Variation among northeast Atlantic regions in the responses of zooplankton to climate change: Not all areas follow the same path

Broad scale climate forcing can interact with local environmental processes to affect the observed ecological phenomena. This causes potential problems of over-extrapolation for results from a limited number of sites or the averaging out of region-specific responses if data from too wide an area are combined. In this study, an area similar in extent to the Celtic Biscay Large Marine Ecosystem, but including off-shelf areas, was partitioned using clustering of satellite chlorophyll (chl-a) measurements. The resulting clusters were used to define areas over which to combine copepod data from the Continuous Plankton Recorder. Following filtering due to data limitations, nine regions were defined with sufficient records for analysis. These regions were consistent with known oceanographic structure in the study area. Off-shelf regions showed a progressively later timing in the seasonal peak of chl-a measurements moving northwards. Generalised additive models were used to estimate seasonal and multiannual signals in the adult and juvenile stages of Calanus finmarchicus, C. helgolandicus and the Paracalanus–Pseudocalanus group. Associations between variables (sea surface temperature (SST), phenology and annual abundance) differed among taxonomic groups, but even within taxonomic groups, relationships were not consistent across regions. For example, in the deep waters off Spain and Portugal the annual abundance of Calanus finmarchicus has a weak positive association with SST, in contrast to the pattern in most other regions. The regions defined in this study provide an objective basis for investigations into the long term dynamics of plankton populations and suggest suitable sub regions for deriving pelagic system indicators.

Research highlights
► The northeast Atlantic was divided into regions using satellite chlorophyll data.
► Inter regional variation was examined using Continuous Plankton Recorder data.
► Patterns of zooplankton abundance and phenology varied among regions.
► The regions represent a coherent scale for ecosystem evaluations and management.