Copepods are the most abundant and diverse group of marine zooplankton on Earth. They can be found in every part of the ocean where they constitute the main grazers of the phytoplankton and the most abundant secondary producers, meaning that they produce organic matter for their predators (e.g., sardines, anchovies and other small pelagic fishes) from the phytoplankton they eat. The copepods play a pivotal role in food-webs and biogeochemical processes such as the biological carbon pump. These tiny crustaceans exhibit an extraordinary diversity of morphologies, sizes (from less than 0.5mm to more than 2cm), life strategies and physiologies, and these parameters dictate their fitness and contribution to ecosystem functioning and biogeochemical processes. In other words, planktonic copepods show a high diversity of functional traits and studying the distribution of such functional diversity across species and ocean basins is crucial for understanding the large scale contribution of zooplankton to ecosystem functioning and services. However, studying the spatial distribution of copepods functional traits over large spatial scales has been historically challenging because information about copepod functional traits and their spatial distributions were scattered across the scientific literature and regions of the globe. As a result, we still lacked a clear picture of what copepods do and where they are in the ocean.
In a very recent study published in Journal of Biogeography, a group of scientists from ETH Zürich (Switzerland) compiled an exhaustive list of functional traits from the literature together with copepod field observations from various surveys to tackle this question. The authors retrieved information about the body size, the feeding habits, the spawning strategies and the quality of the nervous system for more than 340 copepod species spanning different lineages. This unprecedented compilation enabled them to cluster those species into 11 functional groups (FGs) that represent coherent bundles of species that have the same roles in marine ecosystems. This finding is key because it implies that the complexity of traits and functions ensured by a multitude of copepod species can actually be summarized through smaller and more parsimonious groups.
Then, the authors used their compilation of field copepod observations to build an ensemble of distribution models based on various types of statistical algorithms. These models allowed to describe how suitable the regions of the ocean are for the 11 copepod FGs (i.e. where these organisms are in the ocean), as a function of environmental parameters like temperature, phytoplankton biomass or seawater turbulence that describe the conditions of the ocean. This way, Benedetti and colleagues could show that FGs composed of rather small, passive- or active-feeding and carnivorous copepods occur primarily in tropical areas, whereas large-bodied active-feeding herbivorous copepods occur more often in colder and more productive regions such as the high latitudes.
The statistical distribution models were also used to map the spatial distribution of traits values instead of the distribution of the FGs. This analysis revealed that regions of the ocean that share similar traits expression within copepod communities (e.g., median body size or the relative proportions active feeders to passive feeders) showed boundaries that matched the position of known oceanic currents like the Gulf Stream or the Kuroshio. This implies that the biogeography of copepod traits is driven by large-scale ocean features and by the changes in conditions that come with them (i.e., the « seascape »). This study is a first step towards a better understanding of the link between the different roles played by marine zooplankton and the functioning of the ecosystems they live in.
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