Oxygen dynamics and plant-sediment interactions in isoetid ...

Paper 315*15 cm plots homogenously vegetated withboth Lobelia dortmanna and Littorella uniflorawere subjected to organic enrichments withorganic pellets in the sediments as described forthe laboratory experiment over a gradientincluding 0, 0.2 and 0.8 % added organic DW.During the experiment temperature andirradiance were measured every 5 minutes at 0.4m depth at the site by a combined Lux andtemperature logger (Onset ComputerCorporation, Bourne, Massachusetts, USA).Averages for the 90-day long experiment were:20.6 o C (range 12-34), 217 µmol m -2 s -1 (PAR)and a day length of 17.4 hours.At the end of the in situ experiment anintact sediment core was sampled from eachplot in a Perspex tube (5 cm diameter, 30 cmhigh) equipped with rubber stoppers as bottomand lid. The cores were brought back to thelaboratory and stored at 16 o C in a 12-h light: 12-h dark cycle until measurements of pore-waterand sediment characteristics within 1-3 days.All remaining plants from each plot were rinsedand brought to the laboratory in sealed plasticbags holding some water and plenty of air toensure water and O 2 supply. In the laboratory,shoot number of each species, wet weight (WW)and DW of aboveground biomass weredetermined. Leaf samples from each species andplot were then analysed for chlorophyll, TN, TPas described in Møller & Sand-Jensen (2011).Pore-water concentrations of DIC, NH + 3-4 , PO 4and Fe 2+ and sediment content of organicmatter, TN and TP were also determined aspreviously described (Møller & Sand-Jensen2011).O 2 and H 2 O exchange and across leaf surfacesLeaf O 2 exchange for a standard O 2 gradientwas measured as O 2 loss from leaf surfaces tohypoxic water surrounding them with the baseof leaf lacunae exposed to atmospheric air. Sixleaves were cut off and mounted in a PVCcylinder (volume 45.3 ml) through small holesin the lid leaving the most basal 1 mm of theleaf outside the cylinder and leaf surfacesexposed to water in the cylinder. Holes orcracks between leaves, lid and cylinder werecarefully sealed with silicone grease. An O 2electrode (Ox 500, Unisense, Århus, Denmark)connected to a computer for continuouslylogging was introduced to follow O 2concentration in the water of the cylinder. Amagnetic stirrer provided a slow, steady mixing.Measurements were performed undertemperature constant conditions at 5.0 and 15.0o C by submerging the PVC cylinder in water butleaving the exposed leaf bases with air contact.Prior to measurements, the cylinder was flushedwith anoxic water and left for 20 minutes toequilibrate before the linear rate of increase ofO 2 was measured for 10 minutes. Allmeasurements were preformed with a low O 2partial pressure of 1-2 kPa in the watersurrounding the leaves. Similar measurementswithout leaves were used to correct for physicalleakage of O 2 into the PVC cylinder not causedby transport over the leaves. Physical leakagewas very constant (1.45±0.03 and 0.43 ± 0.05nmol O 2 h -1 at 5 and 15 o C respectively). Afterexperiments, leaf dimensions were measured at45 x magnification with a dissectionmicroscope. Three measurements of leaf widthand thickness were made along the length ofevery leaf to calculate surface area. Lobelia hasapproximately rectangular leaves in crosssection while Littorella has cylindrical leaves.Fluxes of O 2 were normalized to leaf surfacesarea and time for a mean gradient of about 19kPa between lacunae at the leaf base and O 258