Dataset: mesozoo grazing by size class
Deployment: MV1008

Mesozooplankton net collections
Mesozooplankton sampling was conducted using a standard 1-m2 ring net with 202-um Nitex mesh, towed obliquely for 20 min at a ship speed of 1-2 kts. A General Oceanics flowmeter was attached across the net mouth to record volume filtered, and a Vemco depth meter was fastened to the net frame to record tow depth and duration. The target depth of tows was 150m. Once on deck, the net was washed down with seawater and the contents of the cod end placed in a bucket with carbonated water to prevent gut evacuation. Separate splits (each of typically 1/8th of the sample) were used for biomass and gut-fluorescence determinations, with the latter done first to minimize pigment degradation. Each of these subsample splits was wet sieved into five size classes of 0.2-0.5, 0.5-1, 1-2, 2-5, and >5 mm.

Gut pigment and grazing estimates
Each size-fractioned sample for gut pigment analysis was concentrated onto a 47-mm 202-um Nitex filter and immediately frozen in liquid nitrogen for processing on shipboard or in the lab. The filters were subsampled for the three smallest size classes, by placing the frozen filters centered under a plastic template and sectioning into 8 pie-shaped fractions with a thin knife blade. Analyses were carried out in duplicate. The replicate 1/8th samples were ground in 90% acetone using a sonicator to extract pigments, and the homogenate was centrifuged to remove particulates. For the 2-5 mm size class, typically the whole sample was processed, although if very dense a smaller subsample was taken. The >5 mm size class was always processed in its entirety. Concentrations of chlorophyll a (Chl a) and phaeopigments (Phaeo) were then measured using a Turner 10AU fluorometer.

For each size-fraction analyzed, the investigators computed the depth-integrated grazing rate (GR) using phaeopigments only to obtain conservative rates. The GR was calculated in the euphotic zone as:

GR = [ (pig * D) / (vol * f) ] * K

where GR is the grazing rate (ug m-2 h-1), pig is the measured pigment value (ug, based on phaeopigments), f is fraction of sample analyzed, D is depth of tow (m), vol is the volume of water filtered (m3), and K is the gut evacuation rate constant. For K, the investigators used the rate constant of 2.1 h-1 derived from shipboard gut evacuation experiments at 140°W during the JGOFS EqPac program, which had similar temperatures as those encountered in the CRD.