Over the past 50 years, phosphorus (P) has been added each year to Lake 227, making it the world’s longest experiment in P fertilization. Located in Canada’s Experimental Lakes Area, Lake 227 conclusively demonstrated that excess phosphate in lakes causes algal blooms, in turn leading to worldwide bans on phosphates in detergents, improvements in wastewater P removal, and reductions in fertilizer applications. A key question, however, is: Where did the P added to Lake 227 end up? This is where David O’Connell and Philippe Van Cappellen of the Ecohydrology Research Group, together with colleagues from Canada, Netherlands, and USA, turned to examining phosphorus in the sediments accumulating at the bottom of the lake. The results show that over 90% of all the P added to the lake since 1969 is retained in the sediments, mostly bound to organic matter, hence challenging the paradigm that phosphate binding to, and release from, minerals modulate the fate of P in lake sediments. Moreover, the organically-bound P is labile, meaning that it can be easily remobilized. This labile legacy P pool may potentially return phosphate back to the overlying waters and continue to fuel algal growth even decades after artificial fertilization is terminated. Sediment legacy P must therefore be recognized when assessing the effectiveness of reducing external P inputs to lakes to control algal blooms.
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