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.

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

Description: Particulate organic matter (POM) distribution in the water column as on several cruises determined after filtration onto pre-combusted, acid-washed GF/F filters (Franz et al., 2012a). Particulate organic phosphorus (POP) collected on GF/F filters was determined colorimetrically. Samples for phytoplankton pigment concentrations were collected by filtration of seawater from the CTD/rosette through GF/F filters, and stored at -80°C immediately after filtration. Transparent exopolymer particles (TEP) and Coomassie stainable particles were filtered under low pressure (〈 150 mbar) onto 25 mm Nuclepore membrane filters (0.4 μm pore size, Whatman Ltd.) and stained with Alcian Blue and Coomassie Brilliant Blue, respectively. Export flux of was characterized using surface-tethered sediment traps (Engel et al., 2017), with Particle Interceptor-Traps (PIT) following Knauer et al. (1979).

Description: Dissolved Organic Matter, Cell Abundance and Extracellular Enzyme Rates and Bacterial Production from several cruises and experiments from 2008-2018

Description: Two 7-day mesocosm experiments were conducted in October 2012 at the Instituto Nacional de Desenvolvimento das Pescas (INDP), Mindelo, Cape Verde. Surface water was collected at night before the start of the respective experiment with RV Islândia south of São Vicente (16°44.4'N, 25°09.4'W) and transported to shore using four 600L food safe intermediate bulk containers. Sixteen mesocosm bags were distributed in four flow-through water baths and shaded with blue, transparent lids to approximately 20% of surface irradiation. Mesocosm bags were filled from the containers by gravity, using a submerged hose to minimize bubbles. The accurate volume inside the individual bags was calculated after addition of 1.5 mmol silicate and measuring the resulting silicate concentration. The volume ranged from 105.5 to 145 L. The experimental manipulation comprised addition of different amounts of inorganic N and P. In the first experiment, the P supply was changed at constant N supply in thirteen of the sixteen units, while in the second experiment the N supply was changed at constant P supply in twelve of the sixteen units. In addition to this, “cornerpoints” were chosen that were repeated during both experiments. Four cornerpoints should have been repeated, but setting the nutrient levels in one mesocosm was not succesfull and therefore this mesocosm also was set at the center point conditions. Experimental treatments were evenly distributed between the four water baths. Initial sampling of the mesocosms on day 1 of each run was conducted between 9:45 and 11:30. After nutrient manipulation, sampling was conducted on a daily basis between 09:00 and 10:30 for days 2 to 8.