I. Definitions
epilimnion - in a thermally stratified lake, the upper layer of water
metalimnion - in a thermally stratified lake, the middle layer of water that is characterized by a sharp change in temperature with depth
hypolimnion - in a thermally stratified lake,
the bottom layer of water; temperature is usually low and uniform
oligotrophic - lakes with minimal nutrients required for plants and animals
mesotrophic - lakes with an intermediate amount of nutrients required for plants and animals
eutrophic - lake with abundant nutrients required for plants and animals
II. Stresses on lake ecosystems – much the same as for rivers
A. Eutrophication
1. cause - increased nutrients (mostly P) from water and air pollution
2. characteristics
a. overabundance of phytoplankton
b. reduced water clarity
c. low oxygen levels
d. depends on geology and stratification of lakes
B. Water diversion
C. Siltation/sedimentation
1. reduced water storage capacity
2. increased turbidity
3. nutrients and toxics adsorbed to sediments
D. Acidification - low pH and increased solubility of heavy metals
E. Toxic substances
F. Exotic species - plants and animals
III. Restoration of lakes
A. Water level - reduce water diversion
B. Nutrient and sediment source reduction
1. waste minimization
2. waste treatment
3. sediment ponds - sediment bound nutrients settle out
4. wetland creation
C. In-lake methods for nutrient reduction - method to use depends on lake size, depth, chemistry
1. phosphorus inactivation - alum Al2(SO4)3 reacts to become aluminum hydroxide which binds P to sediments
a. aluminum may be toxic if lakes are acidic
b. generally effective, but may need to be repeated
2. sediment skimming (dredging) - top nutrient rich layer of sediments is removed by mechanical means - commonly used
a. generally effective, but expensive
b. disturbs bottom dwelling fish
3. deep water discharge - removal of nutrient rich waters from hypolimnion
a. translocate problem to streams
b. little testing
4. hypolimnetic aeration - hypolimnetic waters are raised to the surface, aerated, and returned to the lower layers
a. encourages breakdown of organic matter,
b. lowers algal densities
D. Control of exotic plants and animals (as well as algal overgrowth from eutrophication)
1. biological agents - fish, insects, pathogens, may cause more problems
2. water-level drawdown - lower water level during winter so plants are more likely to freeze - doesn=t work for all species
3. harvesting/removal - not very effective
4. chemicals
a. potential environmental damage
b. problem with decaying plant material consuming O2
E. Acidification
1. reduce SO2 and NO x emissions
2. liming - short-term
F. Toxic substances/contaminants
1. source reduction
2. sediment removal
3. plant harvesting
4. bioremediation
G. Altered food chain - reintroduction of predatory fish