NOAA Long Line Cruises : N92F
Methods and Materials

CLEAN WATER FOR PRODUCTIVITY AND GRAZING EXPERIMENTS

Water for the productivity and grazing experiments was collected at six fixed depths, representing 100, 50, 30, 15, 5 and 1% of the surface irradiance (as determined with a secchi disk), with dedicated, Teflon-coated Go-Flo bottles lowered on a Kevlar cable and closed with Teflon messengers. The type of sampling system and cleaning of components, as well as bottle handling and filtration, was modeled after the recommendations of FITZWATER et al. (1982). In addition to samples from the Kevlar casts, measurements of chlorophyll and particulate carbon and nitrogen were made on samples collected in the upper 200m with the rosette sampler on the CTD.

PRIMARY PRODUCTIVITY

The stable isotope ¹³C (HAMA et al. 1983; SLAWYK et al. 1984) rather than the radioactive isotope 14C was used to measure primary production because of concerns by NOAA investigators collecting samples for measurement of natural levels of ¹⁴C. Samples were drawn into 280 ml polycarbonate bottles which had been washed using the FITZWATER et al. (1982) procedure that is used for cleaning the Go-Flo bottles. To each sample bottle Na213CO3 (minimum 99.9%13D; Cambridge, US) was added to reach a concentration of 7.2% of the total inorganic carbon in the ambient seawater (KANDA et al., 1985). An initial sample was inoculated with the tracer and filtered immediately with no incubation to determine abiotic particulate 13C incorporation and the initial isotopic ratio. The bottles were encased in nickel screens (Perforated Products) that acted as neutral density filters reducing the light intensity to the same level as that occurring at the depth from which the sample was collected, and were incubated on-deck, in surface seawater-cooled Plexiglas incubators. All samples were incubated for 24 hours under natural sunlight, however samples were collected and incubations started at various times of the day. At the lower light intensities parallel incubations using blue filters were conducted but no significant difference between "white" and "blue" conditions was found (data not shown). For determination of particulate carbon fixation, the water was filtered onto Whatman GF/F filters at <250 mm mercury. The filters were dried at 60ºC and stored in a dessicator for latter mass spectrophotometric analysis ashore. The analysis was carried out on a Europa Mass Spectrometer. The calculation of production follows the rationale of DUGDALE and WILKERSON (1986) for 15N:

Rates of primary production were similar for in situ and simulated in situ incubations on the NSF cruises (BARBER, 1993). Differences between these incubation methods, however, were encountered when integrating over the water column using the in situ depths and those estimated using an optical model (MOREL, 1988; CHAVEZ et al., 1990). The in situ integrated values were 1.39 times the simulated in situ integrated values (BARBER, 1993). For the present set of calculations, water column production measurements calculated using the optical model were multiplied by 1.39.

CHLOROPHYLL

Chlorophyll a and phaeopigments were determined by the fluorometric technique using a Turner Designs Model 10-005 R fluorometer that was calibrated with commercial chlorophyll a (Sigma). Samples for determination of plant pigments were filtered onto 25-mm Whatman GF/F glass fiber filters and extracted in 90% acetone in a freezer for between 24 and 30 hours (VENRICK and HAYWARD, 1984). Other than the modification of the extraction procedure, the method used is the conventional fluorometric procedure of HOLM-HANSEN et al. (1965) and LORENZEN (1966). Additional samples were also filtered onto 0.2, 1.0 and 5.0 mm pore size Nuclepore membrane filters.

POC AND PN

Samples for the analysis of particulate organic carbon (POC) and particulate nitrogen (PN) were collected on precombusted GF/F filters. Volume filtered was 1L but on occasion replicate volumes (0.5, 1 and 2L) were filtered. At one station water was also filtered through 0.2 m Anopore filters as described by ALTABET, et al. (1989). Filters were dried at 60ºC, fumed with concentrated HCL, dried at 60ºC and packed into nickel sleeves. The analysis was carried out on a Leeman Labs CHN analyzer using acetanilide as a standard.

PROTISTAN BIOMASS

Phytoplankton and small heterotrophs were sized and counted with epifluorescence microscopy (CHAVEZ et al. 1990, 1991). In addition to enumerating organisms on 0.2 mm pore size filters, larger and rarer organisms were enumerated on 5.0 mm pore size filters through which 200 ml was filtered.

In CHAVEZ et al. (1991) we erroneously reported the use of STRATHMANN (1967) volume to carbon conversions for flagellates and diatoms. The conversions used are the modifications reported by EPPLEY et al. (1970). Comparison of these formulae with those of VERITY et al. (1992) shows these two are equivalent (slope not significantly different than 1, intercept not significantly different than zero). For Synechococcus we have again used the GLOVER et al. (1988) (94fgC cell-1) biomass estimate. This is substantially lower than that reported by CUHEL AND WATERBURY (1984) (290fgC cell-1) or VERITY et al. (1992) (246fgC cell-1). As in CHAVEZ et al. (1991) we estimate prochlorophyte biomass from chlorophyll. Our estimate of 24 fgC cell-1 is substantially lower than the 53 fgC cell-1 used by CAMPBELL et al. (1994). Large differences are also found between our estimate of 495 fgC cell-1 for eukaryotic picoplankton and 2100 fgC cell-1 of CAMPBELL et al. (1994).

MICROZOOPLANKTON GRAZING

Grazing rates on the small phytoplankton and their growth rates were measured using minor modifications of the dilution technique by LANDRY AND HASSETT (1982) and following JGOFS protocols. Samples from the 30% light level (as determined with a secchi disk) were diluted with filtered water from that depth. The 30% light level fluctuated between 20 and 30 meters. The dilutions were at 100, 75, 50, and 25% of the initial sample. Changes in biomass over 24h incubations were followed with chlorophyll extractions. Incubations were for 24 hours in simulated in situ deck incubations with neutral density filters.

UNDERWAY MEASUREMENTS

A mapping system for continuous measurement of fluorescence using a Turner Designs 10-005 and a SeaTech fluorometer, photosynthetically available radiation sensors (PAR), using two sensors: a LiCor 192-SA and a Biospherical QSR-240, location using a Magellan GPS board, and nitrate using a prototype nitrate analyzer based on a Kloehn syringe that was developed at MBARI (SAKAMOTO et al. 1995) and was used throughout the NOAA survey cruises.