Wabash River TMDL Development Final Report
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(respiration). The entire process is part of the natural cycle of most plants, and this cycle causes dissolved
oxygen concentrations to fluctuate throughout the water column in a day. This is called a diurnal oxygen
cycle. Various other processes also produce and consume dissolved oxygen in the water column.
Processes that consume oxygen include organic decomposition, respiration by fish and invertebrates, and
sediment oxygen demand. Additional dissolved oxygen is produced through atmospheric exchange. The
amount and timing of oxygen production and consumption depends on several of the following factors
(Thomann and Mueller, 1987; Wetzel, 2001).
• Solar radiation and water clarity
• Air and water temperature, wind speed
• Flow
• Algae and macrophyte growth and death/decay rates
• Presence or absence of essential elements
• Type of algae present in the water column
• Amount of dissolved oxygen present in the water column
Oxygen depletion occurs when the balance between oxygen consumption and production is altered, either
causing excessive oxygen consumption or reduced oxygen production. The dissolved oxygen
concentration in a waterbody becomes too low, thereby threatening oxygen breathing aquatic life.
Because algae are typically the largest producers and consumers of oxygen in a river, a shift
in that community can greatly affect the dissolved oxygen. The basic processes linking excessive algal
biomass to altered pH and dissolved oxygen concentrations are summarized below.
• Most algae communities have natural, seasonal succession. The timing between growth (oxygen
producing) and decay (oxygen consuming) can be very different. This shift causes periods when
there is excessive decomposition and little new growth, resulting in extreme oxygen depletion.
• Excessive algae and macrophytes cause the diurnal oxygen cycle to expand. Dissolved oxygen
becomes extremely high during the daytime, often resulting in oxygen supersaturation. Dissolved
oxygen then falls to extremely low concentrations during the night (plant respiration), causing
fatal conditions for aquatic life (Thomann and Mueller, 1987).
• As a consequence of photosynthesis, plants utilize carbon dioxide in the day time (removing it
from the water) which causes alkaline carbonates and bicarbonates to predominate in the water
and the pH to rise. The opposite occurs at night. In the case of heavy algae blooms, the pH of the
water can fluctuate quite dramatically through a 24 hour period. While many large fish can
survive these fluctuations, small fish can become quite stressed by these rapid pH changes.
• Natural and anthropogenic sources can cause the sudden death of algae and macrophytes. This
results in a situation with excessive decay and no biological oxygen production, again causing
fatal conditions for aquatic life
2.1.2 Wabash River Impairment Status in Ohio
This TMDL report does not directly address the Wabash River within Ohio because sediment and nutrient
TMDLs were previously developed in 2004 (USEPA, 2004). The impact of the Ohio portion of the
Wabash River on downstream water quality is further discussed in Section 4.0.
2.1.3 Thermal Modification Impairments
Although thermal modifications were initially evaluated during this study as a possible reason for the
impaired biotic community listings, the available temperature data (summarized in Appendix G) do not
suggest that in-stream temperature criteria have been exceeded in the Upper, Middle, or Lower Wabash
