Water worlds: Ocean Chlorophyll levels

The ocean is the last frontier that has not been discovered by cartogram techniques before. As such, it was an inevitable step in my PhD research some years ago to test the creation of a gridded ocean cartogram, a cartogram that is limited to the extent of the world’s oceans (also linking nicely to my past research on coastal ecosystems).
Chlorophyll concentrations in the world’s oceans are important indicators for the presence of algae and other plant-like organisms that carry out photosynthesis. As such, phytoplankton (which contains the chlorophyll) is an essential element of the food chain in the seas as it provides the food for numerous animals. Variations and changes in the chlorophyll levels are also relevant for the study of the ecology of the sea. Changing chlorophyll levels can indicate changing sea temperatures and other conditions in the oceans that cover about 72 percent of the planet’s surface.

Gridded cartogram visualisation of Ocean Chlorophyll concentrations
(click for larger version)

In this visualisation, the data used was derived from NASA’s MODIS (MODerate resolution Imaging Spectroradiometer), which shows long-term chlorophyll concentration estimates in mg per cubic metres. The absolute concentration was then calculated for each of the equally-sized grid cells that cover the sea areas. These were resized according to the chlorophyll levels.

The visualisation therefore shows areas that have the highest density of algae and other photosynthetic organisms largest, while those regions with low concentrations disappear into the dark black areas where the grid cells merge.

The map also highlights the increasing levels towards many of the land areas. The land areas are omitted from the cartogram transformation and therefore appear as bizarrely distorted white islands in the sea. Investigating the changing grid patterns in more detail, higher levels can be seen along the west coasts of the Americas, as well as Africa, where rising cold water streams lift nutrients supporting phytoplankton growth from the ocean floor. Coastal upwellings also influence the high chlorophyll concentrations of the surface waters in the Baltic Sea and other zones along land areas where larger grid cells emerge. Meanwhile, the band along the Equator eastwards from the coast of South America is influenced by the easterly trade winds that also help the upwelling of deeper water layers.

A modified version of this story was published in the February 2015 issue of Geographical Magazine. The content on this page has been created by Benjamin Hennig. Please contact me for further details on the terms of use.

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