ORTNER ET AL.: SARGASSO SEA ZOOPLANKTON BIOMASS DISTRIBUTION 



RESULTS 



Regional Biomass 



The biomass collections obtained in the same 

 ring 3 mo apart in August and November 1975 

 corroborate differences already noted in 0-800 m 

 zooplankton biomass between the Sargasso Sea, 

 cold core rings, and slope water (Figures 1, 2). In 

 both months the slope water 0-800 m biomass was 

 larger than either the Sargasso Sea or ring 0-800 

 m biomass {Mann- Whitney U-iest, P<0.01). In 

 August, the contrast between slope water and the 

 two other regions was particularly marked with 

 average concentration in the upper 800 m approx- 

 imately 10-12 times larger than in the Sargasso 

 Sea and ring (Table 2, top;. At several depths in 

 the August slope water stations the zooplankton 

 biomass was dominated by Salpa aspera.^ Differ- 

 ences rn abundance of this salp accounted for a 

 large part of the variation between slope water 

 stations. The high water content of these animals 

 undoubtedly caused our estimate ofTnomass by 

 displacement volume to be considerably higher 

 than had we measured dry weight or organic car- 

 bon content. It is clear, nonetheless, that the 

 standing stock of zooplankton was exceedingly 

 large. 



Relative to the Sargasso Sea, 0-800 m ring 

 biomass was on the average 1 .36 tim es^larger in 

 August and 1.33 times larger in November (Table 



■•The significant biomass contribution of S. aspera is discussed 

 in Wiebe, P. H., L. P. Madin, G. R. Harbison, L. R. Haury, and L. 

 M. Philbin. 1977. Diel vertical migration by Salpa aspera and its 

 potential for large-scale particulate organic matter transport to 

 the deep-sea. Submitted to Mar. Biol. 



Table 2. — Comparison of slope water, ring, and Sargasso Sea 

 zooplankton biomass (cm^/m^) based on weighted averages of day 

 and night samples collected at 0-800, 200-800, and 0-200 m depth 

 intervals. Number of tows used to make average given in paren- 

 thesis. 



August 1975 



November 1975 



Region 



0-800 200-800 0-200 0-800 200-800 0-200 



Sargasso 



Sea 

 Ring fringe 

 Cold core 



ring 

 Slope water 

 Warm core 



ring 



16.6(4) 8.8(4) 8.1(8) 



22.5(3) 

 256.2(4) 



15.6(3) 6.4(6) 

 95.6(4) 121.8(6) 



10.2(2) 

 91(1) 



13.6(4) 

 33.8(4) 



5.6(2) 

 4.3(1) 



11.4(4) 

 23.0(4) 



6.0(4) 

 48(1) 



3.4(5) 

 12.8(6) 



27.9(2) 17.2(2) 10.8(2) 



Month 



Ratio ring/Sargasso Sea 

 0-800 200-800 0-200 



August 

 November 



1 36 



1.33 



1.77 

 2.03 



0.79 

 0.56 



2, bottom). These differences are consistent with 

 data from previous cruises; using paired regional 

 biomass averages observed on all cruises to date 

 (Table 3) we see that the mean zooplankton 

 biomass in rings and in the Sargasso Sea is sig- 

 nificantly different (Sign test, P<0.05). 



In November zooplankton standing crop was 

 consistently lower than in August with the most 

 pronounced change occurring in the slope water. 

 Although S. aspera was still present at the slope 

 water stations, it no longer dominated the bio- 

 mass. Comparing August and November 0-800 m 

 biomass averages, segregating day and night 

 samples for each region, we see that the overall 

 seasonal decline is statistically significant (Sign 

 test, P<0.05, computed using cubic centimeters 

 per square meter in Figures 1 and 2). Indeed only 

 one November 0-800 m biomass value was as large 

 as the smallest 0-800 m biomass in the same re- 

 gion in August. 



Table 3. — Average zooplankton biomass in slope water, ring, and Sargasso Sea — dry weight (mg/m*) in colurmi 

 =750 m deep. Numbers of stations per area and range of biomass values (after colon) given in parenthesis. This 

 table is an expanded version of table 3 in Wiebe, Hulburt, Carpenter, Jahn, Knapp, Boyd, Ortner, and Cox ( 1976). 

 Note that the biomass units in the original table were incorrectly presented as milligrams per square meter. 



'Each of the two values is Integrated for the water column based on three stratified bongo net tows. 



^Two values are from oblique meter net tows: one is an Integrated value based on three stratified bongo net tows. 



^Two values are from oblique meter net tows; one is an integrated value based on three bongo net tows; one is an oblique bongo net tow. 



"One value is an integrated value based on three stratified bongo net tows; two values based on two stratified bongo net tows. 



^Cubic centimeters per cubic meter converted to milligramsper cubic meter using equation 4 table 2 in Wiebe etal. (1975). Note that this 

 conversion affects the regional biomas ratios (see Figure 2) because the relationship between displacement volume and dry weight is not 

 linear. 



^Salp-rich tows. MOC 39 and MOC 40, excluded. 



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