Hydrographic Measurements
The R/V Hespérides carried three CTDs, two belonging to the Instituto Español de Oceanografía (IEO) and one belonging to Woods Hole Oceanographic Institution (WHOI). The two EG&G/NBIS Mark IIIb CTD underwater units belonging to IEO were each equipped with pressure, temperature, conductivity, and polygraphic oxygen sensors and were used throughout the cruise. Each CTD was configured identically with the same data scan length, variables, and scanning rate of 31.25 Hz. A detailed description of the Mark IIIb CTD can be found in Brown and Morrison (1978). Both instruments were modified at WHOI to add a titanium pressure sensor with a separately digitized resistive temperature device (RTD) (Millard et al. 1993). The third EG&G/NBIS Mark IIIb CTD was provided by WHOI but was not used during this expedition. A General Oceanics (GO) rosette fitted with 24 10- or 12-L Niskin bottles was used with the CTD for collecting water samples. The GO rosette bottles were mounted approximately 0.5 m above the CTD sensors. The conductivity and oxygen sensors were also calibrated at sea using the analysis of the water samples collected at each station. The depths of the sampling stations were based on classical standard depths, although they varied on a station by station basis according to participants' needs to sample a particular layer, provided there was no impairment of the in situ calibration activities. Because of delays inflicted by Hurricane Andrew on the equipment shipment from Miami to Woods Hole, the post-cruise calibrations were not performed on the CTD sensors until December.
All water sample measurements for bottle salinity and bottle oxygen were made in a constant temperature laboratory soon after each cast was completed. Water samples for salinity were collected from the Niskin bottles in Ocean Scientific International glass bottles, and the measurements were made within 24 hours after the station was finished. In total, 2,294 samples were measured. The bottle salinities were measured with a Guildline Autosal Model 8400A salinometer. The manufacturer claims a precision of 0.0002 and an accuracy of 0.003 when the instrument is operated at a temperature between +4 and -2°C of ambient temperature. All salinity measurements were made in a temperature-controlled laboratory about 1-3°C below that of the salinometer water bath.
Bottle oxygen determinations were carried out following the Winkler method and using the reagents prepared according to Carpenter (1965). On this cruise, the modified Carpenter's equation as given by Culberson et al. (1991) was used. The endpoint of tritration was determined visually using starch as the indicator. Reagents were dispensed with 0- to 2-ml capacity Dispensette glass and teflon dispensers from BRAND GMBH & CO. The dispensers had a certified accuracy of <0.6% and a coefficient of variation of <0.1%. The tips of the dispensers were lengthened up to 6 cm with thin plastic tubing to avoid the precipitation of manganese hydroxide in the neck of sample flasks. Titration was done with a Metrohm Dosimat E.412 automatic burette.
Samples for nutrient analyses (silicate, nitrate, nitrite, and phosphate) were collected in 150-mL acid-rinsed polythene flasks directly from the Niskin bottles, following the protocol established by the WOCE Hydrographic Program. Analyses were performed onboard with a four-channel Skalar, Inc. segmented flow autoanalyzer. Analyses were carried out immediately without any treatment of the samples. When necessary, samples were kept in the cold room (unfrozen and never for more than 10 hours) without additives. The analytical techniques followed were those described by Whitledge et al. (1981) with minor modifications to adapt them to the particular conditions of the instrument used and concentration ranges observed. Primary standards were prepared at the beginning and in the middle of the cruise following Whitledge et al. (1981). Secondary standards were prepared every two days and preserved with several drops of chloroform in the refrigerator. Running standards of various concentrations were prepared daily, and calibration curves were run at the beginning of each session. Standards were interleaved with unknown samples in order to provide a measure of analytical stability. Whenever changes in sensitivity were noticed (particularly in the case of nitrate), the standards allowed for a correction to be applied. All concentrations were referred to double distilled water prepared by reverse osmosis. No seawater sample has ever given a concentration negative with respect to this double distilled water. Phosphate analyses were corrected for the change in absorbance as a result of the salinity effect. Surface seawater was used as a carrier and, except for silicate, it always showed the minimum concentrations in the water column. Silicate concentrations below the surface were often found to be lower than those at the surface and very close to the values given by double distilled water. Replicate samples were analyzed at various depths.
More detailed information on hydrographic measurements can be found at http://whpo.ucsd.edu/data/onetime/atlantic/a05/index.htm.
