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 emerge
nt aquatic sedge (Eleocharis sphacelata).
2 We compared internal pressurization, convective gas flow, changes in inte
rnal 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 wer
e similar at all depths, but the convective inflows increased with depth du
e to lower resistances to convective flow in the plants growing in deeper w
ater. In contrast, rates of flow returning up the efflux culms decreased wi
th 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 l
owest 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 la
rge volume of stagnant gas that had to be expelled relative to the flow rat
5 By 14.00 h, the stagnant gas had been flushed from the plants at all dept
hs, and oxygen concentrations were close to atmospheric values throughout.
The two factors apparently responsible for maintaining high oxygen concentr
ations 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 intern
ally over a 24-h cycle by convective flow decreases significantly with dept
h, suggesting that oxygen stress due to gas transport limitations may be on
e of the factors limiting depth penetration in this species and other emerg
ent aquatic plants.