In a submerged bed of eelgrass, newly formed leaves in winter and spring had their maximum levels of total organic matter (90% of dry weight), soluble organic fraction (45%), carbon (42%), and nitrogen (4.8%). These components all decreased as the leaves matured, aged, and died. Soon after death, a leaf had only 70% total organic matter, 28% soluble organic matter, 30% carbon, and 1.5% nitrogen. Intact dead leaves showed little further change in chemical composition. The commonly used crude protein determination (6.25 × nitrogen content) overestimated true protein by up to 180%. The carbon to nitrogen ratio (C:N) was an unreliable index of the nutritional value of the plant. Two growth forms were present, most probably in response to wave action and substrate composition. Daylength, not temperature, most probably controls the seasonal cycle of growth.