What is Sea Colour?
The "colour" of the sea is determined by the interactions of light with the water. The substances in seawater which most affect the colour are, phytoplankton, inorganic particles, dissolved organic chemicals, and the water itself. Satellites using instruments that are more sensitive than the human eye, can measure a wide array of colours, which reveal the presence of varying amounts of phytoplankton, sediments, and dissolved organic chemicals.

What are phytoplankton?
Phytoplankton are the microscopic plant life that float in the lighted surface waters and form the base of the marine food chain. This plant life is the primary food and energy source of the ocean’s ecosystem. When a great number of the microscopic plants are concentrated in an area, the colour of the sea surface will change. This is called a "bloom." Phytoplankton convert nutrients into plant material by using sunlight with the help of the green pigment chlorophyll. The chlorophyll pigments in the plants absorb light, and the plants themselves scatter light. Together, these processes change the colour of the sea. Very productive water with a lot of plankton appears blue-green. Very pure (barren) water appears deep-blue, almost black.

The connection between sea colour and phytoplankton
When wind-driven surface currents carry water away from continents or when underwater obstacles disrupt circulation patterns, an upwelling of deep, nutrient-rich sea water occurs. These cold waters have high concentrations of nutrients and when they mix with warm surface waters, phytoplankton grow rapidly. Because they form the basis of the food chain, phytoplankton blooms attract feed fish which, in turn, draw fish species such as tuna, marlin, and other species which are the target of sport and commercial fishermen. If phytoplankton blooms are occurring in a location, the fish species of interest to commercial and sport fishermen are not far away. Also because phytoplankton drift with the water, patterns in sea colour can also be used to study sea currents.

By using sea colour images to locate areas rich in phytoplankton you can work out where the fish are feeding.

The sea colour images are processed estimates of the average chlorophyll concentration in each 500 metre by 500 metre area of ocean. Chlorophyll is the active ingredient in the phytoplankton.

Sea colour information is only available from the Terra and Aqua satellites and only from day-time passes (because even satellites have trouble seeing colour In the dark!). So in practice this means that at most there will be 2 sea colour datasets per day over each part of the Australian coast. Some days there may be none at all.

Selecting a Sea Colour Image
When a sea colour image exists for an area there will be an additional button to the right of the thumbnail titled Sea Colour. To select and view this data set you use the same techniques as you would use for looking at sea surface temperatures i.e you click on either the thumbnail, the Sea Temperature button or the Sea Colour button. These are explained by clicking here or on Selecting and Viewing a Data Set section in the column to the left.

 Sea Temperature Image                               Sea Colour Image

For daylight Terra and Aqua satellite passes, where both sea colour and SST data are available, you can now easily view both charts. When 2 charts are available a slide will appear down the left of your window (see the images above). To progress from one view to the other all you need to do is, left button click on the red dot, and whilst holding down the button, slide it either up or down to fade from chart to the other. This allows you to quickly see what is happening with regard sea colour under those very interesting temperature gradients and vice versa. If no slide appears then this means there is no corresponding sea colour chart available for that satellite pass.

The two images above have been generated by just sliding the red button on the side from SST to Sea Colour. You can see some high chlorophyll concentrations close inshore off Port Stephens and inshore of those temperature gradients off Lake Macquarie.

Relationship between sea colour and sea surface temperature
Remember, in the sea colour images green areas contain the most life, while the blue areas are nearly empty of life. It is as simple as: more food and the right temperatures = more fish.