Subtropical zooplankton assemblage promotes the harmful cyanobacterium Cylindrospermopsis raciborskii in a mesocosm experiment

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Harmful algal blooms (HABs) with public health impacts threaten freshwater ecosystems, including drinking water reservoirs, globally. Subtropical systems are often dominated by filamentous and colonial cyanobacteria, algae that are potentially less accessible for consumption by resident meso-zooplankton grazers. Less understood than selective grazing is the role of zooplankton in regenerating nutrients and facilitating growth of algae with efficient uptake strategies, such as the toxin-producing cyanobacterium, Cylindrospermopsis raciborskii. Using ∼800-L bags suspended in the upper 3 m of the water column, we examined the growth of C. raciborskii under four treatments: 3× ambient zooplankton biomass, 10× zooplankton, 10× zooplankton plus inorganic P addition and a no amendment control (3Z, 10Z, 10ZP, control, respectively). After 4 days, C. raciborskii relative abundance doubled in the 10Z and 10ZP treatments compared with the control and 3Z treatments, and after 7 days P addition resulted in ∼20% higher relative C. raciborskii biomass compared with other treatments, and an order of magnitude increase in N-fixing phytoplankton. The particulate C : P ratio declined in the 10Z and 10ZP mesocosms, indicating that meso-zooplankton facilitated P transfer to algae. Overall, the copepod dominated subtropical meso-zooplankton assemblage promoted C. raciborskii abundance and biomass over the short-term, demonstrating their facilitation of subtropical freshwater HAB formation.

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