Methodology:

Abundance and distributional sampling of pteropods was conducted in the western Gulf of Maine (GoM) at Murray Basin (42 21’ N and 69 47’ W) on various cruises aboard the R/V Tioga. During each cruise, vertically stratified net and hydrographic sampling was conducted using a standard 1/4-m2 Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS; Wiebe et al., 1985) with 150-m mesh nets that was towed at 8 discrete depths throughout the water column. All net tows were conducted during daylight hours. The upper nets consistently targeted 0–25, 25–50, 50–75, 75–100, and 100–150 m. The depths of the lower three nets were chosen adaptively during each cruise based on transmissometer profiles such that the deepest two nets sampled exclusively within the benthic nepheloid layer, in order to examine any associations of pteropods with the particular chemistry of this bottom resuspension zone (see Wang et al. 2017). Zooplankton samples were preserved in 70% ethanol for later enumeration and size classification. In the lab, splits of each MOCNESS net sample were examined under a lighted stereomicroscope and L. retroversa were enumerated based on size class (<0.5 mm, 0.5-1 mm, 1-3 mm and >3 mm) based on the generalized developmental stage size categories developed by Hsiao (1939).

CTD casts were also routinely conducted at the Murray Basin site using 3-L Niskin bottles and a SBE3/SBR4 sensor set, to characterizing the local hydrography (salinity, temperature, fluorescence, dissolved oxygen, and beam transmission) and carbonate chemistry. Depths for bottle sampling were chosen based on station water depth with a typical profile sampling the upper 100 m depths at 10 m intervals, the 100-200 m depths at 20 m intervals, and less frequently below. Samples for dissolved inorganic carbon (DIC) and total alkalinity (TA) were collected in 250 mL Pyrex borosilicate glass bottles following the best practice of seawater CO2 measurements (Dickson et al. 2007). Air head space of about one percent of the bottle volume was left to allow room for expansion. Each sample was then poisoned with 100 L of saturated mercuric chloride, capped with an Apiezon-L greased stopper, thoroughly mixed, and then tied with a rubber band over the glass stopper.

Sampling and analytical procedures:

For lab measurements of carbonate chemistry, DIC was measured using an Apollo SciTech DIC auto-analyzer, while TA was measured using an Apollo SciTech alkalinity auto-titrator, a Ross combination pH electrode, and a pH meter (ORION 3 Star) based on a modified Gran titration method (detailed in Wang et al., 2017). pH and aragonite saturation state (ΩAr) were calculated from bottle sample measurements and concurrent temperature and salinity measures from the CTD cast using the CO2SYS program by Pierrot et al. (2006) with constants from Mehrbach (1973) as refit by Dickson and Millero (1987).