Quantification of long-term changes in the German Bight using an ecological development index

During recent decades, remarkable long-term changes have taken place in many coastal ecosystems of north-west European shelf seas. These changes have been described in many papers, but an overall quantification is lacking. An ecological development index (EDI) is proposed here, consisting of two parts - a statistical description of the state of the ecosystem and a metric in a multi-dimensional space of statistical descriptors of states for quantitatively characterizing the change of any number of state variables by only one characteristic number. The concept of the EDI is generally applicable by yielding a measure of the distance between system states. Indices are comparable between different ecosystems and within different subspaces of the ecosystem's state space, provided the same characterization is used. The concept is applied to the 32-year time series (1962-1993) of measurements of nine physical (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 statistical descriptors, elucidates the long-term perturbations of the coastal marine system in the German Bight. It is shown that the states of the ecosystem in the 1970s and 1980s have abandoned the region of the 1960s, but are possibly returning in the 1990s to the situation of the 1970s. (C) 1998 International Council for the Exploration of the Sea.