Effects of water temperature and nutrients on the winter blooming diatom Stephanodiscus hantzschii (Bacillariophyceae): Implications for the blooming mechanism

Abstract

In order to determine blooming mechanisms of freshwater diatom, Stephanodiscus hantzschii, we investigated the relationships between fluctuations in environmental factors and blooms of the diatom by bi-weekly monitoring from February 2001 to February 2002 and from February 2004 to February 2005, in the eutrophic lower Han River (LHR). Using short-term batch culture methods, the physiological responses of the diatom were examined to the limitation and then reintroduction of nutrients (silicon, phosphorus, and nitrogen) with various temperature ranges of 5°C to 20°C. In field monitoring, with the exception of summer the rainy season when nutrients were lower, diatoms were abundant. Initial development of S. hantzschii populations (the most dominant diatom species in the LHR) started in late autumn, peaked in winter to early spring, and then gradually decreased in mid-spring. The peak of S. hantzschii abundance occurred at low water temperatures (approximately 5-10°C) and caused a significant decrease in silicon concentration, whereas other nutrients remained at eutrophic levels. In the culture experiment, the maximum growth of S. hantzschii was highest at 10°C. The growth of S. hantzschii under phosphorus limitation did not differ from that of the control group, but the diatom had high requirements for nitrogen and silicon for growth. After the reintroduction of nutrients, the maximum growth rate was highest for diatoms to which silicon had been reintroduced and second highest after the reintroduction of nitrogen. The dynamics of S. hantzschii under the perennially eutrophic conditions of the LHR appears to be primarily affected by changes in water temperature, but growth of the diatom may be restricted by depletion of silicon. Therefore, The results suggest that silicon is more important than other potentially limiting factors with high temperature over 15°C.