NIWA was contracted by Property and Land Management Services (PALMS) LTD to assess phytoplankton depletion due to the extension of marine farms 8431, 8432 and 8433 in East Bay, Port Underwood. This was achieved by running a hydrodynamic model over a period of 100 days (11 April to 20 July 2005) and was validated by the associated ADCP data. Results revealed that the overall depletion footprint was very similar to the existing-farms case, which was expected given the modest increase in the total effective number of mussels from the extension.