Lechtenbörger, Anna K. (2017) The Effects of Ocean Acidification on the Gelatinous Zooplankton Species Oikopleura dioica. (Master thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 119 pp.

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Abstract

The continuous rise of concentrations in atmospheric CO2 let the seawater pH decrease at a rate not seen on the planet for several hundreds of million years. The question, what effect ocean acidification (OA) has on the fitness and physiology of gelatinous zooplankton is still unanswered. The gelatinous zooplankton Oikopleura dioica are appendicularians and are widely used as model organism. Many studies on O. dioica have been done on the molecular biological level. Currently the whole organism and combined ecosystem studies are rare. Appendicularians are known to have a high clearance rate, fast generation- time and high reproductive rates, depending on the circumstances. We studied the OA (elevated pCO2 / lowered pH) effects on a natural O. dioica population during a large scale mesocosm study in the Norwegian Raunefjord, from May to end of June 2015. Eight Kiel-Off-shore Mesocosms for Future Ocean Simulations (KOSMOS) from GEOMAR were adjusted to two different pCO2 levels, ambient 280 μatm and manipulated (acidified) 2200 μatm. Overall the abundances were higher in acidified mesocosms. The fecundity of female appendicularians was also increased in the acidified treatments. The size distribution of the trunk length (TL) and trunk width (TW) showed a tendency to longer but slimmer individuals in the acidified mesocosms than in the ambient mesocosms. The results indicate that O. dioica populations benefit from changes initiated by OA. Additional microcosm experiments were conducted at Sars International Centre for Marine Molecular Biology, Bergen, Norway. The microcosm experiments were to identify the general seawater pH levels in which O. dioica can survive. O. dioica populations were well capable of surviving pH conditions ranging from pH 9.0 (54.8 % survival) and down to pH 7.0 (53.6 % survival) after a 24 hour incubation. Further a generation-time (GT) experiment was conducted under similar conditions as seen during the KOSMOS Bergen 2015 experiment (temperature 9 - 10 °C, pH levels 7.3 and 8.0; a normal/standard diet used by Sars and a low diet – 1/6 of the standard diet). Under low pH and normal diet O. dioica had a GT of eleven days, whereas under low pH and limited diet the GT took around twelve days. The current seawater pH of around 8.0 and a limited diet showed a GT of twelve days. With a normal diet given at pH 8.0 the appendicularians completed their life cycle within eleven days. These findings indicate that the food availability also plays an additional role in the success of these animals. The findings and observations agree to an extend with the published literature that OA has beneficial effects on O. dioica. The potential for an increasing importance in the marine food-web and trophic interactions are likely and need further investigations. Further analyses of the other data sets from the KOSMOS experiment could contribute to make connections within a complex system and deepen the understanding of drivers and connections of marine ecosystems under OA.

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