Seasonal change in urea decomposition by epiphytic and planktonic algae in a reed zone of Lake Biwa, Japan

A dense biomass of emergent and submerged macrophytes and associated epiphytic microorganisms on submerged stems are often observed in shallow near-shore areas. Epiphytic microorganisms contribute to biogeochemical cycling in macrophyte zones. In Lake Biwa, Phragmites communis covers an area of3 km (including lagoons), which comprises 60% o f submerged macrophytes. Throughout the year, Phragmites has a large substratum for epiphytic microbial colonization in the reed zone. In the littoral zone, the standing crop o f these epiphytic algae is generally larger than that of phytoplankton. The reed zone in Lake Biwa is an important subecosystem for the study ofbiogeochemical dynamics in a lake ecosystem. The contribution of the epiphytic microorganisms to the biogeochemical cycling is related to the available surface area of reed stems as the epiphytic substratum. The measurement of epiphytic primary production on reed stems has been studied by several investigators. Some available biological and chemical data on the reed zones of Lake Biwa has been reported (TANIMIZU et al. 1981, ÜHTSUKA et al. 1996, MITAMURA & TACHIBANA 1999, MITO et al. 2002, MITAMURA et al. 2005). There is, however, little information on the biogeochemical cycling in the reed zone. Much information has been accumulated to elucidate the distribution and metabolism o f urea in natural water bodies because of its significance as an important nitrogen source for phytoplankton and the appreciation o f its ro le in the biogeochemical nitrogen cycle. Urea decomposition by natural phytoplankton has been demonstrated using e labeled urea (MITAMURA & SAuo 1986, IRMISCH 1991, MITAMURA et al. 1994, 1995, 2000, 2005). Previous workers reported that urea in the pelagic waters was presumably decomposed more effectively by phytoplankton than by bacteria. Knowledge of urea decomposition by epiphytic microorganisms, however, is quite limited. The purpose of this study was to provide information on the contribution of epiphytic algae to the urea decomposition in a shallow littoral area of Lake Biwa.