Long-term data are derived from the Continuous Plankton Recorder (CPR) survey that has been towed in oceanic waters in the northern part of the ecoregion. Data was first collected at a monthly resolution by the CPR survey in the 1960s to the early 1980s. After a decadal gap the route went into operation again around the Azores ecoregion from 1997 onwards (see Figure 10).
Long-term trends suggest that at the decadal scale zooplankton populations are mainly influenced by large-scale natural climate variations, such as the Atlantic Multidecadal Oscillation and the North Atlantic Oscillation. Phytoplankton trends for this region show a general increase in smaller phytoplankton, and a decrease in larger phytoplankton (e.g. large diatoms and dinoflagellates).
For zooplankton, the abundance of Euphausids and Chaetognaths have generally declined over the decadal period whereas the abundance of copepods has remained relatively stable. Of the main zooplankton community, the Appendicularians (larvaceans) have shown the largest increase in abundance over the last 50 years; this may mirror the change in size structure seen in the phytoplankton.
Figure 10: Sampling effort and long-term monthly zooplankton data from the Continuous Plankton Recorder (SAHFOS) survey north of the Azores Islands (standard CPR regions F7/F6)