Journal
FRESHWATER SCIENCE
Volume 37, Issue 2, Pages 264-275Publisher
UNIV CHICAGO PRESS
DOI: 10.1086/697530
Keywords
nitrogen fixation; nutrient limitation; supply ratios; cyanobacteria; HABs
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Funding
- National Science Foundation [DBI 1359188]
- Bella Vista Property Owners Association
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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.
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