Ecosystem time-series analyses in the Black Sea

Human-induced factors such as eutrophication, heavy fishing, and the introduction of exotic species have been evoked to explain the Black Sea ecosystem changes in recent decades. However, having recognized that natural factors are responsible for the basic physical, chemical and biological processes in the sea several authors have recently explored possible explanations based on natural abiotic and biotic factors. To elucidate the causes of the ecosystem changes, several questions must be asked, e.g. what are the relative roles of the natural and anthropogenic factors (including fishing pressure) for the observed patterns?, how do the different scales of temporal change - seasonal, annual, decadal - affect productivity?, are natural populations predominantly controlled by resource availability (bottom-up) or by predation /competition (top-down) factors; does the recent change in the B1ack Sea ecosystem result from transition between favorable and unfavorable environmental regimes? In order to address some of these questions, the present paper explores the synergistic nature of the change by analysing multiple time-series of physical, biological and anthropogenic data. It attempts to identify long-term patterns, providing possible explanations for the changes, which occurred over the past few decades. The study explores different temporal scales: multidecadal, decadal, interannual and seasonal variations, revealing the crucial importance of the physical processes and climate change. Evidence of decadal regimes resulting in a rise in productivity in the 1980s is presented. Anthropogenic effects upon the ecosystem are linked to physical and biological factors. Three types of time-series data were used in exploring the environmental variations: hydroclimatic, biological, and anthropogenic. The hydroclimatic series possessed the best temporal resolution (monthly data). These were derived from COADS together with published marine environment monitoring data. Biological series were of generally lower quality than the hydroclimatic series (lower resolution, missing values), but were the best series available as indicators of the system's change over time. It was difficult to describe anthropogenic influences by means of historical time-series, because in many studies only mean values, qualitative or semi-qualitative information were presented. As with the biological series, the best available data were included in this study.