MOLLUSK SHELL δ15N VALUES AS INDICATORS OF DENITRIFICATION AND TROPHIC POSITION IN CONTRASTING SITES ACROSS CENTRAL AMERICAN ISTHMUS

Nitrogen is a bio-limiting macronutrient required by all organisms that limits primary productivity across large areas of the ocean. Biological assimilation of nitrogen is accompanied by a predictable kinetic isotopic fractionation which governs the distribution of nitrogen isotope ratios (¹⁵N/¹⁴N) in tissues and shells. The resulting natural abundance of ¹⁵N has been previously used to track changes in dominant source of nutrients, flux of nitrate, and reconstruct food-webs. Most studies, however, are limited to the use of tissue or skeletal material from primary producers or primary consumers to avoid confounding effects related to trophic enrichment in secondary consumers and predatory organisms.

Here, we report a new record of d¹⁵N of carbonate-bound organic nitrogen (CB- d¹⁵N) in two modern mollusk genera – carnivorous Conus and herbivorous Strombus – from localities that represent end-member oceanographic settings across the Central American Isthmus. While the Burica Peninsula in eastern equatorial Pacific (EEP) is largely influenced by seasonal blooms in plankton productivity related to upwelling, San Blas in the southwest Caribbean (SWC) is characterized by oligotrophy and hydrologic stability.

Our results indicate 8.4‰ difference in CB- d¹⁵N of Conus from EEP and SWC which is consistent with seasonal influx of nutrients and amplified denitrification in EEP that causes preferential enrichment of ¹⁵N. Higher average d¹⁵N (13.2‰) in Conus sp. than Strombus peruvianus (11.7‰) from Burica Peninsula corroborates the utility of these genera as reliable indices of trophic position, although the larger size (and thus, older age) of Strombus peruvianus may explain the higher d¹⁵N than what is commonly reported (9.2-7.5‰) for this primary consumer. High-resolution C-N-O isotope measurements across the shells show similar structure, but weak correlation of CB-d¹⁵N with O- and C- isotope values in Strombus and O-C in Conus is noted. Thus, seasonal upwelling within the EEP may be captured by d¹⁵N, with the potential to reveal timing and extend of individual productivity episodes. Natural variation in d¹⁵N of EEP specimens suggests d¹⁵N of original pool of NO₃^-1 to be ~5.5‰ after accounting for trophic enrichment and small fractionation during N₂ fixation.