Lake Omapere, Northland, New Zealand, contributes significant inputs of freshwater to Hokianga Harbour, and is subject to toxic cyanobacterial blooms. During summer 2004, ADDA‐ELISA analysis indicated microcystins were present in Pacific oysters ( Crassostrea gigas ) in Hokianga Harbour, leading to farm closures. Once the bloom had abated, continued detection of low microcystin levels suggested toxin production may be from cya...