Macroecology of Calanus finmarchicus and C. helgolandicus in the North Atlantic Ocean and adjacent seas

Abstract

Global climate change is expected to modify the spatial distribution of marine organisms.
\nHowever, projections of future changes should be based on robust information on the ecological
\nniche of species. This paper presents a macroecological study of the environmental tolerance and
\necological niche (sensu Hutchinson 1957, i.e. the field of tolerance of a species to the principal factors
\nof its environment) of Calanus finmarchicus and C. helgolandicus in the North Atlantic Ocean and
\nadjacent seas. Biological data were collected by the Continuous Plankton Recorder (CPR) Survey,
\nwhich samples plankton in the North Atlantic and adjacent seas at a standard depth of 7 m. Eleven
\nparameters were chosen including bathymetry, temperature, salinity, nutrients, mixed-layer depth
\nand an index of turbulence compiled from wind data and chlorophyll a concentrations (used herein
\nas an index of available food). The environmental window and the optimum level were determined
\nfor both species and for each abiotic factor and chlorophyll concentration. The most important parameters
\nthat influenced abundance and spatial distribution were temperature and its correlates such
\nas oxygen and nutrients. Bathymetry and other water-column-related parameters also played an
\nimportant role. The ecological niche of C. finmarchicus was larger than that of C. helgolandicus and
\nboth niches were significantly separated. Our results have important implications in the context of
\nglobal climate change. As temperature (and to some extent stratification) is predicted to continue to
\nrise in the North Atlantic sector, changes in the spatial distribution of these 2 Calanus species can be
\nexpected. Application of this approach to the 1980s North Sea regime shift provides evidence that
\nchanges in sea temperature alone could have triggered the substantial and rapid changes identified
\nin the dynamic regimes of these ecosystems. C. finmarchicus appears to be a good indicator of the
\nAtlantic Polar Biome (mainly the Atlantic Subarctic and Arctic provinces) while C. helgolandicus is
\nan indicator of more temperate waters (Atlantic Westerly Winds Biome) in regions characterised by
\nmore pronounced spatial changes in bathymetry.