MOCNESS Sample Analysis (From Ashjian et al., submitted, DSRII)

Biomass (wet weight) and abundance were determined principally by silhouette digitization of taxa lengths and conversion to equivalent wet weights according to the equations and methods adapted from Davis and Wiebe (1985) and Wiebe et al. (2004). New formulas were calculated for Southern Ocean krill because the length-weight relationship (Wet Weight = 0.0054*Length^3.214) differed from that of the North Atlantic euphausiids (Wet Weight = 0.0138*Length^3.071; Davis and Wiebe, 1985).

Silhouette photographs first were obtained from each sample; this photograph then was scanned with an Epson Expression 1600 scanner at 1200 dpi to produce a Tagged Image File Format (TIF) image. Identification and measurement of organisms from the tiff image were accomplished manually using the specialized MATLAB (Mathworks, Inc.) routine, the WHOI Silhouette DIGITIZER (v 1.0) (Little and Copley, 2003). A scanned photographic image of an aliquot of a sample was displayed with a superimposed reference grid on the computer screen. The operator identified each organism to taxonomic and/or size category by selecting the appropriate category button. Organism lengths (both straight and curved) then were measured by the operator using the cursor on the computer screen. Although the nominal resolution of the measurement is 0.02 mm, in practice the resolution was ~0.05 mm. In instances where there were too many organisms of a given type to justify measuring every individual organism, the DIGITIZER routine generated a list of randomly located cells to systematically sub-sample the overall image. Each organism’s biomass was calculated automatically from its length measurement using the appropriate length-weight relationship.

References

Ashjian, C.J, Rosenwaks^, G.A., Wiebe, P.H., Davis, C.S., Gallager, S.M., Copley, N.J., Lawson, G.L., Alatalo, P. Distribution of Zooplankton on the Continental Shelf of Marguerite Bay, Antarctic Peninsula, during Austral Fall and Winter, 2001. In revision, Deep-Sea Research II.

Davis, C.S., Wiebe, P.H., 1985. Macrozooplankton biomass in a warm-core Gulf Stream ring: Time series changes in size structure, taxonomic composition, and vertical distribution. Journal of Geophysical Research 90, 8871-8882.

Little. W.S., Copley, N.J. 2003. WHOI Silhouette Digitizer version 1.0 User's Guide. Woods Hole Oceanographic Institution Technical Report # 2003-05, 63 pp.

Wiebe, P.H., C. J. Ashjian, S. M. Gallager, C. S. Davis, G. L. Lawson, and N. J. Copley. 2004. Using a high powered strobe light to increase the catch of Antarctic krill. Marine Biology. 144 (3): 493 - 502.

Table 1. Summary of locations, times, and depths for the twelve net tows. Light conditions are designated as “D” = day, sunlight present; “N” = night, darkness; “T” = twilight

CruiseRegionTowDateTimeLightLatitude LongitudeTow BottomIce CoverNumber(Local)(°S)(°W)Depth (m)Depth (m)(tenths)          FallOffshore605/04/011332D66° 40.1773° 22.080-100136500Midshelf 1104/30/012050N66°10.8069° 10.470-3063600Midshelf 2305/02/010320N66° 48.5370° 21.420-3486000Midshelf 31205/13/011830N68° 23.6272° 18.440-3564000Midshelf 41605/18/010854N68° 59.4374° 55.770-3403500Marguerite2405/29/010300N67° 55.3368° 30.690-6026900BayWinterOffshore1108/11/010918T67° 14.6974° 25.270-989291710Midshelf 11708/26/010306N66° 11.1369° 06.217-3053466Midshelf 2407/31/010949D66° 45.1870° 09.6411-4135018Midshelf 31308/17/012117N68° 13.2172° 56.530-23032510Midshelf 41408/21/011136D69° 15.5575° 34.0020-290~34071508/22/010327N69° 12.3075° 43.900-91~3509Marguerite908/08/010215N67° 53.7468° 10.820-488600+9Bay