Water column stability and Calanus finmarchicus

Many authors have suggested that the abundance of the subarctic species Calanus finmarchicus can be influenced by the structure of the water column. Unfortunately, to date, such a link has never been confirmed either experimentally or statistically. By using a macroecological approach, we investigated this hypothesis and showed that it varies with the developmental stage of the species. First, we implemented a new statistical procedure, based on an exponentially weighted moving average, to identify and quantify the depth and intensity of the thermocline. We applied the technique on 1 005 619 temperature profiles over the North Atlantic Ocean and provide a mapping of these two descriptors at a seasonal scale. Second, we studied the relationships between the depth and the intensity of the thermocline and C. finmarchicus using a biological data set of 99 599 sampling stations. Our results suggested that the characteristics of the water column influence the spatial distribution of C. finmarchicus. The frequency in the occurrence of this species decreases when stratification rises. Our results further revealed that the effect is more pronounced for young copepodite stages. Such findings are of interest, since, in a warmer world, water stratification is expected to increase, making more likely a reduction in the abundance of this key species in the North Atlantic.