2. Methods

1. FIELD SITES

All field sampling was conducted in the Southern Ocean, in close proximity to the South Shetland Islands and the Antarctic Peninsula. Samples were taken from 10 different field sites overall; conductivity-temperature-depth (CTD) measurements were taken at eight sites, Secchi depths at three sites (see Table 1, Figure 1), and water samples for nutrient analysis at seven sites. At two of the sites analyzed for nutrients, this was the sole sampling procedure, with no CTD analysis or Secchi measurements conducted. Sites where CTD casts were conducted are described in further detail in later sections of this website (see "Site Descriptions" in Observations). Sampling locations within each site will henceforth be referred to as "stations".

Table 1: Location, time, and types of sampling conducted at various field sites around the Antarctic Peninsula, in February of 2011.

Figure 1: Satellite map of all field sites where CTD sampling occurred

2. SAMPLING METHODS

2.1 CTD Measurements

A conductivity-temperature-density (CTD) instrument is one of the most commonly-used oceanographic tools. The CTD sensor measures water conductivity (i.e. salinity) and temperature at the depth at which the instrument is situated. Thus, by slowly lowering and raising the CTD through the water column, data can be obtained for several depth increments at the same location. The conductivity and temperature data is later uploaded onto a shipboard computer, allowing for the calculation of water density at each depth for which measurements were taken.

CTD casts were the primary method by which we obtained oceanographic data. CTD casts were conducted at multiple stations within each of eight primary sampling sites (see table 1). Prior to arriving at each field site, the CTD was activated via the shipbound computer, and set to take continuous measurements (one measurement per second). For shipbound CTD casts – taken in Antarctic Sound and the Gerlache Strait – the CTD was lowered and raised over the side of the ship using a winch. For all other locations, CTD casts were conducted from a zodiac, whereby the CTD was manually lowered and raised over the side of the vessel. Care was taken to ensure that the speed at which the CTD was lowered and raised was as consistent as possible, both within individual casts and between casts. All vessels – both ship and zodiacs – remained stationary for the entirety of each cast so as to ensure that the CTD path through the water column was as vertical as possible. Post-sampling at each site, the CTD was connected to the shipboard computer and all data uploaded to Ruskin v1.4.2.

2.2 Secchi Disk Measurements

A Secchi disk is a simple oceanographic tool designed to measure water turbidity. It consists of a circular disk, with a geometric black and white pattern (see figure 2), mounted on a rope. The depth at which the pattern is no longer visible is recorded as the Secchi depth.

Secchi depths were taken at 3 of the zodiac sampling sites – Brown Bluff, Errera Channel, and Neko Harbour – by manually lowering the Secchi disk over the side of the vessel. At the Gerlache Strait site, an attempt was made to conduct a Secchi depth from the ship, at which point the current proved too strong and damaged the instrument, preventing any further Secchi depths from being obtained.

Figure 2: Secchi disk pattern

2.3 Nutrient Sampling

Water samples were taken at a number of sites to test for nutrients in the water (see table 1). At Gerlache strait, water sampling was conducted using a Niskin Bottle. A Niskin Bottle is an open ended plastic tube, with each end outfitted with a cap, tensioned with an elastic line. The bottle is mounted onto a rope, and lowered into the water. A messenger – a small weight – is sent down the rope to activate the caps at the desired depth of water sampling, thus closing the tube.

At all other sites where nutrient testing was conducted, water samples were collected manually in plastic bottles. All water samples were tested manually for ammonia (NH3) and nitrate (NO3) content using a basic water quality analysis kit.

3. DATA PREPARATION AND ANALYSIS

The CTD data was subject to ‘cleanup’ prior to use. As the CTD was activated prior to reaching each field site, continuous measurements were taken until the data was uploaded onto the shipbound computer, including the time between individual casts (i.e. measurements taken in air) within a site. Consequently, all measurements that were recorded while the instrument was out of the water had to be removed from each data set. Additionally, only the measurements obtained from the upcasts – when the CTD was raised through the water column, rather than lowered – were used for analysis, thus all measurements taken on downcasts were also removed from each data set. The final data set for each CTD cast contained only measurements obtained from the upcasts at each site ( i.e. ranging from the deepest point measured in the water column, to the surface).

Data analysis was conducted using Ocean Data View software (ODV v.4), with various plots – both stations and sections – created for each CTD cast at each site (see observations).

We are confident with the validity of our CTD measurement results due to that fact that Point 5 and 6 at Brown Bluff were taken in the same spot, a few minutes apart, and achieved very similar results. This indicates the reproducibility of our CTD measurements.

4. SATELLITE DATA COMPARISON

Satellite data was obtained from MODIS-Aqua, SeaWiFS and SSM/I, in order to draw comparisons with our results. All MODIS-Aqua and SeaWiFS analyses and visualizations used in our analysis were produced with the Giovanni online data system, development and maintained by the NASA GES DISC. SSM/I visualizations were produced by Remote Sensing Systems and sponsored by the NASA Earth Science MEaSUREs DISCOVER Project.

4.1 MODIS Aqua

MODIS Aqua was launched in May 2002 with the Delta-II rocket. It is a moderate resolution spectroradiometer that measures precipitation, evaporation and water cycling. It is part of the Earth Observing System, which also has a Terra satellite that focuses less on Earth’s oceans than MODIS-Aqua. Combined with the Terra satellite, MODIS captures an image of the Earth every 1-2 days to measure large-scale global dynamics. MODIS-Aqua has three on-board calibrators and takes measurements with a resolution of 1000 meters (Parkinson 2003).

4.2 SeaWiFS

SeaWiFS is a sea-viewing wide field of vision sensor that was launched in 1997 as part of NASA’s Ocean Biology Processing Group. It is located on the GeoEye’s OrbView-2 satellite and collected data until December 2010. SeaWiFS is specifically designed to measure chlorophyll-a concentrations and water clarity. It uses orbit sensoring to capture images of every square kilometer of cloud-free ocean every 48 hours (Feldman 2011).

4.3 SSM/I

SSM/I are several special sensor microwave/imagers that form part of the United States Air Force Defense Meteorological Satellite Program. SSM/I are located on the Block 5D-2 satellites and measure surface and atmospheric microwave brightness temperatures to receive information on near-surface wind speed, total columnar water vapor, total columnar cloud liquid water and precipitation. The data used in our study is taken from the F15 satellite, which was launched in December 1999 (Atlas, Ardizzone, Hoffman, 2008).

4.4 Argo

Figure 3 : Argo float