DEEPENING OUR UNDERSTANDING OF THE DELTA-13C BLACK BOX IN BIOGENIC CARBONATES THROUGH A COMPARATIVE ANALYSIS OF BIVALVE SHELLS AND FISH OTOLITHS

The carbon isotopic composition of biogenic carbonates such as fish otoliths and bivalve shells can yield relevant information on the environmental conditions experienced by an organism (phytoplankton dynamics) as well as individual life traits (migration). An increasing body of evidence shows that metabolism controls the variations of the δ¹³C at the seasonal level and throughout ontogeny in both otoliths and shells. Yet, it remains a challenge to interpret this signal as the relative contributions of the two carbon sources - dissolved inorganic carbon (DIC) and respired CO₂ derived from food – remain poorly understood.

Time series of δ¹³C in carbonates often show a different pattern in bivalves and fish (decreasing or increasing δ¹³C values through age). Differential metabolic carbon incorporation through age has been proposed to explain these trends but the metabolic contribution is considered differently in fish and bivalves. For bivalves, the metabolic contribution would be the total amount of CO₂ produced. The decrease of shell δ¹³C values can then be explained by an increase of the total quantity of respired CO₂ available relative to a decrease of the carbon demand for calcification through age. For fish, it is a lower “metabolic production rate” (quantity of respired CO₂ per unit of weight) through age that would explain the increase in otolith δ¹³C values.

After showing experimental and field results on two bivalves Pecten maximus and Ruditapes philipinarum confirming that δ¹³C can be successfully used to track environmental parameters, we investigated the underlying mechanisms that control the δ¹³C in bio-calcified structures from a modeling perspective. Based on the Dynamic Energy Budget (DEB) theory, we modeled the relative contributions of respired CO₂ and DIC as a function of the individual state of the organism and its environment. We suggest that entrance of DIC might be different in bivalve and fish to reproduce the observed contrasted patterns, i.e., proportional to a surface area in bivalves and to a volume in fish. Further study of the mechanisms involving DIC entrance is required but we show how modeling fish and bivalve metabolism within the same conceptual and quantitative framework help reveal mechanisms underlying δ¹³C biocarbonate patterns across taxa.