Environmental effects of mussel farming in the Hauraki Gulf, New Zealand: A coupled physical-biogeochemical modelling approach.

The Hauraki Gulf ecosystem, and particularly the Firth of Thames, is one of the most productive marine ecosystems in New Zealand. The Firth of Thames, recognised as a highly significant marine conservation area, also supports some of New Zealand’s largest marine farms. Roughly a quarter of New Zealand’s mussel production occurs in the Hauraki Gulf with most of it concentrated in the Firth of Thames. A three-dimensional hydrodynamic model coupled to a biogeochemical model with the addition of a dynamic energy budget model for the greenshell mussel Perna canaliculus is used to investigate the environmental effects of mussel farms on the Firth of Thames and Hauraki Gulf ecosystems.

Consumption of particulate organic matter (phytoplankton, zooplankton, small and large detritus) by the mussels results in a decrease in chlorophyll concentrations in the immediate vicinity of the mussel farms whereas there is a slight increase throughout the rest of the Firth of Thames. Similarly, there is a decrease in zooplankton and detritus concentrations throughout most of the Firth of Thames, with the strongest decrease occurring in close proximity to the mussel farms. Ammonium excretion by the mussels and nitrification results in increased nitrate and ammonium concentrations around the mussel farms which, depending on the circulation, can extend throughout the Firth of Thames. These results suggest that the effect of mussel farming in the Firth of Thames is strongest in close proximity to the farms but it also extends throughout the Firth of Thames. The effects of mussel farming, however, does not appear to extend beyond the Firth of Thames into the Hauraki Gulf.