Environmental and intraspecific dimorphism effects on the stable isotope composition of deep-sea benthic foraminifera from the Southern Gulf of California, Mexico

We studied the influence of microhabitat, organic matter flux, and metabolism on the stable oxygen and carbon isotope composition of live (Rose Bengal stained) and dead (empty tests) dimorphic generations (microspheric/sexual and megalospheric/asexual) of deep-sea benthic foraminifera from the southern Gulf of California. Despite the range of environmental conditions that exists between the investigated sites, the distribution and live abundance of dimorphic generations of Bolivina subadvena Cushman and B. argentea Cushman were similar: in every site, each dimorphic generation displayed the same relative depth distribution. Maximum abundance of megalospheric B. argentea was observed between 0 and 2 mm, whereas the microspheric generation of this species is most abundant at depths between 1 and 3 mm. For B. subadvena, megalospheric individuals concentrated at the organic-rich boundary layer (0-1 mm), while individuals of the microspheric generation were most abundant deeper in the sediment (4 and 6 mm). Different specific responses to carbon rain variability were indicate by the variation in the proportions of dimorphic generation and by the consistently lighter δ13C and δ18O values observed in B. subadvena compared to B. argentea. Indicating that B. subadvena populations are being maintain, largely, through asexual reproduction (megalospheric generation), which requires high metabolic rates that account for a larger incorporation of metabolic CO2 into their tests. The comparison of isotopic measurements from three size classes (125, 150 and 250 µm) in dimorphic generations of B. subadvena showed that isotopic fractionation related to ontogeny is small compared to the variation due to dimorphism, illustrating the potential use of the isotopic signals of these two species in the reconstruction of past organic matter fluxes, if the species life strategies are considered.