Balance of carbon species combined with stable isotope ratios show critical switch towards bicarbonate uptake during cyanobacteria blooms.

Next to water quality deterioration, cyanobacteria blooms can affect turnover of aqueous carbon, including dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC). We investigated interactions of these three phases and their stable isotopes in a freshwater pond with periodic cyanobacterial blooms over a period of 23 months. This helped to map turnover and sources of aqueous carbon before, during, and after bloom events. During bloom events POC isotope values (δC) increased up to -17.4‰, after aqueous CO (CO) fell below an atmospheric equilibration value of 412 μatm. Additionally, carbon isotope enrichment between CO and POC (ε) ranged from 2.0 to 21.5‰ with lowest fractionations observed at pH values above 8.9. The increase of δC and decrease of ε values at low pCO and high pH was most likely caused by the activation of the carbon concentrating mechanism (CCM). This mechanism correlated with prevalent assimilation of C-enriched HCO. Surprisingly, CO still contributed more than 50% to the POC pool down to pCO values of around 150 μatm. Only after this threshold the reduced ε suggested incorporation of C-enriched HCO.