Phosphorus limitation of lotic periphyton growth rates: An intersite comparison using continuous‐flow troughs (Thompson River system, British Columbia)1

Abstract

Periphyton growth rates and relative degrees of phosphorus deficiency were compared with onsite, continuous‐flow troughs in three parts of the Thompson River system. Soluble reactive phosphorus (SRP) in the lower Thompson, North Thompson, and South Thompson Rivers averaged 3.4, 1.1, and 0.7 µ g liter−1. Several physiological and chemical composition parameters ranked the degree of P deficiency in the rivers in the same sequence as did SRP. Among these were alkaline phosphatase activity, Vmax for 32 PO 43− uptake, and cellular N:organic P, Chl a:ATP, C:ATP, and C:organic P. Specific growth rates (µ) estimated by biomass accrual and by 14 CO 2 uptake usually, but not always, indicated higher µ with greater availability of P. However, relative specific growth rates (µ:µmax) consistently reflected the influence of P limitation. As assessed from N:organic P and by application of the Droop and Goldman‐Carpenter equations, µ:µmax was 0.8–0.9 at the lower Thompson, 0.3–0.6 at the North Thompson, and 0.0–0.3 at the South Thompson sites. Hence, periphyton growth rates in the lower Thompson River were near the maximum set by temperature and light at ambient SRP of only 3–4 µ g liter−1. Evidence of P‐limited growth rates in the South Thompson and North Thompson Rivers was found at temperatures approaching 0°C.