Cladophora glomerata and Stigeoclonium tenue dominate lowland urban creeks in the Yarra River basin of south-central Victoria. In Darebin and Merri Creeks, Cladophora produces extensive mats in summer and autumn, and is mostly replaced by Stigeoclonium in winter and spring. Although Stigeoclonium can grow all year round, it only outcompetes Cladophora in winter and spring, when air temperatures range between a maximum of < 15� C and a nightly minimum of < 10� C (water temperature < 15� C, usually about 10� C). The seasonal composition and abundance of these macroalgae depend on temperature and on the severity of, and time since, the last floods, and the effect that these factors have on interspecific competition. Features of the microhabitat (e.g. photon irradiance, substratum stability and composition, and mean flow rates) determine the range of these variations. The biomass of both macroalgae fluctuates widely; this is due mainly to floods, which can remove almost the entire standing crop. In off-seasons, both species are maintained by small resilient plants or protected populations (in culture, plants remain viable after up to 6 months in complete darkness). Filaments of Cladophora readily produce zoospores and new vegetative growth following dormancy. The prostrate thallus of Stigeoclonium initiates new erect filaments before zoospores are produced. An understanding of the large local and seasonal variations in macroalgal biomass is essential for biological monitoring programmes.
