LAUSR

A major bloom of Chrysosporum ovalisporum occurred in ~2360km of the Murray–Edward–Wakool River System, Australia, during the 2016 austral summer and autumn. Several potential causes were investigated. Although summer air temperatures were among the hottest on record, no significant relationships were found between meteorological factors and bloom abundance. Instead, a weak but significant relationship was found between surface water temperature and bloom abundance downstream of Lake Hume, because the hot weather is likely to have led to higher water temperatures promoting bloom development. Releases of cold water inflows into Lake Hume from its Murray River arm may have stimulated upwelling of nutrient-rich deeper water, promoting the establishment of the bloom in that lake. Total nitrogen and total phosphorus concentrations were within the mesoeutrophic range and significantly related to bloom abundance, although some nutrient would be contained within cyanobacterial cells. During peak bloom abundance, pH was above 8.5 at one site. One unusual feature was that the bloom occurred in very dilute water, at electrical conductivities of 40–50 µScm–1. However, this does not appear to have affected carbon uptake and photosynthesis at high pH. Further blooms may occur if similar hot summers occur in future.