Steemann Nielsen and the zooplankton

E. Steemann Nielsen is remembered by most biological oceanographers and limnologists as having introduced the C-14 method for measuring photosynthesis in 1952. The present paper is to recall that he was interested in the phytoplankton as part of the plankton community and was much aware of the role of grazing in affecting, if not determining, the concentrations of phytoplankton and, thus, also its rate of production. His principal statements to this effect were made with the open, oligotrophic subtropical and tropical oceans in mind where phytoplankton concentrations exhibit little seasonal change. This paper shows that Steemann Nielsen's sentiment also applies to non-static situations, especially phytoplankton blooms. Of the blooms in Cushing's North Sea Calanus patches of 1949 and 1954 and the two low-latitude, open-sea iron fertilization experiments (IronEx I, II) of the 1990s, more than half or even most of the newly formed cells were lost daily. In these examples, the same water was revisited, mixing was considered, and sinking was an unimportant loss term, so that grazing was the principal cause of mortality. Because of the grazing losses and the subsequent regeneration the CO2 draw down in the fertilized water was much lower than the 14 C uptake. Moreover the examples show that over the course of the blooms, the rate and even the sign of temporal change of phytoplankton abundance had little relation to the rate of cell division, as already postulated by Riley's 1946 model of the seasonal cycle of phytoplankton on Georges Bank. Thus, in most situations in the open sea and, presumably, large lakes, the rates of cell division (instead of photosynthesis by itself) and of mortality (most often from grazing) are needed for understanding and predicting the temporal change of phytoplankton abundance, a principal goal of biological oceanography. The mechanism maintaining the actual abundance of phytoplankton in the quasi-steady state prevailing over most of the ocean much of the time is still unclear.