photosynthetically available radiation (PAR) measured with PAR sensors. For the 
Western NCA, the vertical profile PAR data were used to calculate an estimate of the 
percent transmittance of incident PAR at a reference depth of 1 m (Nelson et al. 2005) 
In the present study, a transmissometer attached to the CTD was used to measure in 
situ light attenuation. The instrument measured the percentage of light that reached a 
receiver with a narrow field of view at 25 cm from a light source generating a narrow 
beam. Transmissivity and percent transmittance of PAR are not directly comparable 
measurements. 
Figure 2.2.1. CTD and Niskin bottle rosette sampler on the deck of the NOAA Ship 
mcarthur ii. 
The CTD was lowered into the water until it was completely submerged and held 
just below the surface for three minutes, allowing the water pump to purge any air in the 
system. The unit was then returned to the sea surface to begin the profile, and lowered 
slowly to the bottom at approximately 0.8 m s' 1 , held near the seabed for one minute, 
and then recovered at a similar velocity. To prevent the equipment from hitting the 
seabed due to wave motion, the maximum depth to which the CTD was lowered was 
generally about 3-8 m above the bottom. 
8 
