<p>Cyanobacterial harmful algal blooms (HABs) are a growing challenge for freshwater management, traditionally addressed through phosphorus (P) limitation strategies. However, increasing evidence highlights nitrogen (N) availability as a key driver of bloom initiation and persistence. Here, we report extracellular voltage recordings from Oscillatoria cohorts exposed to four days of N starvation followed by ammonium (NH₄⁺) repletion using 5&#xa0;mg L⁻¹. Under N deprivation, electrical signalling developed progressively with a median of 1–2 events min⁻¹ and amplitudes between 3-4&#xa0;µV on day 1 and 2, following a marked increase in activity by days 3 and 4, with median spike rates of 5 events min⁻¹ and amplitudes up to 17&#xa0;µV. Following NH₄⁺ repletion, signalling activity declined within 24&#xa0;h, indicating rapid restoration of N balance. These results suggest that electrophysiological monitoring can serve as a real-time, non-invasive indicator of nutrient stress in Oscillatoria and provides a potential early warning tool for HAB onset and associated taste and odour (T&amp;O) outbreaks in freshwater systems.</p> Graphical abstract <p></p>