CTD profiles at the BATS site have been collected since the inception of the program in October 1988. Although there have been some changes during the past thirty-five years as a result of new instrumentation or methodologies, the general sampling procedures have been consistent with those detailed in the BATS method manual version #4 (Knap et al., 1997).

In summary, the CTD is operated as per SeaBird's suggested methods with data collection at the full scan rate of 24 hertz (Hz). The CTD is powered up and allowed to stabilize at 12 meters prior to profiling. This stabilization period is important for both the conductivity and dissolved oxygen sensors to warm up prior to making measurements. Once stable (typically 4 minutes) the CTD is brought back to the surface from which point the profile begins with typical descent rates of 0.7 to 1.0 meters per second (m/s), depending on weather conditions. Water samples are collected on the upcast and prior to triggering bottles, the CTD is kept at the desired depth for a minimum of 60 seconds to ensure that entrainment from the following wake has subsided. Once the water sample is taken, the CTD immediately continues with the upcast at an ascent rate of 0.7 to 1.0 m/s.

The basic system used to acquire CTD data is a Sea-Bird SBE 9plus CTD, with an internal Digiquartz pressure sensor, a Sea-Bird SBE-3F temperature sensor, a Sea-Bird SBE-4 conductivity cell and a Sea-Bird SBE-5 pump. Additional sensors include the Sea-Brid SBE-43 dissolved oxygen sensor, the WET Labs C-Star or Chelsea/Seatech
Transmissometer, the Chelsea Aqua 3 Fluorometer, and the Biospherical/Licor PAR/Irradiance sensor. Present configuration also includes a secondary temperature sensor, a Sea-Bird SBE-35 temperature sensor, a secondary conductivity sensor and pump, which are connected independently from the primary units. The temperature and conductivity sensors are connected by a standard Sea-Bird TC-duct (clear, low viscous type), which ensures that the same parcel of water is sampled by both sensors, improving the accuracy of the computed salinity. The dissolved oxygen sensor is connected downstream from the conductivity cell and the flow rate through this sensor configuration is maintained at a steady rate by the inertia balanced SBE-5 pump.