Ecosystems Notes

Lecture 17 Lake Ecosystems (Nov. 11)

1. Ways that lakes form
- Tectonic: formed by the movement of the earth's crust (can be very old and very deep, Lake Baikal, Asia; Lake Tanganyika, Africa)
- Pothole or Kettle: formed when ice left from retreating glacier is buried in till and then melts. (Small lakes/ wetlands, prairie pothole region in Alberta and North and South Dakota)
- Glacial: Glacial ice scour, cirque lakes, morainal dams, other (Most numerous category, mostly small)
- Landslide: Movement of earth dams, a stream or river.(similar to reservoirs)
- Volcanic-Caldera: Volcanic explosion causes hole that is filler with water. (Often round and deep, Crater Lake, Oregon)
- Dissolution Lake: Limestone dissolves and lake forms. (Small, steep sides)
- Oxbow, floodplain: River bend pinches off, leaves lake behind; areas of seasonal inundations. (Shallow, narrow, may be seasonally flooded)

- Most lakes are small , a few lakes are huge.

2. Light and heating of water
- Photosynthesis is driven by light
- Water is heated by light
- Light attenuates with depth
- Hence Ps is greatest and water temperatures warmest in surface layers

3. Thermal Stratification
- Water reaches its maximum density at 4C
- Lakes stratify vertically into layers of roughly the same temperature
- Lake with same temperature from top to bottom is called isothermal
- A common pattern during summer is to develop three layers
- Cause is differences in water density due to temperature or salinity
- Stratification alters biogeochemistry and ecology

4. Lake Typology – Stratification
- Monomictic- mixes once a year
- Amictic- never mixes (e.g. saline lakes)
- Polymictic- mixes several times a year
- Dimictic- mixes twice a year

5. Lake Productivity
- Primary production (algal photosynthesis) controls productivity in most lakes
- Ps depends on light, temperature, nutrients, as well as water movements and other chemical factors
- These limiting factors vary with season, lake depth, lake type, and surrounding landscape

6. Light Attenuation is Logarithmic
- Light attenuates with depth
- Attenuation varies with wavelength, with dissolved compounds, and with particles in suspension (absorption of light is lowest in blue, hence blue penetrates to great depth)
- Assume 1/10th is left after 1 m, 1/100th after 2 m, 1/1000th after 3 m, etc. (Attenuation coefficient h describes how rapidly light is attenuated)

7. Key Nutrients
- N, P, and a lot more
- Inorganic N is available mainly as NH4+ ammonium and NO3- nitrate (Nitrogen-fixing cyanobacteria can be very important in lake N cycles)
- Inorganic P is available mainly as phosphate, PO4
- N and P move into the biota via uptake by algae and microbes. After excretion and death, they return to inorganic form through decomposition (re- mineralization)

8. Uptake of Nutrients
- Michaelis- Menten equation for cell uptake: V= Vmax*[S]/(KS+[S])
- V= uptake, [S]= substrate conc., KS= half saturation constant

9. Nutrient Limitation
- Leibig’s Law of the Minimum
a. growth is controlled not by the total of resources available, but by the scarcest resource (limiting factor).
b. Led to “Paradox of the Plankton” – algae all require same nutrients distributed in a well-mixed environment – a challenge to Gause’s principle
- Complications
a. Algae may differ in their relative dependency on different nutrients
b. Co-limitation
- Two species can coexist if they are limited by different resources.

10. Eutrophication
- The nutrient enrichment of an aquatic ecosystem
- Natural eutrophication – a process that occurs as a lake ages over hundreds of thousands of years
- Underlies lake classification: oligotrophic = nutrient-poor; eutrophic = nutrient-rich
- Cultural eutrophication – a process that occurs when humans release excessive amounts of nutrients, greatly accelerating the process of eutrophication

11. Is N or P limiting productivity?
- Redfield ratio - 106:16:1 C:N:P by moles
- Algal cells have approximately this composition under balanced growth
- Can serve as an index of nutrient limitation
- When N:P > 16 – which is in short supply? P
- And when N:P < 16 - ? N
- Experimental test: add nutrient spikes in different combinations to a bottle of plankton – or to an entire lake

12. Why do excess nutrients and eutrophication matter?
- Taste and odor problems
- Health concerns: “blue baby” syndrome
- Blooms of toxic algae
- Aesthetics
- Dead zones

13. Oxygen- Depleted waters
- Oxygen cannot diffuse into deep layer of stratified waters
- If surface waters are productive, a rain of organic matter fuels bacterial decomposition in deep waters, depleting oxygen
- Hypoxic: < 2 mg/L O2; anoxic: no oxygen
- Mobile animals depart, sessile animals die in these “dead zones”
- Chemistry changes as well (remember “redox”)

14. Causes of Gulf Hypoxia
- Mass balance calculations attribute 80% of Mississippi’s nitrogen load to fertilizers, originating in the agricultural heartland of the USA
- Nitrate loads of the Mississippi are three times higher than in the 1950’s, and chemical fertilizer use in the Midwest has tripled over this period.
- In effect, Iowa farmers and Louisiana shrimpers are in conflict (cultural eutrophication)

15. Major Sources of Excess Nutrients
- Inorganic fertilizers and manure
- Human waste
- N atmospheric deposition (combustion of fossil fuels, acid rain, etc)
- Crop N fixation

16. Trophic Levels, Food webs, and food chains
- Food chains
a. producers, primary consumers, secondary consumers, decomposers
b. simple linear viewpoint
- Food webs
a. allow for more complexity such as feeding at different trophic level
b. more difficult to work with

17. Plankton
- Plankton have limited power of mobility and so are transported mainly with water movements
- Bacterioplankton: very small, heterotrophic and autotrophic
- Phytoplankton: algae, including greens, blue-greens, diatoms, more…
- Zooplankton: animal plankton including rotifers, cladocerans, copepods, more…

18. Fish and other predators
- Planktivores: Bluegills, lake herring, whitefish, salmonids when small…
- Piscivores: Bass, pike, salmonids when large…
- Fish-eating birds and mammals: Mergansers, dippers, otters….

19. The trophic cascade
- Postulated since 1880 that predators can control herbivores and increase biomass of producers
- Top-down versus bottom-up control
- Managers may use biomanipulation to control trophic state

20. Adaptation to predation pressure
- Mechanical
a. size (too small or too large)
b. Spines (chemical cues may induce protection)
- Chemical
a. toxins (must be large food item relative to predator)
b. poor quality
- Behavioral
a. escape (chemical, hydrodynamic, visual cues)

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