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: During two crusies in June (AL510) and September (AL516) 2018, a data set (N=76) from the sea surface microlayer (SML) was compiled in Eckernförde Bay, Germany. SML samples were collected with the glass plate technique. Reference samples from the underlying water (ULW) at an approx. depth of 20cm were collected with the help of a bottle. Total and dissolved hydrolysable amino acids, combined carbohydrates and dissolved organic carbon were analyzed to describe surfactant dynamics (based on phase-sensitive AC voltammetry). Flow cytometry provided additional information on bacteria and phytoplankton community composition. This data set resolves dynamics on short temporal (diurnal sampling ) and local scales (within an area of 50km^2).

Description: Gel particles - a class of abundant transparent organic particles - have increasingly gathered attention in marine research. Field studies on the bacterial colonization of marine gels however are still scarce. So far, most studies on respective particles have focused on the upper ocean, while little is known on their occurrence in the deep sea. Here, we report on the vertical distribution of the two most common gel particle types, which are polysaccharide-containing transparent exopolymer particles (TEP) and proteinaceous Coomassie stainable particles (CSP), as well as numbers of bacteria attached to gel particles throughout the water column, from the surface ocean down to the bathypelagial (〈 3,000 m). Our study was conducted in the Arctic Fram Strait during northern hemispheres' summer in 2015. Besides data on the bacterial colonization of the two gel particle types (TEP and CSP), we present bacterial densities on different gel particle size classes according to 12 different sampling depths at four sampling locations. Gel particles were frequently abundant at all sampled depths, and their concentrations decreased from the euphotic zone to the dark ocean. They were colonized by bacteria at all sampled water depths with risen importance at the deepest water layers, where fractions of bacteria attached to gel particles (%) increased within the total bacterial community. Due to the omnipresent bacterial colonization of gel particles at all sampled depths in our study, we presume that euphotic production of this type of organic matter may affect microbial species distribution within the whole water column in the Fram Strait, down to the deep sea. Our results raise the question if changes in the bacterial community composition and functioning on gel particles occur over depth, which may affect microbial respiration and remineralization rates of respective particles in different water layers.

Description: Gel particles, such as transparent exopolymer particles (TEP) and Coomassie stainable particles (CSP), are important organic components in the sea surface microlayer (SML). Here, we present results on the effect of different wind speeds on the accumulation and size distribution of TEP and CSP during a wind wave channel experiment in the Aeolotron. Total areas of TEP (TEPSML) and CSP (CSPSML) in the surface microlayer were exponentially related to wind speed. At wind speeds 〈 6 m s−1, accumulation of TEPSML and CSPSML occurred, decreasing at wind speeds of 〉 8 m s−1. Wind speeds 〉 8 m s−1 also significantly altered the size distribution of TEPSML in the 2–16 µm size range towards smaller sizes. The response of the CSPSML size distribution to wind speed varied through time depending on the biogenic source of gels. Wind speeds 〉 8 m s−1 decreased the slope of CSPSML size distribution significantly in the absence of autotrophic growth. For the slopes of TEP and CSP size distribution in the bulk water, no significant difference was observed between high and low wind speeds. Changes in spectral slopes between high and low wind speed were higher for TEPSML than for CSPSML, indicating that the impact of wind speed on size distribution of gel particles in the SML may be more pronounced for TEP than for CSP, and that CSPSML are less prone to aggregation during the low wind speeds. Addition of an E. huxleyi culture resulted in a higher contribution of submicron gels (0.4–1 µm) in the SML at higher wind speed (〉 6 m s−1), indicating that phytoplankton growth may potentially support the emission of submicron gels with sea spray aerosol.

Description: Oxygen minimum zones in the ocean increased during the past 50 years and changed microbial biogeochemical cycling; thereby research was focusing on changes in the nitrogen cycle. Earlier studies suggested higher efficiency of carbon export in those regions due to reduced microbial degradation activity. However, previous findings on the effect of oxygen on microbial activity are ambiguous. Here, we present first results on bacterial biomass production (estimated by 3H leucine incorporation) and extracellular enzyme rates (leucine aminopeptidase and ß-glucosidase), for the oxygen minimum zone off Peru, which is part of one of the largest anoxic zones in the ocean. We observed no reduction in bacterial biomass production, or extracellular enzyme rates and no reduced cell abundance in anoxic and suboxic waters, compared to more oxygenated waters at the oxyclines, suggesting that microbial degradation rate does not necessarily slow down under low oxygen conditions. We estimated a mean microbial carbon uptake of 548 µmol m-3 d-1, thereby only an average of 11 % got transformed into bacterial biomass. The remaining part was respired to carbon dioxide (average: 496 µmol m-3 d-1), that was potentially released to the atmosphere and accounted on average for 32 % of the oxygen reduction in the upper 80 m. Our study therewith proposes that microbial degradation of organic matter significantly contributes to the formation of the oxygen minimum zone off Peru and can proceed at relatively high rate within anoxic waters. This indicates that carbon dioxide production by heterotrophic microbial degradation in the OMZ off Peru, is not necessarily reduced under anoxia, and driven by anaerobic heterotrophic respiration pathways like denitrification.

Description: In the ocean, sinking of particulate organic mat- ter (POM) drives carbon export from the euphotic zone and supplies nutrition to mesopelagic communities, the feeding and degradation activities of which in turn lead to export flux attenuation. Oxygen (O2) minimum zones (OMZs) with suboxic water layers (〈 5 µmol O2 kg-1 ) show a lower carbon flux attenuation compared to well- oxygenated waters (〉 100 µmol O2 kg-1), supposedly due to reduced heterotrophic activity. This study focuses on sinking particle fluxes through hypoxic mesopelagic waters (〈 60 µmol O2 kg-1); these represent about 100 times more ocean volume globally compared to suboxic waters, but they have less been studied. Particle export fluxes and attenuation coefficients were determined in the eastern tropical North Atlantic (ETNA) using two surface-tethered drifting sediment trap arrays with seven trapping depths located between 100 and 600 m.

Description: Here, we investigate the composition and vertical fluxes of POM in two deep basins of the Baltic Sea (GB: Gotland Basin and LD: Landsort Deep).

Description: Upwelling systems play a key role in the global carbon and nitrogen cycles and are also of local relevance due to their high productivity and fish resources. To capture and understand the high spatial and temporal variability of physical and biogeochemical parameters found in these regions novel measurement technics have to be combined in an interdisciplinary manner. Here we use high-resolution glider-based physical-biogeochemical observations in combination with ship-based underwater vision profiler, sensor and bottle data to investigate the drivers of oxygen and nitrate variability across the shelf break off Mauritania in June 2014. Distinct oxygen and nitrate variability shows up in our glider data. High oxygen and low nitrate anomalies were clearly related to water mass variability and probably linked to ocean transport. Low oxygen and high nitrate patches co-occurred with enhanced turbidity signals close to the seabed, which suggests locally high microbial respiration of resuspended organic matter near the sea floor. This interpretation is supported by high particle abundance observed by the underwater vision profiler and enhanced particle-based respiration rate estimates close to the seabed. Discrete in-situ measurements of dissolved organic carbon and amino acids suggest the formation of dissolved organic carbon due to particle dissolution near the seabed fueling additional microbial respiration. Our high-resolution interdisciplinary observations highlight the complex interplay of remote and local physical-biogeochemical drivers of oxygen and nitrate variability off Mauritania, which cannot be captured by classical shipboard observations alone.

Description: Here we demonstrate enhanced natural community APase activity following iron amendment within the low zinc and moderately low iron Western North Atlantic. In contrast we find no evidence for trace metal limitation of APase activity beneath the Saharan dust plume in the Eastern Atlantic. Such intermittent iron limitation of microbial phosphorus acquisition provides an additional facet in the argument for iron controlling the coupling between oceanic nitrogen and phosphorus cycles.