Lecture 17: Managing Eutrophication

What are the two main approaches for targetting P concentrations for eutrophication management?

1. Control the nutrient source

2. Treat the problem within the lake

T/F controlling nutrient sources is easy?

False

Non-point sources are over 80% of total inputs and these are difficult to control

What are some methods for controlling nutrient sources into a waterbody?

• Decreasing fertilizer application, and best practise for timing

• Decreasing sediment runoff by changing tillage practises and installing buffer strips

• Livestock control

Can phosphorus be retained long-term in a watershed?

Yes; when P is added in excess it is retained via adsorption and lost slowly to aquatic systems

What method was used for nutrient source control in Lake Washington?

They determined that the Nutrient source was a sewage inflow

Which they diverted to the ocean

Lake is deep, with a large hypolimnion and high oxygen holding capacity; hypolimnion did not become anoxic

Resident time < 3 years

What did P concentrations look like in Loch Leven?

A decrease in spring P concentrations by 60% and winter P concentrations by 75%

No change in (late) summer P concentrations, and in some years an increase

Continued summer time algal blooms and eutrophication issues

Why did eutrophication problems occur in Loch Leven

It is a warm polymictic lake; that stratifies in the Summer

They had lowered P concentrations during oxic (mixing) periods

No change, or increasing concentrations during anoxic (stratification) periods

⇒ Suggests a shift from external to internal loading

When is lake dredging effective // vs. not effective?

Effective in shallow lakes

Prohibitive in larger lakes

How does lake aeration work? is it common?

Somewhat more common remediation method for minimising release of phosphorus and algal blooms in ponds and lakes

Various techniques work off the same principle that elimination of anoxia via aeration reduces the availability of phosphorus

What may aeration innadvertently result in?

May result in a loss of stratification

= Cold water fish in summer

How can lake aeration be effective?

It may:

• Reduce toxic algal blooms

• Stabilize dissolved oxygen levels

How is iron addition effective in Alberta for reducing P?

May be particularly effective in many Alberta lakes, because of the naturally low Fe, but high S

An effective management technique; reduces P levels and thereby toxic algal blooms

What does controlling eutrophication in lakes require?

Both source control and in-lake treatment

Management sediment P stores is often the primary focus of in-lake treatment

What determines salinity in freshwater systems?

The sum of major of anions and major cations

Defines as Ions with concentrations >1 mg/L

• Cations = Ca2+, Mg2+, Na+, K+

• Anions = HCO3-, SO42-, Cl-

What is conductivity?

The ability of current to flow between two electrodes

Conductivity is a simple measure that provides an integrated assessment of total salt concentration

Saline lakes are common in _______’s _____ ____s region

Canada’s Great Plains; Evaporation > Precipitation

What are the 3 main sources of major ions in freshwater ecosystems?

1. Weathering input

2. Atmospheric precipitation or dry deposition

3. Environmental control

What is the main weathering agent?

Carbonic acid

What is the difference in weathering for sedimentary vs igenous rocks?

Sedimentary rocks weather quickly, and can be rich in Ca, Mg & HCO3-

Igneous rocks have diverse composition, but weather more slowly

What regions in Canada are sedimentary vs igenous rocks most commonly found in the underlying bed rock?

Sedimentary rocks = Great Plains

Igneous rocks = Canadian shield

Where do the products of weathering end up?

Flow from land to freshwater

Ultimately to the ocean

What are the sources ions from atmospheric precipitaion and dry deposition ?

Cyclic inputs from ocean water: Na, Cl, Mg & SO4

Dust deposition from upwind soils: Ca, Mg

Atmospheric and other pollution (combustion): SO4

What are the differences between environmental controls wthin lakes (local) vs. across regions (regional)?

Within lakes (~local scale)

• Biological uptake and redox

• Evaporation and evaporative concentration (particularly in closed basins + dry years)

Across regions (~regional scale)

• Precipitation vs. evaporation

• Catchment composition: soil history and rock type