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Resumen:
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We hypothesised that nitrate, rather than phosphorus, is the primary nutrient limiting cyanobacterial bloom development in a large subtropical river. To test this, we analysed a five-year dataset from the Negro River (Uruguay) that included physical and chemical variables and phytoplankton community composition across multiple sites and seasons. Our analysis revealed that blooms consistently occurred during periods of low-nitrate availability combined with warm water temperatures. Under these conditions, Microcystis spp. became dominant, coinciding with reduced phytoplankton diversity and evenness, while Dolichospermum spp. dominated in non-bloom periods. This seasonal shift suggests that nitrogen availability is a key factor structuring phytoplankton succession. The strong negative association between nitrate concentration and cyanobacterial biomass supports nitrate limitation as the principal regulatory factor in this system, challenging the traditional phosphorus-centric paradigm. Based on these findings, we applied the Low-Nitrate-Days (LND) indicator to our dataset, demonstrating its usefulness as a practical tool for anticipating bloom onset in phosphorus-rich freshwater systems. This study provides one of the few long-term records of nitrate limitation in a large subtropical river, with direct implications for monitoring, forecasting, and nutrient management in warm, nutrient-rich freshwater ecosystems.
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