Evidence from plankton for multi-annual variations of Atlantic inflow in the northwestern North Sea

Long-term variations in Atlantic plankton species in the North Sea were investigated in order to find support for the hypothesis of a reduced inflow of Atlantic water into the North Sea during the 1960s and 1970s. The material was derived from samples of the Continuous Plankton Recorder Survey during 1948–1996. Fourteen Atlantic plankton species were selected as potential indicators of Atlantic inflow to the North Sea. Principal Component Analysis allowed the classification of these species into a number of sub-groups, each of which showed a different variation over time. One of these groups, consisting of Candacia armata, Metridia lucens, and Tomopteris spp., showed temporal variations that were apparently related to changes in Atlantic inflow. This group was at a low level in the North Sea from 1965 to 1982, and it increased in subsequent years. These variations correspond in time and direction with observed changes in pelagic fish stocks in the area.