Fluctuations in nitrogen and carbon compounds were examined over an autumn growing season in the submersed marine angiosperm Zostera marina L. (eelgrass). The experimental design included replicated controls (ambient NO 3 − , typically < 2 μM), increased water-column nitrate (8 μMNO 3 − above ambient, pulsed daily), increased environmental temperature (3 to 4 °C above 20-year weekly means), and combined increased water-column nitrate and temperature. Above- and belowground tissues were collected weekly to biweekly and assayed for total soluble carbohydrates, non-reducing carbohydrates, starch, α-cellulose, lipids, free amino acids, total protein, tissue nitrate, tissue nitrite, and tissue ammonium. Tissue nitrate declined, and amino acids, proteins, lipids, and cellulose increased as the growing season progressed in both control and treated plants. In addition, there were seasonal quadratic responses for tissue ammonium, soluble carbohydrates, and non-reducing sugars, with maxima during periods of optimal plant growth (mid- to late September). Increased temperature promoted periodic increases in amino acids and soluble carbohydrates, but decreased accumulation of α-cellulose by the end of the experiment. Moreover, increases in water-column nitrate led to periodic increases in tissue ammonium and amino acids, as well as decreases in non-reducing sugars. Toward the end of the experiment, increases in soluble carbohydrates for plants grown under higher temperatures may have been associated with an extension of the growing season. In contrast, decreased non-reducing sugars in nitrate-enriched plants may have resulted from an increased carbon demand during nitrate assimilation/reduction, as well as a reallocation of carbon to enhance amino acid synthesis.