INTERVIEW – François Carlotti is the leader of BioSWOT-Med’s WP4 “From zooplankton to higher trophic levels”, whose objective is to understand how fine scale oceanic structures may affect the patchiness of metazoan organisms from the smallest (zooplankton) to the largest (cetaceans). Here, he describes the hypothesis under test to explain the distribution of different zooplankton groups in different water masses.
THE RESEARCH THEMES – François Carlotti is a researcher at CNRS and works at the Mediterranean Institute of Oceanography. He is a biological oceanographer with strong interests in the structure and functioning of pelagic marine ecosystems, and their responses to climate forcing and anthropogenic impacts. In the BioSWOT-Med campaign he is the coordinator of WP4 “From zooplankton to higher trophic levels”.
What are your research interests besides BioSWOT-Med?
I am a biological oceanographer with strong interests in the structure and functioning of pelagic marine ecosystems, and their responses to climate forcing and anthropogenic impacts. My research focuses on zooplankton, which play a pivotal role in all pelagic food webs, and extends to adjacent trophic levels through research on bottom-up and top-down forcing processes. My main approaches are observation with different types of analyses of collected samples (zooplankton imaging, diversity, size structure, physiological processes) and in situ sensor data, and mathematical modelling. My main interests are (1) the importance of couplings between hydrodynamic processes, their biogeochemical functioning and the behaviour of organisms in the variability of zooplankton distributions; (2) the contribution of zooplankton in trophic and biogeochemical fluxes within ecosystems. This corresponds to the research lines of the international programme Future Oceans – IMBeR. My research is carried out from the polar regions to the tropics and the Mediterranean, and from the open sea to the coastal domain. Over the last ten years, I have participated in the main biogeochemical campaigns in the Mediterranean as part of the MISTRALS-MERMEX programme.
In the BioSWOT-Med cruise you are the coordinator of WP 4 that studies from zooplankton to upper trophic levels. You have several research objectives, can you tell us about them?
Our WP 4 is entitled “From zooplankton to higher trophic levels”. Our overall goal in BioSWOT-Med is to understand how fine scale oceanic structures may affect the patchiness from metazoan organisms from the smallest (zooplankton) to the largest (cetaceans). In practice, we mainly focus on the first trophic levels (zooplankton and their planktivorous predators), but also note opportunistically if larger animals (cetaceans, basking sharks, … ) are present around.
This overall goal can be broken down into different scientific objectives:
– To characterize mesozooplankton and macrozooplankton/micronekton distributions in both horizontal and vertical dimensions (from meso- to fine- scales);
– To define structural and functional variations of zooplanktonic communities in water masses and within the frontal region;
– To estimate the impact of grazing of mesozooplankton on the phytoplankton community;
– To estimate mesozooplankton contribution in carbon fluxes;
In the BioSWOT-Med cruise you will take samples at different depths, at day and night. Why?
All of these metazoan organisms can move through the water column, and all of them over large distances relative to their size. Even zooplanktonic organisms (initially thought to be unable to escape from currents) have very strong vertical swimming capacities, especially as they migrate vertically between day and night. The main reason for this is that many of them descend during the day to the deep ocean (down to hundreds of meters) where it is darker to escape visual predators, but rise to the surface at night to feed on the phytoplankton and associated microorganisms that have grown there in the light during the day. This process is called diel vertical migration and is the largest animal migration on Earth. We will therefore try to understand how the mesoscale surface structures can impact or modulate these migrations, if they induce differences in the migration process depending on the species.
In addition to observations of changes in zooplankton distribution between day and night, we will measure the actual grazing of zooplankton between day and night and try to quantify the additional impact of migrating zooplankton on the phytoplankton stock and the associated microbial community at night through large mesocosm experiments.
One of your research hypotheses is that different groups of zooplankton species will be found in different water masses. Can you explain this?
Zooplanktonic organisms have very diverse diets. Even among copepods, a dominant crustacean group among zooplankton, there are herbivores, omnivores and carnivores. Copepods are selective feeders and thus the distribution, size, behaviour and biochemical quality of their prey (including phytoplankton and smaller zooplankton) will condition the selective process. As the nature and distribution of prey are themselves conditioned by the fine oceanic structures, it is expected that the zooplanktonic assemblages in these structures will be affected as well.
What is the hypothesis that could explain how fine scale structures impact zooplankton distribution?
Since the 1990s (GLOBEC International Program), observations have been accumulating showing that zooplankton develop particularly well in mesoscale structures. New physical and biological instrumentation (platforms and sensors) allow nowadays to explore this topic at scale smaller than the mesoscale. The observations made during the BioSWOT-Med campaign, in particular those from the Zooglider, will make it possible to confirm the links between fine-scale structures and zooplankton patchiness. The hypothesis that we want to test in connection with WP3 (Biogeochemistry and microbial dynamics) and WP5 (Plankton genomics) is that the structure of autotrophic and microbial communities stimulated by a fine-scale physical structure is reflected at the level of zooplankton consumers. Thus, at the mosaic of physical structures correspond mosaics of each of the first trophic levels.
Tosca Ballerini (firstname.lastname@example.org)