Nitrogen-fixing cyanobacterial bloom potential in estuaries

Cyanobacterial blooms are common in freshwaters throughout the world, but relatively rare in estuaries. This study investigated several factors that may control blooms in estuaries, including phosphorus availability, shear stress, and salinity. This study focused on environmental factors controlling growth, productivity, and nitrogen fixation of Nodularia, the dominant bloom-forming, toxin-producing cyanobacterium in the Baltic Sea.; Analysis of monitoring data from the Neuse River Estuary (NRE), North Carolina, suggested that the chemical and physical characteristics (temperature, salinity, ratio of dissolved nitrogen to phosphorus, phosphorus concentration) in the system are suitable for growth of salinity tolerant nitrogen-fixing cyanobacteria, such as Nodularia. Culture experiments showed that the NRE water was suitable as a growth medium for Nodularia if external phosphorus was provided. Increases in phosphorus availability in the NRE could increase cyanobacterial bloom potentials in the system.; Small-scale shear had a negative effect on Nodularia nitrogen fixation and photosynthetic activities in cultures. CO₂ fixation in natural cyanobacterial populations in the Baltic Sea also showed sensitivity to small-scale shear. In the field populations, filament lengths of Anabaena and Aphanizomenon were reduced in response to shear. Negative shear effects were seen for shear rates that corresponded to turbulence generated by moderate to strong wind speeds in the Baltic Sea upper mixed layer. Small-scale shear may regulate cyanobacterial species composition, and inhibit cyanobacterial growth in tidally or otherwise well-mixed estuaries.; Photosynthesis and nitrogen fixation in estuarine cyanobacteria continued at estuarine salinities. Isolates of Anabaena aphanizomenoides and Anabaenopsis from the NRE grew at salinities of up to 15 g NaCl l−1 and 20 g NaCl l−1 or higher, respectively. Both the NRE isolates and two Baltic Sea Nodularia strains increased their maximum photosynthetic rates in response to elevated salinity. This indicates that these organisms are adapted to grow in fluctuating salinities in estuaries, and suggests that elevated salinity (NaCl) representative of estuaries does not inhibit nitrogen-fixing cyanobacterial blooms. Cylindrospermopsis raciborskii isolates tolerated only low salinities (up to 4 g NaCl l−1). Therefore, for this nuisance cyanobacterial species, salinity appears to be a barrier for bloom proliferation in estuaries.