Cold Climate Phosphorus Uptake by Submerged Aquatic Plants in a Sewage Treatment Free Water Surface Wetland

Abstract

In November 2002, biomass phosphorus (P) of submerged aquatic plants with associated epiphyton was measured using P-32 tracer addition in a treatment wetland receiving tertiary treated municipal sewage. The wetland is situated 120 km west of Stockholm, Sweden receiving tertiary treated municipal sewage. During the experiment, inflow water had a total P concentration of 0.3 and an iron concentration of 0.7 mg l^{− 1} with a residence time of 3–3.5 days. Samples of submerged plants mainly slender waterweed (Elodea nuttallii,L.) and sago pondweed (Potamogeton pectinatus, L.) were taken in two shallow areas (0.21 and 0.51 ha respectively) adjacent to the inlet of one of the wetland subbasins. The sampled area represented 27% of the total 2.6 ha area of the basin. The young phytomass contained 0.02kg P and the older vegetation parts and epiphyton 0.04 kg P. The sampling took place 90–96 hours after exposure of a pulse of P-32 (18 GBq), i.e. when 75% had passed. Total phytomass was low, only 2 g m^{− 2} as dry weight. The young phytomass close to the site of injection had higher average beta activity in disintegrations per minute and milligram dry weight (140 DPM mg dw^{− 1}) than the older parts of the submerged plants (70 DPM mg dw^{− 1}) indicating an active plant uptake in the former. The latter was interpreted as epiphyton uptake, but since the biomass of old shoots was higher than of young, more P-32 was removed by processes associated with epiphyton as was removed by the young phytomass. In two shallow central zones, the former was equal to 0.006 ‰ of exposed radioactivity in water, while 0.003‰was found in the young phytomass in the same zones of the basin. Close to the inlet a fast P assimilation by the plant biomass was demonstrated. The role of submerged aquatic plants in phosphorus turnover in sewage treatment basins under winter conditions is discussed.

Key words:

• Aquatic plants
• Phytomass
• Epiphyton
• Phosphorus
• P-32
• Sewage treatment.

ACKNOWLEDGMENT

Our sincere thanks to Håkan Johansson, Anna Lindahl and Anders Wörman, Division of Biometrics and Statistics at the Swedish University of Agricultural Sciences for fruitful cooperation during the tracer experiment. Special thanks to Jonny Darabánt who carried out the vegetation study. Thanks are also due to Roland Alsbro at Eskilstuna Energy & Environment, for help during the investigation. Financial support was given by the Swedish Water Management Program VASTRA, financed by the Swedish Foundation for Strategic Environmental Research (MISTRA).