Eutrophication of the Baltic Sea Foreword

~ Int. Revueges.Hydrobio1. I 76 I I 3 I 293-294 _ _ - - - - ~- .. -

Eutrophication of the Baltic Sea

Foreword

Eutrophication is the term used to describe the enrichment of plant nutrients, mainly nitrogen and phosphorus compounds, in waters. Since about 1950, this natural process in the development of waters has been accelerated to an unprecedented degree by human activities. This also applies to the Baltic Sea, which was formerly oligotrophic, but is now regarded as one of the world’s most severely loaded seas. The expansion of oxygen-depleted bottom zones and occurrence of unusual algal blooms are among the alarm signals.’ Both are obviously directly linked to the increased phytoplankton bio- mass production induced by nutrient inputs. However, this is difficult to prove directly owing to the incompleteness of available data and the gaps in our knowledge of nutrient turnover rates. Commenting on a complex study undertaken in the Bay of Germany (North Sea) and Kiel Bay (Baltic Sea), GERLACH (1990) stated: “We still do not have sufficient basic marine biological knowledge concerning Germany’s coastal waters to satisfactorily answer the apparently simple question of the effects and fate of nutrient inputs.” The main difficulties are our lack of knowledge regarding exchange processes between sediment and water column, vertical transport processes, the proportion of organically immobilized nutrients and nutrient recycling rates. In fact, questions relat- ing to nutrient dynamics and the corresponding biological processes received little attention in the past.

A clear distinction must be made between the sometimes thermally stratified Baltic proper and outer and inner coastal waters (Fjords, boddens, haffs). The latter normally act as an effective filter and buffer system for the Baltic proper. However, as eutrophication increases, these functions are increasingly being transformed into exactly the opposite. The eutrophication process in these areas and the Baltic Sea as a whole must therefore be stopped as a matter of urgent necessity. The HELCOM recommendation for a 50 % cut in nitrogen and phosphorus inputs is an important first step towards stabilizing the state of the Baltic waters.

The purpose of the present special edition of the Internationale Revue der gesarnten Hydrobiologie is to support this process. The contributions have been selected specifi- cally to reveal the whoie spectrum of the eutrophication process. At the same time, importance was attached to linking results from the immediate coastal waters and the Baltic proper. Expectedly, the effects of changes caused by eutrophication are most clearly evident in the coastal region. However, due to the thermal stratification of the Baltic proper, knowledge gained from studies in coastal waters does not directly apply to the Baltic proper. 20 hi. Revue ges. Hydmblol. 76 (1991) 3

294 Foreword

The editors hope that the contributions presented here will provide a new stimulus for the discussion of eutrophication and will provide a better basis for ecologically sound management.

Rostock, January 15th, 1991 U. SCHIEWER