Identity and regulation of nutrients limiting phytoplankton production in the shallow estuaries near Beaufort, N.C.

Inorganic nutrients limiting phytoplankton production in the shallow estuarine system near Beaufort, N. C., were identified by nutrient enrichment techniques. Nitrogen was the primary limiting nutrient; phosphorus also was limiting at times.Samples receiving a complete enrichment medium plus organic substrates poor or lacking in nitrogen and phosphorus showed no significant increase in relative photosynthesis over unenriched controls, even though there was a significant decrease in the nitrogen and phosphorus concentrations in the enriched samples. This suggested that microbial immobilization of nitrogen and phosphorus during decomposition of organic matter may limit nutrient availability to phytoplankton and in part account for the general paucity of inorganic nutrients present in this shallow system. An estimated 6 x 109 g carbon, 1 x 108 g nitrogen and 2 x 107 g phosphorus enters the estuarine system in the form of partially decayedSpartina alterniflora each year, primarily from September-March. To utilize the readily available fraction of this detrital material estuarine microbes would have to immobilize 2.4 x 107 g nitrogen and 1.6 x 106 g phosphorus. The data also suggested that an annual cycle in nutrient concentration in the estuarine system in part may result from shifts in the equilibrium between microbial immobilization and remineralization.