The Seabird 25 CTD instrument is cast in the Weatherport/Polar Haven sampling hole at locations where these structures are available  (e.g., Fryxell, Hoare, East and West Lake Bonney), generally between 10 am and 2 pm on a day prior to or following limnological sampling; CTD profiles at lakes where these structures are not available are made  from an outside hole.  The CTD is deployed at all times using a depth calibrated wire on a manual winch. The instrument is lowered at a rate of 1m sec-1 to a depth approximately 1m above the bottom of the lake and retrieved at the same rate.  Downcast data are compiled using SBE Data Processing software and current configuration files. Data are binned into 0.1decibar intervals based on pressure, which equates to approximately 0.1 m intervals. Depth is derived using the freshwater derivation (d = p * 1.019716) using the SBE Data Processing software.    

 

The thickness of the ice varies between 3 and 6 m among lake and year, and the CTD starts recording data at the surface of the water, which is about 30-50 cm beneath the ice surface. Hence, the top 3-6 meters of each cast (depending on ice thickness) are in the ice melt hole, and are not representative of the liquid water column. It is advised that users of the data review the Lake Ice Thickness dataset in the MCM Database to find where the liquid water column starts for each cast.

 

Because of the high density of the bottom waters of East and West Lake Bonney, and the offset in the pressure sensor output compared to the winch measured depth at the surface of all lakes, a correction equation for each lake is applied to the depth calculated from the CTD pressure sensor to obtain the wire-corrected depth. The equations below relate the pressure-depth output by the CTD to actual depth measured with a calibrated wire, where x = CTD measured depth (m), and y = wire corrected depth (m):

 

FRX: y = 0.9957x + 0.6774

HOR: y = 0.9987x + 0.7478

ELB: y = -0.003x² + 1.0431x + 0.5737

WLB: y = -0.0019x² + 1.0162x + 0.8106

MIE: y = -0.9968x + 0.6932

During the 1617 season, new equations were determined for ELB and WLB to account for lake level rise. These equations were better able to correct the CTD measured depths at the deeper depths of these lakes. The equations are as follows:

ELB: y = -0.0028x² + 1.0437x + 0.5828
WLB: y = -0.0017x² + 1.0179x + 0.6418

Note that the depth calculated from the CTD pressure sensor can vary by up to 0.6 m depending on atmospheric pressure at the time of the cast.

Note that no corrections have been determined for other lakes profiled.

For more details on wire corrected data, see Spigel and Priscu 1996.

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