The effects of nutrient additions and zooplankton excretion products upon carbon fixation rates in the phytoplankton present at Station L 4 in the English Channel during the summer and autumn of 1979 have been studied. Nitrate, ammonium, urea, phosphate, glucose-6-phosphate and the excretion products when added individually all caused photosynthesis to be stimulated, and the result of the simultaneous addition of nitrate and phosphate indicated that their effects were additive. Germanic acid, which inhibits photosynthesis mainly in diatoms, removed the stimulatory effect of the nitrogen supplements, indicating that they were utilized mostly by the diatoms; the higher fixation rates caused by the phosphate enrichments were, however, decreased by the same proportion as the unenriched controls when germanic acid was present, suggesting that the whole of the phytoplankton population was phosphorus-limited. This was supported by the finding that glucose-6-phosphate stimulated carbon fixation in all of the phytoplankton.
The excretion products, even at concentrations likely to be produced in the sea, stimulated carbon fixation, and it has been calculated that zooplankton-regenerated nitrogen and phosphorus compounds could supply the amounts needed to maintain primary production during the summer period.
Nutrient additions and zooplankton excretion products had little effect upon carbon fixation in the autumn samples, presumably because the higher nutrient levels then present in the water exceeded the requirements of the phytoplankton.
It has been concluded that the predominance of the sub-10 μ microflagellates in the summertime is probably due to their ability to utilize more efficiently than the other types of phytoplankton the low levels of nutrients which become available due to regeneration.