Determinations of phosphate, silicate, nitrate, nitrite and ammonia have been made at frequent intervals at two Stations, L4 and El, in the English Channel off Plymouth between the autumn of 1930 and January, 1932. Determinations were also made quarterly at E2 in mid-Channel.

The year 1931 was an early one from the point of view of plankton production. The chemical data show that there was a small mid-winter outburst which gathered momentum during January and February. The main spring outburst started, however, early in March, carrying on fairly steadily until about the middle of May. The summer was characterised by exceptionally low phosphate and nitrate right through the water column due to very efficient utilisation in the spring and to the unusual hydrodynamical conditions. There was also a small autumn outburst.

(1) In general, the changes in pH were similar to those observed in previous years. On six occasions the pH at the surface at Station L4 was lower than at the bottom.

(2) The changes in carbon dioxide in the spring agree with the conclusion, formed in Part I from nutrient salt data, that the early start in plant production in mid-Channel was not maintained and that by the middle of April production had become much greater nearer the shore.

(3) The partial pressure of CO2 during nine months out of twelve has been observed to be lower than that of the atmosphere and equal or slightly higher during the remaining three months.

(4) It is suggested that the fall in (Total CO2+O2) during the course of the summer was due mostly to loss of oxygen to the atmosphere. After allowing for carbon dioxide possibly removed from the sea as calcium carbonate and for the loss of oxygen due solely to rising temperature of the water, a net loss of 110 litres of oxygen was found from a column 1 sq. metre in cross-section and 70 metres deep; or 9 cubic kilometres from the whole of the Channel. If this is regarded as a closed system, the oxygen must have been formed during photosynthesis from an equal volume of CO2 dissolved from the atmosphere. But the required transfer of CO2 across the surface of the sea is too great to be accounted for by existing data on the rate of invasion of CO2 into sea-water as calculated for a small bubble of gas in water.

(5) Estimates of the phytoplankton crop have been made, based on the seasonal consumption of carbon dioxide, of phosphate and of nitrate and on the oxygen lost to the atmosphere. All four are of the order of 1,400 metric tons wet weight per sq. km. of surface, in close agreement with the figure calculated by Atkins. The crop production calculated from consumption of silicate is less than one-tenth of this. This is attributed not only to the presence of planktonic organisms requiring no silica but to the silicate being used several times over in the course of the season.

(6) About 0·06% of the wet weight of phytoplankton produced is harvested as fish.

(7) A small variation in excess base, equivalent to about 0-1% of the total calcium present, has been detected between the surface and bottom in summer. The result requires confirmation.