A Radiotracer Study of Phosphorus Cycling in a Eutrophic Canadian Shield Lake, Lake 227, Northwestern Ontario

Abstract

Phosphoric acid labelled with³²P was added to the epilimnion of experimentally eutrophied Lake 227, northwestern Ontario, in August 1978 to trace the lake's phosphorus cycle during late stratification and fall overturn. Radiophosphate was incorporated into bacteria and microphytoplankton (90% of³²P, dissolved phosphate, and a mobile subcompartment of "dissolved organic" phosphorus. Phytoplankton >10 μm in diameter and zooplankton acquired³²P label very slowly and contained 32P label over 17 d of incubation, indicating that this phosphorus may be functionally inert, at least at the time scale of biological phosphorus exchange. Movement of³²P to the littoral zone of Lake 227 was much slower than in lakes with well-developed macrophyte communities, but losses to deep sediments were similar, about 2% of³²P per day. Particles settled through the hypolimnion at a modal velocity of 28 cm∙d⁻¹, losing 10% or less of their³²P during descent. Most of the released phosphorus was reincorporated into particles and sedimented. Less than 1% of the³²P that entered the hypolimnion in particles between August and October remained in the hypolimnion in late October.³²P release from Lake 227′s hypolimnetic sediments during late summer and fall of 1978 was 32P that sedimented.