Christine Klaas1, Fabian Altvater1, Dorothee Kottmeier1, Rita
Kottmeier1, Theresa Rueger1, Vibe Schourup-Kristensen1
1AWI
Objectives
The Atlantic sector of the Southern Ocean is characterized by two major high productivity areas downstream of the Antarctic Peninsula (along the Scotia Arc and South Georgia) and in the Polar Front region, respectively. The composition of sediment assemblages in these two regions is dominated by distinct key diatom species assemblages: the open ocean Fragilariopsis kerguelensis dominating the eastern sector (Polar Front region) and spores of typical coastal species of the genus Chaetoceros dominating regions influenced by proximity to land masses.
The magnitude and dynamics of iron inputs are thought to drive the differences in dynamics of phytoplankton assemblages and export in these two provinces.
However, a quantitative process-based understanding of the link between higher iron availability and plankton community composition and export is still lacking. In this study, we aim to improve our understanding of the processes driving phytoplankton dynamics and sedimentation through field observations of development and decay of natural phytoplankton blooms in the Atlantic sector of the Southern Ocean in the water column. These goals will be accomplished through the study of the whole suite of factors controlling plankton dynamics and export by combining quantitative description of phytoplankton assemblage and dynamics, determination of key species in-situ growth rates and determination of composition of sinking assemblage (Iversen and Klaas, this volume).
Work at sea
Duplicate 200 ml water samples for microscopic analyses of protist assemblage (phyto- and protozooplankton) were be obtained from Niskin bottles attached to a Conductivity Temperature Depth (CTD) rosette from 8 discrete depths between 10 and 200 m depth at each station. One set of samples was preserved with hexamine-buffered formalin solution and one with acidic Lugol’s iodine at a final concentration
11. Plankton assemblage composition, Chla, BSi, POC, PON, DOC & DON and 3,000 m and concentrated down to 50 ml by pouring the water gently through a 10 µm mesh net and fixed with 2% buffered formalin. Fixed samples were stored at 4°C in the dark until counting back in the home laboratory. Species-specific growth rates of diatoms will be estimated from 24 hours on-deck incubation of undisturbed surface (10 m depth) water samples collected from the Niskin bottles attached to the CTD rosette after staining with the fluorochrome PDMPO that binds to the newly deposited silica during cell division. The difference between in-situ growth rates determined with the PDMPO technique and the actual accumulation rates of individual species populations during Eddy Pump will allow a quantitative estimate of the loss rates acting on individual species populations.
Water samples for Chlorophyll a (Chla), biogenic silica (BSi), particulate and dissolved organic carbon and nitrogen (POC, PON, DOC and DON) determination were obtained from the same bottle and depth as for microscopic analysis. In addition, underway surface-samples for Chla analysis were collected at hourly to 30 min intervals from the ship's pump. Further, larger volumes were collected at depths ranging from 500 to 3,000 m depth for POC analysis in order to calibrate the signal from the CTD-rosette transmissometer.
Chla samples were filtered onto 25 mm diameter GF/F filters at pressures not exceeding 200 mbar. Filters were immediately transferred to centrifuge tubes with 10 ml 90% acetone and 1 cm3 of glass beads. The tubes were sealed and stored at –20°C for at least 30 min and up to 24 hours. Chla was extracted by placing the centrifuge tubes in a grinder for 3 min followed by centrifugation at 0°C. The supernatant was poured in quartz tubes and measured for Chla content in a Turner 10-AU fluorometer. Calibration of the fluorometer was carried out at the beginning and at the end of the cruise. Results of the fluorometer calibration diverged by 2%
between beginning and end of the cruise. Chla content was calculated using the equation given in Knap et al. (1996) using average parameter values from the two calibrations.
1 to 2 liters seawater samples for BSi were filtered onto 25 mm diameter polycarbonate filters and stored in plastic (PE) petri dishes. For samples between the 10 and 200 m depth, a similar volume was filtered onto pre-combusted GFF filters and stored in pre-combusted glass petri dishes for POC and PON analysis.
For deeper samples, 12 l seawater samples were filtered onto 25 mm diameter pre-combusted GFF filters for POC and PON analysis. After filtration filters were dried overnight at 50°C and stored frozen (-20°C) for further analysis on land.
About 60 ml samples for DOC and DON analysis were filtered onto 25 mm diameter pre-combusted GFF filters using a HCl-cleaned glass filtration unit. The procedure was repeated 3 times in order to rinse the vials and the filtration unit, keeping the last filtrate for analysis. The final filtrate was collected directly into HCl-rinsed plastic (HDPE) bottles and frozen (-20°C) for further analysis on land.
Preliminary results
Horizontal distribution of Chla measured on board during the cruise is given in Fig. 11.1. The study focused on two regions of high phytoplankton abundance centred at around 52°12' S and 12°40' W and in the South Georgia basin (inserts in Fig. 11.1). In the first region, needle shaped diatoms (Pseudonitzschia spp. and Thalassiothrix antarctica) seemed to dominate phytoplankton standing stocks (Fig.
11.2). During the experiment a decrease in Chla concentrations was observed
starting northwest of the study area and progressing to the southeast. Temporal evolution of Chla at 52°12' S and 12°40' W is given in Fig. 11.3. During the experiment a decrease in Chla concentrations was observed (Fig. 11.3) starting northwest of the study area and progressing to the southeast while main currents in the area flowed in a northeastward direction. This was indicating that the observed decrease in Chla was due to a sinking event.
Fig. 11.1: Surface (8-10 m depth) Chl a concentrations in µg L-1 during ANT-XXVIII/3 (middle panel). Left and right panel illustrate the two main study areas visited during the
cruise.
11. Plankton assemblage composition, Chla, BSi, POC, PON, DOC & DON
Fig.11.3: Temporal evolution of Chla at the core station (52°12' S and 12°40' W)
Data management
Data collected during the cruise will be copied to the data centre WDC-MARE/
PANGAEA (http://www.pangaea.de/) after post-cruise calibration and validation.
Within two years of the end of the cruise the data deposited at PANGAEA will be made available to the international scientific community and the general public.
Processed data obtained through laboratory analyses after the cruise will be uploaded to the database PANGAEA. It is expected that all data will be made available to the international scientific community three years after the cruise references:
Knap, A., A. Michaels, A. Close, H. Ducklow and A. Dickson (eds.). 1996. Protocols for the Joint Global Ocean Flux Study (JGOFS) Core Measurements. JGOFS Report Nr. 19, vi+170 pp. Reprint of the IOC Manuals and Guides No. 29, UNESCO 1994.