Sampling and Analytical Methodology:
Zooplankton tows were performed using a 1-m2, 202-micron mesh MOCNESS (Multiple Opening/Closing Net and Environmental Sensing System) towed through the upper 150 m or occasionally the upper 500 m of the water column. Only depth intervals 150 m and shallower were used for determination of gut fluorescence. The following discrete depth intervals were sampled on the upcast for the upper 150m: 0-25 (or 0-10 and 10-25), 25-50, 50-100, and 100-150 m. Occasionally at shallow depth stations, a double oblique tow using a rectangular frame (0.8 x 1.2 m) single net with 202-micron mesh was performed in surface waters (within top 25 m). Both day and night tows were performed, and occasionally paired day/night tows at the same station. One-quarter of each sample was used for the determination of gut fluorescence. Each sample was size-fractionated by wet sieving through nested sieves of 0.2 mm, 0.5 mm, 1 mm, 2 mm, and 5 mm mesh. Zooplankton in each size class were transferred onto pre-weighed disks of 0.2-mm nitex mesh, rinsed with deionized water, and frozen at sea (-80˚C).
At our home laboratory, gut fluorescence for each size fraction was determined by the gut fluorescence method similar to Decima et al. 2011. For the 0.2-0.5 mm, 0.5-1.0 mm, 1.0-2.0 mm, and 2.0-5.0 mm size fractions, replicate 1/8 or 1/4 sections of the filter were analyzed. On some occasions with very low biomass the entire 2.0-5.0 mm size fraction was analyzed. For the >5mm size fraction the entire sample was processed. The samples were sonicated in 90% acetone and allowed to extract for at least two hours. Samples were then centrifuged to remove particulate and concentrations of chlorophyll a (Chl a) and phaeopigments (Phaeo) were measured on either a TD-700 or Turner Trilogy fluorometer. The post-acidification phaeopigment values (pig) were used to determine the gut pigment content (GPC).
The GPC (mg m-3) for each net was calculated by:
GPC = 
where pig is the measure pigment value (mg) split is the fraction of total tow, f is the fraction of filter analyzed and vol is the volume of water filtered through the net. Grazing rate (G; mg pig /m^3/d) = GPC * K where GPC is gut pigment content from above and K (69.12/d) is the gut evacuation rate constant (Huskin et al 2002).
Related files and references:
Décima, M., Landry, M.R., and Rykaczewski, R.R. (2011). Broad scale patterns in mesozooplankton biomass and grazing in the eastern equatorial Pacific. Deep Sea Res. Part II Top. Stud. Oceanogr. 58, 387–399.
Huskin, I., Anadón, R., Medina, G., Head, R.N., and Harris, R.P. (2001). Mesozooplankton Distribution and Copepod Grazing in the Subtropical Atlantic Near the Azores: Influence of Mesoscale Structures. J. Plankton Res. 23, 671–691.
BCO-DMO Blog
©2024 Biological and Chemical Oceanography Data Management Office.
