Stable Isotope Values of Nitrogen and Carbon in Particulate Matter: Data
Water samples for stable isotope analysis and chlorophyll content were collected during NOAA Shelf-wide Research Vessel Surveys coordinated by the Oceans and Climate Branch in Narragansett, RI between 2000 and 2005 and 2010 to 2013. Cruises occurred several times per year over the continental shelf from Cape Hatteras, North Carolina to Cape Sable, Nova Scotia, and a subset of stations, encompassing the full geographic area, were selected during each cruise for stable isotope sampling. Carbon isotope data (δ13C) were only available for the Phase II dataset. Sample stations spanned the range from the Gulf of Maine to Cape Hatteras and most were unfixed.
For surface sample collection, an on-board flow-through seawater line was used to collect at least 200 ml of water which was first passed through a 300 µm mesh screen to remove zooplankton, and then through a pre-combusted glass fiber (GF/F, GE Whatman, gelifesciences.com) filter. For deep samples, a 5 L Niskin bottle (General Oceanics Inc., Miami, FL USA) was used to sample the bottom of the water column (to a maximum of 500 m) and the water captured by the Niskin was treated in the same manner as the surface seawater. Filters were frozen while the research cruise was underway. They were then dried in the lab in a 65 °C oven for a minimum of 24 hours. Once fully dry, filters were pelletized and analyzed on an Isotope Ratio Mass Spectrometer for δ15N and δ13C.
For surface chlorophyll samples, 200 ml of water was collected from the flow-through seawater line, passed through a 300 µm mesh screen and then passed through a GF/F glass fiber filter. The glass fiber filter was then placed in 7 ml of 90% acetone and stored in the freezer for 24 hours (JGOFS Protocols 1994). The extracted chlorophyll was read using a Turner Designs 10-AU fluorometer (Turner Designs, San Jose, CA USA). A solid standard was used to correct for instrument drift each time a reading was taken. For deep samples down to a maximum of 500 m, a 200 ml sample was drawn from the Niskin bottle used for the isotope sample and the water captured was treated in the same manner as the surface seawater. To determine the chlorophyll a level, the reading of the blank 90% acetone was subtracted from the extracted sample reading and multiplied by the solid standard calibration value and the ratio of volume of water filtered to the volume of acetone used to extract the chlorophyll.
The δ15N and particulate nitrogen (PN) content was determined using an Isoprime 100 Isotope Ratio Mass Spectrometer interfaced with a Micro Vario Elemental Analyzer (Elementar Americas, Mt. Laurel, NJ). The nitrogen isotope composition was expressed as a part per thousand (permil) deviation (δ15N ‰) from air, where δ15N = [(Rsample-Rstandard)/ Rstandard] × 103 and R is the ratio 15N/14N in either the sample or a reference standard (air). Samples were analyzed randomly in batches of approximately 30. We used laboratory standards to check for instrument drift in each run and to correct for instrument offset. The PN content on the filter was calculated by comparing the peak area of the unknown sample to a standard curve of peak area vs. standard N content. To determine the PN concentration, the N content of the material retained on the filter was divided by the volume filtered.