Evidence from algal bioassays of seasonal nutrient limitations in two English lakes

Abstract

Comparative laboratory bioassays using Asterionella formosa and Rhodomonas lacustris were performed on filtered water samples from the deep, base-poor Windermere South Basin and the shallow, base-rich Malham Tarn, to assess their potential fertility and to identify possible limiting nutrients. Phosphate appeared to be the major limiting element for both species throughout the year, except during the spring diatom maxima when Si usually becomes limiting. On most occasions chelated Fe increased the growth increments, particularly in combination with phosphate. In general, the bioassay results showed correspondence with the nutrient concentrations in the test waters, which showed low (<1 μg l-1) levels of soluble reactive phosphate during all or most of the year and depleted Si levels in late spring. -from Author