The role of Antarctic marine protists in trophodynamics and global change and the impact of UV-B on these organisms - Voyage 3, BROKE-West, Aurora Australis 2005/2006 samples
The corrected fluorometer dataset is a 2 minute average dataset, and has been corrected to match the HPLC surface data and the interpolation includes a correction for irradiance. Taken from the draft paper: Materials and Methods Oceanography Oceanographic profiles were collected at 120 CTD stations...
| Summary: | The corrected fluorometer dataset is a 2 minute average dataset, and has been corrected to match the HPLC surface data and the interpolation includes a correction for irradiance. Taken from the draft paper: Materials and Methods Oceanography Oceanographic profiles were collected at 120 CTD stations from RV Aurora Australis using a Seabird SBE9 plus CTD and 24 bottle rosette frame equipped with 10 L Niskin bottles, as described by Rosenberg (2006). Interpretation of oceanographic features follows Williams et al. (this issue). In this paper, the mixed layer depth is defined as the depth at which SigmaT exceeds that at 8 m by 0.02, and the pycnocline is the depth of maximum Brunt-Vaisala frequency Sample collection Samples were normally collected at twelve depths - 10, 20, 30, 40, 50, 60, 70, 80, 100, 120, 150, 200 metres - although this was reduced to eight depths on deeper casts when fewer bottles were available in the upper water column. Seawater (1-2 L) from Niskin bottles or the ship's clean seawater line was filtered onto 13mm Whatman GF/F filters (0.7 micron nominal pore size) using vacuum less than 0.5 atm, while protected from light. The filters were blotted dry and frozen in 1.2 ml internal thread cryotubes in liquid nitrogen for return to Australia. HPLC analysis The pigments were extracted by a modification of the method of Mock and Hoch (2005). Filters were extracted in the cryotube as follows: 300 micro litre dimethylformamide plus 50 micro litre methanol (containing 140 ng apo-8'-carotenal (Fluka) internal standard) were added, shaken briefly, then filters were soaked for 1 hr in the freezer (-18 degrees C) with approximately 0.6g zirconia beads (0.7mm dia., Biospec) added. Tubes were shaken for 20 sec at 4800 cycles/min (Biospec Products Mini-BeadBeater). This reduced the filter to a milky suspension. The extract was cleared of particulate matter by centrifugation, as follows: the base of the cryotube was pierced with a hot needle, the cryotube was placed on top of a second cryotube inside a 15 ml plastic centrifuge tube, and together they were centrifuged in a Heraeus Multifuge 3 in a swing-out rotor for 3 min at each of 350, 1010 and 2100x g at -9 degrees C. The supernatant was then transferred to an amber autosampler vial with a 350 micro litre micro insert. Extracts (150 micro litre) were automatically diluted to 80% with water immediately before injection to improve peak shape (Wright and Jeffrey, 1997) and analysed by HPLC (Zapata et al., 2000) using a Waters 626 pump, Gilson 233xL autoinjector (with the sample stage refrigerated to -10 degrees C), Waters Symmetry C8 column (150 x 4.6mm, 3.5 micron packing, in a water bath at 30.0 plus or minus 0.1 degrees C), a Waters 996 diode array detector and a Hitachi FT1000 fluorescence detector. Pigments were identified by comparison of their retention times and spectra with a sample of mixed standards from known cultures (Jeffrey and Wright, 1997) that was injected at the head of each daily sample queue. Peaks were integrated using Waters Empower software. All peaks were checked manually and corrected where necessary, and quantified using the internal standard method (Mantoura and Repeta, 1997). Unidentified peaks were roughly quantified using the closest match from our spectral library. CHEMTAX analysis Nine taxa were chosen for CHEMTAX analysis, based on microscopy (Davidson et al., this issue) and previous experience in the region (Wright and van den Enden, 2000a; plus unpublished work). The list of lists pigments and their abbreviations is presented in Table 1. Two types of diatoms were defined: Type A, containing typical diatom pigmentation (Chls c1, c2, FUCO, diadinoxanthin), and Type B, where chl c3 replaces chl c1 (typified by Pseudonitzschia sp., which were commonly observed). Similarly, two types of dinoflagellates were defined initially: Type A, containing peridinin (unambiguous marker), and Type B, containing gyroxanthin diesters and fucoxanthin derivatives (the latter was changed as described below). Prasinophytes and cryptophytes were recognized by the unambiguous markers prasinoxanthin and alloxanthin, respectively. However we cannot distinguish free-living cryptophytes from the ciliate Myrionecta (Mesodinium) rubrum, which appropriates cryptophyte chloroplasts (Hibberd 1977) and is common in the Antarctic SIZ (Bathmann et al. 1997). A chlorophyte category was included due to the presence of lutein, but may include some prasinophytes that lack prasinoxanthin. Categorisation of taxa containing FUCO, 19'-HEX and 19'-BUT was somewhat problematic due to the multiple possibilities (eight types of haptophytes, pelagophytes, some dinoflagellates (Zapata et al., 2004; Wright and Jeffrey, 2006), coupled with the inability to identify many of the taxa containing such pigments by light microscopy. After several trials of different models, two types of haptophytes were defined, Types A and B, and allowed to find their own level. These contained a mixture of Haptophyte types 6, 7, 8, as defined by Zapata et al. (2004), while types 1, 2 and 4 would appear as diatoms, since they lack acyloxyfucoxanthins. Comparison with microscopic results suggested that haptophytes-A mainly reflected Phaeocystis numbers, whereas haptophytes-B probably included Phaeocystis gametes and Parmales, although neither of these categories can be regarded as exclusive, and both probably include other haptophytes. The group originally defined as dinoflagellates-B was based on pigments of the dinoflagellates Karlodinium venificum, Takayama tasmanicum, and Karenia brevis from Southern Ocean isolates (Wright and de Salas, unpublished), all of which contain fucoxanthin and its derivatives, plus three gyroxanthin esters rather than peridinin. Such 19'-HEX containing dinoflagellates were found to be dominant by Gall et al. (2001). Late in the analysis it became apparent that the dinoflagellate-B category also corresponded to the abundance of Phaeocystis antarctica. This species was dominant through all regions of the survey (Davidson et al., this volume) and its abundance could only be explained by summing the groups Haptophytes-A, -B and Dinoflagellates-B. Thus we re-named the final category as Hapto/Dino-B. The pigment content of P. antarctica varies between strains (Zapata et al. 2004), and it appears that CHEMTAX mainly tracked the variants of this dominant species rather the intended targets. Sixteen pigments were chosen for CHEMTAX analysis (Table 1a). Other identified pigments, including major pigments such as chl c2 and diadinoxanthin, were excluded after multiple trials showed they contributed no power to the analysis. Up to 50 randomised pigment tables were tried with each scenario to avoid the possibility of poor starting choices producing unrepresentative results. These tables were produced by randomly adjusting each of the pigment ratios in Table 1a by up to plus or minus 70% of the original ratio. Data were split into five bins according to sample depth to allow for variation of pigment ratios according to irradiance. The depth bins and sample numbers in each bin were 0-15m [129], 15-31m [143], 31-56m [169], 56-92m [282], greater than 92 m [405] (Total = 1128). Each bin was processed separately using CHEMTAX v1.95 (obtainable from the senior author). Table 1b lists the same ratios for the 0-15m bin after optimisation by CHEMTAX from a randomised matrix. Figure 2 shows variation in Pigment : Chl a ratios for various taxa in other depth bins. Ratios not shown in Figure 2 did not change down the water column, presumably because the parent taxon was not sufficiently abundant for its pigments to affect the analysis. Locations of sampling sites for ASAC project 40 on voyage 3 of the Aurora Australis in the 2005/2006 season (the BROKE-West voyage). Samples were collected between January and March of 2008. Three datasets are currently included in this download - an excel spreadsheet and a draft publication providing details on the methodology, etc employed, as well as two copies of corrected fluoro data for BROKE-West (BW_UwayFLuChla - in excel and csv formats). Public Summary from the project: This program aims to determine the role of single celled plants, animals, bacteria and viruses in Antarctic waters. We quantify their vital role as food for other organisms, their potential influence in moderating global climate change through absorption of CO2 and production of DMS, and determine their response to effect of climate change. For more information, see the other metadata records related to ASAC project 40 (ASAC_40). ###### Taken from the abstract of the draft paper: The geographic distribution, stocks and vertical profiles of phytoplankton of the seasonal ice zone off east Antarctica were determined during the 2005-2006 austral summer as part of the Baseline Research on Oceanography, Krill and the Environment-West (BROKE-West) survey. CHEMTAX analysis of HPLC pigment samples, coupled with microscopy, permitted a detailed survey along eight transects covering an extensive area between 30 degrees E and 80 degrees E, from 62 degrees S to the fast ice. Significant differences were found in the composition and stocks of populations separated by the Southern Boundary of the Antarctic Circumpolar Current (SB), as well as a small influence of the Weddell Gyre in the western sector of the zone south of the SB (SACCZ). Within the SACCZ, we identified a primary bloom under the ice, a secondary bloom near the ice edge, and an open ocean deep population. The similarity of distribution patterns across all transects allowed us to generalise a hypothesized sequence for the season. The primary bloom was initiated by release of cells and detritus from melting sea ice, some 35 days before ice melting, with stocks of Chl a ranging from 115-239 mg.m-2, apart one leg (41 mg.m-2), which was sampled late in the season. The bloom was dominated by haptophytes (in particular, colonies and gametes of Phaeocystis antarctica), diatoms and cryptophytes (or Myrionecta rubrum). The detrital material quickly sank from the upper water column, but the bloom of diatoms and, to a lesser extent cryptophytes, continued until 20 days after ice melt. Average Chl a stocks during this bloom ranged from 56-92 mg.m-2 between transects. A bloom of Phaeocystis gametes immediately after ice melt lasted for about 10 days. Grazing activity, as indicated by phaeophytin a, also increased at the same time. The diatom bloom became senescent, probably as a result of iron exhaustion, as indicated by chlorophyllides, which reached 45% of total Chl a. The bloom then rapidly declined, apparently due to grazing krill. Well-defined 'holes' in the chlorophyll distribution of most suggested that the krill were moving southward following the retreating sea ice and clearing the ice edge bloom. There was no evidence that blooms had been terminated by sinking or by vertical mixing. It appears that grazing of the bloom and export of cellular material as faecal pellets stripped the upper water column of iron, preventing its normal recycling via the microbial network. Thus, export of iron by grazing, and possibly sedimentation, created a southward migrating iron front, limiting growth in the upper water column. North of the iron front, a recycling nanoflagellate community developed at depth, sustained by residual iron, as indicated by a close correspondence between distributions of Chl a and profiles of Fv/Fm. Its depth was independent of the mixed layer and the pycnoclines. This community consisted of haptophytes (chiefly Phaeocystis gametes), dinoflagellates, prasinophytes, cryptophytes, and some small diatoms. The community may have derived from, and was possibly sustained by, selective grazing by krill. Average stocks of Chl a ranged from 36-49 mg.m-2 between transects. North of the SB, communities were found in the mixed layer, although they still had low Fv/Fm ratios. Populations were dominated by Phaeocystis gametes (with colonies north of the southern ACC front), diatoms such as Pseudonitzschia sp., Fragilariopsis pseudonana, F. kerguelensis, F. curta, and Gymnodinium sp. Average stocks of Chl a ranged from 40-67 mg.m-2 between transects.These appeared to be recycling communities that had been advected into the BROKE-West study region. These interpretations provide a cogent explanation for the composition and structure of microbial populations in the marginal ice zone during the latter half of the summer. ###### The fields in this dataset are: Peak Pigment name Retention times Visible maxima Comments Leg Zone Latitude Longitude CTD Julian Day Date Ice free days Pigment concentrations Protists |
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