Abstract
Variation in seawater magnesium (Mg) and calcium (Ca) concentrations over the past 500 million years has influenced marine calcite composition. A local field study showed that present-day nearshore waters exhibited localized variation in [Mg], most likely due to freshwater mixing. This observation prompted investigation into how different water chemistries would affect the calcite of invertebrates. Artificial seawater was utilized to create multiple Mg/Ca conditions to carry out this research. A cost-effective yet robust methodology for artificial seawater suitable for invertebrate growth study was developed.
Blue mussels (Mytilus edulis), jingle oysters (Anomia trigonopsis), and blue tubeworms (Spirobranchus cariniferus) were grown for 50 days in artificial seawater with different Mg/Ca concentrations. Skeletal Mg/Ca growth responses were modeled from weight percentage of MgCO3 identified in new calcite growth. All three species were affected when Mg/Ca ratios were at the lowest level of 1.5. At seawater Mg/Ca = 1.5, blue mussel and jingle oyster had lower survival rates. However, the survival rates of blue mussels were not affected by Mg/Ca when held in a seawater with decreasing Mg/Ca levels over a 210-day dynamic treatment experiment. Low levels of Mg/Ca were antagonistic to populations of blue mussels only when abruptly treated and not when gradually introduced.
At Mg/Ca = 1.5, tubeworms precipitated calcite with lower amounts of MgCO3. Low Mg calcite produced by oysters and mussels was not influenced by decreased Mg concentration. Tubeworms became unable to regulate their calcification at low Mg concentrations and produced more brittle calcite. This supports a new model describing a tipping point at which active calcification strategies can be influenced to alter in a stepwise fashion. A species that actively regulates calcite precipitation can be influenced to create a calcite mineralogy with MgCO3 content significantly different than its normal weight percentage of Mg at a tipping point and become semi-active or passive in calcification regulation. The boundary of this tipping point seems to be species specific and is overlooked in experiments with few treatment conditions. Further study with more treatments in small increments would provide better resolution to focus on the exact moment of loss of calcification control.