ABSTRACT: The present study assessed the effect of seawater ammonium enrichment on survival and growth of the seagrass Zostera noltii Hornem. Ammonium enrichment had an inhibitory effect on shoot, rhizome and root elongation rates, as well as onprimary production. The inhibitory effect was partially alleviated by phosphate addition. The frequency of ammonium pulses also affected growth negatively, with a maximum effect at both low and high pulse frequencies. Similar results were obtained whenthe experiment was run in the field during a winter trial. However, when the field experiment was repeated in spring at higher mean temperature and irradiance levels, opposite results were obtained, with ammonium enrichment causing a substantial increasein growth rates and productivity. These responses were related to the internal balance of non-structural carbohydrates (especially sucrose) with respect to C and N cell metabolism. Thus, there was a mobilisation of sucrose in both above- and belowgroundtissues to meet C increased demands arising from ammonium assimilation in winter, with sucrose concentrations reaching critical levels in relation to the total internal C pool. In contrast, sucrose accumulated in the tissues when ammonium pulses wereapplied in spring, indicating an enhanced C turnover that was able to meet the increased demands arising from ammonium assimilation into organic N compounds.