Aquatic Ecosystem Chemistry

Dissolved Oxygen (DO)Temperature

Redox Potential (ORP)Major ions (Hardness; Conductivity; Salinity/TDS)

Carbonate Buffer System (pH & Alkalinity)Turbidity (Suspended Sediments)

Macronutrients (N, P, Si)Organic Carbon (Particulate, Dissolved, Color)

The chemistry of natural waters reflects the watershed surface and subsurface geology; weathering climate; and biotic and human impacts within the watershed (terrestrial and aquatic habitats).

TemperatureGreatly influences:

– DO solubility; hence saturation point & content.– Solubility of other gases and solutes (weathering)– Speed of chemical and biological reactions.– Biological populations (directly and indirectly)

• Measured by:– standardized thermometer or thermal conductor probe. – Like DO; must consider time of day and daily range.

• Influenced by:– Climate / Solar Radiation – Turbulent mixing with other water masses.– Shading (turbidity and adjacent terrestrial canopy)– Watershed dynamics (cover, sheet flow, groundwater)

Redox Potential (ORP)

• Oxidation Reduction Potential (ORP; Eh) reflects the relative abundance of electrons available for chemical/biochemical reactions in the environment.

• Measured by an electrochemical probe that compares the capacity of the environment for donating (reduction) or accepting (oxidation) electrons relative to that of a standard platinum electrode for hydrogen (2H+ + 2e-→ H2 ;Eh = 0 mV).

• OIL RIG (Oxidation Is Losing, Reduction Is Gaining). This is relative to the probes response.

• Oxidation reactions will be inhibited (thermodynamically unfavorable) if the environment is reducing (electron rich).

Fe+++ + e-

(ferric)

Fe++

(ferrous)

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Reducing Environment(Low Redox Potential)

Oxidizing Environment(High Redox Potential)

Oxygen Concentration and ORP

• When oxygen is present, redox potential is high:– Lower numbers of free electrons in solution.– Average Eh value in freshwater ~ + 500 mV– As high as + 920 mV

• When oxygen is depleted to low levels, redox potential drops.– Increased number of free electrons in solution.– Without oxygen redox can get as low as -240 mV

• In lakes that stratify / mix, this can create dynamic redox states and ionic concentrations:

Fe+++ + e- Fe++

• At normal redox conditions, ferrous oxidizes to ferric ion.

• Ferric ion forms an insoluble precipitate, sinks to benthos.

• Stratification occurs, oxygen is lost in hypolimnion.

• Redox potential drops, ferrous ion is reduced from ferric in sediments.

Redox and Biotic Processes• Oxidation reactions yield energy under oxidizing conditions.• Reduction reactions yield energy under reducing conditions.• Reverse reactions require energy inputs.

Yields Energy

Requires Energy

Microbe Mediated Redox

Reactionsreduce to Fe (II)

reduce to Mn (II)

CO2 reduce to CH4

Oxidation and reduction reactions are “Coupled”; one needs the other!

Many reductions here couple with the oxidation of organic matter, even when there is no O2 in the environment.

The change in EH between arrow start point between any oxidation and reduction couple reflects the relative energy yield of the reaction.

This would be ΔEH = +820 mV for aerobic respiration; ΔEH = + 640 mV for nitrate anaerobic respiration; ΔEH = +220 mV for methanogenesis.

Chemical Forms and Sources• Elements and compounds occur in water in either

dissolved or particulate forms.– A more recent classification is based on the ability of the

“particulate” compounds to settle out of solution:• Colloidal = does not settle due to gravitational force

• Gravitoidal = does settle out of solution

• As water moves through terrestrial systems (surface, groundwater, etc) new materials are entrained into the water by weathering:– Chemical weathering releases dissolved matter– Mechanical weathering releases particulate matter

• Amount of runoff correlated to concentration of dissolved materials;

• greater runoff = less time for dissolving

Chemical Weathering of Limestone

Carbon dioxide from the atmosphere, respiration and other microbial decomposition reactions reacts with water to form carbonic acid.

Protons react with calcium carbonate (limestone) to form the soluble salt calcium bicarbonate.

Major Ions of Inland Waters

– Solute concentrations in aquatic system is linked to abundance AND solubility of ions in source area (watershed).

– Biological processes can incorporate some ions, thereby reducing their relative abundance in aquatic systems.

– Inland waters vary greatly!

Hardness: • The capacity of water to precipitate soap.• Total sum of Ca+2 and Mg+2 ions.

– Low concentration = “soft”– High concentration = “hard”

• Measured by titration with EDTA (chelater).

Conductivity* Ability for current to pass through water (conductivity) depends on:

- Temperature- Ion Content

* Current is proportional to ion content in water.

Inland Water Salinity (TDS)

Saline Lake Classification:Chemistry (Dominant Anion or Cations)

Biological Factors (Salinity Ranges)

Origins of Solutes

Carbonate Buffer System

Carbonate Buffer SystemCarbon dioxide reacts with water to form carbonic acid.

Depending on pH, carbonic acid will disassociate to bicarbonate and proton.

At higher pH bicarbonate may also disassociate to carbonate and proton.

More acidic will shift the reaction to carbonic acid (left here).

More basic will shift the reaction to carbonate (right here).

Why is it so soluble?

Is it a gas or is it a salt, or does it react to behave as both?

pH Determines Carbonate System Proportions in Solution

Alkalinity: The ability to absorb protons (buffering capacity).

Turbidity & Suspended Solids• Turbidity: measured optically:

– Secchi Depth– Spectrophotometer: (890 nm); linear path.– Nephlometrically: linear and 90º path.

• Suspended Solids: measured gravimetrically1) Filtration of known volume onto tarred filter.2) Oven dry at 105ºC3) Weight to 0.01 mg and subtract filter weight.4) Divide sample weight by volume filtered (= mg SS/L).

With enough comparisons of both turbidity and SS over different load regimes for a specific site, a relationship may be established. Thereafter, SS may be inferred from turbidity, which is by far a simpler more time-efficient method.