No evidence of Calanus reorganization across three Northeast Atlantic transects during 30 years of monitoring

Abstract Reports have suggested dramatic shifts in the calanoid assemblage of the Northeast Atlantic, with warm‐water species expanding northwards at the expense of cold‐water congeners. This is considered a serious threat to lipid‐fueled higher trophic levels, as warm‐water copepods are typically smaller with lower lipid content. Nevertheless, ecosystem dynamics may vary substantially among this vast region. Here, we analyze 30 years of abundance data for temperate Calanus helgolandicus and sub‐Arctic C. finmarchicus along three oceanographic transects in the Norwegian zone, spanning temperate (northern North Sea) to sub‐Arctic (southwestern Barents Sea) conditions. Contrary to expectations, we find no evidence of a copepod reorganization. Following a decline toward 2010, C. finmarchicus has returned to historical levels, while C. helgolandicus remains roughly an order of magnitude less abundant at all transects. Our results caution against extrapolation of regional plankton dynamics to the Northeast Atlantic, emphasizing the value of sustained, spatially distributed monitoring programs.