Algal bioassay of the water from two linked but contrasting Welsh lakes

A comparison of the potential water fertility of two linked lakes, Llyn Peris and Llyn Padarn, was carried out over a year using Asterionella formosa as the test organism in batch cultures. In unenriched lake water, growth was more rapid in Padarn water during autumn‐winter‐spring, but there was no significant difference between growth in water from the two lakes during the period of thermal stratification. Mean growth rates in treatments with additions of nitrogen and chelated iron were not significantly different from unenriched water samples, both between treatments and lakes during thermal stratification. Growth in water from both lakes was increased greatly by addition of phosphate. Maximum growth rates occurred on addition of nitrogen, phosphate and chelated iron, and there was no significant difference between growth rates in this treatment for the two lakes throughout the year. Comparisons of mean growth rates in bioassays by Tukey's interval estimate showed significant differences between other treatments and the two lakes. The potential fertility of water samples from the two lakes differs more during mixed water conditions than in the period of thermal stratification. Inhibition of A. formosa growth was recorded four times in unenriched filtered water from Peris, once on addition of nitrogen and once on addition of iron. Inhibition was alleviated by the addition of nutrients. The addition of diluted sewage effluent which enters the drainage system between the two lakes, resulted in similar rates of growth of A. formosa in water from both lakes. Results of the bioassays are discussed in relation to differences in phytoplankton between the two lakes. Although A. formosa will grow in samples of unenriched lake water from Peris, other features, including a shorter retention time of water, lower level of incident radiation and, occasionally, inhibitory substances in the lake water, contribute to the rare occurrence of this diatom in Peris. Padarn is enriched by sewage effluent which aids the growth of A. formosa.