This volume contains the keynote lectures, reports on the experimental work, and papers contributed as posters at the Fifth International Workshop of the Group for Aquatic Primary Productivity (GAP). GAP workshops bring together a diverse group of scientists interested in many aspects of aquatic primary productivity and the programmes are designed to promote the sharing of expertise through joint experimental work. This time the Workshop theme was the daily growth cycle of phototrophic micro-organisms, and incorporated different aspects of growth - increase in algal biomass, specific cell components and in cell numbers.