Time series of climate indices and of biomass, abundance, and species number of benthic macrofauna in the southern North Sea are related to each other to investigate the predictability of biological time series in presence of biological regime shifts in 1989/1990 and 2001/2002. The results indicate that a smooth biological regime shift occurred in 1989/1990 caused by positive climate feedback mechanisms. In this case, the benthic community structure remained predictable. In contrast, in 2001/2002 an abrupt biological regime shift caused by a climate regime shift occurred. Here became the biological time series inherently unpredictable.