140 NIELSEN [CHAP. 7 



1 m or below. But in bottles kept at the surface or in tubes on the ship deck air 

 bubbles are ordinarily produced if no special measures are taken. The size of 

 the bubbles varies considerably from bottle to bottle. 



In the open ocean the planktonic algae are distributed within the whole 

 photic layer, which is usually about 50-120 m deep. Although as a rule the rate 

 of production per surface unit is by no means very low, the measurements are 

 difficult because of the considerable vertical extension of the photic layer. Thus 

 the amount of algae found in a single litre of water from the photic zone of the 

 ocean is equal to the amount in the whole water column below a surface area of 

 only about 0.1 cm 2 . If the rate of production per square metre per day is put 

 at 200 mg C — a rate somewhat higher than the average value for the oceans — 

 the average production per litre per day in the photic zone is equivalent to 

 0.005 mg O2. This means that the oxygen production per day is only about 

 0.1% of the oxygen ordinarily found. 



The lower limit of the oxygen technique depends on the sensitivity of the 

 Winkler technique for measuring oxygen dissolved in water. In a single titration 

 it is impossible to determine oxygen with this technique more accurately than 

 ± 0.02 mg O2/I. In many coastal waters it is even impossible to determine O2 

 with this accuracy. The colour shift may become rather indistinct. The facts 

 presented above tell us that the oxygen technique cannot be used in the oceans 

 if the duration of the experiments is only 24 h. 



The enclosure of a water sample in a bottle is in fact a rather severe inter- 

 ference. In prolonged experiments the plankton may settle and thus give rise 

 to considerable complications (difficulty in taking up CO2, for example) 

 especially if the density of plankton is high. By providing an effective stirring 

 of the water this complication may be overcome. In plankton-poor water 

 another difficulty turns up. Theoretically a prolonged experimental period 

 would offer the possibility of counteracting a very low rate of photosynthesis. 

 Unfortunately, bacteria at once start growing in large quantities if sea-water 

 is enclosed in bottles. Due to the solid surfaces now present, organic matter, 

 which is present in concentrations of about 1.2-2.0 mg C/l. in all sea-water, is at 

 once attacked by the bacteria. Apparently because of the low concentration it 

 cannot be attacked when the water is in situ. Solid surfaces seem to be a neces- 

 sary condition (see for example, ZoBell, 1936). Jones et al. (1958) have shown 

 that in plankton-poor oceanic water the number of bacteria may increase over 

 a 24-h period by a factor of nearly 500. In experiments lasting three days the 

 rate of respiration of these bacteria in ocean water is often as high as 0.2 mg O2/I. 

 per day. This rate is of an order of magnitude which is very different from that 

 of the rate of photosynthesis of the planktonic algae in oligotrophic waters. 

 This would be a sufficient cause for rejecting the use of experiments of long 

 duration in plankton-poor water. Unfortunately, however, a distinct difference 

 is often found m*the oxygen contents of light and dark bottles after such pro- 

 longed experiments (see Table III). The table shows further that the reduction 

 of the algal stock to one-fifth — by filtration through paper — increases the oxy- 

 gen consumption in the dark by a factor of 2.5 instead of decreasing it. 



