1. Introduction

Research on the marine CO₂ system has its roots in the Baltic Sea, where it commenced at the beginning of the last century. Physico-chemical studies were performed that resulted in the determination of equilibrium constants describing the dissociation of carbonic acid, and they were still being used 50 years later. Comprehensive field studies in the northern Baltic Sea facilitated the first characterization of the seasonality and regional peculiarities of the CO₂ system. It was not until the 1980s that research on the CO₂ system received greater interest, a product of the growing awareness of the oceans role for absorbing anthropogenic CO₂. Studies of the marine CO₂ system were also revived in the Baltic Sea. However, rather than examining the fate of anthropogenic CO₂, investigations of the CO₂ system were considered as an ideal tool to study fundamental biogeochemical processes, such as the production and mineralization of organic matter, which are connected to the consumption and release of CO₂, respectively. It is the aim of this book to demonstrate this and to present related progress in biogeochemical research in the Baltic Sea.