Short term changes in pore water chemistry in river sediments during the early colonization by Vallisneria spiralis

This study aims to investigate the effects of benthic vegetation colonization on pore water chemistry in river sediments with different organic matter and nutrient loadings. Shoots of Vallisneria spiralis L., surface sediments and water were collected in March 2009 from two fluvial reaches, upstream (U) and downstream (D) an urban wastewater treatment plant. Laboratory microcosms were created with homogenised sediments and macrophytes from each sampling site, half with bare sediments (U-B and D-B) and half with transplanted shoots of V. spiralis (U-V and D-V). Microcosms were then incubated over 25 days in two tanks with water from U and D. Approximately every 4 days, three microcosms from each treatment (U-B, D-B, U-V and D-V) were terminated, and pore water was analysed for Eh, pH, O-2, CO2, CH4, Fe2+, PO4 (3-), NH4 (+), NO3 (-) and DRSi. Significant effects of both vegetation and wastewater loadings were found. Vegetated sediments of both sites, especially U-V, displayed significantly higher Eh and O-2 and significantly lower dissolved CH4, NH4 (+) and DRSi in pore water compared to bare sediments. At site D, despite an elevated nitrate availability, pore water NH4 (+) was the preferred N-source for V. spiralis uptake. Unvegetated sediments downstream the sewage plant (D-B) exhibited the lowest redox potential and CH4, NH4 (+), PO4 (3-) and Fe2+ accumulation in the pore water. Overall, results from this study suggest that early colonisation by rooted macrophytes affects pore water chemistry towards more oxidized conditions, along with nutrient retention within sediments, which are related to the uptake capacity and oxygen release by roots. They also suggest an elevated physiological plasticity for V. spiralis, as higher organic matter content and lower redox potential in downstream sediments did not affect its functions of ecosystem engineer as benthic metabolism regulator.