Description: 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.

Description: The Arctic Ocean is highly susceptible to climate change as evidenced by rapid warming and the drastic loss of sea ice during summer. The consequences of these environmental changes for the microbial cycling of organic matter are largely unexplored. Here, we investigated the distribution and composition of dissolved organic matter (DOM) along with heterotrophic bacterial activity in seawater and sea ice of the Eurasian Basin at the time of the record ice minimum in 2012. Bacteria in seawater were highly responsive to fresh organic matter and remineralized on average 55% of primary production in the upper mixed layer. Correlation analysis showed that the accumulation of dissolved combined carbohydrates (DCCHO) and dissolved amino acids (DAA), two major components of fresh organic matter, was related to the drawdown of nitrate. Nitrate‐depleted surface waters at stations adjacent to the Laptev Sea showed about 25% higher concentrations of DAA than stations adjacent to the Barents Sea and in the central Arctic basin. Carbohydrate concentration was the best predictor of heterotrophic bacterial activity in sea ice. In contrast, variability in sea‐ice bacterial biomass was largely driven by differences in ice thickness. This decoupling of bacterial biomass and activity may mitigate the negative effects of biomass loss due to ice melting on heterotrophic bacterial functions. Overall, our results reveal that changes in DOM production and inventories induced by sea‐ice loss have a high potential to enhance the bacterial remineralization of organic matter in seawater and sea ice of the Arctic Ocean.

Description: Numerical simulations of ocean biogeochemical cycles need to adequately represent particle sinking velocities (SV). For decades, Stokes' Law estimating particle SV from density and size has been widely used. But while Stokes' Law holds for small, smooth, and rigid spheres settling at low Reynolds number, it fails when applied to marine aggregates complex in shape, structure, and composition. Minerals and zooplankton can alter phytoplankton aggregates in ways that change their SV, potentially improving the applicability of Stokes' models. Using rolling cylinders, we experimentally produced diatom aggregates in the presence and absence of minerals and/or microzooplankton. Minerals and to a lesser extent microzooplankton decreased aggregate size and roughness and increased their sphericity and compactness. Stokes' Law parameterized with a fractal porosity modeled adequately size‐SV relationships for mineral‐loaded aggregates. Phytoplankton‐only aggregates and those exposed to microzooplankton followed the general Navier‐Stokes drag equation suggesting an indiscernible effect of microzooplankton and a drag coefficient too complex to be calculated with a Stokes' assumption. We compared our results with a larger data set of ballasted and nonballasted marine aggregates. This confirmed that the size‐SV relationships for ballasted aggregates can be simulated by Stokes' models with an adequate fractal porosity parameterization. Given the importance of mineral ballasting in the ocean, our findings could ease biogeochemical model parameterization for a significant pool of particles in the ocean and especially in the mesopelagic zone where the particulate organic matter : mineral ratio decreases. Our results also reinforce the importance of accounting for porosity as a decisive predictor of marine aggregate SV.