Primary productivity, pulse amplitude modulated fluorometry and nutrient drawdown data from the marginal ice zone: BROKE-WEST survey 30o-80oE, 2006
Taken from the paper: 2. Methods 2.1 Oceanographic Survey The survey area was located between 30 degrees and 80 degrees E longitude off the Antarctic coast, and consisted of 11 parallel north to south (N-S) transects every 5 degrees of longitude (Figure 1). Stations along each N-S transect ranged fr...
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Australian Ocean Data Network
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| Online Access: | https://researchdata.ands.org.au/primary-productivity-pulse-80oe-2006/684843 https://data.aad.gov.au/metadata/records/BROKE-West_primary_productivity https://data.aad.gov.au/eds/2572/download https://secure3.aad.gov.au/proms/public/projects/report_project_public.cfm?project_no=40 http://data.aad.gov.au/aadc/metadata/citation.cfm?entry_id=BROKE-West_primary_productivity |
| Summary: | Taken from the paper: 2. Methods 2.1 Oceanographic Survey The survey area was located between 30 degrees and 80 degrees E longitude off the Antarctic coast, and consisted of 11 parallel north to south (N-S) transects every 5 degrees of longitude (Figure 1). Stations along each N-S transect ranged from 61 degrees 30 minutes S in the north to as far south onto the continental shelf as sea-ice allowed. CTD stations were conducted along every second transect. The survey was conducted from west to east over a three month period, commencing in mid-January and finishing in mid-March 2006. An additional east to west (E-W) transect, perpendicular to the main transects, was conducted along the northern boundary of the survey area in early January at approximately 60 degrees to 62 degrees S. Oceanographic results are described with respect to large-scale circulation by Meijers et al. (this issue) and physical-biological interactions in surface layers by Williams et al. (this issue). For a comprehensive description of CTD operations see Rosenberg (2006). CTD casts were conducted using SeaBird SBE9plus instrumentation and 22 x 10 L General Oceanics Niskin bottles mounted on a SeaBird rosette. Additional probes attached to the rosette included a dissolved oxygen sensor (SBE43), fluorometer (Wet Labs ECO), photosynthetic active radiation (PAR) sensor (LI-COR), transmissometer (Wet Labs C-star), three altimeters (TriTech 200 kHz, TriTech 500 kHz, Benthos model 2110), and an LADCP with upward- and downward-looking transducer sets. Incoming PAR was also measured continually from both port and starboard sides of the ship using LI-COR sensors. Water from Niskin bottles was sampled for dissolved oxygen, salinity, nitrate, nitrite, silicate, dissolved inorganic carbon, primary productivity, pulse-amplitude-modulated (PAM) fluorometry, HPLC pigments, and microbe abundance. Unfortunately there was a problem with phosphate analysis and accurate data were unable to be obtained. Primary productivity incubations and PAM measurements were conducted every second to third CTD station along N-S transects. Six depths were sampled per stations ranging from the surface to 150 m. Sample depths were based on downward fluorescence profiles and two of six samples always included both the surface (approximately 10 m) and the depth of the chlorophyll maximum. For sampling, 400 ml water was removed from appropriate Niskin bottles and stored in darkened polycarbonate jars in a sea-water cooled insulated container, until the commencement of incubations. 2.2 Primary Productivity Measurements Primary productivity incubations were conducted according to the method of Griffiths et al. (1999), based on the small bottle 14C technique of Lewis and Smith (1983). 6.327 x 106 Bq (0.171 mCi) NaH14CO3 were added to 162 ml of sample to produce a working solution of 39.183 x 103 Bq per ml (1.1 micro C ml-1). Seven ml aliquots of working solution were then added to transparent glass scintillation vials and incubated for 1 hour at 21 light intensities ranging from 0 to 485 micro mol m-2 s-1. The temperature of the light incubator was controlled by water-baths set to within plus or minus 0.1 degrees C of in situ values. After 1 hour, 250 micro L of 6 M HCl was added to each vial and they were then agitated for 3 hours to ensure that all inorganic carbon was removed. Gaseous 14CO2 was trapped in Carbosorb cartridges after being pumped through silica gel to ensure the air was dry. For radioactive counts, 10 ml Aquasure scintillation fluid was added to each vial and shaken. Samples were then counted using a Beckman LS6500 scintillation counter with the maximum counting time set at 5 min. In addition, Time 0 counts were taken to determine background radiation and 100% counts were used to determine the specific activity of the working solution. For Time 0 counts, 7 ml aliquots of working solution were subjected to acid addition without any exposure to light, and counted after shaking for 3 hours. For 100 percents, 100 micro L of working solution from each depth was added to 7 ml NaOH (0.1 M) and immediately counted following the addition of scintillation fluid. Carbon uptake rates were corrected for in situ chlorophyll a concentrations measured using HPLC (Wright et al., this issue) and for total dissolved inorganic carbon availability analysed using coulometric procedures (Johnson et al., 1998). Photosynthesis-irradiance (P-I) relationships were then plotted and the equation of Platt et al. (1980) was used to fit curves to data using least squares non-linear regression. Photosynthetic parameters determined included light-saturated photosynthetic rate [Pmax, mg C (mg chl a)-1 h-1], initial slope of the light-limited section of the P-I curve [alpha, mg C (mg chl a)-1 h-1 (micro mol m-2 s-1)-1], light intensity at which carbon-uptake became maximal (calculated as Pmax/ alpha = Ek, micro mol m-2 s-1), intercept of the P-I curve with the carbon uptake axis [c, mg C (mg chl a)-1 h-1] , and the rate of photoinhibition where applicable [Beta, mg C (mg chl a)-1 h-1 (micro mol m-2 s-1)-1]. 2.3 Production Modelling For modelling of primary productivity, 2 m depth interval profiles of chlorophyll a from the surface to 150 m were constructed. This was achieved by the conversion of up-cast fluorometry data measured at each CTD station. For conversions, fluorometry burst data from six to twelve depths was linearly regressed against in situ chlorophyll a concentrations measured at the same depths, as determined using HPLC (Wright et al., this issue). Linear equations established for each station were then used to convert the remaining fluorometry data to chlorophyll a concentrations. In some cases the relationship between fluorescence and chlorophyll a was not significant (e.g. midday non-photochemical quenching or high biomass). For these stations, chlorophyll a was linearly interpolated between measured values. This method provided a lower resolution of chlorophyll a in the water-column, but a comparison using stations that had a significant relationship between fluorescence and chlorophyll a showed that the maximum variation was only 10%. Gross daily depth-integrated water-column production was calculated using chlorophyll a depth profiles, photosynthetic parameters (Pmax, alpha , Beta), 5 minute-averaged incoming PAR, vertical light attenuation (Kd) and mixed layer depth. Vertical light attenuation for each station was calculated through linear regression of ln-transformed PAR data with depth. In cases where CTD stations were conducted at night, Kd was calculated from a linear relationship established between average chlorophyll a concentrations and Kd's determined at CTD stations conducted during the day (see results). 2.4 PAM Fluorometry Fluorometry measurements were taken at the same stations and depths as primary productivity measurements using a Water-PAM (Walz, Effeltrich, Germany). Prior to measurement, samples were stored in a sea-water cooled insulated container and dark adapted for 30 minutes to ensure that non-photochemical quenching was relaxed. PAM measurements were then conducted in a temperature-controlled room set at 4 degrees C. To assess the photosynthetic performance of cells the Fv/Fm ratio was used, where Fv = Fm-Fo (Falkowski and Raven 1997; Schreiber 2004). This is essentially a measure of the quantum efficiency of photosystem II and provides an indication of cell health. 2.5 Nutrient-Derived Production To compare production calculated from seasonal data against gross daily production, nutrient drawdown conversions were used. Drawdown was calculated for both oxidised inorganic nitrogen (nitrate + nitrite, hereafter NO3) and silicate. For drawdown calculations, concentrations in the mixed layer were averaged and subtracted from those in the remnant winter mixed (Tmin) layer, after Jennings et al. (1984, see also Strutton et al. 2000). Nitrogen and silicate drawdown concentrations were then converted to equivalent carbon (by atoms) using the ratios suggested by Redfield (1958; C:N = 6.6) and Copin-Montegut and Copin-Montegut (1978; Si:C = 0.4 or C:Si = 2.5). Concentrations were integrated over the mixed layer depth and converted to daily measurements through division by the number of growing days. The number of growing days was assumed to be from the date of ice-melt at each CTD station to the date that the CTD station was sampled (Schwarz et al., this issue). In some cases sea-ice was still present at the CTD station so that the number of growing days was 0. Data for these sites was excluded as various assumptions for the number of growing days (e.g. 1 to 10 days) yielded unrealistic results. Additionally, some of the northern sites on Transects 9 and 11 never had sea-ice in the previous year and data for these sites was also excluded. Seasonal oxygen changes were also calculated in a similar manner to NO3 and silicate drawdown. Oxygen concentrations in the mixed layer were averaged and subtracted from average values in the Tmin layer. Negative values indicated oxygen production rather than drawdown. This data set contains primary productivity, pulse amplitude modulated fluorometry, and nutrient drawdown numbers associated with the abstract presented below. 14C Primary Productivity Gross column-integrated primary productivity determined through measurement of NaH14CO3 uptake by phytoplankton (1 hour incubations). Primary productivity was modelled from photosynthesis v irradiance curves, chlorophyll profiles, photosynthetically active radiation, and vertical light attenuation. Data for these parameters are also shown. Nutrient Draw-down Data Seasonal depletion of oxidised inorganic nitrogen and silicate in the mixed layer, and production of oxygen. Data was calculated by the subtraction of mixed layer concentrations (uM) from values below the mixed layer. Pulse Amplitude Modulated Fluorometry Data Fv/Fm values determined using pulse amplitude modulated fluorometry (PAM). Samples were dark-adapted prior to measurement so that non-photochemical quenching was relaxed. Values provide an indication of cell health. Abstract Primary productivity was measured in the Indian Sector of the Southern Ocean (30 degrees to 80 degrees E) as part of a multi-disciplinary study during austral summer; Baseline Research on Oceanography, Krill and the Environment, West (BROKE-West Survey, 2006). Gross integrated (0-150 m) productivity rates within the marginal ice zone (MIZ) were significantly higher than within the open ocean, with averages of 2110.2 plus or minus 1347.1 and 595.0 plus or minus 283.0 mg C m-2 d-1, respectively. In the MIZ, high productivity was associated with shallow mixed layer depths and increased Pmax up to 5.158 mg C (mg chl a)-1 h-1. High Si:N drawdown ratios in the open ocean (4.1 plus or minus 1.5) compared to the MIZ (2.2 plus or minus 0.79) also suggested that iron limitation was important for the control of productivity. This was supported by higher Fv/Fm ratios in the MIZ (0.50 plus or minus 0.11 above 40 m) compared to the open ocean (0.36 plus or minus 0.08). As well, in the open ocean there were regions of elevated productivity associated with the seasonal pycnocline where iron availability was possibly increased. High silicate drawdown in the north-eastern section of the BROKE-West survey area suggested significant diatom growth and was linked to the presence of the southern Antarctic Circumpolar Current front (sACCF). However, low assimilation numbers (12.8 to 23.2 mg C mg chl a-1 d-1) and Fv/Fm ratios indicated that cells were senescent with initial growth occurring earlier in the season. In the western section of the survey area within the MIZ, high NO3 drawdown but relatively low silicate drawdown were associated with a Phaeocystis bloom. NO3 concentrations were strongly negatively correlated with column-integrated productivity and chlorophyll biomass which was expected given the requirement for this nutrient by all phytoplankton groups. Regardless, concentrations of both NO3 and silicate were above limiting levels within the entire BROKE-West survey area (N greater than 15.7 micro M, Si greater than 18.3 micro M) supporting the high nutrient low chlorophyll status of the Southern Ocean. |
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