NITROGEN VERSUS PHOSPHORUS LIMITATION FOR GROWTH OF AN ESTUARINE POPULATION OF EELGRASS (ZOSTERA-MARINA L)

The relative importance of nitrogen (N) and phosphorus (P) limitation for growth and biomass accumulation in an estuarine population of eelgrass (Zostera marina L.) was examined by in situ additions of nitrogen (+N), phosphorus (+P) and nitrogen plus phosphorus (+N+P) to sediments at low and high loading rates. Nitrogen treatments resulted in no significant increases in leaf tissue N levels and only a small increase in the N content of root plus rhizome tissues. Phosphorus concentrations were, however, significantly (P < 0.05) higher in both leaf and root plus rhizome tissues with +P and +N+P enrichment. Eelgrass growth and biomass exhibited statistically significant (P < 0.05) increases in response to high +P. Similar increases in mean plant growth and biomass were observed with +N and +N+P enrichment, but large variabilities rendered these responses nonsignificant. Our results are in contrast with those reported for a previous sediment fertilization (+N+P) study at the same site and we attribute this difference to a change in the nutrient status of the study area. Comparison with other sediment fertilization experiments for both freshwater and marine plant species revealed a clear relationship between relative plant growth rates and tissue nutrient concentrations for both N and P enrichment. This relationship suggests a uniformity of submersed plant nutrition, wherein responses to changes in nutrient availability are regulated by alterations in both growth rates and tissue nutrient content.