Modelled surface chlorophyll for Hauraki Gulf. Chlorophyll can be used as an indicator as to how much phytoplankton are in the water. Nutrients that come out of the Firth of Themes act as food for phytoplankton and this can be seen as higher chlorophyll concentrations near the Firth of Themes. These chlorophyll blooms can be seen moving and changing directions as the ocean currents carry them around the Hauraki Gulf.