Food quality: what is "good" and "bad" in the oceans?

The balance between nutrients such as nitrogen and phosphorus changes constantly - on the one hand because of algal growth and bacterial remineralisation of dead biomass, on the other hand because of increased runoffs linked to industrial and agricultural activities. This alteration of nutrient ratios is also reflected in the biochemical composition of phytoplankton. But do herbivorous zooplankton actually know which algal quality best suits their needs? Yes! Dinoflagellates and copepods, for example, choose "good" food. In laboratory grazing experiments, both model organisms were offered a mixture of phytoplankton with high and low nitrogen to phosphorus ratios. Dinoflagellates and nauplii selectively ingest phosphorus-rich algae to satisfy their high phosphorus requirements for growth, while older, slower-growing copepodites prefer nitrogen-rich prey. It remains however unknown how exactly zooplankton distinguish algal quality, this key issue is currently under investigation on Helgoland.

Phytoplankton cultures of different nutritional qualities (Photo: Nadia Maaroufi)