Eelgrass and substrate characteristics in Bellingham Bay, Washington, July 2019

SEDIMENT COLLECTION: The surface 0-2 centimeters of sediment was scooped by hand at the beginning, middle, and end of lower intertidal transects where eelgrass was characterized. Sediment was stored chilled until analysis.

SEDIMENT GRAIN-SIZE AND TOTAL ORGANIC CARBON CONTENT (TOC): Sediment particle size distributions were determined at the USGS Pacific Coastal and Marine Science Center sediment laboratory. To remove organic material, 10 milliliters of 35 percent hydrogen peroxide were added to 20 grams of sediment. The sample was heated to 250 degrees Celsius to remove any remaining hydrogen peroxide and was placed in an ultrasonic bath to liberate the less than 0.063 millimeter-diameter (fine) fraction. The sample was centrifuged for 1.5 hours in deionized water to remove soluble salts. Samples were funneled while wet through 2-millimeter and 0.063-millimeter mesh sieves to separate gravel-sized (greater than 2 millimeter), sand-sized (0.063 to 2 millimeter), and mud-sized (less than 0.063 millimeter) particles. The sand and gravel fractions were dried at 80 degrees Celsius and weighed. Five milliliters of sodium hexametaphosphate was added to the mud fraction and a 20 milliliter aliquot was taken from a 1 liter graduated cylinder to determine the weight of the mud. Total carbon and total inorganic carbon of bulk sediment were determined coulometrically (UIC, Inc.) at the USGS Pacific Coastal and Marine Science Center Sediment Laboratory. Samples were run in duplicate. Coulometer performance was verified relative to a calcium carbonate reference material. Total organic carbon content (TOC) was calculated as the difference between total carbon and total inorganic carbon.

RATIOS OF STABLE CARBON 13/12 ISOTOPES (d13C) and TOTAL CARBON TO TOTAL NITROGEN (C:N): Dried and homogenized sediment was analyzed for d13C and C:N ratios using a CE Instruments NC2500 elemental analyzer interfaced to a ThermoFinningan Delta Plus XP isotope ratio mass spectrometer at the University of California at Santa Cruz Stable Isotope Lab. A decarbonation pre-treatment step consisting of fumigation with sulfurous acid removed inorganic carbonates. Stable isotope ratios were corrected for size, drift, and source stretching effects. d13C values were referenced to the Vienna PeeDee Belemnite.

EELGRASS SHOOT DENSITY- Numbers of eelgrass shoots were counted in whole or fractions of 0.25-square-meter quadrats along a shore-parallel transect at approximately -1.0 m depth (lower intertidal) relative to mean lower low water (MLLW). Eelgrass stems with fully formed sheaths at the sediment surface were counted as individual shoots. Alongshore transect lengths were 100 meters at Marine Park, Boulevard Park West, and Boulevard Park North, and 85 meters at Taylor Bridge. In-field shoot counts were normalized to 1 square meter.

EELGRASS LEAF LENGTH, WIDTH, AND NUMBER- Eelgrass plants with the longest intact leaves inside or adjacent to quadrats were harvested in their entirety, including as much of the subterranean rhizome as could be recovered. Lengths of the longest leaf on harvested plants were measured from the top of the sheath to the tip. Widths of the second-newest leaf on harvested plants were measured 5 centimeters above the sheath. Numbers of leaves in the sheathes of harvested plants, excluding leaves of daughter shoots, were recorded.

EELGRASS INTERNODE LENGTH- Distances between rhizome nodes were measured sequentially on the main rhizome stem using a digital caliper with a resolution of 0.01 mm (Fowler). Rhizome segment 1 is the newest fully formed rhizome with erupted root bundles.

EELGRASS DRY BIOMASS- After collection, eelgrass leaves were rinsed with tap water and scraped with a razor blade to remove epiphytic organisms. Eelgrass rhizomes were rinsed with tap water to remove sediment and epiphytic organisms. Eelgrass shoots and rhizomes were separated at the meristem, daughter plants were removed from shoots and rhizomes, and plants were stored moist for less than 1 week. Shoots and rhizomes were dried at 60 degrees Celsius for at least 24 hours and weighed to the nearest 0.001 gram.