Oxygen release from roots of the submersed macrophyte Potamogeton perfoliatus L.: Regulating factors and ecological implications
Rates of photosynthetic production, respiratory consumption and root release of dissolved oxygen (O2) were measured for Potamogeton perfoliatus L. from an estuarine population. Incubations were conducted in split-compartment chambers, with shoots (leaves and stems) separated from roots (plus rhizomes). Time-course observations of O2 exchanges between plants and filtered estuarine water were made at ambient temperatures in natural daylight and in darkness. Release of oxygen from roots (Lr) to surrounding water was directly proportional to photosynthetic production of oxygen in the shoot compartment (Pa). Lr for plants with medium (20–35-cm) stem lengths ranged from less than zero to 0.28 mg O2 (g dry plant)−1 h−1. The fraction of Pa released from roots was inversely proportional to overall stem length, with Lr approaching 18% of Pa for short plants (10–15 cm). Mass-specific respiration rates of shorter, more actively growing plants were also 1.5–2.5 times greater than those for longer plants (50–55 cm). In addition, relative Lr (% Pa) was inversely related to mass/length, possibly reflecting a higher fraction of stem cross-section as gas space in plants with low mass/length. For natural populations of P. perfoliatus in Chesapeake Bay, Lr was calculated to be 17–22 mg O2 m−2 h−1, representing a relatively small fraction of Pa (3–7%). Potential effects of Lr on bacterial metabolism in sediments were also estimated. For example, oxygen release from roots would be sufficient to support 4–6 times ambient nitrification rates or to oxidize all of the sulfide produced from sulfate reduction in unvegetated sediments.