SUMMARY1. Oligotrophic Lake Waikaremoana, New Zealand, is used for hydroelectric power generation and the lake levels are manipulated within an operating range of 3 m. There was concern that rapidly changing water levels adversely affected the littoral zone by decreasing light availability in two ways: local turbidity caused by shoreline erosion at low water levels; and decreased light penetration to the deep littoral zone caused by high water levels in summer.2. The littoral zone was dominated by native aquatic plants with vascular species to 6 m and a characean meadow below this to 16 m. The biomass and heights of the communities in the depth zone 0–6 m were reduced at a site exposed to wave action relative to those at a sheltered site. However, the community structure below 6 m was similar at exposed and sheltered sites. The lower boundary of the littoral zone was sharply delimited at 16 m and this bottom boundary remained constant throughout the year despite large seasonal changes in solar radiation and the 3 m variation in lake level.3. There was evidence that the deep‐water community consisting ofChara corallinahad adapted physiologically to low‐light conditions. Net light saturated photosynthesis (CO2exchange) per unit chlorophylla(Chla) was reduced to 1.7 μg C (μg Chla)−1h−1at the lower boundary, half of that recorded at 5 m. The concentration of Chiaper gram of biomass (dry weight), was considerably greater at the lower boundary than higher in the profile [c. 7 mg Chla(g dry wt)−1at 16 m vs. 4 mg Chla(g dry wt)−1at 5 m]. Chlbalso increased with depth and there was no change in the ratio of Chlaand Chlbwith increasing depth. The saturation light intensity (Ik) of the community at the lower boundary was only 78 μmol photons m−2s−1. Photosynthetic parameters (Ikand α) as well as the Chlacontent remained relatively constant throughout the seasonal and short‐term changes in radiation.4. The photosynthetic characteristics of the littoral community were therefore not greatly affected by the lake level change caused by the present hydroelectric operations. However, the sharpness of the lower boundary and its extreme shade characteristics imply that the deep‐water community would be sensitive to any further changes in unde