Nitrogen versus phosphorus limitation for growth of an estuarine population of eelgrass ( Zostera marina L.)

Abstract

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 non-significant. 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 plants 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 mersed plant nutrition, wherein responses to changes in nutrient availability are regulated by alterations in both growth rates and tissue nutrient content.