^aNKSHT 

 E3DAY 



EIGHT -PERIOD 

 NIGHT AVERAGE 



NINE -PERIOD 

 DAY AVERAGE 





^ 



i 



m 



SEPT-NOV DEC-MAR 



1957-1958 



JUNE -JULY AUG -NOV DEC 



1958 



FiGUBE 9.— Average valuines of zooplankton during niue periods at the IGT station. 



DECEMBER 1957 TO MARCH 1958 



MontMy changes of the measured properties 

 from July to November 1957 were gradual; the 

 change from November to December 1957, how- 

 ever, was abrupt. The to 60 m. temperature 

 dropped from an average of 25.9° C. in November 

 1957 to 24.6° C. in December 1957 (fig. 6A) and 

 remained low, averaging 24.1° C. for December 



1957 to March 1958 (fig. 8A). The monthly to 60 

 m. salinity likewise changed considerably, drop- 

 ping from 35.12%o in November to 34.82%o in De- 

 cember (fig. 6B). The December 1957 to March 



1958 average was 34.92%o (fig. 8B). 



The 200 to 300 m. salinity (35.10%o) changed 

 slightly (fig. 8C), but the depth to the top of the 

 thermocline showed a considerable deepening from 

 an average of 47.4 m. during September to No- 

 vember 1957 to 84.6 m. (fig. 8D) for this period. 



The volumes of zooplankton declined markedly 

 from the preceding period. The night volume 

 dropped from an average of 56 cc./l,000 m.^ in 

 September to November 1957 to 28 c^'./l,000 m.^ 

 The day volumes of zooplankton declined from 25 

 cc./l,000 m.^" to 17 cc./l,000 m.^ (fig. 9). Except 

 for Halosphaera, Ostracoda, Pelecypoda, and 

 Thaliacea the abundance of plankters was rela- 

 tively low during this period. 



APRIL 1958 



Maximum and minimum values were recorded 

 for the physical and chemical properties during 



April 1958 (figs. 6 and 8). The to 60 m. temper- 

 ature (22.8° C.) was the lowest recorded during 

 the 19 months. The to 60 m. salinity (35.14%o) 

 equaled the September to November 1957 average, 

 the highest average for the nine periods. The 200 

 to 300 m. salinity (35.18%o) was also the highest 

 for the nine periods. The isothermal layer during 

 this period was the shallowest (22.6 m.) of the nine 

 periods. 



The zooplankton showed marked differences 

 from the preceding {period. The day and night vol- 

 umes of 30 cc./l,000 m.^ and 50 cc./l,000 m.^ for 

 April were much higher than those for December 

 1957 to March 1958. With the exception of Pele- 

 cypoda, all of the organisms were more numerous 

 than in the preceding period. 



MAY 1958 



In May the physical and chemical characteristics 

 of the water differed markedly from those of April 

 1958. The to 60 m. temperature increased from 

 the low of 22.8° C. in April to 24.6° C. in May. 

 The most important change, however, was in the 

 to 60 m. salinity, which decreased from 35.14%. 

 in April, the highest for the nine periods, to 

 34.78%o in May, the lowest for the nine periods. 

 The 200 to 300 m. salinity decreased slightly from 

 35.18%o in April to 35.13%o in May. These dif- 

 ferences suggest that the major change from April 

 t-o May was in the upper watei-s. The depth to the 

 top of the thermocline (62.8 m.) was slightly 

 greater than the nine-period mean (59.4 ni.). 



96 



U.S. FISH AND WILDLIFE ,SERVIOB 



