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

Paper 1method of Andersen (1976) and total Fe (TFe)by the modified phenantroline method (Møllerand Sand-Jensen 2008).Effluxes of DIC (DIC eff ) during the course ofthe experiment were calculated from depth-gradients as the sum of CO 2 and HCO 3 effluxesby integration and linear interpolation betweenmeasuring days. Fe 2+ efflux (Fe 2+ eff) wascalculated from the Fe 2+ concentration gradient.Effluxes were estimated as the product of: 1) theconcentration gradient from 10 mm depth to thesediment surface (assuming that the latter hadthe same concentration as the lake water), 2) thediffusion coefficient of the element at 15 o C (Liand Gregory 1974), 3) the porosity of thesediment and 4) the inverse of turtuositypreviously measured for these sediments(Pedersen et al. 1995). Accumulation of DICand Fe 2+ (DIC acc and Fe 2+ acc) in the pore-waterwas calculated by the increase of depthintegratedsediment concentrations from beforeto termination of the experiment. Effluxes plusaccumulations are minimum estimates ofprocess rates because exchangeable ions werenot included and this pool can be considerablefor Fe 2+ . Moreover, we did not necessarily findthe maximum concentration gradient. Finally,effluxes of CO 2 from the sediment to the watervia the plant and effluxes of Fe 2+ to andprecipitation of Fe 3+ on root surfaces were notincluded either.Data treatment and statistical analysisData were processed in Excel 2007 andstatistical analysis and graphs were made inGraph Pad Prism 5. Data are presented as means± SD of pseudo replicates from the same turfwhen possible. Experiments were a gradientstudy with no replication of treatments butsuited for regression/correlation analysis acrossthe treatment gradient. Pore-waterconcentrations were measured in duplicate atdifferent time points and in triplicate attermination of the experiment.ResultsFig. 1. Changes in sediment biogeochemistry during c. 200days following organic enrichment of Lobelia sediments(left column) and Littorella sediments (right column) at 0%(○), 0.1% (●), 0.2% (□), 0.4% (■), 0.8% (Δ) and 1.6% (▲)of sediment dry weight. First panel: O 2 penetration depth;second panel: DIC concentration at 1 cm depth; thirdpanel: Fe 2+ concentration at 1 cm depth; fourth panel: NH 4+concentration at 1 cm depth. Mean values of triplicates. SDomitted for clarity.Concentrations of O 2 , DIC, Fe and NH 4 NaturalFig. 1. Changes in sediment biogeochemistry during c.200 low-organic days following Lobelia organic and enrichment Littorella of sediments Lobeliasediments maintained (left deep column) O 2and penetration Littorella sediments depth (>(right40column) at 0% (○), 0.1% (●), 0.2% (□), 0.4% (■), 0.8%mm), low DIC concentrations and negligible(Δ) and 1.6% (▲) of sediment dry weight. First panel: O 2penetrationFe 2+ anddepth;NH + 4 secondat 40-mmpanel:depthDIC concentrationthroughoutatthe1cm 200-days depth; third long panel: experiments Fe 2+ concentration (Fig. 1). at 1 Increasing cm depth;fourth addition panel: NH of +4 concentration organic matter 1 cm depth. stimulated Meanvalues of triplicates. SD omitted for clarity.degradations rates and rapidly diminished O 2penetration depths to just a few mm andincreased DIC concentrations up to 5-20 timesrelative to control sediments. DIC accumulationpeaked only 5-15 days after organic additionand a second peak appeared after some 100days. Fe 2+ appeared in the pore-water after arelatively long delay (ca. 10 days) and slowly32