Kane. Zooplankton biomass and species abundance on Georges Bank 



465 



r~*l- 



i 



/ 



Depth area 

 <61m 



61-100m 

 >100m 



« _ / 



 ^ /y- 



Figure 1 



Location of standard MARMAP (Marine Resources Monitoring, Assessment, and Prediction program) stations on 

 Georges Bank off the U.S. Northeast coast, 1977-86. 



Plankton samples were also collected on trawl and 

 dredge surveys at randomly selected locations that 

 changed yearly. Areal coverage and sampling spacing 

 on these surveys were similar to plankton cruises. 

 Samples from different surveys, closely overlapping in 

 time and space, were sometimes combined to ensure 

 adequate coverage of the survey area. 



Zooplankton was collected with a 61-cm bongo fitted 

 with a 0.333-mm mesh net towed obliquely to a maxi- 

 mum depth of 200 m or 5 m from the bottom and back 

 to the surface. Ship speed varied between 1 and 2 

 knots to maintain a 45 degree wire angle. Winter sur- 

 veys in 1977 and 1978 towed bongos at 3.5 knots. A 

 flowmeter was positioned in the center of the bongo 

 frame to measure volume of water filtered during the 

 tow. Samples were preserved in 5% formalin. At all 

 stations, sea-surface temperature was measured with 

 a stem thermometer to the nearest 0.1°C. Detailed 

 sampling procedures, cruise tracks, and survey logis- 

 tics are summarized by Sibunka and Silverman ( 1984, 

 1989). 



Biomass was measured by displacement volume in 

 the laboratory. Initially, organisms larger than 2.5 cm 

 were removed. The plankton sample with preserving 

 liquid was then measured in a graduated cylinder, 

 poured through a mesh cone into a second cylinder, 

 and drained until the interval between drops from the 

 cone increased to 15 seconds. The liquid in the second 

 cylinder was measured and the displacement volume 



of the sample was the difference between readings. 

 Samples with high concentrations of gelatinous organ- 

 isms were eliminated because the interstitial water 

 retained by these animals leads to gross overestimates 

 of zooplankton biomass. Samples were later subsampled 

 by aliquoting to about 500 organisms and identified to 

 species. Volumes («=1937) are expressed as cc/100m :) 

 of water filtered, and abundance (rc=1839) as number/ 

 100 m 3 . 



The adults and late stage copepodites of the cope- 

 pods Calanus finmarchicus, Pseiiducalanus minutus, 

 Centropages typicus, Centropages hamatus, and 

 Metridia lucens were the dominant species analyzed. 

 Depth distribution of biomass and species abundance 

 was examined by subsetting the data into three geo- 

 graphic subareas according to bottom depth: 1) central 

 shoal (<61m); 2) intermediate (61-100 m); and 3) deep 

 (>100m). Seasonal shifts in biomass and community 

 structure were investigated by grouping the data into 

 the six seasons defined in Table 1. Extenuating cir- 

 cumstances prevented adequate areal coverage in only 

 one season: winter 1979. 



The Shapiro- Wilk test for normality was applied to 

 each seasonal biomass and species data set. The null 

 hypothesis that the data values were a random sample 

 from a normal distribution was rejected (P<0.01). Zoo- 

 plankton data are often log transformed to normalize 

 zooplankton distributions. However, Roesler and 

 Chelton ( 1987 ) found that transformation of zooplank- 



