| Summary: | Clear Lake represents an economically significant mesotrophic lake located in southwest Manitoba. In an attempt to describe the biogeochemical factors and mechanisms controlling the cycling of phosphorus in this waterbody various physio-chemical and chemical attributes of the lake water, seston and sediments were measured. Within the water column clear seasonal summer stratification was evident. Hypolimnetic water remained oxic throughout the ice-free period and pH remained neutral to alkaline throughout the entire period of observation. Chemical analysis of the lake indicated the water is enriched in Ca²⁺ and Mg²⁺, likely as a result of the calcareous Cretaceous shale in the surrounding watershed. During summer stratification in 2008, large concurrent increases in dissolved oxygen, pH and CaCO₃ saturation index were observed to occur simultaneously with large reductions in total and dissolved P. This was interpreted as an incidence of biologically-mediated pH shift during an algal bloom leading to a precipitation of CaCO₃ in association with occluded organic and inorganic P.
Fractional P analysis, conducted on particulate seston and sediment, separated the total P (TP) into Organic P (OP), Non-Apatite Inorganic P (NAIP), and Apatite P (AP). Significant amounts of AP in the seston and sediment provide support for the interpretation that authigenic CaCO₃ scavenged P from the water column. Within the sediment cores, large and increasing amounts of AP indicate that all co-precipitated P is retained long term within the sediment as refractory AP.
The combination of efficient scavenging during co-precipitation events and long-term storage of P as AP in the sediments suggests that the calcareous nature of the lake is playing an important role in the biogeochemical cycles.
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