Engel, Anja (2015) Short Report Alkor Cruise AL 458 [AL458] (Kiel-Riga-Kiel), 03.06.2015-19.06.2015 . . GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, 18 pp.

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Abstract

Objectives of the cruise

The Baltic Sea is a marine ecosystem exposed to multiple stressors. Eutrophication sustains high phytoplankton productivity that fuels high oxygen demands in deeper waters. As a consequence of increasing biological oxygen consumption and high CO2 production, the expansion of hypoxic zones in the Baltic Sea has been predicted. A major process contributing to the consumption of oxygen is the microbial respiration of dissolved organic matter. Hence, the microbial production of organic matter and its subsequent remineralization are biological processes with a high potential to co-determine the direction and magnitude of future oxygen and pH changes in the Baltic Sea. However, little is known about how high nutrient loads and seawater CO2 concentration affect phytoplankton productivity. Furthermore, the relevance of organic matter composition, seawater pH and oxygen availability for carbon remineralization are largely unexplored. The proposed cruise aims at studying the production, composition and degradation of particulate and dissolved organic matter along natural gradients of inorganic nutrients, oxygen and seawater CO2 in the Baltic Sea. The cruise AL458 crossed the Southern Baltic Proper, the deep Gotland Basin and the Gulf of Riga. Sampling of depth profiles along transects was combined with onboard experiments that investigated (i) effects of oxygen concentration and organic matter composition on the bacterial turnover of dissolved organic matter (ii) effects of labile organic matter and nutrients on microbial activities under oxic and anoxic conditions (iii) degradation of halocarbons by microbial communities. The combination of field work and onboard experiments will help to better explain the environmental control of microbial processes and the contribution of microbial activity to organic carbon cycling in the Baltic Sea. In addition this cruise was part of the master program Biological Oceanography MNF-bioc.-201 (C2). Four students participated in the cruise.

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