Concentrations and Kinetics of Phosphorus Fractions Along the Trophic Gradient of Lake Memphremagog

Abstract

Published relationship between chlorophyll and total P concentrations show that the amount of chlorophyll developed per unit of total P increases as total P increases. If the availability of P changes with lake trophy, this should be reflected in the proportion of total P found in different analytical fractions. From June 1975-Sept. 1976, the concentrations of total P, soluble P, Pi, large MW P (> 5000 MW) and small MW P (< 400 MW) were measured at 6 sites along the P production gradient of Lake Memphremagog. Radioactive 32P-PO4 was used to follow short-term exchanges between available Pi and other fractions. Total P declined 3-fold among sites but the proportion of different fractions remained relatively constant, suggesting that any disproportionate loss from one compartment is made good by exchanges with the others. Soluble P formed 1/3 of total P, soluble reactive P formed 14% and PO4 8%. Gel filtration showed that .apprx. 40% of soluble P was associated with large MW P and .apprx. 40% with small MW P. Within this basic similarity was a trend to decreased PO4 concentrations and increased concentrations of soluble P and soluble unreactive P at more oligotrophic sites. Tracer experiments showed that all fractions exchanged some P with 32PO4; soluble fractions, escpecially large MW P, interacted more slowly with Pi than did particulate P. Faster Pi turnover suggested greater P deficiency in more oligotrophic sites. This deficiency may promote the formation of a refractory soluble P which may explain a part of the reduced ratio of chlorophyll to P when total P is reduced.