45. Geographic variability in eco-physiological traits of benthic calcifiers along natural temperature and pH gradients: the Chilean coast as a natural laboratory

Nelson Lagos (1)*, Laura Ramajo (2) & Marco Lardies (3)

1 Centro de Investigación e Innovación para el Cambio Climático (CiiCC), Universidad Santo Tomás, Santiago, Chile
2 Centro para el estudio de forzantes multiples en sistemas socioecologicos (MUSELS), Universidad de Concepción, Chile
3 Departamento de Ciencias, Universidad Adolfo Ibañez, Santiago, Chile

Background
Chilean coast include a nested hierarchy of environmental processes modulating the abundance and distribution patterns of the benthic species. At biogeographycal scale there is an latitudinal gradient in sea surface temperature and pCO2 fluxes, and this gradient interact with the regional influence of upwelling and river discharge dynamics. In this study, we explore individual and population level responses of benthic calcifiers confronting this natural variability.

Methods
We use field survey along the Chilean coast and in situ reciprocal experiments to expose benthic calcifiers to natural changes in temperature and carbonate system parameters. We measured spatial variation in biomineralization, metabolism, abundance patterns in recruitment and carbonate production by benthic calcifiers (gastropods, mussels, barnacles).

Findings
Along the Chilean coast, we found changes in the proportion of calcium carbonate forms precipitated by the gastropods C. concholepas across their shells: the calcite:aragonite ratio in the pallial shell margin (i.e. newly deposited shell) increase significantly from northern to southern populations and this increase in calcite precipitation in the shell of juveniles snails was associated with a decrease in oxygen consumption rates in these populations. In the case of mussels, we determined that exposition to river–influenced conditions increased metabolic rates and reduced growth rates, as compared to mussels experiencing marine conditions. While the energy investment strategies of the two local populations resulted in similar net calcification rates, these rates decreased significantly when mussels were transplanted to the river- influenced site. Stressful conditions at the river-influenced site were reflected evidenced by decreased survivorship across treatments. Freshwater inputs modify the organic composition of shell periostracum through a significant reduction in polysaccharides. Spatial variation in barnacles metabolism showed non–clinal plasticity in the thermal reaction norm along the latitudinal gradient of the Chilean coast and suggest a biogeographic–scale dependence of the thermal sensitivity in the metabolism of the studied populations.

Conclusions
Our result suggests that along the Chilean coast calcite secretion may be favoured when metabolic rates of C. concholepas are lowered, as this carbonate mineral phase might be less energetically costly for the organism to precipitate. In the case of mussels, although our field experiment did not identify specific environmental factors underlying these biological differences, the observed phenotypic changes imply that plasticity plays a strong role when P. purpuratus are exposed to river–induced environmental variability in their habitats, and suggests that the lack of this exposure may promote less tolerant mussels with potential for local adaptation. Barnacles showed evidenced plasticity in metabolism and intra-population variability in thermal responses, with a strong role of thermal safety margins in determining the extent of these population differences.