The suitability of Mytilus edulis as proxy archive and its response to ocean acidification

Past climate changes can be used as indicators of future scenarios, however past climatic changes can not be directly observed. Therefore, the reconstruction of past abiotic conditions can approximated using chemical or isotopic proxies. These proxies can be measured in natural archives (e.g. bivalve shells and coral skeletons). One aspect of current climate change is the acidification of the oceans, a phenomenon caused by the oceanic uptake of anthropogenic CO2 and a resulting shift in the marine carbonate system. As a result of this, a drop of mean ocean surface pH by ~0.3-0.7 units can be expected until the year 2100. In relation to geological timescales this drop occurs very fast (~0.1-0.2 units per 100 years) and causes species specific reactions which are not fully studied yet. For example, elevated [CO2] disturbs the acid-base status of extracellular body fluids and the degree of disturbances depends on animals metabolic rates. Especially marine calcifying organisms are influenced in their ability to form CaCO3-shells and skeletons by this decline in pH. The blue mussel (Mytilus edulis) is an important calcifier in many marine ecosystems and in aquaculture.

In this Thesis I investigated the impact of ocean acidification on the acid-base status and the calcification of M. edulis in experiments conducted under different seawater pCO2 levels (380-4000 µatm). Furthermore, investigations of M. edulis shells as proxy archive have led to contradictory results. Hence, the impact of elevated pCO2, as well as temperature and salinity on the inorganic shell composition have been investigated in this study to test the suitability of M. edulis shells as a proxy archive in general and for pH construction in particular. Physiological experiments can provide knowledge about acclimation reactions of marine organisms to abiotic stressors but not about their adaptation potential, as the relevant timescales cannot be simulated in laboratory studies. Thus, experiments using already pre-adapted animals from challenging habitats may provide a more accurate picture of potential CO2 impacts on M. edulis. Therefore, in all experiments of this study, M. edulis from Kiel Fjord (Western Baltic Sea, Germany) were investigated, as during the summer months, high CO2-concentrations have been observed in this habitat.

In Chapter 1 the suitability of Mytilus edulis shells as proxy archive has been proven. For this, the environmental (temperature and salinity) and the biological influence on the elemental ratios (Mg/Ca and Sr/Ca) in the calcite layer have been modeled. The results showed physiological and individual differences having a significant impact on the Mg/Ca distribution in M. edulis calcite (~45 and ~34 % respectively). Sr/Ca seemed to be less affected (~24 and ~17 %). A more detailed understanding of the mechanisms of biomineralization is necessary to use biominerals as proxy archives. As shell formation occurs in the extrapallial space, the contained fluid (extrapallial fluid, EPF) should be considered even if the detailed mechanisms of precipitation are still unknown. Thus, in Chapter 2 the mineralization of the shell and the elemental ratios in the body fluids (hemolymph and EPF) have been observed in long-term experiments conducted under different pCO2 (380-4000 µatm) values. Elemental concentrations were not influenced by different pCO2 levels however they were modified during shell formation. The inorganic composition of the body fluids was very variable between individuals what may explain the results of Chapter 1. Calcification rate measurements indicated net dissolution in the highest pCO2 treatment (3352 µatm) and inner shell surfaces were corroded while length growth did not differ between treatments. This effect could be reduced by high food levels. To better understand past changes related to ocean acidification, B/Ca ratios in the extrapallial fluid and boron isotopes (delta11B) in the shell of M. edulis were investigated in Chapter 4. For this purpose a new in situ method using LA-MC-ICP-MS for the determination of stable boron isotope ratios (delta11B) in carbonates was developed and described in Chapter 3. delta11B was highly variable between different individuals but also within single shells. This corresponded to a high individual variability in fluid B/Ca ratios. Unless the mean delta11B values showed no trend with pH they appeared to represent internal EPF pH rather than ambient water pH. I also demonstrated that extracellular body fluid pCO2 values of M. edulis are high due to metabolic CO2 and pH was significantly lower than seawater pH. In contrast to Chapter 2, growth rates were much higher due to higher food availability. Elemental ratios (B/Ca, Mg/Ca and Sr/Ca) in EPF increased slightly with pH which was in accordance with increasing growth and calcification rates at higher seawater pH values.

In summary, the results of this study showed a very large biological impact on the elemental and isotopic composition of M. edulis shell. To use M. edulis shells as proxy archive, several abiotic as well as biological factors have to be considered and the whole animal has to be investigated. Thus, it is questionable if a sufficient calibration for the proper use of M. edulis as proxy archive is feasible at all.

Heinemann A., 2011. The suitability of Mytilus edulis as proxy archive and its response to ocean acidification. Doctoral thesis/PhD, Christian-Albrechts-Universität, Kiel. Thesis.

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