Didymosphenia geminata (Didymo) is a nuisance, bloom forming, invasive freshwater algae. Through an ecological survey I examined habitat associations and effects, identifying bloom preferences for stable, low nutrient rivers, often downstream of still waterbodies; with major observed changes to algal and invertebrate communities. Experimental examination of the functional significance of blooms, provided strong support for the hypothesis that blooms are an adaptive, plastic ecophysiological response to nutrient limitation, maximising internal mat cycling and phosphatase based uptake. Further experimentation identified Didymo absence from higher nutrient waterways, is explained by competition with other algae and suppression of the 'bloom' response. Research suggests Didymo is here to stay, but spread management and mitigation will minimise damage to New Zealand's waterways.