9/4/02

Lake Background Chemistry

pH, Conductivity, Calcium and Buffering, Humic Materials, and Clay Minerals

Lake Water Chemistry

• Major ions - those in higher concentrations - determine background ecology of lakes

• Measurement is often indirect - by pH, buffering (alkalinity), or conductivity, rather than concentrations of specific chemicals

• Nutrient chemicals are also important, but considered later

pH = -log10[H+]

Increases in H+ cause decreases in pH.

• pH < 5 and > 9.5 are toxic to lake organisms

– pH < 5 frees toxic Al+++ from clay and other minerals (silicates with AlO4---)

– Low pH from organic (humic) acids is less toxic, because humics bind Al +++

– pH > 9.5 converts NH4+ to toxic NH3

pH Frame of Reference Fig. 6-4

Factors Affecting pH in Lakes

• Solution from air and biotic fluxes of CO2

– photosynthesis, respiration, decay

• Buffering chemicals from rock and soil weathering

• Inflows of humic (and other) acids from peaty soils or coniferous forest litter

Conductivity (Electrolyte Strength)

• Water’s transmission of electrical current

– Units of micro-Siemens (mS)

– Specific Conductance is corrected for temp.

• Main electrolytes:

Ca++, Mg++, Na+, K+ ; HCO3-, Cl-, SO4--

• Conductivity often correlates with pH buffering, because Ca++ and HCO3- are dominant ions.

Buffering (Alkalinity)

• Resistance to changes in pH when H+ or OH- is added

• Buffers are chemicals that react with and remove H+ or OH-

• Most important buffer in lakes is bicarbonate

HCO3- + H+ ® H2CO3 ® H2O + CO2 ­

HCO3- + OH- ® H2O + CO3--

Divalent Cations

• Especially Ca++ + Mg++

– Stabilize HCO3- in solution

– In combination with dissolved CO2

• Buffer water by precipitation / solution

• Make water “hard” and slightly alkaline

– Combine with detergents

Background Conditions

• Catchment rocks and soils weather to control inflowing water chemistry

• Sedimentary rocks (limestone, dolomite) yield Ca++, Mg++, and HCO3- ... strong buffering

• Our granitic rocks and lateritic soils yield Na+, Mg++, SiO2, Cl-, and SO4-- ... weak buffering

• Peaty soils yield dissolved humics which are acidic and chelate divalent ions


Saline Lakes

• Endorheic basins, dry climates … no outflow

• Salinities > 0.5 ppt (brackish), > 40 ppt (hypersaline), often extremely alkaline

• Restricted biota, often endemic

• Low productivity

• Examples: Caspian, Aral, and Dead Seas, Great Salt Lake, Mono Lake

Humic Materials

• Leached from acidic or waterlogged soils

• Yellowish-brown solutes with low pH

– Humic acids, humins, fulvic acids

• Chelate divalent cations, nutrients, toxic metals

• UV photolyses humics into free radicals

– Toxic to biota, consume oxygen

• Absorb visible and IR, heating the water

Ecological Tendencies

• Lakes with medium strong buffering are often productive and stable

– Extremely high buffering suppresses algae

• Lakes with weak buffering may or may not be productive, but are always unstable

• Humic (“dystrophic”) lakes are unproductive and stable

Effects of Suspended Clay

• Platelets of silica with various metallic ions

– especially Al+++, Fe+++, Na+, Mg++

• Surfaces have electrical charges

– Adsorb NH4+ and PO4--- (nutrients)

– May serve as nutrient reservoirs to plankton

– Scavenge nutrients from water to sediments

• Scatter light and interrupt line of sight

• Interfere with zooplankton filter-feeding

Clay Structure


1