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 <i>a </i>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 = [(R_sample-R_standard)/
R_standard] × 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.