1. The solubility of oxygen in water increases with decreasing temperature. This has led to a general perception of cold, high mountain streams as more oxygen rich than warmer lowland streams, and that macroinvertebrates inhabiting high altitude streams have had no need to adapt to critical oxygen conditions. However, this fails to take into account that oxygen solubility declines with decreasing atmospheric pressure, which may be of importance at high altitudes.

2. Based on samples of macroinvertebrate benthos and in situ measurements of respiratory oxygen demand of macroinvertebrates in small streams from sea level to 4000 m a.s.l. in Ecuador, we determined predicted oxygen availability, oxygen demand and macroinvertebrate assemblage structure along this wide altitudinal gradient.

3. We show that the predicted oxygen availability at 4000 m a.s.l. is only one fifth of that at sea level, whereas the mean weight-specific respiratory rate of macroinvertebrates declined by only 50%, from 400 to 3800 m. We suggest that this disproportionately large gap between availability and demand of oxygen at high altitudes may imply a potential oxygen deficiency for the fauna, and we discuss how oxygen deficiency can be expected to affect macroinvertebrates in high altitude streams.

4. Finally, we present preliminary data on the predicted response to oxygen deficiency of macroinvertebrate assemblages in high altitude streams. Compared with lowland streams, assemblages in natural high altitude streams include relatively few groups normally regarded as oxygen-sensitive. Nevertheless, high altitude assemblages react more strongly to lowering of oxygen saturation, and are thus more sensitive to organic pollution.

5. Oxygen deficiency has been overlooked completely in studies of the altitudinal distribution of species and aquatic communities. We argue that oxygen deficiency may be a potentially important factor, and that more focus on this topic is likely to produce significant new insights in aquatic community ecology.