THE CYCLE OF PHOSPHORUS IN A PLANKTON BLOOM IN THE GULF OF MAINE1

Abstract

The cycle of phosphorus was studied in detail during the spring bloom of phytoplankton in the Gulf of Maine in April 1964. A parachute drogue was placed at a depth of 10 m so that the movement of the surface water could be Followed. Five stations were occupied (luring an interval of 24 hr on 8–9 April when the drogue was launched and another five stations at the location of the drogue were occupied 10 days later on 17–18 April. During this interval, the drogue had drifted to the southward about 63 km. Because the isopycnal surfaces below 50 m sloped upwards from the first to the final set of stations, the distribution of properties is compared on surfaces of equal density. Particulate, dissolved organic, and inorganic phosphorus were determined on all samples. There was a considerable decrease in inorganic and an increase of both particulate and dissolved organic phosphorus in the surface waters during this time. Much of the particulate phosphorus formed had sunk below the 50‐m cuphotic zone to the deeper waters. The data permit estimates of the exerction of organic phosphorus in the euphotic zone and of the decomposition of this fraction in the deeper water. Below about 136 m, there was no net change in any of the fractions of phosphorus and the change in total phosphorus for the entire water column was essentially zero.Measurements were made of the distribution of oxygen and of chlorophyll at these two times, which permit the computation of the primary production from the changes in phosphorus, oxygen, and chlorophyll. The average rate for the 10‐day period was slightly more than 2 g C m‐2 day‐1. The rate of exchange of oxygen across the sea surface has been computed from these data and gives an exchange coefficient somewhat smaller than had been previously observed. The relationship between the changes of phosphorus, oxygen, and chlorophyll in primary production in the euphotic zone and in the decomposition processes in the deeper water are discussed. Both the chlorophyll and the particulate phosphorus data give indications of the sinking of particulate matter during the bloom. The sinking particles contain an unusually high proportion of chlorophyll compared to phosphorus.