NO A A PROFESSIONAL PAPER 11 



were obtained from stock reared at NMFS Milford Lab- 

 oratory (Rhodes et al. 1975). All animals were maintained 

 in the laboratory in running seawater at laboratory am- 

 bient conditions for at least 2 weeks before experimen- 

 tation. Damaged clams or any in obviously poor condition 

 were discarded. No supplemental food was given to any 

 clam beyond that available in the seawater. 



Clams were held in a temperature-controlled bath in 

 3.8-1 (1-gal) glass jars for up to 8 weeks. A biomass no 

 greater than 250 g (equal to one large 12-cm clam) was 

 placed in each jar. The water in each container was 

 changed every other day to eliminate metabolic waste 

 products. Each clam was transferred to a holding jar con- 

 taining the desired low D.O. water, the test container was 

 rinsed and refilled with fresh seawater, nitrogen gas was 

 bubbled through the water until the desired D.O. level 

 was achieved, and the clam was replaced in its test con- 

 tainer. On days without a water change, the D.O. level 

 was readjusted to the predetermined level by bubbling 

 with compressed air. Air pockets were carefully excluded 

 when the jars were sealed. The D.O. levels were moni- 

 tored with a Beckman fieldlab oxygen analyzer whose 

 accuracy was periodically checked by Winkler titration. 

 Although temperature, D.O., and animal condition were 

 monitored daily, there were occasional weekend lapses. 

 Animals in control jars were treated the same as the ex- 

 perimentals, except that D.O. was maintained at satura- 

 tion with compressed air continuously bubbled through 

 aquarium airstones. Test conditions included 0.7, 1.4, and 

 2.1 ml/I D.O. (1,2, and 3 ppm) at 10° C and 0.7 ml/I at 

 20° C. Unfiltered Milford Harbor seawater at 26 ± 2%o 

 salinity was used throughout this study. 



An all-glass mixer was used to mix natural seawater 

 with nitrogen gas. A D.O. level of 1.4 to 1.8 ml/I was 

 maintained by adjusting the flow of nitrogen and seawater 

 in this counter-current system. This water was continu- 

 ously fed into holding trays of surf clams throughout the 

 8-week experimental period. Oxygen-saturated seawater 

 was delivered to the control clams from the mixer line 

 before contact with the nitrogen gas supply. 



Because high levels of hydrogen sulfide were found in 

 certain areas of the New Jersey low oxygen zone, some 

 preliminary tests were made to examine the role of hy- 

 drogen sulfide interaction with low D.O. and test orga- 

 nisms. Sediments containing hydrogen sulfide were ob- 

 tained from Milford Harbor. Glass jars (3.8-1) were filled 

 to 4 cm with this sediment, then topped with seawater 

 either saturated with oxygen or maintained at a D.O. level 

 below 0.7 ml/1. Again, no more than 250 g of biomass 

 were added per jar, and each container was monitored 

 daily as in static water tests. 



Metabolic studies were made on 3.5- to 3.7-cm clams 

 held in 0.7, 1.4, or 2.1 ml/I D.O. water at 10° C. Two 

 clams were placed in each jar, and oxygen levels were 



measured at the same each day to determine a daily oxy- 

 gen-consumption value for the pair. D.O. levels were 

 readjusted each day, with water changes every other day 

 as described above. During the period of this study, Mil- 

 ford Harbor had a heavy phytoplankton bloom. To com- 

 pensate for oxygen consumed by these planktonic orga- 

 nisms and for oxygen used during decomposition of dead 

 plankton, a series of 10 jars was monitored as "blanks" 

 to obtain a representative value for oxygen use by material 

 other than the clams. This value was subtracted from the 

 daily oxygen depletion values before calculating oxygen 

 as microliters of oxygen consumed per hour per gram (wet 

 weight), including shell. Oxygen consumption values ob- 

 tained at each D.O. level were calculated by averaging 

 100 daily readings for 10 to 16 clams. 



RESULTS 



Table 11.2-1 summarizes results of static water tests at 

 10° C. Clams in the smallest size range (group 4) survived 

 for 8 weeks at 0.7 ml/1 D.O. Three experiments dealt with 

 slightly larger clams (groups 2, 3, and 8) at 0.7 ml/I. The 

 clams in group 2 died in 7 to 9 days. Those in group 3 died 

 in 4 to 8 days. Clams in group 8 were initially exposed to 

 2.1 ml/I D.O. for 8 weeks before being placed in 0.7 ml/ 

 1 D.O. water, and survived an additional 8 weeks at 0.7 

 ml/I. One group of large clams (group 1) was held in 0.7 

 ml/I water; clams began dying on day 8, and all were dead 

 by day 30; 50 percent were dead by day 15. Two experi- 

 ments (groups 5 and 6) were run for 8 weeks each at 1.4 

 ml/1 D.O. In both experiments 50 percent of the clams 

 died within 3 weeks (five died in 10 to 21 days in the first 

 experiment, and five died in 11 to 25 days in the second). 

 One experiment (group 7) was run for 8 weeks at 2.1 ml/ 

 I D.O.; all 12 clams survived. These clams were subse- 

 quently held at 0.7 ml/I D.O. for 8 weeks, with 100 percent 

 survival. 



A series of 42 clams (2.4-4.8 cm, 4/jar) was tested at 

 0.7 ml/I D.O. and 20° C. These died within 8 days. Held 

 under these same conditions, 20 clams (8-11 cm, 1/jar) 

 died within 13 to 14 days. 



Twelve small clams (3.1-5.0 cm) and 12 adult clams 

 (6.0-12.1 cm) were held in flowing water at 1 .4 to 1 .8 ml/ 

 I D.O. for 8 weeks at 20° C. This group had no mortalities. 



Two experiments were performed with hydrogen sul- 

 fide-laden sediment; one with oxygen-saturated seawater 

 and one with seawater below 0.7 ml/I D.O. The first ex- 

 periment included 19 clams (5 large clams of 10.8-12.3 cm 

 and 14 small clams of 4.1^.9 cm) held in open 3.8-1 

 aerated jars. Four clams died in 20 to 36 days (3 adults, 

 1 juvenile); the remaining 15 clams survived for 68 days. 

 The second experiment included 10 clams (3.6-3.9 cm, 1/ 

 jar) that were subjected to D.O. levels below 0.7 ml/I in 



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