• emergent macrophyte;
  • flooding;
  • oxygen;
  • respiration;
  • wetland


1 The objective of this study was to examine how increasing water depth in the field affects the aeration achieved by convective gas flow in an emergent aquatic sedge (Eleocharis sphacelata).

2 We compared internal pressurization, convective gas flow, changes in internal gas composition, and differences in plant morphology at three depths (0.75–0.90 m, 1.40 m, and 2.15–2.65 m) in an oligotrophic lake.

3 Internal pressures generated in the aerial tissue of the influx culms were similar at all depths, but the convective inflows increased with depth due to lower resistances to convective flow in the plants growing in deeper water. In contrast, rates of flow returning up the efflux culms decreased with depth, due to a decrease in ratio of influx : efflux culms with depth.

4 Respiration lowered the oxygen concentration in the stagnant internal gas at night, with predawn concentrations highest at the shallowest site and lowest at the deepest site. Low oxygen concentrations also persisted longer after dawn in plants at the deep site than at the other sites due to the large volume of stagnant gas that had to be expelled relative to the flow rates.

5 By 14.00 h, the stagnant gas had been flushed from the plants at all depths, and oxygen concentrations were close to atmospheric values throughout. The two factors apparently responsible for maintaining high oxygen concentrations when convective flow was operating were (i) morphological changes at the deep site that reduced the resistance to flow, and (ii) dissolution of carbon dioxide produced in respiration into the external medium.

6 The data indicate that the average oxygen concentration maintained internally over a 24-h cycle by convective flow decreases significantly with depth, suggesting that oxygen stress due to gas transport limitations may be one of the factors limiting depth penetration in this species and other emergent aquatic plants.