Experimental additions of aluminum sulfateand ammonium nitrate to in situ mesocosms toreduce cyanobacterial biovolume and microcystinconcentration

Recent studies suggest that nitrogen additions to increase the total nitrogen:total phosphorus (TN:TP) ratio may
reduce cyanobacterial biovolume and microcystin concentration in reservoirs. In systems where TP is >100 μg/L,
however, nitrogen additions to increase the TN:TP ratio could cause ammonia, nitrate, or nitrite toxicity to terrestrial
and aquatic organisms. Reducing phosphorus via aluminum sulfate (alum) may be needed prior to nitrogen additions
aimed at increasing the TN:TP ratio.We experimentally tested this sequential management approach in large in situ
mesocosms (70.7 m3) to examine effects on cyanobacteria and microcystin concentration. Because alum removes
nutrients and most seston from the water column, alum treatment reduced both TN and TP, leaving post-treatment
TN:TP ratios similar to pre-treatment ratios. Cyanobacterial biovolume was reduced after alum addition, but the
percent composition (i.e., relative) cyanobacterial abundance remained unchanged. A single ammonium nitrate
(nitrogen) addition increased the TN:TP ratio 7-fold. After the TN:TP ratio was >50 (by weight), cyanobacterial
biovolume and abundance were reduced, and chrysophyte and cryptophyte biovolume and abundance increased
compared to the alum treatment. Microcystin was not detectable until the TN:TP ratio was <50. Although both
treatments reduced cyanobacteria, only the nitrogen treatment seemed to stimulate energy flow from primary
producers to zooplankton, which suggests that combining alum and nitrogen treatments may be a viable in-lake
management strategy to reduce cyanobacteria and possibly microcystin concentrations in high-phosphorus systems.
Additional studies are needed to define best management practices before combined alum and nitrogen additions are
implemented as a reservoir management strategy.