Research undertaken on the South Finger wetland demonstrated that the wetland has been exporting orthophosphate every year since 2005, and possibly since 1996. The source of the excess orthophosphate is the settlement lagoon, while the rest of the constructed wetland has been unable to reduce the excessive fluxes (Stratford et al, 2010 and Palmer-Felgate, 2011a and 2011b. Observations similar to those made at the settlement lagoon have been reported in shallow lakes. For example Lake Blankensee in Germany has an average depth of 1.2 meters, and the majority of the TP in the water column in summer was reported to be
generated within the lake (Ramm and Scheps 1997). Loch Leven in Scotland has a mean depth of 3.9 meters and an area of 13.3 km2. Water column SRP is depleted in spring due to the uptake of the spring algal blooms, while in late summer the peaks are produced by intense release from the sediments (Spears et al 2007). Further examples of shallow lakes that release phosphorus internally are given from lakes in Denmark (Søndergaard et al 2003), Sweden (Ryding 1985), Italy and the US, (Marsden 1989).
1.2.3.1 The settlement lagoon as the source of excess P
As discussed above, the settlement lagoon is the source of excess P within the South Finger wetland. P cycling in such shallow bodies of water has some
characteristics that differentiate them from deeper stratified lakes. Given the surface area to depth ratios of shallow lakes, the pool of P in the sediments is often more than 2 orders of magnitude larger than the pool of P in the water column. Therefore the levels of P in the latter depend largely on the fluxes through the SWI (Søndergaard et al 2003). Lake Ontario, in North America, has a mean depth of 89 meters, and the internal loading of P has been estimated to be 11% of the external load. On the other hand, in a number of shallow lakes in Sweden, the internal load of P is up to 4 times larger than all other external loads, averaged annually (Boström et al 1988a). The passage of P from the sediments into the layers in the water column where photosynthesis takes place is rapid and direct (Shaw and Prepas, 1990; Søndergaard et al 2003), therefore it has a critical impact on the primary productivity in the water column (Boström and Pettersson 1982).
The release of P from the sediments in shallow lakes happens mainly from aerobic sediments (Jensen et al 1992, Marsden 1989), and the rates of release from sediments in well oxygenated waters are often of the same magnitude as for anaerobic sediments (Boström et al 1988a). This disagrees with the classical model of Mortimer (1941) for deep lakes, but the inherent characteristics of shallow lakes help explain this:
freshly produced organic matter in shallow lakes reaches the sediment surface quickly and almost intact
the water column is subject to rapid changes in its physical and chemical conditions
resuspension events are frequent
Large amounts of freshly produced organic matter falls onto the sediments of shallow lakes before decomposing. This is a rich source of organic matter for sediment microorganisms, which have a significant role in the uptake, storage and release of P, as long as the oxygen levels and other oxidisers like nitrate are present (Søndergaard et al 2003). It has been reported that in shallow lakes, up to 50% of the primary production is mineralised in the bottom waters or in the
sediments (Caraco et al 1990). Release of P originated from the decomposition of algae on the sediments surface of Lake Grevelingen and from the Loosdrecht Lakes in the Netherlands (Marsden, 1989). Mass balance calculations indicated that the release of SRP occurred on the top few centimetres of sediments, and it has been suggested that the P loading of the lakes was caused by the
mineralisation of organic matter (Marsden, 1989). An additional effect of high concentrations of SRP near the sediment surface is that they lower the Fe:P ratios in those top layers, and it has been suggested that at Fe:P ratios below 15,
orthophosphate would cross the oxic layer into the water column without being adsorbed by iron minerals (Jensen et al 1992, and Ramm and Scheps 1997).
Sediments in shallow lakes are exposed to more heterogeneous conditions than in deep lakes, because of the combined effects of weather, the circulation of water, shelter from nearby trees, buildings and fauna (Boström et al 1988a). That heterogeneity causes the formation and destruction of micro chemical gradients in the sediments and in the pore waters, related to temperature, pH or levels of oxygen (Boström et al 1988a). The chemical gradients change rapidly and unpredictably, and loosely-bound P or P in solution in the pore waters are continuously recycled into and out of the water column (Boström and Pettersson 1982) and wide variations in the release of P can occur in shallow lakes
(Marsden 1989). Water column and sediment temperature in shallow lakes can span 20°C rapidly. This causes great variations in the rates of microbial activity and the releases of P associated with it (Sinke et al 1990). An increase in primary productivity, resulting from changing environmental conditions, causes an increase in pH (Sinke etal 1990) and higher pH trigger the release of
orthophosphate from sediments, by ion exchange between OH-and PO43-on the surfaces of Fe oxy-hydroxides (Jensen and Andersen, 1992). Although the water column of shallow lakes is usually well mixed and oxygenated (Søndergaard et al 2003), it has been reported that under special weather conditions in summer, anoxia can set in the top millimetres of the sediments (Shaw and Prepas, 1990), or that even the water column can become anoxic overnight during warm and calm weather (Correll 1998). When this occurs, P stored in the oxic top
millimetres of the sediments can be liberated as Fe minerals are chemically and microbially reduced (Søndergaard et al, 2003).
It has been reported that in shallow lakes, the top 10 cm of sediments can be subject to disturbance and mixing by water turbulence (Forsberg 1989), and that the release of dissolved P from pore waters after the resuspension of sediments or the mineralisation of resuspended sediments is significant (Boström et al 1988b). Also, the constant turbulence in the water column maintains steep concentration gradients between the SWI and the overlying water, accelerating the process of molecular diffusion between the two phases (Forsberg 1989). A shallow lake in Denmark showed increases of between 5 to 10 times the background levels of TP in the water column within days of high wind events (Søndergaard et al 2003). The same authors discussed that yearly variations in the internal P loading in other shallow lakes were controlled by wind mixing (Søndergaard et al 2003). Internal P loading of Lake Blankensee in Germany was also shown to be the result, in part, of disturbance of sediments by water turbulence (Ramm and Scheps 1997).