Abstract
Due to low salinity and excessive nutrient inputs, oligohaline estuaries are often subject to cyanobacterial blooms that commonly occur in eutrophic freshwater systems. Albemarle Sound and its tributaries in northeastern North Carolina comprise the largest oligohaline lagoon system in the United States. In recent summers, cyanobacterial blooms dominated by diazotrophic taxa have threatened the ecosystem and its value as an important commercial fishery. We conducted nutrient addition bioassays on natural phytoplankton communities from Albemarle Sound and its tributaries to determine which nutrient(s) limit phytoplankton growth, and measured N 2 fixation to assess the ability of cyanobacteria to fulfill their own nitrogen requirements. Results showed that nitrogen was the most common limiting nutrient for phytoplankton. Nitrogen addition together with phosphorus often stimulated growth more than nitrogen addition alone. Phosphorus limitation was observed once in a tributary of Albemarle Sound. In situ cyanobacterial N 2 fixation approached half of the riverine nitrogen load during summer through fall and was further increased by experimental phosphorus addition. The predominance of nitrogen limitation clearly indicates that nitrogen management should be implemented to control blooms. However, the finding of the importance of N 2 fixation in the Albemarle Sound nitrogen budget and evidence for stimulation of N 2 fixation by phosphorus input suggest that controlling both nitrogen and phosphorus may be more effective than controlling nitrogen alone for Albemarle Sound.