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 (<10 μm in diameter) within minutes of introduction. Thereafter, the bulk of32P exchange was between the microplankton, which typically held >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 <5% of the radiotracer. In addition, about half of dissolved organic phosphorus failed to acquire a32P 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 <5% of the32P that sedimented.