Phytoplankton N2-fixation efficiency and its effect on harmful algal blooms

Toxin production during harmful algal blooms (HABs) depends on N availability. However, the role of N-2 fixation as a mechanism to fuel 'new' N into HABs and increase their toxicity has not been well studied. We quantified the effects of N : P supply ratios on N-2-fixation efficiency in HABs from 3 warm-temperate man-made reservoirs. We enriched mesocosms with the same concentration of P (112 mu g P/L) but differing amounts of N (50-2500 mu g N/L) labeled with a N-15 tracer to simulate HABs growing along a large molar N : P gradient (1-50). N-2 fixation increased significantly at low N : P but generally did not alleviate N limitation and lead to accumulating N and phytoplankton biomass efficiently unless the magnitude of stoichiometric imbalance was low. Furthermore, microcystin concentrations >1.0 mu g/L occurred only in mesocosms receiving N : P = 50 supply and only in the reservoir with detectable concentrations of microcystin at the beginning of the experiment. These results suggest that HABs in P-rich reservoirs may yield significantly more biomass and potentially become more toxic when reactive N is plentiful in the water column relative to P. Thus, reducing N concentrations can be useful as a supplement to the primary P reduction strategies used to minimize the harmful effects of algal blooms.