10.3.1. The physical properties of oxygen
Oxygen comprises 200,000 ppm (parts per million) of air, but in aqueous solution its concentration is only about 15 ppm in saturated cool water. Energy at the cellular level can be provided by anaerobic respiration but it is inefficient, providing 19 times less energy per unit of substrate respired than aerobic respiration. Although insects such as bloodworms (certain chironomid midge larvae) survive extended periods of almost anoxic conditions, most aquatic insects must obtain oxygen from their surroundings in order to function effectively.
The proportions of gases dissolved in water vary according to their solubilities: the amount is inversely proportional to temperature and salinity, and proportional to pressure, decreasing with elevation. In lentic (standing) waters, diffusion through water is very slow; it would take years for oxygen to diffuse several meters from the surface in still water. This slow rate, combined with the oxygen demand from microbial breakdown of submerged organic matter, can totally deplete the oxygen on the bottom (benthic anoxia). However, the oxygenation of surface waters by diffusion is enhanced by turbulence, which increases the surface area, forces aeration, and mixes the water. If this turbulent mixing is prevented, such as in a deep lake with a small surface area or one with extensive sheltering vegetation or under extended ice cover, anoxia can be prolonged or permanent. Living under these circumstances, benthic insects must tolerate wide annual and seasonal fluctuations in oxygen availability.
Oxygen levels in lotic (flowing) conditions can reach 15 ppm, especially in cold water. Equilibrium concentrations may be exceeded if photosynthesis generates locally abundant oxygen, such as in macrophyte- and algal-rich pools in sunlight. However, when this vegetation respires at night oxygen is consumed, leading to a decline in dissolved oxygen. Aquatic insects must cope with a diurnal range of oxygen tensions.