Conrady, Xemina (2018) C:N:P stoichiometry of POM during a simulated upwelling event in a mesocosm experiment. (Bachelor thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 62 pp.

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Abstract

The ocean is presently the second largest sink for anthropogenic carbon dioxide (CO2) emissions and might serve as an opportunity to counteract the increase of atmospheric CO2 concentrations, that occur since the Industrial Revolution. Artificial upwelling (AU) introduces an option to enhance the carbon export ability in oligotrophic regions via biological pump. Upwelling of deep ocean water (DOW) is considered to enhance the primary production of nutrient depleted ocean surfaces. Since DOW is enriched with dissolved inorganic carbon (DIC), an upwelling event transports nutrients as well as DIC to the surface ocean which has to be compensated by enhanced CO2 uptake of the phytoplankton community and by additional DIC export to depth. Thus, the efficiency of the biological pump is directly related to the carbon:nitrogen (C:N) ratio. The DIC input to the surface of different upwelling depths (~650m and ~1000m) was previously surveyed in an in situ mesocosm study offshore Gran Canaria in 2014 (see Taucher et al. 2017). In the summer of 2017 a four-week mesocosm study was conducted in Gran Canaria, Telde, to examine the impact of AU on a pelagic food web, being part of the OceanArtUp experiment. In the study a whole pelagic community was observed and its biological responses to a simulated artificial upwelling event analyzed, e.g. changes in particulate organic matter (POM). The central question of this study was whether the C export potential is influenced by a simulated upwelling event of the pelagic community with a particular focus on the elemental stoichiometry of C and N. Eight mesocosms were simultaneously filled with oligotrophic water from the surface at ~10m depth off the harbour. In order to simulate an upwelling event we added essential macronutrients (nitrate (NO3-), silicate acid (Si(OH)4) and phosphate (PO43-)) to the particular mesocosm in 3 different concentrations from low to high with a Si:N:P ratio of 8:16:1. The study was divided into 3 phases according to the highest growth response. We expected a linear relationship between the different nutrient addition (NA) and its effect on the elemental stoichiometric development, e.g. an increase of C:N ratios with increasing nutrient input. However, our results implied a non-linear relationship for most of the phases. The C:N ratios of suspended POM (C:NSUSP) for the moderate and high treatment displayed significant variances in contrast to the control during all phases (t-test: P<0,05). As expected the high treatment showed the largest POM production together with the highest export strength compared to the other treatments. The C:NSUSP ratios of the high treatment remained mostly below Redfield values and did not seem sufficient to compensate the DIC input to the surface at the upwelling depth of neither ~650m nor ~1000m. In contrast the moderate treatment exhibited a similar biomass production as the high treatment towards the end of the experiment but resulted in much lower sedimentation. However, the C:NSUSP ratios of the moderate treatment were highest among all treatments, thus displaying the potential to counterbalance the DIC input of DOW to the surface at both AU depths. Since the C:NSUSP ratios of the low treatment did not diverge significantly from the control at any phase after the NA, we assume that the NA was insufficient to cause significant changes in the elemental composition and food web dynamics compared to the control. The pilot study in 2017 provides valuable data concerning the right range of NA to achieve elevated C:N:PSUSP ratios and demonstrates that AU could potentially enhance ocean carbon sequestration if nutrient inputs are optimized for sufficiently high C:N ratios. Clearly the elemental composition is also determined by seasonal and regional variations that cause non-Redfield proportions and need further investigation. Since the environmental impact of AU is still unknown precise predictions are difficult to forecast. Data concerning area- and species-unique elemental composition and seasonal shifts is required for regions AU is meant to be applied in.

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