Surge (non-linear) uptake of ammonium, measured by incorporation of 15N, was investigated in three species of macroalgae (Ulva lactuca Linnaeus (Chlorophyta), Soliera robusta (Greville) Kylin (Rhodophyta) and Dictyota dichotoma (Hudson) Lamouroux (Phaeophyta)) from Kavaratti atoll (Lakshadweep, India). Addition of ammonium (up to 20 mmol L-1) led to pronounced uptake within 4–6 min, with the amount of ammonium taken up during surge phase (<4 min) accounting for from about half to 10 times that taken up during the remaining period of incubation (5-30 min). Amount of ammonium taken up during surge related linearly to the concentration of ammonium given. Surge uptake in dark was also substantial, averaging 80% of that in light. Capability for rapid uptake of pulses of ammonium released by heterotrophs during day or night could thus be an important mechanism of survival and proliferation of macroalgae in the N-impoverished atoll waters.