Vertical hauls between surface and 45 metres depth were made with a measuring net, throughout 1934, at a position 5 miles S.W. of Plymouth Breakwater in circa 50 metres depth of water.The phytoplankton organisms were counted and also assessed from their content of plant pigments.A close relation was found between numbers of diatoms and their pigment contents after making allowance for the small size of three species.The phosphorus content of the phytoplankton was estimated on several occasions.The zooplankton organisms per cubic metre of sea were counted and estimations were made of their phosphorus content.The diatom population differed from that found in 1933, in which year the spring outburst was shorter but more intense, the sporadic summer outbursts were more intense, the autumn one lasted longer; and more phosphate was utilised by the plants during the first half of the year.

The rates at which Calanus finmarchicus eat both carmine particles and the diatom Nitzschia closterium have been investigated by Fuller &Clarke (1936) and Fuller (1937). They found that the number of Nitzschia or of carmine particles, eaten by a Calanus in unit time, was proportional to the concentration of Nitzschia or of particles in the water. Lucas (1936) also found that Neomysis and Eurytemora ate Nitzschia at rates which were roughly proportional to the concentration of diatoms in the water over fairly wide limits.During 1935 I had made, as part of another investigation, three experiments on the rate at which Calanus eat diatoms of larger size. This happened much more rapidly than Fuller found to be the case with suspensions of Nitzschia. Meanwhile Lowndes (1935) had concluded, from direct observations, from the anatomy of their mouth-parts and from observations by Lebour and Marshall, that Calanus should be able to catch and select food. He concluded that Calanus does not depend entirely upon its (automatic) filtering mechanism for all the food it obtained.The quantitative data which I had collected in these three experiments, in conjunction with those published by Fuller & Clarke (1936, 1937, were suitable for examining this contention. They suggested that the animal when fed with diatoms of moderate size selected by catching the species it preferred, while it automatically filtered the minute Nitzschia. With these possibilities in view, experiment N 9°was made in order to link up the three experiments with the numerous experiments made by Fuller (1937). EXPERIMENTALStages V and VI of Calanus finmarchicus were recognized by their larger size, transferred from a tow-net catch into filtered sea water, to which some particular species of diatom was added as food, and were kept for some days. At the start of the experiment the required number were transferred to a litre beaker half-filled with sea water to which diatoms had been added from a culture. These diatom suspensions were made the previous day and kept overnight in the dark, in order that the number of diatoms increasing by division during the course of the experiment should be reduced to a minimum.After adding the Calanus the water in the beaker was kept stirred by means of an oscillating glass plate. This proved an efficient method of stopping any JOURN. MAR. BIOL. ASSOC.vol. XXI!, 1937 7

A simple photoelectric meter is described which allows the molybdenum blue formed in sea water, due to the presence of phosphate, to be estimated within that due to ± 0·25 mg. phosphate-P per m.3.The effects of concentration of acid, molybdate, reductant, temperature and suspended particles on the rate of formation, fading and amount of molybdenum blue, formed in sea waters containing phosphate are detailed.Intramolecular changes taking place during storage of molybdate solutions, and while being mixed with acidified sea water, have been investigated.The hydrolysis of organic phosphorus compounds in acidified sea water at 140° C , and the prevention of arsenate formation, are described.Procedures, resulting from these investigations, for the estimation of phosphate, and of total phosphorus, are described.The growth of bacteria and the physical adsorption of organic phosphorus compounds in solution on the walls of glass vessels used for storage of sea water have been investigated, and a method of prevention evolved.

The nature of iron occurring in sea water, and its utilization by diatoms, is discussed.Diatoms in the sea obtain many thousand times more iron than calculation shows they can obtain by diffusion of iron ions from the surrounding water.Evidence is presented that ferric hydroxide is readily adsorbed on the surface of diatoms.It is shown that colloidal and larger particles of ferric hydroxide or phosphate can be utilized by, and support the growth of diatoms.Experiments show that the diatoms Nitzschia closterium and Lauderia borealis require, for continued growth, a very small quantity of iron compared with that found on, and in, diatoms taken from the open sea.It is contended that iron hydroxide adsorbed on diatoms is in contact with an interface where its solution, and subsequent passage into the cell, is probable.