Seasonal variation in phosphorus removal processes within reed beds--mass balance investigations

The phosphorus (P) removal processes in two pairs of High and Low Loaded reed beds were investigated during five periods within a 27-month study. The uptake/release of P was measured in seven mass balance compartments. With the exception of the first year of operation, the reed beds consistently removed over 96% of the influent P load, with total phosphorus (TP) concentrations being reduced from 0.5 mg/L to generally less than 0.005 mg/L across the range of loading rates and seasons studied. During the first year, uptake by Phragmites australis accounted for greater than 75% of P removed, and was equally distributed between above and below-ground biomass. During the second and third years, three seasonal stages were identified in the uptake and cycling of P by P. australis. A period of rapid above-ground growth and uptake occurred during spring fuelled partly by P reserves accumulated in rhizomes during the previous year. During summer, uptake by above-ground biomass was governed by the influent P loading rate, while the amount of P held in below-ground biomass remained relatively stable. During autumn and winter, P appeared to be translocated from senescent shoots to reserves in the rhizomes. Approximately 85% of the below-ground biomass P occurred in the top 20 cm of the substrate. Gravel fixation increased in importance from 12% in the first year to approximately 30% of P removed in the second year, with a highly significant correlation between the influent P loading rate and P fixed by the gravel. The weakly-bound P fraction from a sequential extraction was the dominant form of P fixed by the gravel. HCI extracts were inappropriate for the examination of sorption processes as they dissolved large amounts of mineral P from within the basaltic gravel. The bottom 30 cm of the substrate became the most important site for gravel fixation during the second year. Incorporation of P into the detritus/microbiota/other compartment increased after the first year to become one of the most important P removal processes, probably consisting mainly of leaf litter and slowly accreted organic sediments.