The Science Behind Springwatch

Last Wednesday Pixalytics made it’s TV debut on the BBC2 Springwatch programme, where they showed a video we’d made on phytoplankton blooms.  The video was based on NASA MODIS-Aqua daily images. MODIS, or the Moderate-Resolution Imaging Spectroradiometer, is an optical sensor that’s used for mapping the both land and the oceans. It can be thought of as a digital camera that operates at a number of different wavelengths of light.

Spring 2014 phytoplankton image

Spring 2014 phytoplankton image, MODIS data from NASA with movie animation by Pixalytics Ltd.

As an ocean colour sensor it detects the change in colour of the ocean caused by what’s both dissolved and suspended in the water, e.g. the microscopic plants of the sea that are called phytoplankton. The chlorophyll pigments in plants (both on land and in the oceans) absorb light at blue and red wavelengths making waters high in phytoplankton appear green in colour. This colour change is picked up by chlorophyll algorithms (mathematical equations) and equated to changes in concentration that are displayed using a rainbow colour palette, which goes from purple to blue, green, yellow and red as the concentrations go from low to high values. Black on the imagery is where there’s no data, which for optical imagery is primarily due to cloud cover.

MODIS is on both the Aqua (travels south to north over the equator in the afternoon) and Terra (north to south across the equator in the morning) satellites, which orbit the Earth several times a day collecting strips of imagery 2330 km wide at a spatial resolution of around 1 km. The strips from a day are combined to create a daily composite image, and by looking at images over time we can see the changes in the phytoplankton concentrations as we as we move out of the winter through months into spring. The ‘spring bloom’ is an increase in phytoplankton concentrations as the days become lighter and the phytoplankton make use of the nutrients mixed into the surface waters over the winter.