Blue versus green ocean

Ulf Riebesell by Ulf Riebesell on 21.11.2018

The equivalent of desert on land is the blue ocean in the sea. We love its turquoise to deep blue colours, but for marine organisms it means life at the minimum. The beautiful shades of blue signal crystal clear water with low densities of plankton creatures. This is because the water is void of life-supporting inorganic nutrients, needed by the phytoplankton to grow and nourish the marine food web. In the absence of a dense plankton community, the process painting the ocean blue works at its fullest: The clear water absorbs long-wavelength colours at the red end of the light spectrum. The remaining light that we see is then mostly made up of blue.

What happens when deep-water nutrients are supplied to the hungry plankton community at the surface can be seen in our mesocosms, particularly in those at the high end of simulated upwelling. The water turns from blue to green, indicating that the phytoplankton is having a feast. How much of the suddenly abundant food is eaten by the zooplankton and is thereby channelled higher up the food web is a key question of our study. The efficiency of this trophic transfer very much depends on the time it takes the diverse community of grazers to grow up in numbers and keep pace with the exponentially multiplying phytoplankton. If the zooplankton develops fast enough to keep the phytoplankton in check, we call it a match. If the phytoplankton outgrows the zooplankton, it’s a mismatch. Make your own bet whether our simulated upwelling is going to trigger a match or a mismatch.

Blue vs green ocean View into M1, the mesocosm with the highest deep water addition (Photo: Joaquin Ortiz)