Seasonal variation in the concentrations of nutrients in two green macroalgae and nutrient levels in sediments in the Rías Baixas (NW Spain)

Seasonal monitoring of the levels of carbon, nitrogen and phosphorus in two green macroalgae (Ulva and Enteromorpha) was carried out at 22 sampling sites in four embayments (rı´as) on the northwest coast of Spain. Sediments were also analysed to determine the concentrations of these elements as well as organic matter, organic carbon, iron and texture. In addition, accumulations of Ulva were monitored. Nitrogen levels in algae were similar to those found in comparable studies, whereas carbon concentrations were generally higher and those of phosphorus were lower. As a result the macroalgae were relatively enriched by carbon and nitrogen compared with phosphorus. Seasonal variations in algal tissue nitrogen and phosphorus followed the usual pattern for temperate zones, with minimum levels in summer and maximum levels in winter. Variations in carbon concentrations were much less accentuated. The low levels of P in the algae appear to indicate limitation by this element, however, comparison of the monthly changes in nitrogen and phosphorus in Ulva with the critical concentrations of these elements suggests dual nutrient limitation in this alga in summer. By contrast, the sediments were more enriched by phosphorus; high retention of this element by the sediment may explain the low levels in algae. The highest accumulations of Ulva occurred in spring and summer; hydrodynamic factors appeared to be important in determining the extent of these accumulations