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Subproject 2.2

Role of plankton derived exopolymeric substances for air-sea exchange processes in the Anthropocene

Indoor Mesocosm Experiment Summer 2013

Combined effects of temperature and CO2 on organic matter dynamics in the surface microlayer and consequences for air-sea gas exchange

The extent and abundance of gel particles in the surface microlayer (SML) are suggested to influence gas exchange at the ocean surface as well as the formation and emission of primary organic aerosols. Hence, gel particles are assumed to play an important role in exchange processes between the surface ocean and the lower atmosphere. Gel particles may be of polysaccharidic or proteinaceous composition and are therefore referred to as transparent exopolymer particles (TEP) or coomassie stainable particles (CSP), respectively. They find their origin in dissolved organic matter (DOM) precursors released by phyto- and bacterioplankton. At the same time, gels represent hotspots for intense microbial activity, which may be an important process implied in the continuous recycling of organic matter in the SML.

In order to study the formation and degradation of gel particles as well as their effect on air-sea gas exchange under future ocean conditions, an indoor mesocosm experiment was carried out from August to September 2013. Twelve mesocosms were filled with ~1400L of fjord water including the natural plankton community up to mesozooplankton and adjusted to two temperatures (16.5 and 22.5°C) and six pCO2 levels (500 to 2500 µatm CO2).

Samples were taken twice per week to analyse DOM (sugars, amino acids, coloured DOM) and gel particles dynamics as well as bacterial biomass production, both in the SML and the underlying bulk water. The size and abundance of TEP and CSP were analysed microscopically. Simultaneously, gas exchange rates over the sampled SML were quantified using N2O as a tracer gas. N2O is considered to behave similarly to CO2 in the atmosphere and therefore enables estimations concerning future CO2 uptake in the ocean via air-sea gas exchange. Gas chromatography (GC) measurements were carried out weekly using SML samples from two selected mesocosms.


Effect of bubbling on organic matter enrichment in the surface microlayer

In the ocean, dissolved organic compounds and TEP may adsorb on rising bubbles formed during the breaking of waves. Bubble bursting at the ocean’s surface is responsible for major emissions of sea-spray droplets to the overlying atmosphere. Furthermore, ascending bubbles enrich the SML with organic compounds scavenged in the water column. To simulate this natural process of organic accumulation in the SML, an additional experiment was performed in continuation of the four-week temperature-CO2-mesocosm experiment. The mesocosms were bubbled for 12h with ambient air at bubble sizes of 25 – 1000 µm. Before and after bubbling, samples were taken to analyse DOM and gel particles dynamics as well as bacterial biomass production both in the SML and the underlying bulk water. The results of this experiment may help to understand physical and biological processes in the ocean that lead to the formation of an organic-rich SML. Furthermore, they will give estimates how these processes may change in the future due to ocean warming and acidification.




PI: A. Engel
Contact: Anja Engel (aengel@geomar.de)
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