Dataset: CN isotopes seawater and zooplankton

Dissolved inorganic carbon (DIC) – 20 ml of seawater was filtered (0.45 µm pore size) and preserved in a glass crimp-top bottle with 200 µl of dried mercuric chloride for subsequent isotope analysis. The remaining seawater collected from each site was pre-filtered through 55 µm nylon mesh to remove large particulates and zooplankton. The d13C of DIC samples was analyzed at the Duke University Environmental Isotope Laboratory using a Thermo Finnigan Delta Plus XL continuous flow mass spectrometer via a ThermoFinnigan GasBench II and reported in permil relative to V-PDB. Repeated measurements of internal standards had a standard deviation of ± 0.2 per mil.

Dissolved organic carbon (DOC) – 40 ml of seawater was filtered through a quartz microfiber filter into a polytetrafluoroethylene bottle and acidified using 10% HCl to reach a pH below 2 to preserve the sample. The d13C of DOC samples was analyzed using an OI Analytical 1030D Total Organic Carbon Analyzer in the Aquatic Biogeochemical Laboratory at North Carolina State University and reported in permil relative to V-PDB. Repeated measurements of external standards had a standard deviation of ± 0.4 per mil.

Particulate organic matter (POM) – Between 3 – 4 L of the seawater was filtered through a quartz microfiber  filter until the filter became light brown in color and was immediately stored at –20 degrees C for subsequent isotopic analysis of particulate organic carbon (POC) and particulate organic nitrogen (PON). The POM filters were fumigated with 12 M hydrochloric acid to remove carbonates (Moyer et al. 2013). All samples were combusted using a PDZ Europa ANCA-GSL elemental analyzer interfaced to a PDZ Europa 20-20 isotope ratio mass spectrometer (Sercon Ltd., Cheshire, UK) at the University of California (UC) Davis Stable Isotope Facility. The d13C and d15N values are reported in permil relative to V-PDB and air, respectively. The standard deviation of replicate analyses for these POM samples was ± 0.31 per mil for d13C. The standard deviations of replicate analyses for d15N of POM was not able to be calculated due lack of additional sample material.

Zooplankton – Zooplankton were collected using a bucket with an illuminated dive torch affixed to the bottom, which was placed on the seafloor near the reef at 1 m depth for 5 min. The zooplankton were separated into 150 – 400 µm and 400 – 800 µm size fractions in the field, stored on ice, and then isolated onto a glass fiber filter and stored at –20 degrees C upon return to the lab the same day. The zooplankton samples were processed separately for d13C and d15N. For d13C, zooplankton samples were fumigated with 12 M hydrochloric acid to remove carbonates. The samples were processed for d15N without pretreatment to ensure that these isotope values were unaffected by acidification. All samples were combusted using a PDZ Europa ANCA-GSL elemental analyzer interfaced to a PDZ Europa 20-20 isotope ratio mass spectrometer (Sercon Ltd., Cheshire, UK) at the University of California (UC) Davis Stable Isotope Facility. The d13C and d15N values are reported in permil relative to V-PDB and air, respectively. The standard deviation of replicate analyses for these zooplankton samples was ± 0.55 per mil for d15N of zooplankton. The standard deviations of replicate analyses for d13C of zooplankton was not able to be calculated due lack of additional sample material.