1. In a nitrogen (N)-limited river subject to Mediterranean summer drought hydrology, the colour of macroalgal proliferations changed with successional and seasonal changes in epiphyte assemblages. New growth of the dominant macroalga, Cladophora glomerata, was green, as were proliferations of Oedogonium, Mougeotia and Spirogyra, which did not become heavily colonized with diatoms. Green Cladophora turned yellow as Cladophora filaments became colonized by diatoms that were not N fixers, and turned rust-coloured as later-successional epiphyte assemblages became dominated by dense Epithemia turgida and E. sorex, which both contain N-fixing cyanobacterial endosymbionts.
2. The rate and composition of insect emergence from floating algal mats differed among proliferations of different colour. The rates of emergence (individuals day−1 500 cm−2) of nematoceran flies were three to 25 times greater from yellow or rusty-coloured Cladophora mats than from green Cladophora, Oedogonium or Mougeotia mats that had lower epiphyte densities. Biomass emergence from Cladophora mats that were rusty in colour was eight to 10 times greater than from yellow Cladophora mats, because larger nematocerans dominated in rusty mats (Chironominae versus Ceratopogonidae in yellow mats).
3. Proliferations of Epithemia-infested Cladophora occur at and above drainage areas of about 100 km2 (channel widths of 25–30 m) in this river network, coinciding with the drainage area threshold where a step increase in concentration of total dissolved N is observed during summer.
4. In rivers under Mediterranean climate regimes, algal succession during the prolonged low flow season is less subject to stochastic interruption by spates than in rivers under more continental climates. Under these summer drought conditions, photogrammetric detection of colour changes in algal proliferations may help us track reach or basin-scale change in their ecological functions.