The exclusion experiment to measure zooplankton grazing
During BioSWOT-Med, Alice della Penna is in charge of studying zooplankton grazing. She explains what it is, why it is important to measure it and describes a new 'exclusion experiment' to study it during the campaign.
Alice della Penna - Credits: Nicole Estaphan.
OCEANOGRAPHERS' INSTRUMENTS - Alice della Penna is a biological oceanographer currently working as a lecturer at Waipapa Taumata Rau University in Auckland, New Zealand. Her research focuses on how ocean currents affect marine life, from microbes to large animals such as sharks, seabirds and marine mammals. During the BioSWOT-Med campaign, she will be studying the grazing of zooplankton.
First of all, what is zooplankton grazing?
Zooplankton grazing is the process by which zooplankton (an extremely diverse group of floating animals that can be microscopic or as large as jellyfish) feed on phytoplankton (floating microalgae). One of the aspects that interests me more and more is not only how currents affect the distribution of marine organisms themselves, but also how features such as eddies, meanders and fronts can have an impact on the relationships between organisms (e.g. grazing and predation).
Why is it important to measure it?
Phytoplankton play a key role in marine ecosystems and biogeochemical cycles. However, their role is strongly influenced by what happens to them. For example, if phytoplankton are grazed by zooplankton migrating through the water column, the carbon that makes up the phytoplankton will be transported away from the ocean surface and 'exported' to the deep ocean. This is an important component of the carbon cycle, but it is very difficult to quantify, and it is particularly difficult to assess how it varies in space and time. It is also a mechanism that, to my knowledge, is not well represented in climate models.
During the BioSWOT-Med campaign, you will be carrying out an 'exclusion experiment' to measure zooplankton grazing. Can you explain what this is and how it works?
There are many methods for measuring zooplankton grazing, focusing on different types of zooplankton of different sizes. My objective for the BioSWOT-Med campaign is to try out a new method for estimating grazing by zooplankton migrating vertically on phytoplankton living in the epipelagic (the upper layer of the ocean that receives sunlight during the day).
The main idea is to have two mesocosms (two very large aquariums aboard the R/V L'Atalante) and to monitor how the plankton community they contain changes during the night. The 'diurnal mesocosm' will contain the phytoplankton and zooplankton that live permanently in the epipelagic ocean, while the other, the 'nocturnal mesocosm', will contain the organisms that live in the epipelagic during the day and the zooplankton that migrate to the epipelagic when the sun sets from the depths of the ocean. Consequently, the migratory animals whose grazing we want to estimate are excluded from the "diurnal mesocosm" but not from the "nocturnal mesocosm". We will then monitor these two mesocosms at night and study the differences we obtain between the two. Given that everything else should be the same in the two mesocosms (for example, the physiological cycles of phytoplankton, the digestion of zooplankton, which is influenced by sunlight, etc.), we think that the differences between the abundance and size of phytoplankton in the two mesocosms will be the result of grazing by migrating zooplankton.
More information