Rates of nitrogen uptake by cyanobacterially-dominated assemblages in Lake Taihu, China, during late summer

Lake Taihu has suffered an increasing number of cyanobacteria harmful algal blooms (CyanoHABs) over the past three decades, bringing about formidable ecological and economical losses. Efforts to control phosphate (P) and/or nitrogen (N) have been applied to mitigate these blooms, but there has been much less attention paid to N and its different forms on the ecology of the blooms. Therefore, kinetic and nutrient enrichment experiments were conducted to assess N uptake rates under differing conditions, and to examine effects of changes in N forms (NH4+, NO3− and urea) and P availability on phytoplankton community physiology. In 2014 these experiments involved mesocosm enrichments; in 2015 these experiments were conducted over a diurnal period. Both involved measurements of short-term N uptake. The kinetic results showed that the utilization of NH4+, NO3− and urea by Microcystis-dominated communities was not efficient at low ambient substrate concentrations. Maximum uptake rates by these phytoplankton was achieved on NH4+ and these rates were significantly higher than those on NO3− or urea with or without nutrient pretreatment. Moreover in the presence of PO43− enrichment, the maximal uptake velocity of NH4+ substantially increased without evidence of saturation. High amounts of NH4+ may have inhibited or repressed the uptake of NO3− at certain times in these studies. In the diurnal study, dissolved inorganic carbon and pH changed substantially throughout the day. The resulting high pH altered N and P in ways that may help to sustain nutrient cycling for the blooms.