Data Dictionary - L. Legendre, S. Demers, G. Ingram, A. Mucci, T. Packard, S. Roy, J.-C. Therriault, A. Vézina, and B. Klein

Variables

1. Legendre, L. and Gosselin, M. 1997. Estimation of N or C uptake rates by phytoplankton using 15N or 13C: revisiting the usual computation formulae. J. Plankton Research 19(2): 263-271.

2. Savenkoff, C., Vezina, A.F., Packard, T.T., Silverberg, N., Therriault, J.-C., Chen, W., Berube, C., Mucci, A., Klein, B., Mesplé, F., Tremblay, J.-E., Legendre, L., Wesson, J. and Ingram, R.G. 1996. Oxygen, carbon and respiratory activity measurements in the deep layer of the Gulf of St. Lawrence and their implications for the carbon cycle. Canadian Journal for Fisheries and Aquatic Science 53: 2451-2465. (note: an excerpt from this paper is included as the sampling methods for this data set)

3. Tremblay, J.-E., Legendre, L., Klein, B., and Therriault, J.C. In press. Size-differential uptake of nitrogen and carbon in a marginal sea (Gulf of St. Lawrence, Canada): significance of diel periodicity and urea uptake. Deep-Sea Res.II (Special volume JGOFS-Gulf of St. Lawrence)

Sampling Methods - Dr. L. Legendre

Field Sampling

This study is based on data collected at three stations in the Laurentian Channel during nine cruises (not all stations were visited during each cruise). Sampling occurred between July 1992 and June 1994 in all months except January, February, and March. Station 1 is located in the Anticosti Gyre, about 100km east of the boundary between the St. Lawrence Estuary and the Gulf of St. Lawrence, and is characterized by high primary production (Steven 1975, see review of de Lafontaine et al. 1991; this study: 1057 mg C m^-2d^-1). Station 2 is located in the Anticosti Channel and Station 3 is in the Laurentian Channel west of Cabot Strait. These stations are in regions of low primary production (Steven 1975, see review of de Lafontaine et al. 1991; this study: 522 and 227 mg C m^-2d^-1 for station 2 and station 3, respectively).

During the first four cruises, a Secchi disk was used to estimate the light attenuation coefficient and the optical depths. During the subsequent five cruises, underwater irradiance was measured at stations sampled during the daytime with a photosynthetically active radiation (PAR) quantum meter (QSP 200PD, Biospherical Instruments). Samples from six optical depths (100,50,25,15,10, and 1% of the surface irradiance) and up to 11 depths below the photic zone (defined as extending to the depth where 1% of the surface incident radiation remains) were collected at sunrise using 8-L Niskin bottles mounted on a CTD-rosette. The depths sampled in the aphotic layer were usually as follows: 50, 75, 100, and 150m (intermediate water) and 200, 250, and, station depth permitting, 300, 350, 400, 450, and 500 m (deep water). An additional sample was collected at 10-20 m above the bottom. Samples for the determination of dissolved oxygen, DOC, and dissolved inorganic carbon (DIC) were drawn directly from the Niskin bottles. Samples for the measurements of phytoplankton biomass, primary production, inorganic nutrient concentration, CHN content of suspended particulate matter, and microorganism respiratory activity were prefiltered through a Nitex screen (200-um mesh size).

Physical and chemical variables

At each station, vertical profiles of irradiance, temperature, salinity, density, and turbidity were determined at 2-h intervals down to 5-10 m above the bottom with a SeaBird model SBE25 CTD instrument.

The dissolved oxygen concentration was determined by the Winkler method as described by Aminot and Chaussepied (1983) using the automated titration system developed by Jones et al. (1992). Nitrate (NO₃) concentration was determined on fresh samples using a Technicon autoanalyser II (Parsons et al. 1984). Water samples for phosphate (PO₄) and silicate (SiO₄) concentrations were frozen for later onshore use.

Primary Production

Primary production was determined using the ¹⁴C method described in Parsons et al. (1984). Subsamples from the six euphotic depths were incubated with 370 kBq of NaH¹⁴CO₃ for 24 h, starting at sunrise, in two deck incubators. Water temperatures were maintained at those of the upper and lower parts of the euphotic zone, and irradiances at the six sampling depths were stimulated using combinations of blue and neutral-density filters. At the end of the incubation, samples were filtered through 25-mm Whatman GF/F glass fiber filters. After the addition of 0.05 N HCl to the filters and subsequent evaporation, 10 mL of Ready-Safe scintillation cocktail was added. The activity, corrected for quenching and background radiation, was determined after the cruise using a LKB Minibeta scintillation counter.

Respiratory electron transport system (ETS) activity in the water column

For respiration measurements, we used the ETS method, an enzymatic approach that determines the capacity of the respiratory chain to transport electrons from the physiological substrates (NADH, NADPH, succinate) to oxygen (Packard 1971, 1985). The sample volumes were 4 and 11.5 L for the euphotic and aphotic layers, respectively. Samples for the ETS activity measurements were vacuum filtered (30 kPa or 3 atm) through 47-mm Whatman GF/F glass fiber filters. The filters were then stored in liquid nitrogen to preserve the enzyme activity (Ahmed et al. 1976). The assay procedure in the shore laboratory was performed following the methods described in Packard and Williams (1981) and Packard et al. (1983), respectively, for the euphotic and aphotic samples. The ETS activity was corrected from the incubation temperature (18 C) to in situ temperature using an Arrhenius activation energy of 15.8 kcalmol^-1 found by Packard et al. (1975). However, recent work in polar areas (temperature < 3 C) showed lower activation energy values of 11.5 and 12.0 kcalmol^-1 for the microplankton on the Barents Sea (Martinez 1991) and the Weddell Sea (Martinez and Estrada 1992), respectively. We used the mean (11.8 kcalmol^-1)of these two values for the samples collected in the layer with an in situ temperature below 3 C (the intermediate waters in our study).

To convert ETS activity to respiration in the surface layer, we used the following respiration equation from Aristegui and Montero (1995):

log₁₀ respiration = 0.357 + 0.750 log₁₀ ETS activity

where respiration and ETS activity are expressed in milligrams of O₂ per cubic meter per day. This equation was obtained from a large data set (n=197) on respiration and ETS activity from plankton communities (<225 um) of different surface oceanic areas: the east and central North Atlantic, the Baltic Sea, the Gulf of California, and the Antarctic Ocean (Aristegui and Montero 1995). To facilitate comparison with primary production, we used a respiratory quotient of 1 to convert moles of O₂ uptake to moles of CO₂ produced in the euphotic layer (Packard 1979). Multiplying by 12 converts these units to milligrams of carbon per cubic meter per day. Primary production and ETS activity values were integrated over the euphotic zone using the trapezoidal method.

In the aphotic zone, the ETS activity was converted into respiration rates (R) following the procedure described by Packard et al. (1988; R/ETS activity = 0.09). We then used a delta O/delta C molar ratio of 138/106 (Redford et al. 1963) to convert respiration values to metabolic CO₂ production.

CHN of suspended particulate matter

Samples for CHN determination of suspended particulate matter content were collected on precombusted 25mm Whatman GF/F filters, stored frozen, and analysed ashore with a Perkin-Elmer elemental analyser (model 2400 CHN).

DOC in water column samples

A 45-mL seawater sample was filtered through a 47-mm Whatman GF/F glass fiber filter; a 100-uL aliquot was then used for DOC analysis using the modified high-temperature catalytic oxidation method described by Chen and Wangersky (1993a, 1993b). DOC was only measured during the last two cruises (April and June 1994).

DIC in water column samples

Water column samples were drawn directly from the Niskin bottles into rinsed 125-mL polyethylene bottles and allowed to overflow. A few crystals of mercuric chloride (HgCl₂) were added to the bottles before they were capped leaving no headspace gap. Samples were kept at about 4 C until analysis.

A weighed amount of water (about 10g) was taken from each of the bottles with a syringe, to avoid gas exchange with the atmosphere, and injected directly into the gas stripper of a CO₂ coulometer. The total DIC concentration, sumCO₂, of the sample was determined following acidification with 2 mL of 2 N HCl and coulometric titration of the evolved CO₂. Total alkalinity, A_t, was determined by potentiometric titration with a standardized dilute HCl solution. The precision of the sumCO₂ and A_t analyses is 0.2 and 0.4%, respectively.

The carbonate system speciation and pH were calculated from the analytical data (i.e., CO₂ and A_t) using boric acid and carbonic acid stoichiometric dissociation constants based on the total hydrogen ion concentration scale, pH_t (Hansson 1973; Millero 1979) at in situ salinities, temperatures and pressures (Millero 1983). The borate ion contribution to the total alkalinity was estimated from the total boron concentration at the measured salinity (Culkin 1965). The partial pressure of CO₂ (PCO₂) in equilibrium with the solution was calculated using the carbon dioxide solubility constants of Weiss (1974) at 1 atm total pressure. SumCO₂ was measured and PCO₂ estimated for samples collected only during the last two cruises (April and June 1994).