Opitz, Sebastian (2011) Vergleichende Untersuchungen über den Einfluss von erhöhtem Seewasser pCO2 aif den gemeinen Seestern Asterias rubens L. in der Ostsee. (Diploma thesis), Christian-Albrechts-Universität, Kiel, Germany, 113 pp.

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Abstract

Near-future ocean acidification has been shown to decrease performance of various – especially calcifying – species. Naturally high-pCO2 environments such as Kiel Fjord (Germany) can already today help to estimate possible effects of ocean acidification. This study used a short term (6 weeks) experiment to elucidate effects of ocean acidification on the common starfish Asterias rubens in the Baltic sea. Results showed a significant decrease in growth and feeding rates among juveniles at high-pCO2 conditions (3130 μatm). The same (yet insignificant) trend at intermediate levels (1100 μatm) was found, showing that adaptation of juveniles to lowered seawater pH has not yet taken place in Kiel Fjord. A trend (non-significant) of metabolic depression at high-pCO2 was found, while ammonia-excretion increased significantly with lowered seawater pH. These results express a decreasing O:N atomic ratio, suggesting higher rates of protein degradation as a source of energy. This was suggested to be an energy-expensive means of compensatory proton extrusion via NH4+-excretion to make up for the lack of an efficient ion-regulatory machinery. Using simple energetic transformations, this study shows a trend of changing patterns of energetic investments with decreasing pCO2: While more energy needs to be channeled into maintenance and hypercapnia-compensation processes, the amount of energy left for growth (Scope for growth) decreases with higher pCO2. It was found that animals at high pCO2 with catabolic energy balances showed no resources to grow because they invested proportionally large amounts of energy into respiration and excretion (and probably other means of hypercapnia-compensation), leaving no energy for growth. This work suggests that reduced growth rates of A. rubens individuals at high pCO2 are caused by changing patterns of energy investment. Further research is needed to investigate whether the process of feeding and its efficiency is affected by lowered seawater pH and whether it is linked to a general decreased size of the animals. Calcification rates were found not to be effected quantitatively. Yet, significant difference were observed for structural parameters and the density of the calcified tissues in a low-replicate (N=3) pilot-study using X-ray and CT analysis. Even though these results may not be representative for a larger number of animals, the analysis still shows that subtle effects on calcification can be detected and may pose a tool to better understand the causes for the observed reductions in growth and feeding. The outcome of the study shows that about 1/3 of the juveniles tested are not able to grow at high pCO2-conditions, while the general response of A. rubens to ocean acidification is highly variable. Even though longer multi-generation experiments with focus on ecosystem interactions are needed to find clues for effects on an ecosystem scale, this study suggests that during prolonged high-pCO2 upwelling-events, the ecosystem-structure in Kiel Fjord might be liable to changes in the near future due to decreasing recruitment of juveniles into the adult life stage.

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