Controlling cyanobacterial blooms in hypertrophic Lake Taihu, China: will nitrogen reductions cause replacement of non-N2 fixing by N2 fixing taxa?

Abstract

Excessive anthropogenic nitrogen (N) and phosphorus (P) inputs have caused an alarming increase in harmful cyanobacterial blooms, threatening sustainability of lakes and reservoirs worldwide. Hypertrophic Lake Taihu, China's third largest freshwater lake, typifies this predicament, with toxic blooms of the non-N2 fixing cyanobacteria Microcystis spp. dominating from spring through fall. Previous studies indicate N and P reductions are needed to reduce bloom magnitude and duration. However, N reductions may encourage replacement of non-N2 fixing with N2 fixing cyanobacteria. This potentially counterproductive scenario was evaluated using replicate, large (1000 L), in-lake mesocosms during summer bloom periods. N+P additions led to maximum phytoplankton production. Phosphorus enrichment, which promoted N limitation, resulted in increases in N2 fixing taxa (Anabaena spp.), but it did not lead to significant replacement of non-N2 fixing with N2 fixing cyanobacteria, and N2 fixation rates remained ecologically insignificant. Furthermore, P enrichment failed to increase phytoplankton production relative to controls, indicating that N was the most limiting nutrient throughout this period. We propose that Microcystis spp. and other non-N2 fixing genera can maintain dominance in this shallow, highly turbid, nutrient-enriched lake by outcompeting N2 fixing taxa for existing sources of N and P stored and cycled in the lake. To bring Taihu and other hypertrophic systems below the bloom threshold, both N and P reductions will be needed until the legacy of high N and P loading and sediment nutrient storage in these systems is depleted. At that point, a more exclusive focus on P reductions may be feasible.

Figure 1. Map of Lake Taihu showing major tributaries and nearby cities.

The location of Taihu in China is shown on the inserted map.

Figure 2. Photographs of mesocosm array, located on the shore of Taihu near the Taihu Laboratory for Lake Ecosystem Research (TLLER), at Wuxi, China.

Figure 3. Time series of dissolved and total nitrogen and phosphorous forms and phytoplankton biomass as chlorophyll a from the summer 2013 mesocosm experiment.

Solid lines connect means of triplicate mesocosms. Error bars are one standard deviation.

Figure 4. Time series of biomass of the dominant algal classes from the summer 2013 mesocosm experiment.

Solid lines connect means of triplicate mesocosm tanks. Error bars are one standard deviation.

Figure 5. Pie graphs depicting proportion of total chlorophyll a comprised by each of the dominant four algal classes at the initial sampling of the mesocosm experiments (average of all samples collected) and final sampling (average of triplicate mesocosms) for each treatment and the ambient lake water.

Figure 6. Time series of cell abundances for Microcystis spp. and Anabaena spp., the dominant non-nitrogen fixing and nitrogen fixing genera during the summer 2013 mesocosm experiment.

Solid lines connect means of log₁₀ values of triplicate mesocosm tanks. Error bars are one standard deviation.

Figure 7. Representative microscopic observations (X200, brightfield) of dominant cyanobacterial genera in the mesocosm experiment conducted during 2013.

Samples were collected towards at the end of the experiment, 31 July 2013. (A) Control (no nutrient addition). (B) N-only addition. (C) P-only addition. (D) N+P addition. Note the overall dominance by Microcystis spp. colonies. However, an increase in number of filaments of the N₂ fixing genus Anabaena was observed in the phosphorus and phosphorus and nitrogen treatments. No significant N₂ fixation was observed in either of these treatments however. Biomass-wise, Microcystis spp. clearly dominated the cyanobacterial community in all treatments.

Figure 8. Time series of light and dark acetylene reduction rates from the summer 2013 mesocosm experiment.

Solid lines connect means of triplicate mesocosms. Error bars are one standard deviation.