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

Abstract

Phosphoric acid labelled with 32P 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 of 32P exchange was between the microplankton, which typically held >90% of 32P, dissolved phosphate, and a mobile subcompartment of "dissolved organic" phosphorus. Phytoplankton >10 μm in diameter and zooplankton acquired 32P label very slowly and contained <5% of the radiotracer. In addition, about half of dissolved organic phosphorus failed to acquire a 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 of 32P 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% of 32P per day. Particles settled through the hypolimnion at a modal velocity of 28 cm∙d−1, losing 10% or less of their 32P during descent. Most of the released phosphorus was reincorporated into particles and sedimented. Less than 1% of the 32P 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 the 32P that sedimented.

Internal phosphorus loading in Canadian fresh waters: a critical review and data analysis

“…In experimentally eutrophied Lake 227 at the Experimental Lakes Area in northwestern Ontario, additions of radiotracers indicated that P tends to be retained by sediments, even under anoxic conditions. Less than 5% of newly sedimented P was remobilized over a 10 week period (Levine et al 1986). Over the longer term, high retention of P in the lake has continued, again indicative of low rates of internal P loading (Schindler et al 1987).…”

Recent advances in the understanding and management of eutrophication

“…On the other hand, lakes of the Precambrian Shield, where precipitation is normally the only external source of phosphorus, have very low internal loading of phosphorus, even under anoxic conditions (Schindler et al 1977;Levine et al 1986). …”

New and recycled primary production in an oligotrophic lake: Insights for summer phosphorus dynamics

“…This term, Pexport, is equivalent to the oceanographic term, new production (Eppley and Peterson 1979), and is the component of primary production that cannot bc supported by P regeneration within the system. Particle sinking is the dominant loss term of P from surface waters (Lawacz 1985;Levine et al 1986). Mass balance dictates that Pexpor, be equal to the loading of P to surface waters (Pi,,,,) plus change in standing stock of P (Pdelm; positive values of Pdclta denote a decrease in P standing stocks and a negative value denotes an increase).…”