QUANTIFICATION OF LONG-TERM CHANGES IN THE GERMAN BIGHT USING AN ECOLOGICAL DEVELOPMENT INDEX

During recent decades, remarkable long-term changes have taken place i n many coastal ecosystems of north-west European shelf seas. These cha nges have been described in many papers, but an overall quantification is lacking. An ecological development index (EDI) is proposed here, c onsisting of two parts - a statistical description of the state of the ecosystem and a metric in a multi-dimensional space of statistical de scriptors of states for quantitatively characterizing the change of an y number of state variables by only one characteristic number. The con cept of the EDI is generally applicable by yielding a measure of the d istance between system states. Indices are comparable between differen t ecosystems and within different subspaces of the ecosystem's state s pace, provided the same characterization is used. The concept is appli ed to the 32-year time series (1962-1993) of measurements of nine phys ical (sea-surface temperature, salinity), chemical (phosphate, nitrate , nitrite, ammonia, silicate), and biological (biomass of diatoms and flagellates) state variables at Helgoland. The analysis of consecutive 5-year quantiles of monthly mean values, which are used as statistica l descriptors, elucidates the long-term perturbations of the coastal m arine system in the German Bight. It is shown that the states of the e cosystem in the 1970s and 1980s have abandoned the region of the 1960s , but are possibly returning in the 1990s to the situation of the 1970 s. (C) 1998 International Council for the Exploration of the Sea.