Seasonal nitrogen metabolism in an intertidal population ofGelidium latifolium(Gelidiaceae, Rhodophyta)

The nitrogen status of Gelidium latifolium on the north coast of Spain was monitored for a 2 year period from 1990 to 1991. Each summer, the population showed obvious symptoms of nitrogen (N) limitation, such as thallus ‘bleaching’. Minimum N content (as % of dry weight), highest C/N ratio (above 45) and lowest protein (< 10% dry weight) and phycobilin contents were observed during summer, when nitrate availability in seawater was lowest, and temperature and light levels were highest. However, tissue N content did not respond to a short local upwelling period that increased nitrate levels in seawater more than ten-fold. The effect of seawater N concentration and increased light levels on pigment content was established experimentally. N-starved plants placed in N-enriched seawater during summer 1990 showed increased levels of phycobilin and chlorophyll a compared with an unenriched control. Screens which transmitted different proportions of natural light placed over a Gelidium latifolium population during summer 1991 resulted in a smaller reduction in both phycobilin and chlorophyll a contents. We conclude that N limitation in this population occurs as a response to increased irradiance and decreased seawater nitrate, typical of summer conditions.