Phosphorus cycling in the settlement lagoon of a treatment wetland

The South Finger treatment wetland at Slimbridge, UK, was designed to treat water that has been impacted by the faeces of a dense population of waterfowl. The wetland system has been failing consistently in retaining phosphorus (P). It has been suggested that the settlement lagoon of the wetland is the cause for its failure regarding P, because the lagoon exports P in the summer months. The aim of this project was to understand the importance of the settlement lagoon in the overall budget of P, and to understand the mechanisms that result in such
behaviour. This was achieved by measuring the fluxes of P in and out of the lagoon, as well as measuring the fluxes through the sediment water interface and the consumption/release of P by water column process. Also, an exhaustive study of the chemistry of the pore waters and of the different species of P in the sediments was carried out.
The data showed that the role of the settlement lagoon in the loading of P of the treatment wetland is minimal. The sediments of the settlement lagoon release dissolved P in the early summer, but this flux is much smaller than the mass of P that enters the lagoon at the same time. The failure of the treatment wetland is not related to the settlement lagoon, which has been performing satisfactorily in retaining suspended solids and particulate P, but to the inadequate retention time of the reed beds. This in turn is related to the original design of the wetland system.
The source of the P that is released in the early summer is the bird faeces that accumulate at the bottom of the lagoon through the winter. The accumulated faeces are consumed rapidly in the early summer when temperature increases and
oxidisers are present in the pore waters the right conditions are present, releasing their P through the sediment water interface (SWI). This process commences in
the early spring, with the appearance of an algal bloom, accompanied by high levels of dissolved oxygen and the deposition of fresh algae onto the lagoon sediments. Biodegradable algae is consumed by aerobic respiration above the SWI at this time. The faeces, buried within the anaerobic sediments, are not consumed significantly at this time however, because temperatures are still too low. The bacterial activity within the sediments, during the early summer, is carried out mainly through iron and sulphate reduction. At some time between March and June, temperatures increased and the degradation of freshly deposited algae accelerates. This releases large quantities
of ammonium above the SWI, which triggers the combined process of nitrification-denitrification, with nitrate reaching deep into the sediments. The supply of nitrate into the sediments, accompanied by the increased temperatures, accelerates the consumption of the buried bird faeces and the release of their associated P through the SWI. By June, dissolved P is still released through the
SWI, although the consumption of the labile fraction of the bird faeces slows down the rate of release. A small fraction of the released P is precipitated as apatite within the sediments, without reaching the water column.