Lake Rotoiti, North Island, New Zealand, is a deep (maximum depth, 126 m) lake surrounded by a mostly forested and farmland catchment. However, it currently demonstrates several attributes characteristic of enriched waters. Algal production is high relative to other deep basin lakes of the central North Island. The hypolimnion deoxygenates completely each year, and a bloom‐forming species of blue‐green algae is numerically important in the summer phytoplankton. These features appear to be recent rather than longstanding properties of the Lake Rotoiti ecosystem. A detailed comparison of limnologieal measurements from 1955 onwards reveals a consistent decline in water clarity, a large and possibly exponential increase in hypolimnetic oxygen demand, and a conspicuous shift in the summer phytoplankton community towards blue‐green algal dominance. This recently accelerated change in the trophic status of Lake Rotoiti is probably a response to the Ohau Channel inflow from Lake Rotorua.