1. On storing, sea water suffers a decrease in pH value. The amount of change varies from that produced by adding 1·0 c.c. of N/100 acid to 100 c.c. of sea water, up to that due to adding about 2·5–3·0 c.c. The decrease is due to the production of carbonic acid by organisms.

2. The change corresponding to 1·0 c.c. of acid, as above, is equivalent to that produced by the complete oxidation of 3 milligrams per litre of a hexose sugar, which requires 3·2 mgrms. per litre of oxygen. This is the minimum value, from 8·0–9·6 mgrms. corresponds to the higher values of acid. Figures for oxygen consumption by estuarine waters, which are available for an approximate comparison, show that 1·5 mgrms. of oxygen is not often exceeded in estimations by means of alkaline permanganate. The highest of this series is 5·6 mgrms. It is suggested that the different results given by the two methods are due to the fact that respiratory changes taking place in the water during storage set free much of the organically combined carbon before the oxidation by permanganate has been started. Determinations on freshly drawn filtered sea water give, according to Raben, 7·5 mgrms. of hexose or 8·0 mgrms. of oxygen consumed.

3. It is probable that the change in pH value on storing indicates the amount of plankton present, at any rate when sewerage contamination is negligible. It appears that water near the surface, at 20–25 metres and sometimes at the bottom, 70 metres, is particularly subject to change during storage. Four cases out of seven showed marked decreases in pH value at 20–25 metres, and two others exhibited the change to a less marked degree.

4. The total amount of carbon, reckoned as hexose, which is set free during storage by respiration in sea water at E1 is about twice that photosynthesised between July and December, taking for this the minimum value 3 mgrms. per litre, namely a total of 6 mgrms. per litre. Considering the column of water from bottom to surface, this is equivalent to about 500,000 kilograms per square kilometre in the English Channel off Plymouth.