Photosynthetic organisms on Earth convert about 10 Pmol of carbon dioxide to organic carbon in net primary productivity each year. Almost half of this carbon is assimilated by phytoplankton living in the top 100 m or so of the ocean. Since more then two-thirds of the Earth’s surface is occupied by water, the ocean has a lower productivity per unit area than the land surface, despite the presence of significant areas of desert of very low productivity. Much of my work relates to the factors restricting primary productivity in the ocean. The constraints can be conceptually separated into ‘top down’ and ‘bottom up’ factors. ‘Top down’ factors, such as grazing and parasitism, remove primary producer biomass. ‘Bottom up’ factors limit the growth rate of primary producers, such as limitation of the supply of essential resources such as light and carbon dioxide for photosynthesis and of the nitrogen, phosphorus, iron and zinc needed to produce cells from photosynthetic products. Almost all of the work I have been involved in has involved ‘bottom up’ factors, and especially the results of interactions among resource supply limitations. As far as possible the laboratory experiments and mechanistic modelling relate to real world situations and ecologically important organisms.