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 (11B) 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 (11B) in carbonates was developed and described in Chapter 3. 11B 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 11B 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

The influence of increased pCO2 on the calcification of Mytilus edulis

One of the most important and abundant calcifying organisms in several marine ecosystems is the blue mussel, Mytilus edulis. It has a wide geographic distribution (Gosling 1992 Developm. Aquacult. Fish. Sci. 25, 1-20) and tolerates a broad range of environmental conditions (Seed and Suchanek 1992 Developm. Aquacult. Fish. Sci. 25, 87-170). Blue mussel beds are also common features in the Kiel Fjord (Baltic Sea), a habitat dominated by low salinity (10-20 PSU), low alkalinity (1900-2150 μmol kg-1), low pH (minimum values < 7.5) and high pCO2 (maximum value of 2340 ppm). The resulting calcium carbonate saturation state (min. values: Ωarag = 0.34 and Ωcalc = 0.58) is significantly lower than in the open ocean (Thomsen et al. submitted). Therefore, pCO2 in Kiel Fjord during summer is already higher than what is predicted for the future (e.g., Caldeira and Wickett 2003 Nature 425, 365). Additionally, Meier (2006 Clim. Dyn. 27, 39-68) projected an increase of temperature (2.6 to 5.0 °C) in the next 100 years for the Baltic Sea. To contribute to the understanding of the ability of calcifying organisms to live under ocean acidification conditions and of biomineralization mechanisms, M. edulis from this naturally CO2-enriched habitat were cultured in a flow-through system. Experiments were conducted using CO2 concentrations ranging from 380 ppm to 4000 ppm and temperatures ranging from 5° to 25°C. At the end of the experiments, hemolymph and extrapallial fluid (EPF) were taken and analyzed for pH, pCO2, bicarbonate and elemental ratios. Fluids showed decreased pH and increased CO2 with increasing water pCO2. Elemental ratios (Mg/Ca and Sr/Ca) in the fluids did not show pCO2 or temperature-related systematic changes. Furthermore, boron isotopes ([Delta]11B), used in isotope geochemistry as a pH proxy, were investigated by LA-MC-ICP-MS in shell portions precipitated during the experimental treatment. We observed high [Delta]11B variability between different individuals, but also within single shells. Average [Delta]11B values showed a weak positive correlation with pH. When comparing our results to published studies, boron isotopes appeared to represent internal pH conditions (EPF) instead of ambient water pH (Kasemann et al. 2009 Chem. Geol. 260, 138-147; Reynaud et al. 2004 Coral Reefs 23, 539-546; Sanyal et al. 2000 Geochim. Cosmochim. Acta 64, 1551-1555)

Untersuchungen zur Eignung von <em>Mytilis edulis</em> als Proxyarchiv

Different environmental conditions have measurable impacts on the shells of calcifying marine organisms. The degree of these impacts can be investigated by establishing proxies. Proxies, e. g. elemental ratios and isotopic compositions, serve to construct and improve models of the recent and past climate. In this study, shells of the blue mussel Mytilus edulis, cultivated (1) at different salinities (17, 20, 29, 34 psu) and (2) collected from stations in the Schwentine River, the Kiel Fjord and the North Sea, were analyzed for their suitability as proxy-archives. Mg/Ca and Sr/Ca ratios, two already established proxies, were determined within the calcitic parts of the shells from the cultivation experiments. The ratios were measured using LA-MC-ICP-MS. The lowest ratios were obtained from the 20 psu treatments, where the highest growth rate had occurred. A new proxy, d44/40Ca, was measured with TIMS in the aragonite as well as the calcite of the fieldsampled mussels. Paired analysis of calcite and aragonite in all samples showed a consisten offset, with aragonite featuring 0.15 to 0.31 ‰ lower d44/40Ca values. The highest degree of fractionation between shells and seawater was found in the samples from the Schwentine Delta and the North Sea. ICP-OES measurements of the Sr/Ca and Mg/Ca ratios as well as the d44/40Ca of the extrapallial fluid (EPF) showed values which correspond to more saline seawater and not to the surrounding brackish Schwentine water. The pronounced variability of the elemental ratios of biological replicates as well as the different composition of surrounding water to that of the EPF and of the shell underline a strong biological control and show the complexity of the calcification process. The suitability of Mytilus edulis as a proxy-archive could not be confirmed in the present study and should be subject to further investigation

Die Eignung von Mytilus edulis als Proxy-Archiv und deren Reaktion auf Ozeanversauerung

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 (11B) 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 (11B) in carbonates was developed and described in Chapter 3. 11B 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 11B 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.Die Klimageschichte kann einen Blick in die Zukunft ermöglichen, aber selbst nicht mehr direkt untersucht werden. Daher sind chemische oder isotopische Stellvertreter (sogenannte Proxies), welche in natürlichen Archiven (z. B. Muschelschalen und Korallenskelette) gemessen werden, für die Rekonstruktion des Klimas notwendig. Ein wichtiger Aspekt des Klimawandels ist die Ozeanversauerung, die durch die Aufnahme des anthropogen emittierten Kohlenstoffdioxids (CO2) durch die Ozeane verursacht wird. Dies hat eine Veränderung im Kohlensäuregleichgewicht zur Folge. Daraus resultiert eine Senkung des pH-Wertes im Meerwasser um ~0.3-0.7 Einheiten bis zum Jahre 2100. Relativ zu geologischen Zeitskalen verläuft die Versauerung der Ozeane mit ~0.1-0.2 pH Einheiten pro einhundert Jahre sehr schnell und löst in verschiedenen marinen Tierarten unterschiedliche Reaktionen aus, die noch nicht ausreichend verstanden sind. Die Auswirkungen der erhöhten CO2-Konzentrationen auf den Säure-Base-Haushalt hängen z. B. vom Metabolismus der Tiere ab. Besonders kalzifizierende Organismen sind in ihrer Fähigkeit, Schalen und Skelette aus Kalziumkarbonat zu bilden, durch den sinkenden Wasser-pH gefährdet. Die Miesmuschel (Mytilus edulis) spielt in vielen marinen Ökosystemen, sowie in der Aquakultur eine wichtige Rolle. Um die individuelle Gefährdung dieser wichtigen Art besser erfassen zu können, habe ich im Rahmen meiner Dissertation in verschiedenen Experimenten mit unterschiedlichen CO2-Konzentrationen () die Auswirkungen der Ozeanversauerung auf den Säure-Base-Haushalt, als auch die Kalzifizierung von M. edulis näher untersucht. Weiterhin wird M. edulis als Proxy-Archiv genutzt, was aber in zahlreichen Studien zu kontroversen Ergebnissen führt. In dieser Arbeit wurden deshalb die Auswirkungen der Ozeanversauerung, sowie von Temperatur und Salinität auf die Zusammensetzung der Schale näher betrachtet, um so ein Fazit über die Eignung der Schale von M. edulis als Proxy-Archiv, insbesondere im Hinblick auf pH-Rekonstruktionen, ziehen zu können. Im Allgemeinen erlauben physiologische Experimente nur einen Einblick in die Kurzzeitreaktionen auf Veränderungen bestimmter Umweltparameter und liefern keine Aussage über mögliche Adaptionen. Tieren aus Lebensräumen, die bereits heute Bedingungen ausweisen wie sie für die Zukunft vorausgesagt sind, weisen daher bereits eine zielgerichtete Anpassung auf. Somit können Experimente mit diesen voradaptierten Individuen genauere Abschätzungen liefern. Diesem Ansatz folgend stammten die Miesmuscheln dieser Arbeit alle aus der Kieler Förde (westliche Ostsee, Deutschland), da in diesem Habitat in den Sommermonaten hohe pCO2-Werte gemessen wurden. In Kapitel 1 wurde die Eignung der Schale von M. edulis als Proxy-Archiv anhand von Elementverhältnissen (Mg/Ca und Sr/Ca) in der Kalzitlage getestet. Dafür wurden das Ausmaß des Einflusses der Umweltparameter (Temperatur und Salinität) und der Biologie modelliert. Die physiologischen, sowie individuellen Unterschiede zeigten mit ~45 bzw. ~34 % einen signifikanten Einfluss auf die Verteilung von Mg/Ca in der Schale, während Sr/Ca weniger beeinflusst (~24 bzw. ~17 %) waren. Um die Schalen der Miesmuscheln als Proxy-Archiv nutzen zu können, müssen die Mechanismen der Schalenbildung erst besser verstanden werden. Auch, wenn die genauen Mechanismen noch ungeklärt sind, muss hierbei das extrapalliale Fluid (EPF) berücksichtigt werden, da es den Raum füllt, in dem die Schalenbildung stattfindet. Aus diesem Grund wurden in einem Langzeitexperiment (3 Monate) sowohl die Kalzifizierung, als auch die Elementzusammensetzung der Hämolymphe und des EPF unter Einfluss verschiedener pCO2-Level untersucht (380-4000 µatm, Kapitel 2). Die Elementkonzentrationen wurden nicht durch die pCO2-Level beeinflusst, aber durch die Schalenbildung und waren sehr variabel zwischen verschiedenen Individuen. Dies könnte eine Erklärung für die Ergebnisse aus Kapitel 1 sein. Obwohl im höchsten pCO2-Level (3352 µatm) Längenwachstum der Schale beobachtet wurde, deuteten die Messungen der Kalzifizierungsraten eine Auflösung der Schalen an und die Innenseite der Schalen wies angelöste Flächen auf. Unter optimalen Futterbedingungen wurde dieser Effekt vermindert. In Kapitel 4 sind die B/Ca Verhältnisse im EPF, sowie die Borisotopie (11B) der Schale untersucht worden, um zu testen, ob sich so vergangene pH-Bedingungen rekonstruieren lassen. Hierfür wurde zuerst eine in situ Messmethode für 11B in den Schalen entwickelt (Kapitel 3). Sowohl 11B, als auch B/Ca waren sehr variabel zwischen verschiedenen Individuen und 11B auch innerhalb einer Schale. Die mittleren 11B-Werte zeigten keine Abhängigkeit vom pH Wert des Umgebungswassers, sondern scheinen den internen pH (extrapalliale Flüssigkeit, EPF) wiederzuspiegeln. In Kapitel 4 wurde ebenfalls gezeigt, dass der pH-Wert der Körperfluide (HL und EPF) signifikant niedriger ist, als der des Wassers. Dies ist ein Effekt des durch Atmung produzierten Kohlenstoffdioxids. Im Gegensatz zum 2. Kapitel waren die Wachstumsraten in den Experimenten aus diesem Kapitel aufgrund von optimierten Futterbedingungen deutlich höher. Die Elementverhältnisse (B/Ca, Mg/Ca, Sr/Ca) wurden mit steigendem pH leicht erhöht. Dies scheint eher ein Effekt der steigenden Wachstumsraten unter hohem pH zu sein, als der direkte Einfluss des pH. Zusammenfassend lässt sich sagen, dass eine große biologische Kontrolle die Zusammensetzung der Schale beeinflusst. Um die Schale von M. edulis als Proxy-Archiv nutzen zu können, müssen viele abiotische, als auch biotische Faktoren berücksichtigt werden und es sind Untersuchungen am ganzen Tier notwendig. Daher ist es fragwürdig, ob eine ausreichende Kalibration für die einwandfreie Nutzung von M. edulis als Proxy-Archiv möglich ist

Conditions of Mytilus edulis extracellular body fluids and shell composition in a pH-treatment experiment: Acid-base status, trace elements and delta B-11

Mytilus edulis were cultured for 3 months under six different seawater pCO(2) levels ranging from 380 to 4000 mu atm. Specimen were taken from Kiel Fjord (Western Baltic Sea, Germany) which is a habitat with high and variable seawater pCO(2) and related shifts in carbonate system speciation (e. g., low pH and low CaCO3 saturation state). Hemolymph (HL) and extrapallial fluid (EPF) samples were analyzed for pH and total dissolved inorganic carbon (C-T) to calculate pCO(2) and [HCO3-]. A second experiment was conducted for 2 months with three different pCO(2) levels (380, 1400 and 4000 mu atm). Boron isotopes (delta B-11) were investigated by LA-MC-ICP-MS (Laser Ablation-Multicollector-Inductively Coupled Plasma-Mass Spectrometry) in shell portions precipitated during experimental treatment time. Additionally, elemental ratios (B/Ca, Mg/Ca and Sr/Ca) in the EPF of specimen from the second experiment were measured via ICP-OES (Inductively Coupled Plasma-Optical Emission Spectrometry). Extracellular pH was not significantly different in HL and EPF but systematically lower than ambient water pH. This is due to high extracellular pCO(2) values, a prerequisite for metabolic CO2 excretion. No accumulation of extracellular [HCO3-] was measured. Elemental ratios (B/Ca, Mg/Ca and Sr/Ca) in the EPF increased slightly with pH which is in accordance with increasing growth and calcification rates at higher seawater pH values. Boron isotope ratios were highly variable between different individuals but also within single shells. This corresponds to a high individual variability in fluid B/Ca ratios and may be due to high boron concentrations in the organic parts of the shell. The mean delta B-11 value shows no trend with pH but appears to represent internal pH (EPF) rather than ambient water pH

Disentangling the Biological and Environmental Control of M. edulis Shell Chemistry

Blue mussel individuals (Mytilus edulis) were cultured at four different salinities (17, 20, 29, and 34). During the course of the experiment, temperature was gradually increased from 6°C to 14°C. Mg/Ca and Sr/Ca ratios of the shell calcite portions produced during the 9 weeks of experimental treatment as well parts that were precipitated before the treatment phase were measured by laser ablation–multicollector–inductively coupled plasma–mass spectrometry. Mg/Ca ratios show a positive correlation with temperature for individuals cultured at salinity 29 and 34 (Mg/Ca (mmol/mol) ∼ (0.2–0.3)*T (°C)), while for individuals cultured at low salinities (17, 20) no trend was observed. Sr/Ca ratios were not affected by temperature but strongly by salinity. The data show very strong biological influence (“individual differences” and “physiological variability”) on elemental ratios (79% on Mg/Ca and 41% on Sr/Ca) in M. edulis calcite. The results challenge the use of blue mussel shell data as environmental proxies

Calcifying invertebrates succeed in a naturally CO2 enriched coastal habitat but are threatened by high levels of future acidification

CO2 emissions are leading to an acidification of the oceans. Predicting marine community vulnerability towards acidification is difficult, as adaptation processes cannot be accounted for in most experimental studies. Naturally CO2 enriched sites thus can serve as valuable proxies for future changes in community structure. Here we describe a natural analogue site in the Western Baltic Sea. Seawater pCO2 in Kiel Fjord is elevated for large parts of the year due to upwelling of CO2 rich waters. Peak pCO2 values of >230 Pa (>2300 μatm) and pHNBS values of 400 Pa (>4000 μatm). These changes will most likely affect calcification and recruitment, and increase external shell dissolution

Boron isotope ratio determination in carbonates /via/ LA-MC-ICP-MS using soda-lime glass standards as reference material

A new in situ method using LA-MC-ICP-MS (193 nm excimer laser) for the determination of stable boron isotope ratios (δ11B) in carbonates was developed. Data were acquired via a standard sample standard bracketing procedure typically providing a reproducibility of 0.5‰ (SD) for samples containing 35 ppm of boron. A single ablation interval consumed about 5 µg of sample corresponding to about 0.2 ng of boron. The major finding was the similar instrumental fractionation behaviour of carbonates, soda-lime glass and sea salt with respect to boron isotopes. As no matrix induced offset was detectable between these distinct materials we propose the use of NIST glasses as internal standards for boron isotope ratio measurements via LA-MC-ICP-MS. This finding overcomes the problem of a missing matrix matched carbonate standard for in situ boron isotope studies. As a first application a set of coral samples from a culturing experiment was analysed. δ11B values range from 19.5 to 25‰ depending on the pH of the water used in the particular treatment. This is in good agreement with the results of earlier studies

Modification of Ca isotope and trace metal composition of the major matrices involved in shell formation of Mytilus edulis

In this study we present the first combined investigation into the composition of the major matrices involved in calcification processes (surrounding water, extrapallial fluid, aragonite, and calcite) of Mytilus edulis with respect to their calcium isotope (d44/40Ca) and elemental compositions (Sr/Ca and Mg/Ca). Our aim was to examine the suitability of Mytilus edulis as a proxy archive and to contribute to the understanding of the process of biomineralization. Mytilus edulis specimens were live collected from the Schwentine Estuary, Kiel Fjord, and North Sea (Sylt). d44/40Ca was determined by thermal ionization mass spectrometry (TIMS) accompanied by measurements of Mg/Ca and Sr/Ca using inductively coupled plasma–optical emission spectroscopy (ICP-OES). The elemental and isotopic compositions of the investigated matrices showed systematic offsets. The carbonates are strongly depleted in their magnesium and strontium concentrations and fractionated toward lighter calcium isotope compositions relative to the surrounding Schwentine Estuary water. The opposite is observed for the extrapallial fluid (EPF). Our findings extend the results of previous studies reporting a strong biological control and the interaction of different environmental conditions influencing biomineralization. Future studies should focus on the temporal development of the interrelation between the different matrices