Macronutrient and B vitamin dynamics of Chowan River (North Carolina, U.S.A.) CyanoHABs

The Chowan River (CR)-Albemarle Sound Estuary, NC has been plagued by a resurgence of
toxigenic cyanobacterial harmful algal blooms (CyanoHABs) since 2015. Both N2-fixing and
Non-N2-fixing cyanobacteria blooms have occurred in the CR blooms and, in summer 2020,
potentially harmful N2-fixing Dolichospermum spp. was dominant. To identify the nutritional
drivers of the summer 2020 CyanoHAB, we conducted nitrogen (N), phosphorus (P), and B
vitamin addition experiments and measured N2-fixation rates using acetylene reduction assays.
Cyanobacteria are generally thought to not require external B vitamins, though the opportunistic
use of external vitamins may increase metabolism directly or through stimulation of members
of the cyanobacterial ‘microbiome’. The CR phytoplankton community exhibited N-limitation
with a secondary vitamin limitation in NH4 addition treatments. B vitamin additions influenced
the community composition of the CR CyanoHABs by increasing chlorophyte biomass and
reducing cyanobacterial biomass. Furthermore, given the high N2-fixation rates (240 nmol N
L-1 h-1), further research is required to evaluate the budgetary importance of N2-fixation relative
to external N inputs and determine the effectiveness of reducing external N loads in reducing
the severity of the blooms. The results demonstrate the importance of N in sup-porting CR
CyanoHABs and the need to better understand multiple nutritional drivers of the recurring
blooms.