How to detect Regime Shifts and their causes in the European Seas?

During the late 1980s air and sea surface temperature increased in many European regional Seas (Baltic Sea, North Sea, Mediterranean Sea). This was accompanied by a longer growing season and by increases in phytoplankton biomass as well as changes in the zooplankton and fish communities. By many authors these changes are considered to represent regime shifts in the ecology of the Baltic Sea, the North Sea and the Mediterranean Sea. Further it is speculated that the regime shift in the Baltic Sea and the North Sea could be caused by a related sign change in the North Atlantic Oscillation index (NAO). The aim of this investigation is to inspect physical and ecosystem variables for trends and structural breakpoints in the time series using sound statistical methods that include confidence tests at the 5% error probability level. For this purpose we investigated a broad range of physical variables including air temperature, wind speed, sea surface temperature, ice cover, precipitation, oxygen as well as ecosystem variables such as phytoplankton biomass, zooplankton and several fish species from the different European regions. We found that most of these time series do exhibit a statistical significant linear trend. But tests for structural breakpoints in these time series reveal only for some investigated variables the existence of a breakpoint in the 70-80ties of the last century. In contradiction to the seemingly well established “regime shift” in the different Seas no clear breakpoint can be identified in many physical variables and also not in most ecosystem variables including fish. Finally also the proposed reason for the supposed ecological regime shift in the Baltic Sea and North Sea, the change of the NAO sign at around 1987, is not statistically significant. Therefore most physical and ecosystem time series data from the European Regional Seas are statistically best described by a linear trend and not by a regime shift.JRC.H.1-Water Resource